THE MANUFACTURE OF
EARTH COLOURS
• THE
MANUFACTURE OF
EARTH COLOURS
BY
DR. JOSEF BERSCH
TRANSLATED FROM THE THIRD GERMAN EDITION (AS
REVISED BY PROF. DR. WILHELM BERSCH)
BY
CHARLES SALTER
WITH THIRTT-ONE ILLUSTRATIONS
3o, to- 2
LONDON
SCOTT, GREENWOOD & SON
8 BROADWAY, LUDGATE, E.C. 4.
1921
[The sole rights of translation in English remain with Scott, Green-wood £? Sen.\
PRINTED IN GREAT BRITAIN BY
RICHARD CLAY & SONS, LIMITED,
PARIS GARDEN, STAMFORD ST., S.E. I,
AND BUNGAY, SUFFOLK.
PREFACE
ORIGINALLY issued as a volume of the series on
pigments and colouring matters by the present author's
father, the necessity for a new edition afforded a welcome
opportunity of revising "Earth Cofours." Although,
in the nature of things, little progress has been made
in this subject itself, there was a good deal to add in
connection with the mechanical appliances for treating
the colour earths and manufacturing them into pigments.
In other respects, too, the work has been carefully
gone through and brought up to date, with new and
additional illustrations.
The author desires to express his thanks to the
various firms who have afforded him assistance in his
task by furnishing illustrations and descriptions of new
machinery, together with other information. It is hoped
that this third edition will meet the approval of those
interested in the subject ; and the author will be glad to
receive supplementary information to render the work
more complete in the event of a future edition being
found advisable.
PROF. DR. WILHELM BERSCH.
1918.
CONTENTS
CHAPTER I
PAGE
INTRODUCTORY ....... i
CHAPTER II
THE RAW MATERIALS FOR EARTH COLOURS . . 8
(A) White Raw Materials and Pigmentary Earths . 1 1
Limestone (Calcite, Limestone, Chalk) . . 1 1
Gypsum (Alabaster) . . . . .18
Barytes, or Heavy Spar . . . 19
Talc, Soapstone, Steatite . 20
Clay . . . . . .21
(B) Yellow Earths . . . . . .23
Brown Ironstone . . . . ,23
Ochre ....... 25
Yellow Earth 26
Terra di Siena . . ... . .27
(C) The Red Earths 27
Red Ironstone . . . . . .28
Bole 31
Alum Sludge . . .. . . .32
Mine Sludge ...... 32
(D) Blue Earths 33
Azurite, or Ultramarine . . . , -33
Vivianite ....... 33
vii
viii CONTENTS
PAGE
THE RAW MATERIALS FOR EARTH COLOURS (continued]
(E) Green Earth Pigments .... 34
Green Earth ...... 34
Malachite . . . . . -35
(F) Brown Earth Pigments .... 36
Umber ....... 36
Asphaltum . . . . . -37
(G) Black Earth 38
Black Schist ... 38
Graphite ....... 38
CHAPTER III
THE PREPARATION OF THE COLOUR EARTHS . . 40
Crushing Machinery . . . . . . 43
Crushing and Sifting . . . . . -77
Calcining . . . . . . .81
Mixing and Improving . . . . .81
Moulding ....... 85
CHAPTER IV
WHITE EARTH COLOURS . . . . .87
Caustic Lime ....... 87
Pearl White ,94
Vienna White ....... 95
Chalk 98
Precipitated Chalk . . . . . . 107
Calcareous Marl . . . . . .no
Gypsum . . . . . . . .in
Kaolin, Pipeclay . . . . . .112
Bary tes, or Heavy Spar . . . . .119
Carbonate of Magnesia . . . . .123
Talc . . . . . . , .124
Steatite or Soapstone . . . . .125
CONTENTS ix
CHAPTER V
PAGE
YELLOW EARTH COLOURS . . . . .127
The Ochres 128
Calcining (Burning) Ochre .... 132
Ochres from Various Deposits . . . .136
Artificial Ochres . . . . . .138
Ochres as By-products . . . . .146
CHAPTER VI
RED EARTH COLOURS . . . . . -151
Bole 152
Native Ferric Oxide as a Pigment . . 154
Iron Glance ....... 154
Hematite . . . . ... 155
Raddle . . . . . . . .155
Burnt Ferric Oxide and Ochres . . . .158
(a) Burning in the Muffle . . . .158
(b) Caput Mortuum, Colcothar . . .160
(c) Calcining Ferric Oxide . . . .161
Ferric Oxide Pigments from Alum Sludge . .164
CHAPTER VII
BROWN EARTH COLOURS . ... . .168
Terra di Siena 168
True Umber . . . . . . .170
Cologne Earth (Cologne Umber) . . .173
Asphaltum Brown (Bitumen) . . . .174
CHAPTER VIII
GREEN EARTH COLOURS . . . . .176
Green Earth, or Celadon Green . . .176
Artificial Green Earth (Green Ochre) . . .180
Malachite Green . 181
x CONTENTS
CHAPTER IX
PAGE
BLUE EARTH COLOURS . . . . . .183
Malachite Blue (Lazulite) 183
Vivianite or Blue Ochre . . . . .184
CHAPTER X
BLACK EARTH COLOURS. . . . . .185
Graphite . . . . . . .185
Black Chalk 194
CHAPTER XI
THE COMMERCIAL NOMENCLATURE OF THE EARTH
COLOURS . . . . . . -197
White Earth Colours 198
Yellow Earth Colours . . . . .200
Red Earth Colours . . . . . .200
Brown Earth Colours ..... 200
Green Earth Colours ..... 201
Blue Earth Colours ...... 202
Grey Earth Colours . . . . . . 202
Black Earth Colours . . . . .202
INDEX . . . . . . . , 203
EARTH COLOURS
CHAPTER I
INTRODUCTORY
BOTH from the chemical and practical standpoint
it is necessary to divide pigments into clearly denned
groups, the following classification being adopted on
the basis and natural history of the substances con-
cerned : —
(i) Pigments occurring native in a finished con-
dition, and only requiring mechanical preparation to
fit them for use as painters' colours. (2) Pigments
which are not ready formed in Nature, but contain
some metallic compound as pigmentary material,
which requires certain chemical treatment for its full
development. (3) Pigments which, in contrast to
these two groups, contain only organic, and no in-
organic, constituents. This last class comprises all
the natural vegetable pigments, together with the
large group of colours obtained artificially from tar
products, fresh groups of which are being continually
introduced. Nowadays, there is no longer any strict
line of demarcation between the natural and artificial
organic colouring matters, it being possible to produce
even those of the vegetable series, such as madder
and indigo, by artificial means.
i
2 EARTH COLOURS
Whilst this group of colours exhibits the greatest
variety, and is constantly being enriched and increased
by the progress of colour chemistry, the case is different
with the first group, the natural earth pigments. Here
we have chiefly to do with the preparation of materials
occurring in Nature, or with bringing about certain
chemical results, so that, consequently, the range of
variety is far more restricted, and there is little or no
possibility of increasing the number of these colours
by the manufacture of really new products. The
earth colours nevertheless have a high technical and
economic importance, on account of their extremely
valuable properties, coupled, for the most part, with
low cost.
If the term " earth colours " were, strictly adhered
to, the present work would have to be confined to a
description of the physical and chemical properties
of the various pigments, and of the various means by
which they can be brought into suitable condition
for use in paints.
However, of late, the term has found wider applica-
tion than formerly, since it has been found practicable
to modify (shade) certain of the earth colours by simple
operations, and thus considerably increase the range
of tones of the substances known as earth colours.
The progress of chemical industry has also largely
increased the number of the so-called earth colours,
certain methods of chemical treatment having enabled
substances that are of little use for other purposes,
to be employed, in large quantities, as pigments. The
application of these — usually cheap — by-products is
still further facilitated by the fact that they can be
transformed, by a simple chemical treatment, into
pigments which are distinguished by their beauty of
INTRODUCTORY 3
colour and at the same time possess the great advantages
of durability and cheapness.
As an example of this, mention may be made of
iron oxide, which occurs in Nature in the form of various
minerals which can be made into pigments by mechanical
treatment. In many cases, this treatment has already
been carried out by Nature, and deposits of iron oxide
are found in which the material has only to be incor-
porated with a vehicle to make it fit for immediate
use as a painters' colour.
Moreover, the same oxide is obtained, in large quan-
tities, as a by-product of the treatment of other minerals.
From the point of view of chemical composition, this
by-product is of very low value, by reason of the large
supplies of native oxide available. By means of a
very simple chemical treatment, however, this by-
product oxide can be considerably improved in com-
mercial value, being, in many cases, convertible, by
merely heating it to certain temperatures, into a variety
of colours which sell at remunerative prices.
Consequently, in view of the present condition of
the chemical industry, the term " earth colours "
can be enlarged to include a number of waste products
which fetch good prices as colours, though otherwise
practically valueless in themselves.
The number of earth pigments is very large, and
comprises representatives of all the principal colours.
For painting purposes, few pigments beyond the earth
colours were known to the ancients ; and most of the
colours in the paintings which have come down to us
from antiquity are pure earth pigments, thus affording
proof of their great durability, having retained their
freshness unimpaired for hundreds — and some for
thousands — of years.
4 EARTH COLOURS
The earth colours might be divided into such as
occur ready-formed in " Nature, and require only
mechanical preparation, and which either require
special treatment (e. g. calcining), or are artificial
products (like the iron oxide mentioned above). Since,
however, such a classification would not advantage
our knowledge of the nature of this class of colours,
it appears useless and superfluous, and we will therefore
simply confine ourselves to arranging the earth pig-
ments according to their colour — white, yellow, red, etc.
Adopting this classification, the following minerals
and chemical products may be considered as earth
colours :—
White. — These include the varieties of calcium
carbonate, such as chalk, marble, precipitated chalk,
calcium phosphate, calcium sulphate (in the form of
gypsum, alabaster, muriacite and the precipitated
gypsum produced as a by-product in many chemical
works), heavy spar, the different varieties of clay, and
magnesia.
Yellow. — This group comprises ferric hydroxide
(hydrated oxide of iron) in the form of the various
minerals known as ochre ; all the preparations chiefly
composed of this hydroxide, and all those prepared
by artificial means. A very important member of this
group is orpiment ; the other arsenical compounds
frequently met with native, being however, on account
of their poisonous properties, no longer used as pigments.
Red. — Chief among the red earth colours are those
consisting of ferric oxide (iron oxide), under various
names. The only other member of the group is the
far rarer vermilion.
Blue. — The blue earth colours are few in number
and of no particular beauty ; but they are of importance
INTRODUCTORY 5
on account of their cheapness and because all the
artificial blue pigments are rather expensive. Two
products in particular merit attention in this con-
nection, namely, ultramarine, and the mineral known
as blue ochre or blue ironstone. The latter, as a matter
of fact, cannot be used for anything else than a painters'
colour, and can be obtained at a low price; whereas
ultramarine also forms a valuable raw material for
the recovery of copper, and is therefore dearer.
Green. — This group, again, contains only two mem-
bers, viz. malachite green (chrysocolla), and the green
earths (seladonite), known as Verona, etc. , green. These
occur fairly often in Nature, and the green earths in
particular find a wide industrial application by reason
of their low price. Malachite green is very similar,
in chemical constitution, to ultramarine ; and both
form sources of copper and are consequently expensive.
It should be mentioned that both ultramarine and
malachite green can only be profitably made into
pigments where the minerals can be obtained cheaply,
since both of them can be manufactured where arti-
ficial pigments are produced, and are put on the market
under the same names as the native articles. The
very low price of the green earths makes them highly
popular as colouring matters in certain branches of
industry, and they are very largely used by wall-paper
manufacturers.
Brown. — This is a large group, and the pigments
composing it are specially distinguished for their beauty
and depth of colour, on which account they are used
in the finest paintings. Here, again, it is ferric oxide,
in combination with water — and therefore ferric
hydroxide — that furnishes a large number of the mem-
bers of the group. Like the renowned Siena earth,
6 EARTH COLOURS
the artists' colours known as Vandyck brown, bole,
Lemnos earth, umber, etc., mainly consist of more
or less pure ferric hydroxide. These minerals are,
moreover, specially important to the colour manu-
facturer, inasmuch as most of them enable a large
number of different shades to be obtained by a simple
method of treatment consisting merely of the applica-
tion of heat in a suitable manner; and these colours
are among the most excellent we possess, by reason
of their beauty and permanence. Amongst this series
must also be classed native manganese brown, which
chiefly consists of a mixture of manganese oxide and
the hydrated peroxide of the same metal.
Black. — There is really only one member of this
class, which, however, is frequently used, viz. that
form of carbon occurring as hexagonal crystals and
known as graphite. Another natural black natural
product, occasionally used as a painters' colour is
the so-called black chalk. However, since black
pigments can be produced very cheaply by artificial
means, the natural colours find only a limited applica-
tion ; and only in one instance is graphite used alone,
viz. for making blacklead pencils.
As already mentioned, certain chemical industries
furnish by-products which are of very little value
in themselves, and many of them, indeed, may be
classed as worthless, since chemical manufacturers
naturally endeavour to get everything possible out
of their materials in the course of manufacture.
Some of these by-products, however, can advantage-
ously be used as pigments, a good example of this
being afforded by the iron oxide formed as a by-product
in the manufacture of fuming sulphuric acid (Nord-
hausen oil of vitriol), by the old process, from green
INTRODUCTORY 7
vitriol (ferrous sulphate). In itself, this oxide is
practically valueless, but, by very simple treatment,
it can be converted into very valuable pigments which
have a market value far in excess of the original material.
Although it has hitherto been the custom to confine
the term earth colours to such as occur ready-formed
in Nature and only require simple mechanical treatment
to make them ready for immediate use as pigments,
the author is nevertheless of opinion that a book dealing
exhaustively with earth colours should also make some
mention of all the mineral colouring matters which
can be easily made into pigments by simple processes,
such as calcination or bringing into association with
other substances. In accordance with this view, the
present work will describe all the pigments that are
obtainable in this manner. Most of the earth colours
consist of decomposition products of certain minerals ;
and this applies particularly to such of them as contain
iron oxide. According as the decomposition of the
original mineral has been more or less extensive, the
natural product exhibits different properties; and
the manufacturer must consequently endeavour to
treat them in such a manner as to ensure that the pig-
ment obtained will be as uniform as possible in shade
and permanence. In order to accomplish this it is
essential to have an accurate knowledge of the origin
of the raw material under treatment, and of its chemical
and physical properties. In view of this, the author
considers it necessary to deal more fully with the pig-
mentary earths forming the raw materials of the earth
colours, before passing on to the preparation of the
colours themselves.
CHAPTER II
THE RAW MATERIALS FOR EARTH COLOURS
THE minerals constituting the raw materials for the
preparation of the earth colours occur under very
divergent conditions in Nature. Some of them, such
as chalk, form immense deposits, even whole mountains,
whilst in other cases, e. g. the blue ferruginous earths,
the occurrence is connected with certain local con-
ditions, and many are found only in isolated deposits,
as pockets or beds. This last is the case, for instance,
with the handsome brown iron pigments ; and indeed
the names by which they are known indicate that they
are only found in well-defined localities, or that they
are met with of special quality there. The brown
earth colour known to all painters as Terra di Siena,
is found at many other places as well as near Siena,
but the product from that city acquired aforetime
a special reputation for beauty, and therefore all
similar earths, provided they are equal to that from
Siena, also bear the same name in commerce.
A number of raw materials for the preparation of
earth colours are found, it is true, in many deposits,
but their utilisation depends, in turn, on local con-
ditions. For example, many copper mines contain,
in addition to the other cupriferous minerals, those
used, in the powdered state, as ultramarine or ultra-
marine green, and not infrequently lumps of mineral
RAW MATERIALS FOR EARTH COLOURS 9
are found containing both blue and green together.
However, it is only when these minerals occur in
sufficient quantity to make the necessary sorting
profitable that their manufacture into pigments can
be regarded as practicable.
Before commencing to work a deposit it is essential
to make sure whether the raw material, or pigmentary
earth, is actually suitable for the manufacture of earth
colour. Even the general character of the material
is important, those of soft, earthy consistency being
much easier to treat, and the cost of preparation
smaller, than if the raw material be hard, tough and
crystalline.
The extent and thickness of the deposit, and the
ease with which it can be worked, also play an im-
portant, and even decisive part, since, other conditions
being equal, it will not pay to erect a colour works
unless the raw material is available in sufficient quantity
and is cheap. Generally, the deposit is not homo-
geneous throughout, the mineral being purer in some
places and more contaminated with gangue in others.
The percentage of moisture also varies, and in short,
a number of circumstances must be taken into con-
sideration in forming a conclusion as to whether a
deposit is workable or not.
In order to arrive at a reliable opinion on all these
conditions, sampling is indispensable. If the samples
are of uniform character, they can be mixed together
to make an average sample. But if they differ con-
siderably in appearance, general character, proportion
of gangue, etc., it is preferable to examine them
separately, more especially when the area which each
represents is large.
The examination should extend, on the one hand,
io EARTH COLOURS
to the natural percentage of moisture, and, on the
other, to the purity of the material. The water con-
tent is determined by thoroughly drying a weighed
sample, bearing, however, in mind the fact that
pigmentary earths of a clayey nature vary in water
content according to the time of year, besides changing
in accordance with the weather when the won material
is stored in the open.
The purity can only be ascertained by an examination
in which a sample of the soft, clayey material is crushed
and passed through a narrow-mesh gauze sieve, the
amount of the coarse particles — sand, small stones,
etc. — remaining on the sieve being determined. A
more accurate method, of course, is to separate the
true pigmentary earth from the gangue by levigation.
For this purpose, a weighed quantity of the crushed,
air-dry sample is placed in a relatively narrow glass
vessel and thoroughly mixed with water, the turbid
supernatant liquid being poured off after a short
interval. The residue is repeatedly treated in the
same way, until no more fine particles remain in
suspension, the residue then consisting of impurities,
or gangue. Of course, the washings can be collected,
the suspended matter allowed to settle, and finally
weighed in an air-dry condition. By this means an
approximate idea of the yield of earth colour can be
obtained at the same time.
Raw materials which are not amorphous, soft and
clayey must first be crushed, an operation facilitated
by heating to redness and quenching in cold water.
Oftentimes the heating causes a change of colour
and improves the covering power — a point to which
reference will be made later on.
In the following description of the various raw
RAW MATERIALS FOR EARTH COLOURS n
materials, the chemical composition of the pure
minerals will be given, together with an enumeration
of the most common impurities.
(A) WHITE RAW MATERIALS AND PIGMENTARY
EARTHS
Limestone (Calcite, Limestone, Chalk)
The number of materials furnishing white earth
colours is comparatively large, and these colours are
particularly important, because, not only are they
extensively used by themselves, but they also serve
as adjuncts to other colours and for the production
of special shades. The chief raw material for the
preparation of white earth colours is the mineral
calcite in its numerous modifications.
Calcite, or calc spar, occurs very frequently in
Nature, and is one of the most highly diversified
minerals known. In its purest state it appears as
" double spar " (calcite), in the form of water- white
crystals, which are very remarkable, for certain optical
properties. White marble is also a very pure variety
of calcite, in which the individual crystals are very
small. The various coloured marbles owe their appear-
ance to certain admixtures of extraneous substances,
chiefly metallic oxides.
No sharp line of demarcation separates marble from
ordinary limestone, the difference between them
really consisting only in the degree of fineness of
grain ; and all limestones which grind and polish well
may be classed as marble. As is the case with marble,
there are also limestones of various colours, grey being,
however, the most common. This grey limestone
forms huge mountain masses which, in Europe, follow
12 EARTH COLOURS
for example, the Alpine chain on its northern and
southern edges.
A few other examples of calcite may be mentioned
which occur in certain localities and, in part, are still
in course of formation. To these belong the stalactites
and stalagmites, which sometimes consist of extremely
pure calcite. They are formed by the action of water,
containing carbonic acid in solution, which trickles
through cracks and cavities in limestone rock and
dissolves out calcium carbonate from the adjacent
stone. On prolonged exposure to the air such water
gives off its free carbonic acid again ; and as the calcium
carbonate is insoluble in pure water, it separates out
in crystalline form. The masses formed in this way
usually resemble icicles in shape, and the finest
examples are to be found in the well-known stalactite
grottoes at Krain, whilst the grotto at Adelsberg is
renowned for its beautiful stalactites. Occasionally,
stalactites have an opaque yellow or brownish tinge,
which they owe to the presence of iron oxide.
A formation similar in its origin to stalactites is the
so-called calc sinter and calcareous tuff. The former
often occurs in cavities as irregular masses which, in
some places, enclose large quantities of fossil animal
bones, in which case they form " bone breccia "
(crag breccia). Calcareous tuff is deposited from
numerous springs, occasionally in very large quantities,
enveloping plants and sometimes forming thick deposits
in which the structure of the plants can be clearly
recognised.
In some places a more or less pure white, extremely
friable variety of calcite is met with under the name
"mountain milk" or "mountain chalk" (earthy
calcite), which seems to be a decomposition product,
RAW MATERIALS FOR EARTH COLOURS 13
and consists of a mixture of arragonite and chalk.
Arragonite — which will be referred to later — is com-
pletely identical, chemically, with calcite — both being
composed of calcium carbonate — the sole difference
being their crystalline form.
The most important for the colour-maker, however,
is the variety known as chalk. This is really a fossil
product, i. e. it consists of the microscopic shells of
marine animals united into solid masses. Despite
the smallness of these animals, their epoch lasted long
enough for their shells to form entire mountains which
are encountered all over the world. A large part of
the coast of England, the island of Riigen, and many
other localities, consist entirely of chalk.
In many cases, chalk is found interspersed with
nodular masses of flint, and in some places it also
contains great quantities of the remains of other
marine animals, such as sea urchins, the spines of which
occur in such numbers in certain kinds of chalk as to
unfit them entirely for use as a pigment.
The foregoing varieties of calc spar are the most
important, and also occur in large quantities ; but, to
complete the tale, it is necessary to mention also a few
others which, however, are only found in small amounts.
To these belong, for example, anthracolite, a limestone
stained quite black by coal ; the oolithic limestones or
roe stones, which are composed of granules resembling
fish roe ; muschelkalk, which is also of fossil character
and is almost entirely composed of mussel shells
cemented together with lime ; the marls, which consist
of calc spar mixed with varying quantities of clay and
consequently often bear a great resemblance to loam
in their properties. A few of these varieties find
extensive employment for certain purposes, some
14 EARTH COLOURS
marls for instance being used for making hydraulic
lime, whilst all modifications of calc spar that are
sufficiently pure can be burned for quick lime.
It has already been stated that the mineral arragonite
is. identical, chemically, with calc spar, since both
consist of calcium carbonate, but differ in their crystal-
line habit. Thus, whereas the crystals of calc spar
belong to the rhombohedral or hexagonal system, those
of arragonite are always rhombic. This occurrence of
one and the same substance in two different crystalline
forms is known as dimorphism, and calcium carbonate
is therefore dimorphous. Whether calcium carbonate
assumes the form of calcite or arragonite depends
entirely on physical causes. When the deposition of
the carbonate takes place from a cold solution the
shape of the crystals is always one belonging to the
hexagonal or rhombohedral system; but when it
is from hot solution, rhombic crystals are invariably
formed, calc spar resulting in the former case and
arragonite in the latter.
These different methods of formation which can be
carried out in the laboratory by producing the re-
quisite conditions, occur on the large scale in many
parts of the world. Wherever a hot spring comes to
the surface, containing considerable amounts of lime
in solution, this separates out in the form of arragonite,
which received its name from the circumstance that
specially handsome crystals of this mineral are found
in Arragon.
One of the best-known places where the formation
of arragonite can be observed at the present time is
Carlsbad in Bohemia. The hot springs there deposit
a very large amount of lime, which is stained more
or less yellow or red by the presence of varying quan-
RAW MATERIALS FOR EARTH COLOURS 15
titles of iron oxide, and, under the name of" sprudel-
stein " is used for producing various works of art.
When the hot springs bring up particles of sand, the
lime substance incrusts these sand grains, forming
globular masses resembling peas, and consequently
named pisolite.
In chemical composition, calcite and arragonite
consist of a combination of calcium oxide (lime) and
carbonic acid, the formula being expressed by CaCO3.
Calcium carbonate is insoluble in pure water, but
dissolves somewhat freely in water charged with free
carbonic acid. It is assumed that a compound is
formed, which is known as calcium bi- (or acid) car-
bonate, is very unstable and can only exist in a state
of solution. When a solution of calcium bicarbonate —
which can be prepared by passing carbonic acid gas
through water containing finely divided calcium
carbonate in suspension — is exposed for some time to
the air, it soon becomes cloudy, and a deposit of calcium
carbonate settles down at the bottom of the vessel,
because, in the air the dissolved calcium bicarbonate
is decomposed into free carbonic acid gas and calcium
carbonate, which latter, as has been mentioned, is
quite insoluble in water. It has already been stated
that this phenomenon goes on in Nature in the forma-
tion of stalactites, lime sinter and calcareous tuff.
Calcium carbonate is readily soluble in acids, the
contained carbonic acid being liberated (as carbon
dioxide) with effervescence. WTien such acids are
employed for solution as form readily soluble salts
with lime, such as hydrochloric, nitric, acetic, etc.
acids, a perfectly clear solution is obtained; but if
sulphuric acid is used, a white pulpy mass is formed,
consisting of calcium sulphate, or gypsum, which,
16 EARTH COLOURS
owing to its low solubility, separates out as small
crystals. Any sandy residue left when calcium
carbonate is dissolved, mostly consists of quartz sand.
In dissolving dark -coloured limestones, grey, or even
black, flakes are left, which consist of organic material
very high in carbon. On limestone being subjected
to fairly strong calcination, all the carbonic acid is
expelled, leaving behind the so-called quick or burnt
lime, which is, chemically, calcium oxide :—
CaCO3 = CaO + CO2
Calcium Quick Carbon
carbonate lime dioxide
If burnt lime be left exposed to the air for some
time, it again gradually absorbs carbon dioxide and
is reconverted into calcium carbonate. When burnt
lime is sprinkled with water it takes up the latter
avidly, becoming very hot and finally crumbling down
to a very friable white powder, consisting of slaked
or hydrated lime (calcium hydroxide, Ca(OH)2). The
considerable rise of temperature in quenching the lime
is due to the chemical combination of the calcium oxide
and water.
Both quick and slaked lime dissolve to a certain
extent in water, and impart strongly alkaline properties
thereto, lime being one of the strongest of bases. On
exposure to the air, the solution of quick lime in
water (lime-water) quickly forms an opalescent super-
ficial film of calcium carbonate, and in a short time no
more lime is present in solution, the whole having been
transformed into calcium carbonate, which settles
down to the bottom of the vessel as a very fine powder.
Limestone that consists entirely of calcium oxide
and carbon dioxide is of rare occurrence in Nature,
RAW MATERIALS FOR EARTH COLOURS 17
foreign substances being nearly always present. Since
the nature of these admixtures is of the greatest
importance to the colour-maker, owing to the consider-
able influence they exert on the suitability of the
minerals for his purposes, it is necessary that these
extraneous substances occurring in limestone should
be more closely described.
Nearly all varieties of limestone contain certain
proportions of ferrous and ferric oxides. The presence
of ferrous oxide, when the relative amount is but
small, cannot be detected by mere inspection ; and
even many limestones containing really appreciable
quantities of ferrous oxide are pure white in colour
so long as they are in large lumps. If, however, such
a limestone be reduced to powder and exposed to the
air for a short time, it gradually assumes a yellow
tinge, the depth of which increases with the length
of exposure.
The cause of this change is due to the fact that
ferrous oxide has a great affinity for oxygen, by absorb-
ing which it changes into ferric oxide. (Ferrous oxide
consists of FeO, ferric oxide of Fe2O3.) Ferrous oxide
and its compounds are of a pale green colour which
is not very noticeable, whereas ferric oxide has a very
powerful yellow colour, and consequently the lime-
stone, when its superficial area has been greatly
increased by reduction to powder, assumes the yellow
tinge due to ferric oxide. A limestone exhibiting
this property can evidently not be used for making
white earth colours, but is, at best, only suitable for
mixing with other colours.
Occasionally, limestone contains varying quantities
of magnesia, and when this oxide is present in large
amount, changes into another mineral known as
2,
i8 EARTH COLOURS
dolomite. In many places this dolomite forms large
masses of rock, which, however, is not employed for
making colours, owing to the yellow shade imparted
by the fairly large amount of ferric oxide present.
Gypsum (Alabaster)
This mineral occurs native in many places, and is
frequently worked for a number of purposes. Gypsum
occurs in Nature in a great variety of forms. The
purest kind is met with either as water-clear crystals,
which cleave readily in two directions, or as trans-
parent tabular masses (selenite) which also cleave easily.
Micro-crystalline fine-grained gypsum is milk-white in
colour, highly translucent and is largely used, under
the name of alabaster, in sculpture. Owing to its
low hardness, alabaster can be readily cut with a knife,
and on this account is frequently shaped by planing or
lathe-turning.
Gypsum is generally met with in dense masses,
which may be of any colour, grey, blue and reddish
shades being the most common, whilst pure white is
rarer. The dark-coloured varieties can only be used
for manurial purposes ; but the white finds a two-fold
application as a pigment, and, in the calcined state,
for making plaster casts.
In point of chemical composition, gypsum consists of
sulphate of lime, or calcium sulphate (CaSO4 -f 2H2O).
It is soluble in water, but only in such small quantity
that over 400 parts of the latter are needed to dissolve
one part of gypsum. On being heated to between
120° and 130° C., gypsum parts with its two molecules
of combined water and becomes anhydrous calcium
sulphate or burnt gypsum. When this 'latter is stirred
with water to a pulp, it takes up the water again, with
RAW MATERIALS FOR EARTH COLOURS 19
considerable evolution of heat, swelling up considerably
and setting quickly to a solid mass.
The number of substances exhibiting this property
being small, burnt gypsum is very frequently used for
making casts of statuary, and for stucco work in
building. Finely ground white gypsum can also be
used as a pigment, but is inferior to calcium carbonate
in covering power, and is therefore seldom employed
for this purpose, though frequently added to other
colours. The mineral known as muriacite or anhydrite
consists of anhydrous calcium sulphate ; and is there-
fore similar in composition to burnt gypsum; but it
lacks the property of combining with water when
brought into contact therewith.
Barytes, or Heavy Spar
The mineral known as heavy spar occurs in very
large quantities and in numerous localities. It forms
rhombic crystals, which are very often extremely
well developed and form flat plates of considerable
size. A remarkable peculiarity of this mineral is its
high specific gravity, which is due to the barium
content. It is found native in all colours, white being
the most common.
Chemically, heavy spar is barium sulphate, BaS04.
It can be used as a pigment per se, but only when
prepared artificially, the trade name for the product
being permanent white, or blanc fixe. Powdered
native heavy spar, even when ground ever so fine, has
not enough covering power, this property being
peculiar to the artificial product.
When it is desired to mix other pigments with a
white substance, to lighten the shade, permanent
white can be specially recommended, since it is quite
20 EARTH COLOURS
insensitive to atmospheric influences and has no
chemical action on the colour, so that it can be used
with even the most delicate colours without risk. In
this way, not only can the colours be considerably
cheapened, but over -dark colours can be shaded to
the desired extent. Another advantage of such
mixtures is that a smaller quantity of oil or varnish
is required, barytes only needing about 8% of its own
weight of vehicle to form a workable mixture, whilst
other pigments take five times as much, or even more.
In many cases the low covering power of barytes
enables large quantities to be added, and this reacts
favourably on the consumption of varnish.
Another barium mineral is witherite, or barium
carbonate. This is not used direct as a pigment,
but — in contrast to heavy spar — is readily soluble in
hydrochloric acid, and therefore serves as raw material
for the preparation of artificial barytes and other
barium compounds, the first -named being obtained
by treating a solution of barium chloride with sulphuric
acid, insoluble barium sulphate being precipitated.
Talc, Soap stone, Steatite
Talc occurs in Nature either as a pure white' mass,
of greasy lustre, or occasionally as yellow, green or
grey masses, all distinguished by a peculiar greasy
appearance and a soapy feel. This appearance is
common to all the minerals of the steatite group, and
is the cause of their generic name, soapstone. Although
the steatites have a very low degree of hardness —
most of them can be scratched by the finger-nail-
some difficulty is encountered in reducing them to
fine powder. Calcination usually increases the hard-
ness considerably, so that, in some cases, the calcined
RAW MATERIALS FOR EARTH COLOURS 21
mineral gives off sparks when struck with a steel
instrument.
Soapstone is composed of magnesium silicates,
containing varying proportions of magnesia and silica,
together with a small quantity of water, apparently in
a state of chemical combination, a very high tempera-
ture, approaching white heat, being required to effect
its complete expulsion, the residue then attaining the
aforesaid high degree of hardness. The composition
of talc can be expressed by the symbol H2Mg2(SiO3)4,
corresponding to 63-52% of silica, 31*72% of magnesia,
and 4*76% of water. In some varieties of talc, a
portion (1-5%) of the magnesia is replaced by ferrous
oxide. Talc is quite unaffected by the action of
dilute acids, boiling concentrated sulphuric acid being
required to decompose it, with separation of silica.
Owing to its low specific gravity and chemical
indifference, talc is suitable for lightening the shade of
certain lake pigments. It can also be used as a pig-
ment by itself, and also as a gloss on wall-paper, for
mixing with paper pulp, and for various other purposes.
Clay
The mineral known as clay is, in all cases, a product
of the decomposition of other minerals, mainly felspar.
This substance is a double silicate of alumina and
potash, K2O.Al2O8.(SiO2)6. Pure kaolin is Al2O3(SiO2)2
+ 2H2O, or 46-50% silica, 39-56% alumina, 13-9%
water.
Clay may be supposed to have been formed by the
conversion of felspar, under the action of air and water,
into silicate of alumina, the silicate of potash being
dissolved out. Being insoluble, the silicate of alumina
would be transported by the water, in a very fine
22 EARTH COLOURS
state of division, and finally deposited as a sediment,
which in course of time became a solid mass. This,
when again brought into contact with water, forms a
very plastic pulp which, when dried and baked, forms
a solid mass, brick, which is no longer affected by
water. Perfectly pure clay forms a white mass,
which, under the name of China clay or kaolin, is used
for making porcelain, and is only occasionally met
with in large quantities.
Pure kaolin is characterised by its great chemical
indifference, being decomposed only by strong alkalis
and sulphuric acid. At the high temperature of the
pottery kiln, kaolin sinters to a very compact mass,
but cannot be fused, except when small quantities are
subjected to the intense heat of the oxyhydrogen
flame, whereupon it fuses to a colourless glass of great
hardness.
In an impure state, silicate of alumina occurs
frequently in Nature, and then forms the minerals
known under the generic names of clay, loam, marl,
etc. These impure clays contain varying proportions
of extraneous minerals which produce changes in the
physical and chemical properties. They are grey,
blue or yellow in colour, the grey and blue varieties
mostly containing appreciable quantities of ferrous
oxide, whilst the yellow kinds contain ferric oxide.
When fired, all of them become yellow or red, the
ferrous oxide being transformed into ferric oxide by
the heat. Some fairly white clays are high in lime,
which makes them fusible at high temperatures. In
some very impure kinds, even the comparatively low
heat of the brick-kiln is sufficient to cause partial
fusion. For colour-making, the white clays, especially
kaolin and pipeclay, form a highly important material,
RAW MATERIALS FOR EARTH COLOURS 23
being procurable at very low prices and fairly easy to
prepare.
The white clays aie either used as pigments by
themselves, or for mixing with other colours of low
specific gravity.
(B) YELLOW EARTHS
The number of yellow earths is large, but most of
them exhibit a certain similarity in chemical composi-
tion, the pigmentary principle in the majority being
either ferric oxide or ferric hydroxide. The former is
yellow, the latter brown, and the colour of the minerals
resembles that of the preponderating iron compound.
Brown Ironstone
The mineral known as brown ironstone consists of
ferric hydroxide, and usually forms compact masses,
no decided crystals having, so far, been observed.
The lumps have an irregular or earthy fracture, a
hardness of 5-5*5, and a sp. gr. between 3*40 and 3-95.
The colour ranges, in the different varieties, from
yellowish (rusty) brown, through cinnamon to blackish-
brown. The chemical composition of the pure lumps
may be expressed by the symbol 2Fe2O3 -j- 3H2O;
but a little manganese oxide and silica is generally
present even in the pure kinds.
The chief varieties of this mineral are : —
(a) Fibrous brown iron ore, or brown hematite,
mostly forming reniform or stalactitic masses.
(b) Compact brown ironstone, usually in dense
masses, and not infrequently also appearing in pseudo-
morphs of other minerals.
(c) Ochreous brown ironstone. This variety is the
most important to the colour-maker, for whose purposes
24 EARTH COLOURS
it is preferably used. It nearly always forms very
loose, earthy masses, yellow or brown in colour.
(d) Clay ironstone. This consists of a mixture of
the above-mentioned varieties with variable propor-
tions of other minerals, clay being the most common
ingredient. Nodular iron ore, oolitic, bog and siliceous
ore belong to this class, as also the minette ores that
are found in great abundance in Alsace-Lorraine,
Belgium and Luxemburg, and are classed with the
oolitic brown ironstones.
In most cases, the varieties enumerated are found
together, and are used for the production of iron.
The ochre constituting the most interesting member
to the colour-maker often occurs as deposits embedded
in dense masses of brown ironstone, though in many
places it is found by itself.
CHEMICAL COMPOSITION OF VARIOUS . BROWN IRONSTONES
The following analyses of brown ironstone from different deposits
will give an idea of the composition of these minerals.
Ordinary Brown Ironstone
i.
2.
3-
4-
5-
6.
7-
!
8. 9.
10.
Ferric oxide .
76-76
\ j
73-75 77-54 78-50
78-42
4?'25
„
„
Manganese oxide
16-56
10-50
2-70 1-95
1-30
24-73
—
| . .
—
Iron
—
. —
— . — .
33-9 37-88 54-80
55-04
Manganese
. —
—
. —
. —
• —
—
0-15 0-17 0-57
O-2O
Alumina
—
. —
—
. —
1-13
2-33
10-03 0-88 1-15 , 2-50
Lime
0-60
2-75
0-48 5-08 3-55 2-85
0-41 0-32 0-50 0-34
Magnesia
6-44
1-25 4-50 0-18 0-90
0-67
0-02 0-38
Silica .
• —
3-55
0-85
5-48
"•35
28-29
33-38 O-O2 ' 0-38
S03
. —
. —
. —
.
—
0-09
• —
" — i — '
P205 .
Sulphur .
• — •
~
• —
~
0-09
0-08
o-33
0-06 0-04 Trace
Phosphorus
. —
—
—
. —
—
—
0-04
0-56 0-02
O-O6
Loss on incin- \
eration /
5-65
14-00
I4-5I
9-12
9-10
9-80
9-88
7-77 10-55
10-71
Deposits: (i) Hamm; (2) Schmalkalden ; (3) Hiittenberg (Carynthia) ; (4) Styria
(5) and (9) Bilbao; (6) Algeria; (7) Schwelm (Westphalia); (8) Elbingcrode (Harz) ;
(10) Pennsylvania.
RAW MATERIALS FOR EARTH COLOURS 25
Argillaceous Brown Ironstone
a. b.
c.
d.
e.
/•
g-
h.
,'. ^
Ferric oxide .
80-76 19-4
55-39
66-33
57-32
52-50
39-50
75-67
__
Iron
— ' —
40-90 2I-69
Manganese oxide
— 8-2
—
6-42
5-49
6-12
0-72
•
Manganese
— • — —
— .
— .
• —
• — -
Zinc oxide
0-92 1-6
—
—
0-47
—
. —
. — .
Alumina
2-36 ii-o
12-80
7-74
1-68
5-23
9-89
3-10
4-95 3-88
Lime
— 2-6
Trace
0-41
0-13
3-36
20-34
5-oi
5-59 21-25
Magnesia
0-2
. — .
o-37
0-36
• —
0-49 0-30
Silica .
4-58 48-61
22-73
12-97
30-64
8-64
5-22
8-70
16-63 14-71
PaOg •
Phosphorus
~ ~
0-02
0-32
3-86
2-19
3-68
1-13 0-48
S03
. — - '
—
0-03
Trace
—
—
— —
Sulphur .
__ '
—
—
. —
;
—
o-io 0-05
Loss on incin- \
eration /
1271 ; 9-1
8-50
11-77
12-70
20-55
25-74
—
16-04 28-70
(a) Oolitic (pea) ore from Elligserbrick (Brunswick) ; (b) from Durlach (Baden) ;
(c) and (d) Ore from Esslingen ; (e) Oolitic ore from Siptingen (Baden) ; (/) from Adenstedt,
nr. Pirna (argillaceous) ; (g) Ibid, (calcareous) ; (h) Minette from Esch ; (i) Red minette
from Dolvaux; (k) Brown minette from Redange.
Limonite (Bog Iron Ore)
i.
2.
3.
4-
5.
6.
Ferric oxide
61-00
67-46
65-66
67-59 7°'°5
62-2O
Manganese oxide
7-00
3-19 3-87
1-45 178
7-60
P205 .
2-00
0-67 I'I3
0-18 0-34
1-60
S03 .
3-07 Trace
0-21 Trace
Trace
Silica
6-00 7-00 7-15
7-89
8-03
1 6-60
Alumina
. — •
5-09
4-18
1-50
2-20
Lime .
. — .
0-90
0-82
0-47
2-31
I-OO
Magnesia .
—
— 0'I5
0-23
O-I2
373
Water and \
organic acids /
19-00
I7-00
16-22
17-81
15-87
19-90
(i) Limonite from Lausitz ; (2) Limonite from Auer, nr. Moriz-
burg ; (3 to 6) Swedish limonite.
Ochre
Ochre, or yellow Terra di Siena, forms earthy -looking
masses, fawn, reddish -yellow to brownish-red in
colour. Whilst not infrequent in Nature, ochre is
only found in small quantities, as pockets, and not as
extensive deposits. The discovery of a bed of good
26 EARTH COLOURS
coloured ochre is, however, always a very valuable
find, bright natural ochres being somewhat rare, and
most kinds requiring special preparation before they
can be used as painters' colours. Owing to the com-
parative scarcity of good coloured ochres, they are
often called after the place of origin, such as Thuringian,
Italian (Siena), English, etc., ochre.
In nearly every case, ochre is a decomposition product
of various ferruginous minerals, which has been
transported by water, often in admixture with other
minerals, and finally deposited in the places where it
is now found. Most ochres consist of varying mixtures
of clay, ferric hydroxide and lime ; and, as a rule, the
higher the proportion of ferric hydroxide, the deeper
the colour. Thus, for example, the ferric hydroxide
may amount, in the dark grades, to 25% of the entire
mass, whilst in the lighter kinds it may be as low as
3%. It is very rare that ochre is put on the market
in its native condition, being mostly subjected to
chemical treatment enabling a definite shade of colour
to be obtained. This will be gone into more fully
later.
Yellow Earth
Yellow earth is found in many places as compact
masses, and less frequently as schistous deposits. It
has a fine earthy fracture, and is mostly devoid of
lustre, except for a faint shimmer on the surface of
fracture ; slightly greasy feel ; and a tendency to
crumble, in water, to a non-plastic powder. It con-
tains silica, ferric oxide and water in varying pro-
portions, and the yellow earths from different deposits
always vary slightly in percentage composition. These
differences are clearly shown in the following analyses
RAW MATERIALS FOR EARTH COLOURS 27
of two varieties from the vicinity of Amberg
(Bavaria) : —
i. ii.
Silica . . 33-23% 35-io%
Alumina . . 14-21 I4'4°
Magnesia
Ferric oxide
Water
1-38
3776 36-80
13-24 13-60
When heated, the colour changes gradually to red,
and the earth becomes extremely hard. There are
several recognised commercial grades, the price of
which varies mainly in accordance with the colour
and fineness. The Amberg variety is specially
esteemed, the Hungarian and Moravian kinds being
less valuable.
The colour not being particularly good, this earth
is never used for fine work, but is largely employed as
a yellow wash for houses and as ordinary, distemper.
It may also be used as an oil paint.
Red Ochre is a less important, cheap variety of
ochre, chiefly used in cheap paints and for low-priced
wall-papers. It occurs in the deposits as clayey
masses.
Terra di Siena
Terra di Siena is a very pure form of ferric hydroxide.
When ground, the light to dark brown lumps furnish
a pale to dark yellow powder, which can be transformed
into a number of gradations by burning. In spite of
its handsome colour, this pigment is deficient in cover-
ing power, in addition to which it darkens when mixed
with varnish, and dries slowly.
(C) THE RED EARTHS
Apart from the small quantities of native vermilion
handsome enough for direct use as painters' colours-
28 EARTH COLOURS
when reduced to powder, the red earths, with practically
no exception, consist of ferruginous minerals, and it
is only within a recent period that red painters' colours
have been prepared from certain chemical waste
products from manufacturing processes. In all cases,
however, compounds of iron and oxygen constitute
the bulk of the red earths. In addition to ferric oxide,
which is the chief material used for making the im-
portant red colours, are compounds of ferric oxide and
water, i. e. ferric hydroxides. The ferric oxide pig-
ments are among the most important in the entire
series of earth colours, being on the one hand very
cheap, and on the other so handsome in colour that
ferric oxide can be used for the finest paintings.
Ferric oxide can also be shaded very extensively by
a fairly simple treatment, so as to furnish a whole
range of very handsome shades.
In nature, ferric oxide occurs in numerous varieties
of one and the same mineral, red iron ore, which is
also known as hematite, blood stone, raddle, etc.
Red Ironstone
Red hematite occurs native as rhombohedral crystals,
which mostly consist solely of ferric oxide, and may be
considered as pure oxide for the purposes of the colour -
maker. The difference between the several varieties
is due, not to any chemical variation, but entirely to
changes in physical structure. The varieties with a
radial, fibrous structure are known as red hematite,
the colour of which ranges from blood red to dark
brown and is frequently accompanied by metallic
lustre. The scaly modification of this mineral forms
micaceous iron ore, and is usually a deep iron black.
RAW MATERIALS FOR EARTH COLOURS 29
In the neighbourhood of volcanoes it is frequently
found as particularly handsome crystals.
Iron cream (frosty hematite) is the name given to
a beautiful cherry red variety, which easily rubs off,
has a greasy feel and is composed of extremely fine
scales.
The so-called raddle occurs in Nature as a readily
pulverulent earthy mass of ferric oxide contaminated
more or less with extraneous substances. On account
of its abundance and low market price, it is largely
used in painting.
Although mixed with numerous foreign substances,
certain clay ironstones, oolitic ironstones and siliceous
ironstones may be regarded as ferric oxide in the
sense understood by the colour -maker, all these minerals
having a deep red to deep brown colour and being
capable of rinding advantageous employment as
pigments.
Ferric oxide is distinguished by two properties
which render it specially valuable to the colour -maker.
When combined with water, its colour is no longer
red, but a handsome brown ; and, on the other hand,
when heated, the colour passes through brown into
a permanent dark violet. By suitable treatment of
such minerals as consist mainly of ferric hydroxide,
mixtures can be obtained which contain the oxide and
hydroxide in variable proportions and give a whole
range of shades between brown and red.
The preparation of these colours is easy when very
pure red ironstone is available. The somewhat ex-
pensive pigment, Indian red, is — when pure — really
nothing but a very pure ferric oxide of Indian origin.
Ferric oxide, however, often contains impurities which
considerably influence the colour of the product.
30 EARTH COLOURS
Owing to the fact that large quantities of ferric oxide
are formed as by-products in certain chemical processes
which are carried out on a very extensive scale, this
oxide, which is very pure, can be advantageously used
for making iron pigments, especially as its application
for other purposes is very restricted, and it can there-
fore be had at a very low price.
The following analyses show the composition of
a number of red ironstones, Nos. i, 2 and 3 being
hematite from Froment, or Wetzlar, No 4 from Wetzlar,
Nos. 5 and 6 hematite from Whitehaven, No. 7 from
Thuringia, No. 8 from Bohemia, No. 9 from Spain,
No. 10 from N. America, and No. n from England.
No.
Ferric
oxide.
Manganese
oxide.
Silica.
P205.
Alumina,
lime and
magnesia.
Water.
I
94-00
Trace
2-OO
___
Trace
2 -OO
2
80-95
—
16-74
0-51
0-97
0-83
3
73-77
—
23-16
o-45
1-41
I-2I
4
92-45
—
5-63
0-19
0-65
1-08
5
96-27
4-20
- —
o-59
4-40
6
63-05
5-29
22-76
—
0-49
4-40
7
85-00
1-601
3-304
o-457
8-795
0-633
|
No.
T ^ Man-
Ir0n- ganese.
1
3Z. "-•
Mag-
nesia.
Silica.
Sulphur.
Phos-
phorus.
Loss on
calcina-
tion.
8
33-64 o-io
7-58 8-10
0-82
17-80
Trace 0-19
9-82
9 31-38 0-19
0-06 29-95
o-35
0-87
— 0-09
23-68
10 62-54 1-93
1-71 —
0-50
3-80
0-02
0-04
2-23
ii 62-91 Trace 1-39 0-70
0-42
5-89
0-05 o-n
—
1 1
There are certain other minerals closely allied, both
chemically and mineralogically, to red ironstone,
namely, the brown hematites or ironstones used in the
RAW MATERIALS FOR EARTH COLOURS 31
manufacture of iron. Brown hematite consists of
ferric hydroxide, Fe2O3H2O, and occurs in a variety
of forms in Nature, the most frequent being pea
(oolitic) ore, which owes its name to the spherical
shape of the grains. Some brown hematites are
decomposition products of other minerals, and contain
sulphur and phosphorus in addition to ferric hydroxide.
Like the pure hydroxide, they are biown in colour,
but differ therefrom considerably in their chemical
behaviour when heated. This is particularly the case
with the so-called bog ore, which is mostly found, as
spongy yellow-brown to black masses, in swamps, and
owes its origin to the decomposition of various ferrugi-
nous minerals. It varies greatly in chemical composi-
tion and occasionally contains up to about 50% of sand.
The amount of ferric oxide in bog ore varies between
20 and 60%, and it also contains 7-30% of water,
up to 4% of P2O5, small quantities of ferrous oxide
and manganese hydroxide, together with, in most cases,
mechanically admixed organic residues.
The phosphorus content makes bog iron a very
inferior material for smelting, the resulting iron being
of low quality. Nevertheless, it can sometimes be
advantageously used in making earth colours, though
the products cannot lay much claim to beauty of
colour.
Bole
The native earth pigments known by this name
form masses of the colour of leather to dark brown,
with a conchoidal fracture and an earthy appearance.
Bole chiefly consists of iron silicate combined with
water, some varieties containing small quantities of
alumina. The composition fluctuates very considerably,
32 EARTH COLOURS
most varieties containing 41-42% of silica, 20-25%
of alumina, and 24-25% of water, the remainder
consisting of ferric oxide. Some kinds, such as
Oravicza and Sinope bole, contain only 31-32% of
silica and 17-21% of water.
Bole is used as a paint for walls, clapboards, etc.,
and is only mentioned here because of its relationship
to the ferric oxide pigments.
Alum Sludge
Large quantities of clarification sludge are produced,
in alum works, as the sediment from the red liquors.
This sludge consists mainly of ferric oxide, with small
quantities of other oxides and sulphuric acid (basic
ferric sulphate, and would be an entirely worthless
by-product except for the fact that it can be manu-
factured into pigments, some of them of great beauty.
All alum makers should treat this residue and con-
vert it into pigments, which they could put on the
market at a low rate, the cost of preparation being
small. Since the material is chiefly composed of ferric
oxide, the resulting colours are very similar to those
obtained from iron ores ; and all shades, from yellow-
brown, through red, to the darkest brown, are
represented.
Mine Sludge
The water frequently present in iron mines some-
times contains large quantities of sediment, which
consist mainly of iron ochre and can be advantageously
worked up into pigments. There is scarcely any
need to mention that all substances containing ferric
oxide can be used for making any of the pigments
obtainable from the oxide itself, the only difference
RAW MATERIALS FOR EARTH COLOURS 33
between the various raw materials being their degree
of purity, so that it is not always so easy to obtain a
certain desired shade from a given material in such
beauty as is furnished by another material, the small
quantities of impurities associated with the ferric
oxide having, in many instances, an important influence
on the colour.
(D) BLUE EARTHS
Only two minerals are known which are capable
of direct use as blue pigments, viz. vivianite (native
Prussian blue) and copper carbonate (azurite, ultra-
marine), and as neither of them is particularly hand-
some, they are only used for unimportant work. Lapis
lazuli is no longer employed.
Azurite, or Ultramarine
This mineral, which is of frequent occurrence with
malachite and other cupriferous minerals, forms small
crystals of a beautiful deep azure blue consisting of
cupric oxide in combination with carbon dioxide and
water, expressed by the formula 2CuCO8,Cu(OH)2,
or Cu3(OH)2(CO3)2, and containing 69-19% of cupric
oxide, 25-58% of CO2 and 5-23% of water. The
colour of the powdered mineral is much paler than
that of the crystals. The pigment, which is used for
cheap paints, is not particularly stable, and loses much
of its beauty when applied to plaster.
Vivianite
This mineral occurs in many places as crystalline
masses, but also forms earthy deposits, some of which,
especially in certain bogs, attain considerable thickness.
3
34 EARTH COLOURS
The colour is between indigo and blackish blue ;
and the freshly won mineral often has an unsightly
whitish appearance, which, however, soon changes
into the pure blue. The cause of this peculiarity is
due to the fact that vivianite originally consisted of
hydrate d ferrous phosphate, which is white, this com-
pound being transformed, under the influence of the
air, into the blue ferric phosphate.
Vivianite contains ferric oxide, phosphoric acid and
water, but in variable proportions. The original
composition, expressed by Fe2(PO4)2 + 8H2O2, corre-
sponds to 43-03% of ferrous oxide, 28-29% of P2O5
and 28-68% of water; but, in the air, part of the
ferrous phosphate is oxidised to basic ferric phosphate,
so that the content of ferrous oxide may range from
9-75 to 42-71%, and that of ferric oxide between 1-12
and 38-20%. Vivianite is also sold as blue ochre, and
is now seldom used as a painters' colour, owing to the
introduction of a large number cf artificially prepared
blues, which are superior to vivianite in colour and are
cheaply made. However, it can still find application
in localities where it is obtainable in quantity.
(E) GREEN EARTH PIGMENTS
The green earth pigments comprise green earth
(Verona green) and malachite. Like the blue earths,
they cannot lay any particular claim to beauty, but
they are very cheap, and consequently are largely
used where low price is the chief consideration.
Green Earth
In Nature, green occurs as an entirely non-crystalline
earthy mass, which is probably a decomposition
RAW MATERIALS FOR EARTH COLOURS 35
product of augite. It has a close, earthy fracture,
a colour between seladon and olive green, and a slightly
greasy appearance. In point of chemical composition
it consists of silica, alumina, magnesia, sodium,
potassium, ferrous oxide and water, the usual repre-
sentative formula being ROSiO2 H2O, in which RO
symbolises a metallic oxide.
The colour is due to ferrous oxide ; and if left exposed
to the air for a long time, or subjected to powerful
calcination, the great affinity of ferrous oxide for
oxygen causes the colour to turn red and red-brown.
Green earth is found in many localities, e. g. Bohemia,
Hungary, the Tyrol and Cyprus, the finest, however,
occurring near Verona, on which account it is known as
Veronese earth.
Malachite
The commercial pigment consists of powdered
malachite, a mineral which usually occurs in compact
masses of a handsome emerald green colour, though
isolated lumps exhibit considerable variation in shade,
some of them being dark green and others very pale.
In chemical composition, malachite is closely allied to
azurite, consisting of cupric oxide, carbon dioxide
and water, and the difference is entirely one of per-
centage proportions. The formula is CuCO3, Cu(OH)2,
or Cu2(OH)2CO3, corresponding to 71-90% of cupric
oxide, 19-94% of carbon dioxide and 8-16% of water.
Powdered malachite (even the dark green varieties)
is always rather light in colour, and for this reason is
not much used. Furthermore, the mineral is rather
hard (3-5), and is consequently difficult to grind; in
addition to which the mineral is fairly expensive, on
account of its employment as a source of copper,
36 EARTH COLOURS
particularly fine pieces being also used as ornaments
or for making works of art. Moreover, like all copper
compounds, it is very sensitive to the action of sul-
phuretted hydrogen, and liable to discoloration in
course of time.
(F) BROWN EARTH PIGMENTS
Numerous minerals are adapted for the manufacture
of brown pigments. On the basis of chemical com-
position, they may be classed in two groups; those
consisting of ferric hydroxide, and those in which the
brown colour is due to organic substances.
The first group comprises the minerals which have
already been mentioned in connection with the red
earth pigments, bole and brown ochre (umber), Terra
di Siena, Cologne earth and a number of other earths
rich in ferric hydroxide belonging to this category. The
second, or organic group, includes compounds that
are very rich in carbon and are therefore of a very
dark colour, the shades ranging from light brown to
black, e. g. the true umbers and asphaltum.
Umber
As already mentioned, the term " umber " was
formerly applied to brown varieties of ochre, whereas
at present it is extended to certain masses of brown-
coal character, often interspersed with iron ochre and
sometimes containing manganese. Umber generally
consists of fairly dense, earthy masses, which are
dried and ground — after crushing and levigation, if
necessary.
Valuable varieties are Cappagh brown and Cale-
donian brown, both with a reddish tinge.
RAW MATERIALS FOR EARTH COLOURS 37
It is thus evident that " umber " now implies two
different kinds of materials, organic masses and iron-
manganese compounds, 'which can also be used as oil
paints. These umbers can also be extensively shaded
by burning, the final colour being particularly influenced
by the amount of manganese compounds present.
The carbonaceous umbers (Cassel brown, Carbon
brown) are combustible, and mostly leave behind a
merely small residue of ash. An important property
of these umbers is their partial solubility in alkalis, a
peculiarity which is utilised for the preparation of
brown wood stains.
A sphaltum
Asphaltum forms very friable dark brown to black
masses, which, in contact with a light, easily ignite
and burn with a bright, but very smoky, flame, dis-
engaging a peculiar, " bituminous " smell, and leaving
only a very small quantity of ash.
Extensive deposits of asphaltum are found at the
Dead Sea, the Pitch Lake on the island of Trinidad,
in Dalmatia, and many other places, where, however,
it is in an impure condition and frequently contains
large quantities of sand. In many localities the rock
is impregnated with asphaltum, which makes it dark
brown to black in colour and gives rise to a bitu-
minous odour when rubbed.
Peat beds sometimes contain pockets of a mass with
a handsome brown colour and consisting of a mixture
of humic acids and other organic substances which
may be ranked with the humin bodies that are always
formed when organic matter decomposes in presence
of an insufficient supply of oxygen. These bodies
are dark coloured, mostly deep brown, rich in carbon,
38 EARTH COLOURS
and, to some extent, similar to brown coal or peat in
chemical composition.
Their high carbon content renders these substances
very inert towards chemical reagents, and therefore
particularly adapted for the preparation of painters'
colours. Genuine Vandyke brown, which is the
handsomest brown known, is an earth rich in humin
compounds; and Cassel brown also belongs to this
group.
(G) BLACK EARTH
The colour of these earths is entirely due to carbon,
and pure carbon, a certain form of which occurs native,
is itself used as a pigment. Actually, there are only
two minerals that require to be mentioned in this
connection : black schist and graphite.
Black Schist
In most cases this is a clay shale, so rich in carbon
as to appear deep black. In commerce, this mineral
is also erroneously called "black chalk"; but at
present it is seldom used as a pigment or drawing-
material, black chalks being produced far more cheaply
than the expense of preparing the natural article.
Grey clay shales are used for making grey earth
pigments (stone grey, and mineral grey).
Graphite
This mineral is found, in a very pure state, in many
localities, celebrated deposits occurring in England,
Siberia, Bohemia and Bavaria, whilst North American
graphite has lately come into prominence.
Graphite is a modification of pure carbon, and is
RAW MATERIALS FOR EARTH COLOURS 39
met with in the form of hexagonal (rhombohedral)
crystals, usually occurring as hexagonal plates with a
lustrous, iron-black colour. It rubs off easily, and
readily burns away, leaving a very small amount of
ash, when subjected to a very high temperature in
presence of air.
The principal uses of graphite are as an anticorrosive
paint for iron, and for making lead pencils.
As already mentioned, the term " earth colours "
has been considerably broadened of late. Whereas,
formerly, it was restricted to colours prepared ex-
clusively from minerals by a simple treatment, limited
to crushing, levigation or calcination, it now includes
the pigments obtainable from large by-products of
certain chemical processes. This latter class is
especially important as affording an opportunity of
utilising products formerly considered worthless and
whose removal often entailed heavy expense.
By drawing on these materials the industry of the
earth colours has greatly enlarged its scope. At
present, many colours of this kind are on the market,
and it is to the interest of many manufacturers to
endeavour to utilise certain waste products in the
same direction. The advantage of such a course
hardly needs emphasising ; but, to give only a single
example, it may be mentioned that the manufacture
of fuming sulphuric acid from green vitriol, by the old
process, produces residues which were formerly looked
upon as quite worthless, and sold at .very low prices,
but are now worked up, in a number of factories, into
very handsome and durable pigments.
CHAPTER III
THE PREPARATION OF THE COLOUR EARTHS
THE preparation of the raw materials for the purpose
of making earth colours is a very important matter,
because many minerals or pigmentary earths merely
require mechanical treatment to render them at once
fit for use. The mechanical preparation differs con-
siderably, in accordance with the raw material under
treatment, substances that are found native in a finely
powdered condition only needing, for the most part,
to be levigated.
It rarely happens, however, that the raw material
occurs in condition for use direct, an example of this
kind being afforded by the finest clays or ochres.
Whilst these are found in a state of extremely fine
powder, they nearly always contain certain quantities
of sandy ingredients or even large lumps of foreign
minerals, and therefore require levigating. Sometimes
they need crushing as well, the small particles cohering
so strongly that mere treatment with water (levigation)
is unable to separate them. Mechanical force is there-
fore necessary, a passage through grooved rollers being
generally sufficient to crush the lumps; but in some
cases stamps have to be used.
When solid materials have to be treated, mechanical
appliances must always be used, their selection depend-
ing on the materials in question. Thus, gypsum, for
40
PREPARATION OF THE COLOUR EARTHS 41
example, can be crushed with ordinary rolls or mill
stones, its degree of hardness being so very low (2)
that it can be scratched with the finger-nail.
If, however, the material to be reduced is limestone,
which belongs to the third degree of the scale of hard-
ness (can only be scratched with an iron nail), or heavy
spar (hardness 3-3*5), very powerful stamps or edge-
runners must be employed to break it down into small
lumps, which can then be further reduced, without
any special difficulty, in an ordinary mill.
It is thus evident that a great variety of mechanical
appliances are used in the manufacture of earth colours.
Before going into their construction it is necessary to
point out that, whatever the mechanical treatment
employed, a considerable expenditure of mechanical
force is entailed ; and more power is needed when
mixtures have to be prepared. It is therefore essential,
in planning a factory for making earth colours on a
large scale, to make provision for ample motive power.
This power may be supplied by a steam engine ;
but it must not be forgotten that the prime cost and
running expenses of such an engine are considerable,
and form an important item in view of the low value
of most earth colours. Consequently, it is highly
important to be able to generate motive power as
cheaply as possible.
Now, the cheapest and most uniform source of power
is water; and therefore, wherever the conditions allow
of the erection of the colour works near a stream or
river, which can supply the power to run the various
machinery, the most favourable circumstances will
have been secured, the power being obtained at mini-
mum cost, whilst the upkeep of the motor cannot be
very great. If there is sufficient head for the water
42 EARTH COLOURS
to be run through a trough over the top of the leviga-
tion tanks, the conditions will be ideally favourable.
Wind power costs nothing, once the motor has been
installed; but unfortunately, one is dependent on
the weather, and sometimes there is not enough wind,
for days together, to drive the sails at all, and therefore
all the operations have to be stopped, including leviga-
tion, the water for which has to be raised by a windmill
pump.
In districts where the winters are severe, water
power may also fail and work have to be stopped;
and consequently, even when water power is the prime
source of energy, a steam engine must be installed as
a stand-by, being, of course, only used when the main
source of power gives out or proves insufficient.
The machines employed for preparing the raw
materials in the manufacture of earth colours may be
divided into the following groups :—
Machines operating entirely by pressure : crushers ;
machines acting by impact : stamps ; those acting
by impact and pressure : vertical mills (edge -runners),
ball mills, centrifugal mills; and, finally, machines
with a frictional action : grinding mills. Then there
are the levigating machines, which do not reduce the
material but separate the coarser particles from the
finer. The construction of the foregoing machines
is a matter for the machinery manufacturer rather
than the maker of earth colours ; but as the business
of the latter is dependent on them, a short description
is considered necessary. The selection depends, on
the one hand, on the nature of the materials to be
treated, and, on the other, on the size of the works,
since a manufacturer who has to deal with large quan-
tities of a given raw material will require different
PREPARATION OF THE COLOUR EARTHS 43
machines from those used on a small scale. The sole
purpose of the following description is to indicate to the
colour maker the way in which the reduction of the
raw material can be accomplished.
CRUSHING MACHINERY
Crushers and Breakers. — Crushers usually consist
of grooved iron rollers revolving on horizontal axes.
One of the rollers is fixed, the other being adjustable
by screws, in order that lumps of different sizes may
be treated in one and the same machine, which may
be employed either to turn out a roughly crushed
product, or to reduce it to a certain degree of fineness.
If several pairs of crushing rollers be mounted in
series, and each set a little closer than its predecessor,
the material can be reduced progressively from large
lumps to a fairly fine powder.
Each pair of rollers is geared together by pinions,
and is turned in such a way as to draw the material
in between. If the gear pinions have the same number
of teeth, the two rollers will revolve at the same speed
and will then merely crush the material into lumps
of a size depending on the distance at which the rollers
are set apart.
Nevertheless, by simply altering the gear ratio of
the pinions, the crushing action of the rollers can be
supplemented by a grinding action, a much finer powder
being then obtainable than otherwise, the one Droller
running at a higher speed than the other.
These crushers differ in strength of construction,
very strongly built machines being required for dealing
with large lumps of hard material, whereas substances
of low crushing strength, such as clay or other earthy
44
EARTH COLOURS
materials, can be treated in much lighter machines.
In any case, however, it is advisable to have the machine
stronger than is absolutely necessary for the work in
view; for, although the prime cost is thus increased,
the outlay on repairs will be reduced, and the machines
can, if necessary, be used on harder material as well.
The framework supporting the rollers should always
consist of a strong iron casting; and the machine
FIG. i.
should be set up as close as possible to the engine or
motor, to minimise the loss of power in transmission
through long shafting, etc.
Fig. i represents a breaker (made by the Badische
Maschinenfabrik, Durlach), suitable for the rough
crushing of clayey materials supplied in large lumps.
It can, however, also crush shale, lime, chalk, as well
as hard, sticky masses which would clog up a stone-
breaker.
The material fed into this breaker is gripped at once
PREPARATION OF THE COLOUR EARTHS 45
by the poweiful projecting teeth, which are connected
together by sharp-edged ridges, and is crushed in such
a way that it can be easily reduced still further by a
succeeding pair of smooth rollers.
The granulator (Fig. 2), made by the same firm, is
an example of a machine for crushing harder materials.
FIG. 2.
It is similar in construction to a stone-breaker, but
differs in the movement of the jaws, and combines
the properties of breaker and grinder, inasmuch as
it tears the material as well as crushes it. The figure
shows the machine adapted for direct electric drive.
If necessary, these granulators can be fitted with
classifying jig screens.
Stamps. — Stamps or stamping-mills have been used
46 EARTH COLOURS
from prehistoric times, and were probably employed
for reducing hard materials long before the introduction
of grinding-mills. The underlying principle of the
stamping -mill is very simple. The material to be
reduced is placed in a trough or mortar, and the ram
or head, which is of considerable weight, is raised by a
mechanical device and then allowed to fall freely, from
a certain height, on to the material underneath, which
it crushes. The heavier the head and the greater
the height of fall, the greater the effect produced. As
a rule, a large number of stamps are mounted together,
and in such a way that half of them are being lifted
while the other half are falling. Either a separate
mortar or trough is arranged under each stamp, or
else the whole drop into a common trough charged
with the material under treatment. Sometimes a
lateral movement is imparted to the material in the
trough, so as to bring it under the action of all the stamps
in succession.
Although the construction of stamping -mills in
general appears simple, various modifications are
employed for different purposes.
As a rule, a single passage through a stamping -mill
is not sufficient to reduce the material completely
to the desired fineness, the first product always con-
taining large and coarse fragments of various sizes,
as well as fine powder.
If the latter were left in with the larger pieces for
the second stamping it would impede the work, and
the stamping-mill should therefore be provided with
means for classifying the material discharged from the
trough, to separate the fine from the coarse and grade
the latter into sizes. This is usually effected by means
of a grading-screen.
PREPARATION OF THE COLOUR EARTHS 47
Stamping -mills are chiefly used for reducing brittle
materials. A number of stamps arranged in a row are
alternately lifted, by means of cams mounted on a
common shaft, and then let fall on to the material
lying on a solid plate, or else on a grating through which
FIG. 3.
the crushings fall. Fig. 3 is a stamping-mill con-
structed by H. F. Stollberg, Offenbach.
These mills are very strongly built, as independent
units, the frame being of cast-iron and the rams of best
wrought -iron with interchangeable chill-cast heads.
In some mills the stamps are rotated during the up-
stroke, in order to equalise the wear on the heads, and
also to economise power.
48
EARTH COLOURS
The grating or trough holding the material is per-
forated with holes, the diameter of which varies with
the material under treatment and the desired degree
of fineness in the product. To increase the efficiency
of the mill, the grating or trough is adapted to move
while the mill is running, in order to clean itself auto-
FIG. 4.
matically. These mills are made in different sizes,
with 2, 4, 6, or 8 heads.
Edge-runners. — This type of crusher is highly suitable
for reducing earth colours in large works. The special
feature of the type is that both stones are mounted
vertically and turn on a common shaft in the same way
that a cart wheel does on its axle. These runners are
particularly useful for reducing clay, chalk and other
earth colours, which have to be dealt with in large
PREPARATION OF THE COLOUR EARTHS 49
quantities. They will also crush fairly large lumps, and
they can therefore be used for the further reduction of
materials roughly crushed in a breaker, etc. The
material may be treated in either the wet or dry state,
only slight alteration being needed to change from one
method to the other.
There are numerous different patterns of edge -runner,
FIG. 5.
but all of them can be divided into two groups, viz. :
mills with stationary troughs, whilst the shaft carrying
the runners rotates; and those in which the trough
revolves, and the stones merely turn on the stationary
horizontal shaft.
Comparison of the efficiency of the two types has
shown that the revolving-trough type is the better,
giving a larger output per unit time with a reduced
consumption of power. Figs. 4 and 5 show a vertical
section and plan respectively of this type of edge-runner.
The trough G is turned by means of a toothed crown
4
50 EARTH COLOURS
gearing with the bevel pinion 0 mounted on an over-
head shaft C driven by a belt pulley N.
The bearings of the vertical shaft / of the trough
are situated at L and M. The runners H are loosely
mounted on the fixed horizontal shaft E and revolve
in consequence of the friction between them and the
material in the trough. As the latter revolves, the
material is continuously pushed aside by the runners,
and is again brought under them by the action of
scrapers.
The great advantages afforded by edge-runners,
in consequence of their simplicity, easy management
and low wear in comparison with other grinding
appliances, have led to their reintroduction on a large
scale. It should, however, be borne in mind that the
edge-runner mill must be of a pattern suitable to the
materials it will have to treat. The method of drive
usually depends on local conditions. The revolving-
trough type is chiefly useful for mixing, on account
of the ease with which the materials can be charged.
The capacity of edge-runner mills depends on the
nature of the material, the diameter and weight of
the runners, the speed at which they are run, and also
on the rate at which the reduced material is discharged
in order to give place to fresh portions of the charge.
This is effected by means of two sets of scrapers, the
individual members of which can be adjusted in any
direction. Their ploughing action also greatly assists
the mixing effect.
Fig. 6 illustrates an edge-runner mill with revolving
trough and overhead drive; and Fig. 7 one with
stationary trough and bottom drive; both made by
the Badische Maschinenfabrik, Durlach. The runners
are of grey cast-iron, chill -castings or cast -steel being
PREPARATION OF THE COLOUR EARTHS 51
used for crushing hard materials. The trough in
all cases is lined with detachable chill-cast plates.
Special attention is bestowed on the lubrication of
all the moving parts, and all the lubricators are easily
accessible.
The main shafts of the fixed-trough machines have
FIG. 6.
forged cranks, and the metal crank bearings are pro-
vided with dust caps. All the shaft journals run in
detachable metal bushes.
A special advantage attaching to this type is the
automatic screening device and the returning of the
screen residue. In some cases, complicated appliances
are employed to return the coarse residue from the
screen, bucket elevators, worm conveyors, etc., all
52 EARTH COLOURS
entailing increased motive power, not inconsiderable
wear, and a higher prime cost; but in this instance
the object is achieved, without extra power or wear,
by very simple means. The dust-proof shell enclosing
the runners and screen is provided with large doors
and charging hoppers.
FIG. 7.
The motive power required to drive edge-runner
mills depends on the dimensions of the mill and on the
class of material to be treated; the larger the mill
and the coarser the material, the more power needed
to drive it.
This type is the more suitable for raw materials
that are of an earthy character, so that all that is
PREPARATION OF THE COLOUR EARTHS 53
FIG. 8.
54
EARTH COLOURS
necessary is to destroy the cohesion of the particles,
as is the case, for example, with clay and all earthy
minerals.
The wet method of crushing with edge runners is
FIG. 9.
particularly suitable as a preliminary to levigation. A
machine arranged for this purpose is shown in Fig. 8.
It consists of two sets of edge runners, one with fixed,
and the other with revolving trough. The material
is introduced by hand, or by suitable charging mechan-
PREPARATION OF THE COLOUR EARTHS 55
ism, into the upper, fixed-trough machine, where it
is continuously sprinkled with water and kneaded
by the one runner, and is passed thence to the second
roller which forces it through the slotted bed into the
bed of the lower set. The slotted beds of the upper and
FIG. 10.
lower set are offset ; and the chief function of the lower
set, with rotating bed, is to secure intimate admixture
of the material which, in most cases, is already
sufficiently reduced.
Ball Mills. — Ball mills are generally used for crushing
dry materials to fine powder. The mill shown in
Fig. 9 is a typical form of grinding drum enclosed in
56 EARTH COLOURS
a dust-proof casing, the latter being provided, at the
top, with an opening connected to the dust exhaust
pipe. The discharge outlet at the bottom can be
closed by a slide.
The drum is provided with two strong lateral shields
or cheeks (Fig. 10), one of which carries the inter-
changeable cross-arm and the charging hopper. Both
cheeks are lined with detachable chill-cast plates,
to take up the wear. The bed is formed of heavy
steel bars (which can be turned round), between which
are arranged adjustable slits for the discharge of the
reduced material. Guard sieves are mounted all
round, and close to, the bed, and interchangeable fine
screens surround these in turn. The mesh of the fine
screens determines the fineness of the product, and the
residue falls down on to a plate which returns it to the
interior of the drum. The reduction of the charge
is effected by a number of very hard, forged steel balls
of various sizes.
The mill must be run in the direction marked by the
arrow on the outer shell, so that the residue on the
screens can be returned to the drum by the plate pro-
vided for that purpose ; and the prescribed working
speed must be maintained. The mill must not be
overloaded. The impact of the balls should be mild,
but distinctly audible. Overloading reduces the out-
put. Idle running causes the most wear, since the
balls then roll directly on the bed, which, of course,
should be prevented as far as possible. The feed is
continuous; and, of course, only dry material should
be introduced.
When the balls have lost size and weight through
wear, they must be replaced by a fresh set.
Pulverisers. — Pulverisers are the best form of crusher
PREPARATION OF THE COLOUR EARTHS 57
for tough and not over-hard materials. They are
simple and strong in construction, of high capacity
with comparatively small consumption of power,
and furnish a good, uniform product, the size of
FIG. ii.
which ranges from fine powder to coarse granules,
according to the screens used and the class of material
treated.
The crushing is effected by a cross-arm beater,
composed of four to six radial steel arms on. a divided,
58 EARTH COLOURS
cast -steel hub, keyed on to the horizontal shaft. The
arms are hardened, and are adjustably and detach ably
mounted on the hub.
The beating action of the arms, which run at high
speed, forces the material against the studded surface
of the hardened cheeks of the machine and also against
the hardened square steel bars forming the periphery,
the repeated impact of the material on itself, as well
as against the arms and bars, progressively reducing it
until small enough to fall through the screen on the
under half of the casing, into a closed receptacle below.
The screen mesh varies according to the degree of
fineness required.
The peripheral bars are mounted in a very simple
manner, and in such a way that when one edge of the
bars is worn, a quarter turn brings a fresh, sharp edge
into operation, so that all four edges of each bar can
be utilised.
To prevent the escape of dust, the machine is pro-
vided with an air-circulation chamber, which maintains
the flow of air in continuous circulation, the resulting
strong draught also drawing the fine material through
the screen and keeping the meshes open. By this means
the capacity of the pulveriser is considerably increased.
The interchange of the crushing organs and screens,
and also the cleaning of the machine, can be effected
without difficulty or loss of time.
The charge is introduced through a feed hopper at the
side, and may vary, according to the size of the machine,
from nut size to lumps twice as large as a man's fist.
If necessary, suitable mechanical feed devices can be
applied.
Disintegrators (Figs. 12 and 13). — This type of machine
is used for reducing medium-hard or soft materials,
PREPARATION OF THE COLOUR EARTHS 59
especially where it is desired to obtain a comparatively
large output of a gritty product.
In the patterns shown, the main shaft is of steel,
with dust- and dirt-proof red-brass bearings with
pad or ring lubrication. The spindle case draws out
to facilitate cleaning. Mechanical feeding attachments
can be provided.
FIG. 12.
According to local conditions, the disintegrator can
be mounted either on a brick foundation, with lateral
discharge outlet into a storage bin, or on a raised grating
of iron joists.
If the product is to be conveyed to a distance, it
is advisable to have a hopper-shaped collector leading
directly to a worm conveyor or bucket elevator.
The arrangement shown in Fig. 13, in which the
disintegrator is mounted on a dust-proof cast-iron
6o
EARTH COLOURS
collector, has been found very suitable for colour works
of various kinds (aniline, lead, mineral and other
colours), particularly on account of the suppression
of dust ; whilst the automatic charging worm greatly
increases the capacity as compared with charging by
hand.
FIG. 13.
LEVI CATION
The effect of levigation is based on the circumstance
that bodies of greater density than water remain longer
in suspension in that medium in proportion as the
fineness of their particles increases. This treatment
consequently enables the finer portions of a substance
PREPARATION OF THE COLOUR EARTHS 61
to be mechanically separated from the coarser. Leviga-
tion is extensively practised in colour works because
it furnishes powder of finer grain than can be obtained
by even the most careful grinding.
The appliances used for levigation may be of a very
simple character, consisting only of several tubs or
tanks, mounted in such a way that the liquid con-
tained in one can be run off into the one next below.
With this primitive plant, the material to be levigated
is stirred up in the water in the uppermost tub and
left to settle until the coarsest particles may be assumed
to have settled down, whereupon the turbid water is
drawn off into another tub, in which it is left to settle
completely. When the clear liquid has been carefully
drawn off, a fine sludge is left in the bottom of the tub,
consisting of the fine particles of material mixed with
water.
When a particularly fine powder is required, a single
levigation does not always suffice, but the liquid in
the second tub must be left to settle for a short
time only, and then run into a third for complete
subsidence.
A well-designed levigator for treating large quantities
of powder is illustrated in Fig. 14. A stirrer R, driven
by cone gearing, is arranged in a wooden or stone vat
G. The levigating water enters close to the bottom of
the vat, through the pipe W. When G is half full of
water, the stirrer is set running, and the substance to
be levigated is added. After a while, the water laden
with the levigated powder begins to run off at A into
the long narrow trough 7\ provided, at the opposite
end from A, with a number of perforations through
which the water runs into the trough TV From this
it escapes through the perforations into the trough T3.
62
EARTH COLOURS
and thence successively into T4 and T5, finally dis-
charging into the large tank S.
The coarsest and heaviest of the water-borne particles
deposit in the trough 7\, finer particles settling down
in T2, and so on in succession, until the water reaching
the tank S contains only the very finest of all in sus-
FIG. 14.
pension, these taking a long time to settle down to
the bottom. The deposit in the upper troughs can
be returned to the vat, whilst that in the lower ones
will be fine enough to dry as it is. The residue in
the vat is discharged through Z when the operation
is finished.
It will be evident that the fineness of the product
depends on the number and length of the troughs T,
the larger these factors the more delicate will be the
PREPARATION OF THE COLOUR EARTHS 63
particles remaining in prolonged suspension in the
liquid.
Many earth colours require no treatment beyond
levigation to fit them for use in paints. This is the case
with, e. g., the white clays ; and certain grades of ferric
oxide, which occur native in the state of fine powder,
may also be included in this category. In many cases,
however, if large quantities of a finely pulverulent
mineral be stirred up with water and left to stand,
the deposited solid matter forms such a highly coherent
mass that it can only be distributed in water with
difficulty, the fine particles adhering so firmly together
that it is hardly possible to stir them up again completely
in the liquid by means of a paddle.
Nevertheless, this can be easily effected by using
a special appliance of the kind employed by starch
manufacturers for a similar purpose, viz. the levigation
of starch. This apparatus is designed in such a way
that the pulpy charge of material is gradually and com-
pletely disseminated in the introduced liquid.
Fig. 15 shows a device of this kind, consisting of a
circular vessel provided with a step bearing for a ver-
tical shaft driven by cone pinions. The lower part
of the shaft is provided with a thread, on which a nut
is adapted to travel up and down. By means of rods,
this nut is connected to a wooden cross-bar provided
with stiff bristles on its lower face. A horizontal handle
is attached to the nut. The water is admitted through
the pipe on the right.
In working the apparatus, the shaft is rotated and
the handle held firmly, thus causing the nut and
attached cross-bar to rise to the limit of its travel.
The levigating liquid, mixed with the material under
treatment, is then admitted, until the vessel is full,
64 EARTH COLOURS
and when the solids have completely subsided, the
clear liquid is drawn off, and the operation is repeated
until a thick layer of sediment has accumulated on
the bottom of the vessel.
To levigate this, the cross-arm carrying the bristles
is lowered until it just touches the surface of the deposit,
and a continuous stream of water is admitted through
FIG. 15.
the pipe at the side. The bristles gradually disseminate
the upper layers of the sediment in the water, which
becomes turbid and is then drawn off into another
vessel, cement-lined pits being used in the case of
large quantities. When- the brushes no longer encounter
any of the sludge, the cross-arm is lowered sufficiently
to stir up another layer ; and in this way, large quan-
tities of solid matter can be distributed in water. If
the cross-arm is rotated at low enough speed, the
PREPARATION OF THE COLOUR EARTHS 65
coarser particles of material keep on settling down again,
and the collecting vessels will receive only the finest
particles.
In addition to the mechanical separation of coarse
and fine particles, levigation accomplishes another
purpose, namely that the prolonged contact of the
treated material with water dissolves out any admixed
soluble constituents which might affect the quality
of the colour, the latter being left in a purified condition.
For successful levigation it is essential that the charge
should be in a sufficiently fine condition at the outset.
Clayey raw materials require no preliminary treatment
other, perhaps, than passing them through a disinte-
grator, whereas hard, crystalline substances must
first be ground in a wet mill, such as an edge-runner
mill with stationary bed, into which the materials are
fed with an admixture of water, provision being made
for keeping the charge under the runners all the
time. The crushed material is screened previous to
levigation.
In the levigation process a few vessels of large size
are preferable to a number of small ones. The nature
of the material will determine whether any stirrers
are required or not, these being unnecessary in the
case of the pigmentary earths, which naturally remain
a long time in suspension and therefore do not require
stirring up.
The pulpy levigated material is taken out of the tubs,
etc., drained (if necessary) and dried. The draining
may be effected in bags, or — in large plants — filter
presses or hydro-extractors. In these latter instances,
pumps will be provided for feeding the sludge direct
to the presses, and conveyors for delivering the pressed
material to the drying-plant.
5
66 EARTH COLOURS
DRAINING AND DRYING
The levigated colour earths form a stiff pulp con-
taining a large quantity of water, which can be elimi-
nated in various ways. Usually, the mass is dried by
spreading it out thinly on boards and leaving it exposed
to the air until it has become solid ; or else it is only
left long enough to acquire the consistence of a thick
paste, which is then shaped into cones or blocks, which
are allowed to dry completely in an airy place. If
the colours are to be sold in the form of powder, the
dried lumps are crushed.
To accelerate drying, the pulp may be put through
a hydro-extractor, or dried in hot-air stoves or rooms.
As, however, this last method entails special appliances
and also expenditure, this acceleration is only resorted
to when rendered necessary by special conditions.
The Hydro-extractor. — When a substance is set in
rapid rotation, it tends to fly away from the centre at
which the rotational force is applied. The centrifugal
force thus coming into action increases with the velocity
of rotation and with the distance of the substance
from the axis of rotation.
The centrifugal hydro-extractor consists, therefore,
of a vessel in rapid rotation; and if a liquid be intro-
duced into such vessel, it is projected with considerable
force against the peripheral walls. If the peripheral
surface be perforated, the liquid portion of a charge
consisting of liquid and solid matters will be ejected
through the perforations, while the solid matter remains
inside. As a rule, a few minutes' treatment in a hydro-
extractor is sufficient to separate the water from a
thin pulp so completely that the solid residue is in an
almost completely dry state. A hydro-extractor which,
PREPARATION OF THE COLOUR EARTHS 67
though of an old pattern, is well adapted for the purposes
of the colour-maker, is shown in Fig. 16.
The drum A, which revolves easily on a vertical
axis, is of metal, and is provided with a large number
of fine perforations on its peripheral surface. It can
be rotated at high speed by means of the crank / and
-
FIG. 1 6.
pinions d, e, or by the fast-and-loose pulley a b con-
nected with a source of power. To prevent any of
the charge from being projected over the rim of the
drum, the upper edge is turned over so as to leave
only a comparatively small opening at the top. The
lower end of the drum shaft carries a strong steel
spindle, which must be carefully machined and enable
the drum to revolve as easily as possible. This is
68 EARTH COLOURS
essential, because even small machines require a com-
paratively large amount of motive power — which is
not surprising in view of the high speed at which the
drum has to revolve in order to perform its functions.
The drum is enclosed in a casing of somewhat larger
diameter, which may be of any convenient material.
The bottom of the casing is preferably tapered slightly
downward, and is covered, at its lowest part — below
the bearing of the drum — with a sieve communicating
with a pipe through which the ejected liquid is drained
off.
When a liquid, containing solid matter, is fed into
the drum, which is already running at high speed, the
liquid is thrown, by the centrifugal force, against the
peripheral surface of the drum and escapes through
the perforations, leaving the solid matter behind.
Where large crystals are in question, as for instance in
sugar factories, the centrifugal machine can be employed
without any additional precautions, the liquid being
expelled and the crystals being practically dried by
keeping the machine running a short time longer.
In the case of the pulp obtained by levigating colours,
however, this procedure would result in failure, because
the fine solid particles would be ejected along with the
liquid and the drum would be left quite empty.
In this case it is therefore necessary to provide means
for retaining the solid matter in the drum, and allow
only the water to escape, with which object the drum
is lined with a bag of closely woven fabric, open at
the top and fitting snugly against the inner surface
of the drum. When the drum is first started, the
ejected liquid is milky, no fabric being sufficiently close
to retain all the extremely fine solid particles present.
In a very short time, however, the liquid will begin
PREPARATION OF THE COLOUR EARTHS 69
FIG. 17.
70 EARTH COLOURS
to run away perfectly clear, this occurring as soon as
the pores in the fabric have become so far obstructed
by the projected solids as to allow water alone to pass
through. The milky water is then returned to the
feed tank and run slowly into the machine. The water
is very quickly expelled, and the colour remains in the
drum as a stiff paste, of sufficient consistence to be
moulded into lumps of any desired shape. The use
of the hydro-extractor may be particularly recom-
mended when ample motive power is available and
accelerated draining is desirable.
Fig. 17 illustrates a modern type of hydro-extractor
with bottom discharge and suspended drum, the shaft
of which is coupled directly to an electro-motor.
Filter-presses. — Whereas the hydro-extractor is
only used in particular cases for the purpose of the
earth-colour manufacturer, the filter-press enjoys more
extensive application. Every filter-press is composed of
a number of closely fitting press frames, held together by
the pressure of a screw. These frames, when assembled,
form chambers provided with inlet and outlet openings.
vSuitably shaped and stitched filter -cloths are secured
inside the chambers, and the sludge to be filtered is
run into the press from a high-level tank. The water
passes through the filter -cloths and runs off, whilst
the colour earth gradually fills the chambers. When
draining is completed, the press is taken apart and
emptied. In this way the earths are obtained in the
form of more or less dry cakes, which are then put
through further treatment or dried.
Fig. 1 8 shows a Dehne filter-press suitable for the
earth-colour manufacturer. Wood internal fittings
are often used, because wood does not affect the shade
of the colours ; but, wherever the nature of the materials
PREPARATION OF THE COLOUR EARTHS 71
admits, iron presses are to be preferred on account
of their greater durability and the certainty of the
joints continuing tight. The finer the grain of the
levigated colour, the more difficult the expulsion of
the water; but as a rule, a pressure of 115-195 inches,
water-gauge, will be sufficient.
If the sludge be run into the press from a tank at
sufficient height, two charges can be worked in a day,
but the cakes will not be as firm as butter of medium
hardness. It is better to pump the charge into the
press by means of a special diaphragm pump. The
drainage is then incomparably quicker, the cakes will
be formed in about an hour and will also be drier. A
good deal, however, depends, naturally, on the nature
of the earth colour.
If the colour contains acid, alkali or salts, the filter-
cloths can be washed by flushing the press with water
under pressure. The cloths are made of specially
fine cotton fabric. The press-runnings, which are
never quite clear, are collected in a clarifying tank,
72 EARTH COLOURS
where they are treated with lime and kieserite, whereby
gypsum is formed, and the mass is put through a filter-
press, which retains the solids and leaves the effluent
clear.
Filter-cloths which have become choked by use are
spread on a table and scrubbed with water, or else
FIG. 19.
washed in a special machine (Fig. 19), consisting of a
rotary drum, with belt drive, the rotation circulating
the water in the interior trough and enabling it to
extract the dirt from the cloths. The flow and dis-
charge of the water are controlled by valves, and the
water may be warmed by admitting steam into the
machine. The size of the washer depends on that
of the filter-cloths.
From the press, the cakes of colour are conveyed
PREPARATION OF THE COLOUR EARTHS 73
to the drying-plant, usually by the aid of automatic
machinery.
Drying Appliances. — The stiff paste or cakes from
the hydro-extractor or filter-press can be shaped, but
require to be dried before they are put on the market.
Drying is a wearisome operation, the finely divided
material taking a very long time to dry completely,
even during the summer months, whilst in winter it
is almost impossible to get certain colours — such as
ferric oxide colours and levigated clay — quite dry in
the air, the inside of the lumps remaining soft and
pasty after lying for months.
The only way in which this troublesome delay in
the completion of the operation can be overcome is
by artificial dr}dng; but as the employment of arti-
ficial heat entails expense, it is necessary to carry on
the process with the smallest possible outlay, in view
of the low commercial value of most earth colours.
Long experience has convinced the author that the
arrangement of the drying-rooms in many colour works
is based on entirely wrong principles, and that a great
portion of the heat furnished by the fuel is wasted.
For this reason the description of a properly arranged
drying-room will be welcomed by a number of readers.
It is a well-known fact that hot air is lighter than
cold. Consequently, when a room is artificially heated,
the highest temperature will be found just under the
roof or ceiling, and articles placed in that part of a
heated room will dry much faster than those near the
floor. If the drying-room is heated by an ordinary
stove, articles placed on a fairly low level will only
dry very slowly, because the hot air flowing from the
stove tends to ascend.
In order, therefore, to utilise the entire space of the
74 EARTH COLOURS
drying-room, it is necessary to place the heating appar-
atus in such a position that the whole of the room will
be warmed as uniformly as possible. The stove should
therefore be situated in a chamber underneath the
drying-room proper.
Because air that is already saturated with moisture
cannot take up any further quantity, care must be
taken to remove the damp air continuously from the
drying-room, and to replace it by dry air. This may
be effected by suitably designed ventilation, on the
lines shown in Fig. 20, which represents a drying-room
arranged in such a way as to provide for all the above-
mentioned contingencies, and ensure continuous drying.
The heating apparatus is located in the cellar, and
consists preferably of a slow-combustion stove com-
prising a cast-iron cylinder, with an air inlet A (with
sliding regulator T), for the air of combustion, and
a shoot F at the top, through which the stove is fed
with fuel — preferably coke, on account of its great
heating power.
The stove is surrounded by an iron or brick shell M,
having two flues R and Ri leading to the chambers
I and II, where they terminate in register cowls K,
which can be adjusted, by turning the handles h, so
that when the slots o in K coincide with corresponding
slots in the end of the pipe, the maximum amount of
hot air from the stove is delivered into the drying-
chambers; and, by suitably adjusting the cowls and
the draught through the fire-door T, it is possible to
regulate the temperature of the chambers to within
one degree of the thermometer scale. When only one
of the drying-chambers is required to be heated, the
register in the other is closed, and the whole of the hot
air is delivered to the first one. With this arrangement
PREPARATION OF THE COLOUR EARTHS 75
none of the heat is wasted, and the contents of one
chamber can be dried while those of the other are being
removed and replaced.
The moisture-laden air from the drying-chambers
can be led direct into the stove chimney. When coke
FIG. 20.
is used, the flue gases consist almost entirely of carbon
dioxide. If the vent pipes are led from the top of the
drying-chambers into the chimney, the hot gases ascend-
ing the latter induce a strong draught in the chambers
and carry off the moist air into the open. These pipes,
also, are fitted with registers, which, when suitably
76 EARTH COLOURS
adjusted, assist in the maintenance of a uniform drying
temperature.
The colours to be dried are spread on trays laid on
suitable racks in the drying-chambers ; and, by carefully
planning out the available space, a very large quantity
of colour can be quickly and completely dried in a
comparatively small plant. The cost of the fuel is so
small as to be more than counterbalanced by the saving
of time.
The heating arrangements in drying-rooms are
capable of improvement in many respects, especially
where steam is at disposal ; and in such cases, it is
better to substitute steam heating for a fire. It will
then be necessary to put in a good fan, or other device,
to ensure the removal of the moist air. An excessive
room temperature — above, say, 50° C. (122° F.)—
is not only superfluous, but in many cases injurious,
because, apart from the fact that some colours change
in shade when over -warmed, an unduly high tempera-
ture causes the surface layers to dry very quickly and
form a crust which prevents the escape of water vapour
from the interior of the material.
Another form of drying-plant for earth colours is
the drying-floor, a large room with a rammed concrete
or stone floor, intersected with brick flues (about one
foot square), covered with iron or concrete slabs and
conveying hot flue gases from a furnace. These floors
are particularly suitable where there is a possibility
of utilising an existing supply of hot flue gases.
Drying-tunnels are specially adapted where large
amounts of material have to be dried. The tunnels
are built of brick and provided with a rail track on
which the trucks carrying a series of trays laden with
colour are run. As the trucks move slowly forward,
PREPARATION OF THE COLOUR EARTHS 77
they are met by a current of hot air which dries the
charge. The tunnel is kept filled with laden trucks,
each fresh one introduced pushing a finished one out
at the further end.
In many cases, drying troughs are also useful. These
are long, semicircular, jacketed troughs of boiler plate,
hot air or steam being passed through the jacket space.
A worm conveyor keeps the contents moved forward,
turned over and mixed to facilitate drying.
Mention may finally be made of vacuum drying-
cupboards, which are heated, air-tight chambers, for
the material, in which the air is partially exhausted,
thus increasing the rate of evaporation of the water
and causing the materials to dry quickly at a much
lower temperature than otherwise.
CRUSHING AND SIFTING
The distributing and covering power of the earth
colours depends — apart from their special properties—
on the fineness of their particles. For this reason,
all the means adopted for the purpose of pulverisation
are of particular interest. The most important crush-
ing and powdering devices have already been described,
and may be referred to, all that needs mention in
addition being the fact that stone mills also are used
for fine grinding.
The ground products, however, are not entirely
homogeneous, always containing, in addition to the
very finest particles, those of a coarser nature which
must be removed by sifting.
Sifting machines are essentially sieves through which
the colour is passed. The sieves are made of wire
gauze or bolting-cloth, stretched on prismatic frames
78 EARTH COLOURS
which are rotated (centrifugal sieves), or superposed
on the flat and reciprocated. In centrifugal sieves,
the material is projected against the sieve, and the
whole apparatus is in a state of vibration, or else beaters
are provided to keep the fine orifices in the sieve from
choking up.
Nowadays there are numerous types of sifting
FIG. 21.
devices, none of which, however, can be considered as
the best for all purposes, since each type of earth colour
behaves differently and requires special treatment.
The proportion of moisture in the material, also, has
an important influence on the method of treatment
required.
A typical flat sif ting-machine, with eccentric jig
motion, is illustrated in Fig. 21. The machine is
fed through a hopper provided with feed rollers, the
PREPARATION OF THE COLOUR EARTHS 79
rate of feed being adjustable. The screened product
is discharged through a shoot at one side of the machine,
and the residue at the opposite side, into boxes, etc.,
placed underneath.
For materials that give off a large amount of dust,
the machine can be enclosed in a dust-proof casing,
in which event the product and residue are delivered
FIG. 22.
into drawers. The machine is easily cleaned and the
sieves quickly changed, and is well adapted for dealing
with a succession of different materials. The hopper
can be fitted with a pair of adjustable crushing
rollers.
Fig. 22 is a drum sifter, which is fed by means of a
hopper and worm ; and the drum can be covered with
wire or silk gauze. The sifted product falls into a
worm conveyor in the bottom of the casing and is
8o
EARTH COLOURS
discharged at the side. This may be replaced by a
series of mouths for discharging direct into bags, or the
machine can be adapted to deliver into an elevator,
worm conveyor or other means of transport to a
distance.
The screenings are discharged through a shoot at
the back of the machine, and can be handled in various
FIG. 23.
ways. A beater is provided to clear the drum and
increase the output.
Fig. 23 illustrates a centrifugal sifting-machine for
producing very fine powder in large quantities without
any escape of dust. It contains a screening drum,
the frames of which are detachable and facilitate chang-
ing the sieves. A beater revolving inside the drum
projects the powder against the sieves, such portions
as pass through being taken up and discharged by a
worm conveyor; this, however, can be replaced by a
bagging device, etc.
PREPARATION OF THE COLOUR EARTHS 81
CALCINING
Colour earths are sometimes calcined at a high tem-
perature in order to modify their structure and shade,
the operation being accompanied, in some cases, by
the destruction of organic admixtures and the expulsion
of volatile constituents.
An important feature of calcining is that it improves
the covering power of many colours, especially heavy
spar and certain ferric oxide pigments. This alteration
is probably due to the heat causing the finest particles
to cohere, and also to the expulsion of chemically-com-
bined water, etc.
The change of shade, which is often dependent on
the degree and duration of the heating, is probably
also connected with cohesion ; but in many instances
it is attributable to chemical modifications produced
by the treatment ; ferric hydroxide, for example, losing
its water of hyd ration when heated and becoming
transformed into ferric oxide.
The details of the calcination process vary with the
nature of the material, and will therefore be described,
together with the appliances used, when we deal with
the colours which require to be put through this
treatment.
MIXING AND IMPROVING
It is very important that the maker of earth colours
should always be able to turn out his products uniform
in shade, and since the raw materials are liable to vary
in character, and the composition of the earths from
one and the same deposit is not invariable, the desired
shade has to be obtained by mixing. For this purpose,
6
82 EARTH COLOURS
standard samples must be prepared, for comparison
in matching.
Mixing is a highly important operation, on the proper
performance of which oftentimes depends the sale of
certain colours and the reputation of the maker. It
may be effected in various ways, such as shovelling
the ingredients together or by combining the work
with grinding in edge-runner mills, ball mills, etc.
Another method is the mixing barrel shown in Fig. 24,
FIG. 24.
a strong cask mounted on an axial shaft driven by a
motor, etc. The barrel is filled about two-thirds
full of the materials to be mixed, and, after closing
the feed door, is slowly rotated, since, if run at excessive
speed, the contents are merely projected against the
sides of the barrel by centrifugal force, and it can then
be turned for hours without result. The mixing effect
can be considerably increased by mounting the barrel
so that the shaft is offset from the longitudinal axis
of the barrel by an angle of about 30°, the contents
being then moved from side to side at each revolution
PREPARATION OF THE COLOUR EARTHS 83
and thus more intimately intermixed by the twofold
motion.
In addition to such home-made appliances, there
are mixing-machines of the type illustrated in Fig. 25,
the body of which is fitted with a distributing worm
at the top, and a pair of rollers at the bottom. Below
the rollers, which are covered by plates that can be
FIG. 25.
adjusted at a convenient angle, is a worm conveyor
delivering into an elevator, outside the machine casing,
which connects the two worms. One or more discharg-
ing-doors, according to the size of the machine, are
provided under the worm conveyor at the end next
the elevator. The feed hopper can be arranged on
the elevator or on top of the machine, according to
local conditions.
In working this machine, the elevator and distributing
worm are started and the full charge is fed into the
84 EARTH COLOURS
hopper. When it has all passed through the distributor
and is lodged on the sloping plates and bottom rollers,
the latter and the worm conveyor are set in motion,
the material being then carried through by the rotation
of the rollers and dropping on to the conveyor, which
delivers it to the elevator, to be returned to the dis-
tributor. In this way the charge is kept in continuous
circulation, and the finely divided particles are repeat-
edly intermingled, a uniform mixture being obtained.
The effect is heightened by the grinding action of the
rollers as the material passes between them.
The serial order of the various ingredients, their
physical condition (granular or powder), and their
density, are all immaterial, the mixing being effected
so intimately that when, for example, a colour is shaded
with aniline dyes, the ingredients are so completely
blended in less than an hour that even the smallest
sample then taken will perfectly represent the bulk.
These machines are made in various sizes, are entirely
automatic, both in charging, discharging and mixing,
and are quite dust-proof, the consumption of power
being also small. If necessary, they can be combined
with a crusher or sifter feeding direct into the hopper.
A simple means of ascertaining whether the mixing
is completed, and one that can also be employed for
judging the character of ground materials, consists
in placing a sample of the product on a sheet of white
paper and spreading it out, under gentle pressure,
with a steel or horn spatula. No irregularities, streaks,
spots or granules should then be discernible either by
the unaided eye or under a magnifier.
Improving, which consists in staining earth colours
with other (usually organic) colouring agents, to improve
the shade, is an operation which is generally resorted
PREPARATION OF THE COLOUR EARTHS 85
to only in case of need, because it means extra expense,
and is of no value unless light-proof colours are used.
No permanent effect can be obtained by merely mixing-
in coal-tar dyes at random. In addition to certain
organic dyestuffs, artificially prepared mineral colours
and colour lakes — artificial preparations of an organic
dyestuff with an inorganic substratum — are also used
for improving.
Another way of improving earth colours is by pre-
cipitating certain coal-tar dyes on them, in presence
of a fixing agent. Of course the dyes used must not
only be fast to light, but also inert towards the sub-
stratum and to any other ingredient, such as lime, that
is subsequently added to the earth colours.
The following dyestuffs (Hochst) are suitable for
direct precipitation on siliceous colours (green earths,
clay, ochres, etc.).
Auramine, cone. O, I, II ; new phosphine extra ;
chrysoidine A cryst., B cryst., C extra; Vesuvine
(all marks) ; cachou brown D, G ; dark brown M, MB ;
safranine G, GS cone., B cone.; rhodamine O extra,
B, B extra ; fuchsine (all marks) ; fuchsine acetate ;
cerise G, R; grenadine O, R, RR; maroon O extra;
new fuchsine O, P ; methylene violet (all marks) ;
peacock blue P ; Victoria blue B, R ; thionine blue GO ;
methylene blue (all marks) ; malachite green (all
marks) ; brilliant green (all marks) ; coal black O, I, II.
MOULDING
The colour pulp can be made into tablets by moulding
it in dry boxes divided into a number of compartments.
The colour shrinks in drying, and the tablets will then
easily fall out of the moulds. Cones are obtained by
86 EARTH COLOURS
placing the pulp in a box, the bottom of which is per-
forated with numerous holes of uniform size, the box
being then tapped against the surface of a stone table.
At each stroke, a certain amount of colour is forced,
in the shape of small cones, through the perforations,
on to a sheet of paper underneath. The cones are
then dried.
Some colours are moulded into blocks by forcing
the partly dried paste into suitable moulds — preferably
of metal, so that they may be engraved with the maker's
name, or other imprint — and left to dry slowly and
without cracking. The cakes may be prevented from
crumbling by incorporating a small quantity of adhesive,
such as a weak solution of dextrin, with the water in
which the colour is suspended.
CHAPTER IV
WHITE EARTH COLOURS
THE white earth colours are important for the
purposes of the colour-maker, because not only are
they used by themselves as paints, but also serve in
the production of light shades of other colours.
The white colours containing clay or lime are the
most abundant and important of all, and will therefore
be described first. The lime colours comprise caustic
lime, carbonate of lime (chalk or powdered limestone),
gypsum and bone ash.
CAUSTIC LIME
Though this product is not used direct as a painters'
colour, it is employed in the preparation of compounds
that are so used. It is made on a large scale for the
preparation of mortar, and there is therefore no need
for the colour-maker to manufacture it himself, since
it can always be bought from a lime-burner. It must
be borne in mind, however, that lime for the colour-
maker's purposes must possess certain properties,
failing which it is of no use to him. What these
properties are and how the product is made will now
be briefly described.
When carbonate of lime, i. e. native limestone, is
87
SS EARTH COLOURS
exposed to strong heat it parts with carbon dioxide
and is transformed into burnt or caustic lime.
CaCO3 CaO + CO2
Carbonate Caustic Carbon
of lime. lime. dioxide.
The limestone is burned either in kilns of very simple
construction, or else in more complicated furnaces
in which a continuous process is maintained. The
ordinary limekiln, which can be found in many parts
of the country, consists merely of four walls, with a
door in the front one for the introduction of the fuel.
Kilns of this kind are usually set up in the vicinity of
the limestone deposits, and are abandoned when they
get worn out.
The limestone is broken to lumps of fairly even
size, about as large as a man's head, and these are piled
up in a domed heap in the kiln, sufficient space being
left between the lumps for the passage of the flame.
A fire is then lighted under the pile, pine wood being
mostly used for this purpose on account of its high
content of resin, which gives a very strong flame. The
fire is kept up until the top of the pile has become
white hot, and only a blue, smokeless flame is visible.
The appearance of this denotes that the burning is
ended, the fire being then allowed to die out and the
lumps left until cool enough to be taken out of the kiln.
This operation is performed with great care, par-
ticular importance being attached to preserving the
lumps as intact as possible and preventing the formation
of dust, which is of little value. The lime made in this
way is endowed with properties that render it valuable
for the purposes of the colour-manufacturer; but, on
the other hand, possesses certain disadvantages.
WHITE EARTH COLOURS 89
Owing to the use of wood as fuel, the caustic lime
obtained in this way is usually a very pure white,
because the burning is continued until the whole mass
is glowing and the firewood has been completely
consumed. If this is not the case, the burnt lime is
greyish in colour, from the finely divided particles of
carbon, which, of course, spoils the lime for colour-
making. The defects existing in lime burned in the
above type of kiln originate in the irregular character
of the product. It will be evident that the limestone
lumps that are nearest the fire will be far more strongly
heated than those in the upper part of the dome ; and
when calcined lime is kept incandescent for a long time,
it becomes so compact in texture that it quenches with
great difficulty when brought in contact with water.
This condition is known as " dead burnt," and such
lime is of little value.
The lumps at the top of the pile are least exposed
to the heat, and very often still contain carbonate, as
is shown by the effervescence produced on treatment
with an acid. Such lime is imperfectly burnt, and
the lumps frequently still exhibit the crystalline
structure of limestone when broken. They quench
rapidly, but when mixed with a little extra water, the
mass is no longer of the buttery consistency typical
of caustic lime, but contains gritty portions consisting
of unaltered limestone.
Owing to the defects of dead burning on the one
hand and insufficient calcining on the other, colour-
makers now prefer lime that has been burned in con-
tinuous kilns, because, when properly made, such lime
is very uniform in character, and is also cheaper than
that burned with such an expensive fuel as wood. In
consequence of the greater capacity of the continuous
90 EARTH COLOURS
kiln, and the more uniform character of the product,
the old-fashioned kilns are more and more falling into
disuse.
The arrangement of the continuous kiln is very
simple. The kiln consists of a fairly high shaft, open
at the top, and provided at the bottom with a small
hole for the removal of the burnt lime. A coal fire is
lighted, and as soon as the kiln is heated up, alternate
layers of limestone and sufficient coal for burning it
are introduced. The burnt lime sinks to the bottom
of the shaft and is pulled out, with iron hooks, from
time to time.
Given the right proportions of coal and limestone,
the lime made in these kilns is burnt to just the right
degree, and is excellent for builders' use. In many
cases, however, it is less valuable to the colour-maker,
and in some quite useless. For example, when the
coal is not completely consumed, carbon, even though
only a very small quantity, is deposited on the lime,
and the burnt lime, instead of being a brilliant white,
as it should be, is grey ; and colour made therefrom is
also greyish white and will spoil the shade of other
colours with which it is mixed.
The chemical composition of the original limestone
also has an influence on the character of the burnt
lime. Limestone consisting entirely of carbon dioxide
and lime is so rare that sufficient is never available
for making burnt lime on a large scale. Even the
purest limestone found native in large quantities—
namely marble — is not pure carbonate of lime, but
contains a certain proportion of extraneous substances.
At the same time it is too expensive to use for technical
purposes.
The ordinary impurities present in limestone are
WHITE EARTH COLOURS 91
ferrous oxide, ferric oxide, magnesia and organic
matter. The presence of ferrous oxide can usually be
detected by the greenish tinge of the raw limestone,
and the reddish cast of the burnt product. Ferric
oxide is revealed by its reddish colour, in both the
limestone and burnt lime.
Magnesia, which is present, for example, in dolomitic
limestone, cannot be detected by the colour, either
before or after burning, this oxide being itself perfectly
white ; but its presence is a drawback because if in
large quantity it makes the lime very difficult to quench,
and such lime is never of a fatty character.
Organic matter betrays itself by the colour, the
lime being dark tinted, varying from grey to black.
Black limestones usually contain carbon in an extremely
fine state of division, and are quite useless to the colour-
maker owing to the impossibility of completely burning
off this contained carbon, which always imparts a
greyish tinge to the burnt lime. The behaviour of
limestones in this respect varies, however, considerably,
and can only be ascertained with certainty by a trial
burning. Many that are rather dark in colour will,
nevertheless, burn perfectly white, whereas others,
much lighter in shade, always give a product that is
not quite pure in tone. This divergent behaviour seems
to have some connection with the chemical composition
of the organic matter in question. If it consists of
coal, or substances analogous thereto, no really pure
white lime can be obtained from a light grey limestone,
it being impossible to burn off the finely divided carbon
completely.
In addition to making a trial burning with a fairly
large sample of material, the behaviour of a limestone
towards hydrochloric acid will afford some information
92 EARTH COLOURS
as to the nature of the grey colouring matter. If the
limestone dissolves completely when suffused with the
acid, the indications are favourable for its usefulness
to the colour-maker. If, on the contrary, a black
residue is left, the coloration is due to finely divided
carbon, and there is then little prospect of the material
furnishing a suitable product. In any event, a trial
burning is the most reliable guide. In addition to
carbon, the presence of any large proportion of ferric
or ferrous oxide is objectionable, since, in either case,
the product will be tinged red with ferric oxide, into
which the ferrous oxide is transformed at calcination
temperature.
In addition to comparing the colour of the product
with a standard sample, the suitability of a burnt lime
for colour-making can be tested by quenching. If a
lump about the size of the fist be placed in a large
porcelain basin and suffused with a small quantity of
water, preferably poured in a thin stream, the lime,
if properly burned, will continue to absorb the water
for a considerable time, like a sponge, and will very
soon give evidence of a brisk reaction by increasing
in bulk and generating such an amount of heat as to
cause the immediate evaporation of a few drops of
water allowed to fall on the surface of the mass.
Finally, the entire lump will crumble down to a very
delicate, voluminous powder, consisting of slaked lime
(calcium hydroxide).
This chemical reaction is expressed by the
equation :—
CaO + H2O Ca(OH)2
Lime Water Calcium hydroxide.
When the amount of water added to burnt lime is
WHITE EARTH COLOURS 93
no more than sufficient to effect its transformation into
hydroxide, this latter, as already stated, forms a deli-
cate white powder. The addition of more water results
in the formation of a homogeneous pulp, of a peculiar
fatty character. Since this fatty appearance is only
possessed by pure lime, it is a criterion of high quality
in burnt lime, and contrasts strongly with that of the
less valued poor (or lean) lime.
Calcium hydroxide acts as an extremely powerful
base, and therefore must not be mixed with colours
that are sensitive to the action of strong bases. As a
matter of fact, its direct use in painting is very small.
Of course, a thin milk of lime is used for whitewashing
walls, etc. ; and if any colouring ingredients are added
they must be such — e. g. ochres — as are not affected
by the lime. Nevertheless, quick and slaked lime are
very important in colour-making, as forming the
originating material for the preparation of a number
of colours.
When slaked lime is mixed with sufficient water to
form a stiff pulp, and is left exposed to the air for some
time, a change will be observed to take place, the mass
solidifying gradually (commencing on the outside) and
finally crumbling to a soft white powder. This change
is due to chemical action, the lime having a great
affinity for carbon dioxide, which it readily takes up
from the atmosphere — a fact which explains the
solidification mentioned. It would be erroneous to
assume that the lime is again completely converted
into calcium carbonate in this way ; for, though such
conversion does ultimately take place, it requires a
very long time for completion.
The resulting compound is, actually, a double com-
pound of calcium oxide and carbonate. Although this
94 EARTH COLOURS
compound has fairly strong basic properties, they are,
nevertheless, far weaker than those of caustic lime,
being partly neutralised by the carbon dioxide absorbed.
If the superficial area of the slaked lime be increased
by spreading it out thinly, so as to offer greater
opportunity for the action of carbon dioxide, the
formation of the double compound in question will be
greatly accelerated.
This double compound is prepared artificially in
special works, and the resulting colours are put on the
market under various names. They, too, must not be
mixed with colours that are sensitive to alkali, and on
this account they cannot be used in fine paints. If
applied as a white priming to the walls of rooms, care
must be taken to cover the coating with some substance
that will protect the topping colour from the action of
the lime. For this purpose, painters use a wash of
milk, soap and water, etc.
An important property of lime is its behaviour
towards casein, the substance forming the curd of
milk. With this body it combines to form a mass
which sets hard -and is highly resistant, viz. calcium
caseate, and is formed when limewash is stirred up
with milk or freshly precipitated casein. Weatherproof
distempers for outside use are prepared in this manner.
PEARL WHITE
The preparation frequently met with in commerce
under this name is nothing more than a burnt lime of
great purity. It is prepared in the coastal districts
by burning oyster shells, the resulting burnt lime being
easily transformed into a fine powder, the pure white
colour of which is due to the absence of iron. It is
used in the same way as pure burnt lime, and is mainly
WHITE EARTH COLOURS 95
of interest in seaside towns where oyster shells are often
accumulated. It may be pointed out that the name
pearl white is often applied also to pure white grades of
white lead.
VIENNA WHITE
This colour is prepared from any kind of burnt lime
that is sufficiently pure ; that is, free from ferric oxide.
The method of preparation is simple, requiring no
special apparatus, and can therefore be carried out
wherever suitable lime is available.
Operations are commenced by carefully slaking
well-burnt lime with water, a sufficient excess of which
is added to produce a fairly thick pulp. To accelerate
the absorption of carbon dioxide, the mass is exposed
to the air in thin layers, by spreading it out on boards,
so as to present a large surface to the air. As soon as
the pulpy character has disappeared, the mass is
detached from the boards, and is pressed and kneaded,
with wooden paddles, into prismatic cakes which are
left exposed to the air — being, of course, protected
from the wet — until the absorption of carbon dioxide
is complete — a condition that can be recognised by
the earthy character of the product. The cakes are
then dried, an operation entailing great care, since
lightness is a sign of good quality, whereas a damp
product is very heavy.
In forming the cakes they must not be touched by
the bare hands, because the lime is so caustic that it
would soon destroy the skin. The foregoing method of
manufacture is capable of many improvements, which
can be introduced without adding much to the cost of
production.
If the lime is formed into large blocks, it will evidently
96 EARTH COLOURS
take a long time for the mass to acquire, all through,
the earthy character indicating combination with
carbon dioxide. This drawback can be easily remedied
by forming the mass into small cakes, which will
become ripe, owing to their larger surface, much sooner
than the bigger blocks.
A very good plan to adopt in moulding is to form the
burnt lime into a stiff paste with water, preferably by
adding enough water to make a viscous mass, and
leaving this in a lime-pit for several weeks, the prolonged
storage enabling the lime to acquire the already
mentioned fatty character, and at the same time to
become highly plastic. Lime treated in this way can
be forced through a nozzle, forming a cylindrical rope,
which can be cut by a knife into convenient lengths
and left on boards for a few days until they have
become firm enough to stand up without breaking.
Cylinders made in this manner, with a length of about
four inches and a diameter of two inches, will absorb
carbon dioxide very quickly.
The absorption can be still further accelerated by
setting up the cylinders in an atmosphere highly
charged with the gas, for instance in the vicinity of a
manure pit, as they will then avidly take up the carbon
dioxide abundantly liberated from the rotting manure.
Similar acceleration will take place if the boards
carrying the cylinders are placed in a stable, or in a
room where wash for making spirits is fermenting,
because large quantities of carbon dioxide are liberated
in both places.
Working the caustic mass by hand is accompanied
by so many inconveniences that it seems highly
desirable to employ some mechanical moulding device
which will render contact with the wet lime entirely
WHITE EARTH COLOURS
97
superfluous. It may be pointed out that such a device
can also be advantageously used for moulding all earth
colours in paste or pulp form, and in particular for
shaping ferric oxide colours into rods or small cylinders.
Such a machine (Fig. 26) is composed of a rectangular
box with semi-cylindrical bottom, a detachable shaft
carrying a sheet-metal worm being arranged in the box
so that the worm is in contact with the rounded bottom
and is continued into the cylindrical extension of the
FIG. 26.
box. This extension terminates in a hollow cone, to
the mouth of which nozzles of varying aperture (square,
rectangular or round) can be attached. A knife,
operated by hand or mechanical means, enables the
extruded soft mass to be cut into convenient lengths,
which drop on to a series of easy running rollers in
front of the nozzle, and are thereby delivered to an
endless-belt conveyor from which they can be trans-
ferred to the drying-boards.
When the box has been charged with the lime pulp
and the worm is rotated, the latter forces the soft mass
into the cone and extrudes it through the nozzle, so
7
98 EARTH COLOURS
that, as long as there is any material in the box, it is
discharged as a continuous rope, of square, rectangular
or cylindrical section, on to the guide -rollers, where it
can be cut off into lengths by the knife.
A fundamental condition for the preparation of a
good Vienna white is the employment of pure raw
material, which must be free from ferric oxide or
earthy impurities, and fully burned. An excellent
material for this purpose is calcined mussel shells,
which furnish a loose, and at the same time very pure,
lime, and are very largely used for lime -burning in
places such as Holland, where they are available in
large quantities.
Vienna white is not much used as a paint colour,
owing to its powerful alkaline properties which have
a destructive effect on many colours. It is, however,
largely employed as a polishing agent, for which purpose
it is powdered and is put on the market — mostly in
bottles — as Vienna lime. Its very handsome white
colour and low price render it particularly suitable for
coarse painting, for example as a prime coating for
painted interior walls. To guard against the danger
of the painted decoration being destroyed by the
alkaline nature of the white, it is advisable to coat the
dried ground with alum solution, the alumina of which
combines with the lime to form an insoluble compound
to which organic colours adhere well. The sulphuric
acid also enters into combination with the lime, the
resulting gypsum having no effect on the paints
subsequently applied.
CHALK
The name chalk is used for a number of commercial
substances which differ considerably in both the
WHITE EARTH COLOURS 99
mineralogical and chemical sense. French chalk, for
instance, is a mineral belonging to the steatite group
and, apart from its name, has nothing in common with
true chalk, except the white colour, and even this
differs altogether from that of chalk properly so called.
It is therefore necessary, in the interests of proper
nomenclature, to differentiate the various kinds of
chalk, commencing with the mineral known by that
name to the chemist and mineralogist.
In chemical composition, true chalk is calcium
carbonate, but of a fossil character, for if chalk dust
be examined under a high-power microscope, it will be
seen to consist of the shells of minute animals, and is
therefore to be regarded as fossil. The organic matter
of the animals has long disappeared, leaving the
inorganic material, a very pure calcium carbonate,
behind.
Such progress has been made that the zoological
status of the animals which inhabited the shells — many
thousands of which are present in a lump of chalk-
has been identified ; and it is known that these animals
were of marine type. Fig. 27 shows the appearance
of the animal remains in Meudon chalk when highly
magnified, the upper half being viewed by transmitted
light and the lower by reflected light.
Notwithstanding the extremely minute dimensions
of the chalk animalculae, their remains form rocks
of great thickness in all parts of the world. In Europe
we find, for example, extensive chalk formations in
England, whose Latin name Albion was bestowed on
account of the white chalk cliffs occupying long
stretches of the coast. The hills of Champagne consist
almost entirely of chalk; and Riigen, together with
many other islands, is nearly all chalk cliffs.
TOO
EARTH COLOURS
It is only in very rare cases, however, that chalk
occurs in sufficient purity to be immediately suitable
for use as a pigment or writing-material. For the most
part it contains other minerals, or large fossils, from
which it has to be separated by mechanical treatment.
Nodular flints are often met with in chalk, and many
deposits contain such large numbers of the petrified
shells of the sea urchin that the chalk really cannot
be used as a pigment at all, by reason of the high cost
of purification. The only places where chalk can be
advantageously worked for the preparation of pigment
is where the mineral is in a high state of purity, and
also contains only very few sandy particles. Such
chalk deposits are worked on a mining scale, and, as
a rule, in the state in which the chalk comes from the
quarry ; it is in the form of a soft mass, easily scratched
with the finger-nail and of fairly high density, owing
to the considerable quantity of water with which it is
ordinarily impregnated.
In order to convert this crude chalk into a product
that can be used as a pigmc nt, it is first left to dry
WHITE EARTH COLOURS 101
until the lumps can be easily broken, and then crushed
into small pieces, from which all the extraneous
minerals, which occur as large lumps, are sorted out
and removed. This picking process is important,
especially when the chalk contains flints, because these
latter are very hard and would injure the millstones
in the subsequent grinding.
The lumps of chalk are reduced by mechanical means,
such as a stamp-mill, or, more frequently, in a mill of
the same type as for grinding flour, since it is impossible
to get the lumps so dry as to produce the degree of
brittleness necessary for a thorough reduction in a
stamp-mill. The millstones are enclosed in a wooden
casing, and the chalk is ground in admixture with
water, the ground mass escaping, through an opening
in the casing, as a thick pulp which is stored for a
considerable time in large tanks.
Experience has shown that this method of prolonged
storage in contact with water greatly improves the
colour. The only explanation of this fact is that the
chalk still contains a very small amount of organic
matter, which gradually decomposes in presence of
water. The evidence in favour of this is the peculiar
smell given off during storage.
Even with the most careful grinding, chalk cannot
be transformed into such a fine powder that is directly
fit for all purposes; and the only way to obtain the
requisite fineness is by levigation. Owing to the large
quantities that are usually handled in this process, the
milky liquid coming from the mill is mostly run into
large brick tanks, where it is left to settle until all the
chalk has deposited and the supernatant water is
perfectly clear. Tapping-off being usually imprac-
ticable, the water is generally drawn off by careful
102 EARTH COLOURS
syphoning, so as not to disturb the fine sludge at the
bottom of the tank.
The deposit in the settling-tanks is shovelled into
wooden boxes, perforated at the sides to enable the
water to drain away, the chalk being prevented from
escaping by lining the boxes with linen cloths. The
pulp soon loses its liquid character and shrinks con-
siderably, the boxes being then filled up with more
sludge, and so on until the contents have ceased to
shrink. When the mass is so far dry that it will no
longer run when lifted, the boxes are covered with
boards and inverted, discharging the contents on to
the boards, on which the mass is left to become quite
dry. Filter-presses are also used.
Large prismatic masses of chalk never dry so
uniformly as to prevent the formation of cracks, and
if the chalk is to be sold in this form the cracks are
plastered up with thick pulp ; this operation, however,
being superfluous when the chalk is to be sold as
powder.
In order to obtain a more compact product and
accelerate the drying of the moulded lumps, some
makers use presses, in which the fairly dry chalk is
subjected to progressive heavy pressure.
Owing to the fineness of the component particles of
chalk, they adhere so firmly together, without any
bind, that a fair amount of force is necessary to break
down a piece of perfectly dry levigated chalk. Some-
times, however, chalk exhibits the unpleasant property
of losing its cohesion almost completely when dry, and
in such cases it can only be shaped into prisms with
great trouble. This peculiarity is specially accentuated
when the chalk contains magnesia; and in order to
mould chalk of this kind into blocks, a binding agent,
WHITE EARTH COLOURS 103
such as ordinary glue, must be added to the water used
in grinding, care being taken not to use too much, or
the chalk will become too hard, when dry, for certain
purposes, e. g. as drawing or writing chalk.
For some purposes, chalk is sold in powder form,
and very high purity is not then essential, an admixture
of magnesia or clay being harmless. Gilders, for
instance, use large quantities of chalk for priming
picture frames, and stir the chalk up with a certain
amount of bind (mostly size), to give the particles the
desired cohesion.
The chief requirement exacted of a good quality
chalk is a handsome white colour; and this depends
entirely on the quality of the raw material, not on the
method of preparation. It is known that a substance
quite devoid of colour will furnish a perfectly white
powder, because the colourless particles reflect the
light in all directions without breaking it up into its
constituent yellow, red and blue rays. Chalk, too, is
in reality a colourless substance, and reflects light with
greater uniformity in proportion as the fineness of the
particles increases. Consequently, when one has a
chalk that is not perfectly white, it can, nevertheless,
be made to furnish a very handsome product by
bestowing great care on grinding and levigation.
Properly prepared chalk should be as fine as the finest
flour.
When the colour of the best grades of chalk are
compared with what may be termed pure white — such
as that of white lead, zinc white, permanent white—
a skilled eye will always detect a greyish or yellowish
tinge in the former, even if obtained from the whitest
Carrara marble.
The grey tinge is due to the presence of organic
104 EARTH COLOURS
matter, which cannot be eliminated by any known
means, but which can be shown to exist by the fact
that when such chalk is heated to incandescence in the
air for a short time, the resulting burnt lime is pure
white, the organic matter having been burned off.
A yellow tinge is caused by minute traces of ferric
oxide, which — as also ferrous oxide — almost invariably
accompanies calcium carbonate ; and limestone free
from determinable quantities of these oxides is of rare
occurrence. Ferrous oxide does not reveal its presence
in limestone unless in large proportion, its pale green
colour being of low tinctorial power, whereas ferric
oxide, which is a very strong colouring agent, can be
more readily detected.
To those who are engaged in the manufacture of
white earth colours, however, it is quite immaterial
whether a limestone or chalk contains ferrous oxide,
because that oxide quickly changes into ferric oxide
in the finely divided product, and a chalk which was
originally pure white will become decidedly yellow in
a short time.
Fortunately, such a yellow-tinged product can be
rendered perfectly white by simple means and at small
cost, all that is necessary being to add a suitable
quantity of a blue colouring matter. When this has
been done, the chalk will seem pure white to even the
most skilled eye.
This result of adding a blue pigment is based on the
well-known physical fact that certain kinds of coloured
light produce white light when combined, the colours
that give this effect being termed " complementary."
A pure blue is complementary to a yellow with a reddish
cast- — e. g. ferric oxide — and therefore a chalk that is
tinged yellow by a small quantity of ferric oxide can
WHITE EARTH COLOURS 105
be changed into a seemingly pure white substance by
the addition of a blue pigment.
The only pigments of use in this connection to the
colour-maker are such as have very intensive colouring
power and at the same time are low enough in price.
Such substances are ultramarine, smalt and coal-tar
dyes. Smalt is the best because its colour is unalter-
able. In point of chemical composition, this substance
is a very hard glass coloured blue by cobalt ous oxide.
For improving the colour of chalk or any other white,
the smalt must be in an extreme state of fine division,
and levigated to an impalpable powder. Ultramarine
can be used for the same purpose, but is not so
permanent.
To ascertain the correct proportion of blue pigment,
it is advisable to make a systematic experiment, which
is easily performed. Exactly 90 parts of the chalk
in question are triturated with 10 parts of blue pigment
in a mortar until the entire mass has become a perfectly
uniform pale blue powder, which contains 10% of the
blue ingredient.
Several samples, each representing one hundred
parts of the white pigment to be corrected are carefully
weighed out, I part of the blue powder being added
to the first sample, 2 parts to the second, 3 to the third,
and so on, and the mixtures are compared with a
standard white substance, such as best white lead or
zinc white, to see which most nearly approaches the
standar4 colour. It is then easy to calculate how
much of the blue requires to be added to 100 or 1000 Ib.
of the material to be corrected.
The correction can be effected in several ways ; for
instance, by grinding the blue pigment directly with
the bulk, by adding it at the levigation stage, or mixing
io6 EARTH COLOURS
it with the dry, finished product. The first two methods
are attended with certain drawbacks which render it
difficult to obtain a perfectly uniform product, owing
to the specific gravity of the blue pigments being higher
than that of the whites. Consequently, when the two
are mixed in presence of water — as is always the case
in grinding and levigation — the heavier blue pigment
settles down more quickly, and several strata can be
clearly distinguished in the sediment. The upper
layers will still have a decided yellow tinge — the
proportion of blue being too small for proper correc-
tion-— whilst the next in order will be pure white—
accurately corrected — and those at the very bottom
will be decidedly blue, because they contain the largest
proportion of the blue substance.
The most satisfactory results are obtained by dry
mixing ; and this can be successfully practised when the
colour-maker has a cheap source of power (such as
water power) available. Where/however, costly power
plant has to be provided, only the finest grades of
white pigments can be improved in this way, the
expense of labour being too high for cheap materials.
As a pigment, chalk possesses many valuable
properties. The organic structure of chalk gives it
high covering power as a wash, a thin layer applied
to a surface sufficing to mask the colour of the under-
lying ground completely. The lime in chalk being
combined with carbonic acid, its basic properties are
so extensively weakened that chalk can be mixed with
even the most delicate colours without fear of their
shade being affected. A coating of pure chalk paint
on any surface will never change colour in the air;
and on this account, chalk is extensively used both as
an indoor wash and by wall-paper manufacturers.
WHITE EARTH COLOURS 107
PRECIPITATED CHALK
Many chemical processes furnish soluble salts of
lime that constitute a by-product of little value. These
salts, however, can be advantageously utilised for the
preparation of an artificial chalk which is preferable
to the native article in many respects. For instance,
where large quantities of calcium chloride solution
are available, and soda can be purchased at a sufficiently
cheap rate, they can be converted into artificial chalk,
because these two substances react on each other,
forming, on the one hand, calcium carbonate, which is
precipitated as a very delicate, insoluble powder, and
on the other, sodium chloride, or common salt, which
remains in solution, according to the equation :—
CaCl2 + Na2CO3 = CaCO3 + NaCl.
If, however, these solutions were mixed together in
a crude state, the resulting product would be of only
low value as a pigment, being of a yellow tinge and
never pure white. This is due to the fact that the
impure lime salts, being waste products from chemical
works, frequently contain fairly large amounts of
ferric oxide, and the soda also is often so high in that
impurity that the colour of the precipitated chalk is
considerably impaired.
Fortunately,, there is no difficulty in eliminating this
ferric oxide by chemical means, and obtaining a product
of superior colour to the best native chalk. This is
effected by treating the perfectly neutral lime-salt
solution with calcium carbonate, which causes the
precipitation of the iron, a corresponding amount of
lime passing into solution.
In order to eliminate the ferric oxide from the lime-
loS EARTH COLOURS
salt solution so completely that not even the most
delicate chemical test known will be able to reveal
any trace remaining, the solution is placed in a vat
and stirred up with finely powdered chalk. If the
solution contains any free acid, effervescence, due to
the liberation of carbon dioxide, will take place ; and
in such event the addition of chalk is continued until
the free acid is all neutralised, and the added chalk
sinks to the bottom undissolved. The chalk should
be in slight excess, so that a decided sediment is visible
at the bottom of the liquid when at rest.
This deposit is stirred up again at intervals with the
liquid for several days. When ferric oxide is present,
the colour of the deposit will gradually change to a
yellowish brown, through the precipitation of ferric
hydroxide by the chalk; and in this way the final
traces of iron can be removed.
The liquid is then carefully drawn off, without
disturbing the sediment, and the soda solution is run
in so long as a precipitate of calcium carbonate con-
tinues to form. The completion of the reaction can be
ascertained by pouring a small quantity of the liquid
into a tall, narrow glass, leaving it to clarify, adding a
little more soda solution and observing whether any
further precipitate is produced. On the other hand,
it may be that an excess of soda has already been added
in the precipitating tank; and this can be determined
by testing a sample with turmeric paper — blotting-
paper soaked in a solution of the colouring-matter of
turmeric root — which is turned brown by alkaline
reagents. Even in very dilute solution, soda will give
this colour change, and the test is therefore very
accurate. The complete precipitation of the lime in
the solution can be ascertained by passing a small
WHITE EARTH COLOURS 109
quantity through blotting-paper and treating it with
a little acid potassium oxalate solution, which, if lime
be present, will at once produce a strong crystalline
precipitate of calcium oxalate, which is only very
sparingly soluble in water. If the oxalate gives
merely a slight turbidity, the residual amount of lime
is so small that the process may be regarded as complete.
Since carbonate of soda is usually much dearer
than the lime-salt liquor, it is preferable to leave a
small quantity of the lime unprecipitated. Given
sufficient care in effecting the precipitation, and
especially when fairly strong solutions are used, a
brilliant white precipitate of calcium carbonate is
obtained, which is in such a finely divided state that
the minute constituent crystals can only be detected
under a high magnifying power.
This precipitated chalk being already in an extremely
fine condition needs no further preparation, and, when
washed, is ready for immediate use, forming a handsome
pigment with excellent covering power.
When precipitation is ended, the deposit is allowed
to settle down, and the clear supernatant liquid is
carefully drawn off so as not to disturb the delicate
sediment, which is then stirred up thoroughly with
clean water, left to subside, washed again, and then
spread out to dry on cloths which are suspended by
the four sides. The surplus water drains away and
the residue gradually assumes the consistency of paste,
in which condition it can easily be moulded to any
desired shape. If left long enough to dry completely,
it forms a very delicate powder, furnishing a pigment
of excellent quality.
If this precipitated chalk be moulded into prisms
for sale, the blocks are laid on one of their broad sides
no EARTH COLOURS
until firm enough to turn over on to one of the narrow
faces, slabs of gypsum being used as the supporting
material, in order to ensure uniform drying. The
gypsum absorbs water with avidity and thus dries the
prisms evenly.
A defect of these prisms is their great fragility ; but
their strength may be improved by mixing a little
very weak solution of dextrin to the mass after the
last washing-water has been completely removed. In
drying, the dextrin binds the material of the prisms
sufficiently to keep them from breaking except under
the influence of a fair degree of force.
CALCAREOUS MARL
As already mentioned, calcium carbonate rarely occurs
in a perfectly pure condition in Nature; and chalk,
also, is frequently contaminated by other minerals.
A variety of limestone occurring as extensive deposits
in many places is that in which calcium carbonate
is associated with clay. Sometimes the clay pre-
dominates, and the mineral is then known as marl,
being really a clay contaminated with chalk. If,
on the other hand, the chalk forms the chief constituent,
the mineral is termed calcareous marl.
Calcareous marls are used in much the same way as
limestone, some modification, however, being necessi-
tated by the presence of the clay. Although limestone
containing a certain amount of clay can be burned in
the kiln, it yields an inferior lime that is of little use to
the builder owing to its low binding power. Marl
of a certain composition finds an important application
in the manufacture of hydraulic lime or cement.
The only kind of marl suitable for pigment is that
WHITE EARTH COLOURS in*
containing clay with very little colour; and this is of
somewhat rare occurrence, because most marls contain
sufficient ferric oxide to give them a yellow shade.
Marl that is fairly free from ferric oxide, however, can
very well be used as pigment ; and many white pigments
sold as " chalk " are really finely ground marl.
In accordance with the general practice, in the colour
industry, of giving colours a great variety of names,
and suppressing the real names, which, so far as the
artificially prepared colours are concerned, should
bear some reference to their chemical composition,
numerous white earth colours bear fancy names, though
really consisting of chalk, lime (generally marl), or
white clay.
In France, where both chalk and clay are of frequent
occurrence — the soil of Champagne, for instance,
being all chalky — the manufacture of the white earth
colours is extensively practised, and a large number are
put on the market, usually named after the place of
origin, and consisting of either calcium carbonate or
marl.
The trade names of the white earth colours include
Cologne chalk, Bologna chalk, Briancon chalk,
Champagne chalk, Blanc de Bougival, Blanc de Meudon,
Spanish white, Blanc d' Orleans, Blanc de Troyes, etc.
All are either more or less pure chalk, marl, or a fairly
white clay, pipeclay — which is also used for making
clay pipes and for removing grease spots.
GYPSUM
The mineral known as gypsum, or alabaster, consists
of calcium sulphate, or sulphate of lime, its composition
being expressed by CaSO4 + 2H./X In gypsum the
'H2 EARTH COLOURS
crystalline structure is just discernible, whilst other
varieties, such as the so-called " marine glass," occur
in considerable quantities as large, perfectly transparent
masses. " Russian glass " consists of large, trans-
parent lumps possessing the specific property of
gypsum, viz. that of cleaving in two directions, in a
high degree. Alabaster is composed of finely granular
masses, which are either quite white, or else yellowish,
or traversed by grey veins. This variety of gypsum is
very abundant in central Italy, and the best blocks
are employed for the production of works of art.
Ordinary gypsum, which frequently occurs in the
vicinity of dolomitic limestones, is found in a great
variety of colours, bluish-grey, yellowish or reddish
tints being the most common. Pure white lumps,
which are plentiful in some deposits, can be used as
white pigment, the method of preparation being simple,
viz. merely reducing the mass to powder. This is
easily effected, the specific hardness of gypsum being
only 2 ; and in many cases it is soft enough to scratch
with the finger-nail.
If the original gypsum is white, the powder forms a
dazzling white flour which, notwithstanding, is of
comparatively little value as a pigment, on account of
its low covering power. For this reason, powdered
gypsum is chiefly used for making plaster of Paris
(calcined gypsum) for plaster casts and stucco. Gypsum
may also be employed to advantage for lightening
various colours, since it is inert towards even the most
delicate.
KAOLIN, PIPECLAY
Large areas of the earth's surface are covered with
clay, which often attains a considerable thickness.
WHITE EARTH COLOURS 113
Nevertheless, the kind of clay that is suitable for use
as pigment is comparatively scarce. The principal
requirement for this purpose is a pure white colour,
but by far the great majority of clays are either yellow
or of a shade between blue and grey (for example the
clay of the Vienna basin).
The character of clay is just as varied as its colour.
In some places, large deposits of extremely fine clay
are found, the material, when mixed with water,
forming a highly plastic mass which, when dried and
subjected to slight pressure, furnishes a very soft
powder. On the other hand, some clays are so inter-
spersed with large quantities of sand, large stones and
the debris of mussels, that they cannot be used until
they have been put through very careful mechanical
treatment.
This great divergence in the physical character of
clays is due to their method of formation. Clay
originated in the weathering of felspar, which chiefly
consists of a double salt, a compound of the silicates
of alumina and potash. Under the influence of air
and water, this compound is decomposed, the potassium
silicate passing into solution, whilst the aluminium
silicate, being insoluble in water, is carried away by
that medium. When the water can no longer carry
the particles of aluminium silicate in suspension — for
example when it reaches a sea or lake — the silicate
settles down to the bottom, and a deposit of clay is
formed.
If the original felspar was very pure, and in particular
very low in iron, the resulting clay will be of a handsome
white colour. An example of this is afforded by
kaolin, or porcelain earth, which is preferably used for
making china. If, however, the felspar contained a
ii4 EARTH COLOURS
considerable proportion of ferric oxide, the resulting
clay is yellow; and if stones or mussel shells became
incorporated with the clay prior to deposition, these
bodies will be found as inclusions in the deposit, and
such clay will require much troublesome preparation-
grinding and levigation — before it is fit for use.
For the purposes of the colour-maker, the most
suitable clay is one that is pure white, free from inclu-
sions, and does not change colour when exposed, in a
finely divided state, to the action of the air. Many
clays that were originally white gradually assume a
yellow tinge on prolonged exposure to air and moisture,
because the clay contained ferrous oxide, which
changes, in the air, to the stronger pigment, ferric
oxide.
Many kinds of clay merely require a simple levigation
to fit them for use as pigment. The lumps of freshly
dug clay are placed in large tanks, etc., filled with water
and stirred up continuously in order that, instead of
forming a plastic mass which is very difficult to dis-
tribute in water, the particles detached from the lumps
may pass at once into suspension. This turbid water
is then transferred to another tank, etc., where the
minute particles of clay are allowed to settle down, and
the water becomes quite clear.
Where this work is carried on on a large scale, it is
advisable to put the freshly won clay into large pits
close to the clay deposit, and to leave it there, covered
with water, during the winter season. The freezing
of the water breaks down the larger lumps of clay,
by the resulting expansion, and this facilitates the
subsequent levigation, the cohesion between the
particles being destroyed.
If the clay contains larger proportions of lime or
WHITE EARTH COLOURS 115
magnesia, a little experience will enable their presence
to be detected at once by the way the clay behaves
on being placed in contact with water. Pure clay
quickly forms a fatty and extremely plastic paste, and
sticks closely to the tongue when applied in the dry
state. On the other hand, clay containing much lime
or magnesia is far less plastic when mixed with water,
and the dry clay hardly adheres to the tongue at all.
These latter clays are classed as poor or lean, in
contrast to the fat, plastic kinds. For certain purposes
for which clay is used as pigment, these admixtures
are not harmful ; whereas others, especially quartz
sand and mica, not infrequently present in white clays,
constitute a serious drawback.
As already mentioned, clay is formed by the weather-
ing of felspar, which is a constituent of granite and
gneiss, both rocks composed of quartz, mica and felspar.
When the clay has been derived from the weathering
of such rocks, it is easy to understand that it may
contain admixtures of quartz and mica, which are
frequently visible to the naked eye, or at any rate
under the microscope. Whereas clay forms a white,
amorphous mass, the grains of quartz sand are decidedly
crystalline, transparent and of vitreous lustre; the
scales of mica, on the other hand, appearing as thin
tabular crystals, mostly of a green or brown colour
and exhibiting, when viewed at certain angles, a
brilliant metallic sheen.
Quartz sand can be eliminated from clay witnout
any special difficulty, quartz being of higher specific
gravity and therefore settling down quickly, leaving
the delicate particles of clay in suspension in the liquid.
The scales of mica are harder to get rid of, their tabular
form retarding deposition from the suspending liquid ;
n6 EARTH COLOURS
and on this account, several washings are often required
to separate them completely.
In all cases where clay is to be used as a white
distemper, the presence or absence of lime is immaterial ;
but where it is to be employed for removing grease,
lime is a drawback. This is also sometimes the case
when the clay is wanted for the purposes of the colour
manufacturer. The author has found, by experience,
that perfectly pure, white clay forms a good paint,
in a vehicle of oil or varnish — a purpose to which it
has, so far, been seldom applied, if at all. Such paint
is of good covering power, and possesses the valuable
property of remaining quite unaffected by atmospheric
influences.
If, however, the clay contains even but a small
quantity of lime, it cannot possibly be used as an oil
or varnish paint, for though the freshly made paint
has a very good appearance, its character soon changes,
turning viscous and suffering a considerable diminution
of covering power. Thinning with turps or boiled
oil results in the formation of small lumps, so that it
is quite impossible to obtain a uniform coating on even
a small surface.
This behaviour is apparently due to the presence of
the lime, the explanation being that the fatty acids
always present in the oils and varnishes used for the
paint combine with the lime to form compounds
which, from the standpoint of the chemist, must be
regarded as soaps. The small lumps already mentioned
really consist of lime soap, and the formation of these
colourless compounds accounts for the lessened
covering power.
Given a fine white clay, otherwise capable of forming
a valuable pigment, it is sometimes possible, by simple
WHITE EARTH COLOURS 117
means, to eliminate accompanying lime, provided
the amount of the latter is not too great, and also
provided that very cheap hydrochloric or acetic acid
is available. The acid need not be pure, and the impure
but very strong pyroligneous acid, which is very
cheap on account of its empyreumatic smell, may be
used.
To eliminate lime from the clay, the still moist
levigated mass is introduced, in small quantities, into
a vat containing the requisite quantity (see later) of
hydrochloric or acetic acid, the addition being con-
tinued until the liquid gives only a faintly acid reaction
with blue litmus paper. When the clay is run in,
effervescence is produced by the liberation of the
carbon dioxide displaced by the stronger acid employed.
The amount of lime present in a clay may be deter-
mined by very simple means. A small sample of the
clay is dried by artificial heat, until of constant weight,
and exactly 100 parts by weight of the dry mass are
placed in a glass and suffused with hydrochloric acid,
sufficient of the latter being used to make the liquid
still strongly acid after effervescence has ceased.
The contents of the glass are transferred to a
filter, and washed with pure water so long as the
washings continue to redden blue litmus paper.
The residue is then dried until of constant weight, and
the difference between the initial and final weights
will give the percentage of substances soluble in
hydrochloric acid.
After performing this simple test on a clay, it is
easy to calculate the quantity of acid needed to extract
all the soluble constituents from a given weight of
the material. All that is necessary is to measure
the volume of acid required to extract a small quantity
n8 EARTH COLOURS
of the clay completely. Thus, if one pint of the acid
at disposal is sufficient to treat one pound of the clay,
the amount needed for a given quantity of clay is a
simple matter of calculation.
Since, on account of the cost of pure hydrochloric
acid, crude acid will always be used, it will be necessary
to remember that this crude acid always contains
ferric oxide in solution — this being the cause of its
yellow colour. If the amount of acid taken is barely
sufficient to combine the whole of the lime, leaving
the latter slightly in excess, the ferric oxide — which
would otherwise tinge the clay yellow — will be
precipitated.
If, on the other hand, the acid is in excess, the
clay is obtained free from all constituents soluble in
the acid. The purified clay must then be freed from
the calcium chloride, formed by dissolving the lime, by
a thorough washing, since the clay would otherwise
always remain moist on account of the hygroscopic
properties of the chloride in question. Moreover, any
small residuum of free acid would constitute a draw-
back on the clay being mixed with other colours.
Calcium chloride is very soluble in water, and there-
fore can be completely removed from the clay by
washing. The purified clay is left to settle down as
completely as possible, and after drawing the liquid off
from the sediment, the latter is suffused with pure
water and left to settle once more. As a rule, two such
washings will cleanse the clay of calcium chloride and
free acid sufficiently to render the product suitable
for any purpose.
When large quantities of clay have to be treated
in this manner, considerable amounts of calcium
chloride solution will be obtained, which can be advan-
WHITE EARTH COLOURS 119
tageously utilised for the production of precipitated
chalk, all that is necessary being to collect the liquor
in a large tank and treat it with a small quantity of
slaked lime, to transform the surplus free acid into
calcium chloride and precipitate the ferric oxide present
in solution. At the end of a few days the liquor in
the tank will consist of a very pure solution of calcium
chloride which will furnish an excellent precipitated
chalk when treated in the manner already described
under that heading.
BARYTES, OR HEAVY SPAR
This mineral — chemically, barium sulphate, BaSO4 —
occurs native, as extensive deposits, in many places —
England, Bohemia, Saxony, Styria, etc. It sometimes
forms handsome tabular crystals, but more frequently
compact masses, which may be pure white, grey yellow,
etc., in colour, and are distinguished by high specific
gravity (usually 4-3-4-7), to which the mineral owes its
name. This high density also limits the application
of the mineral, and it cannot be used as a pigment,
in the true sense of the term, being only suitable as an
adjunct to artificially prepared colours.
The employment of barytes in the colour industry
is often regarded as adulteration, which, however, it
is not when the case is considered from the right point
of view. For instance, the only preparation which can
properly be termed white lead consists of basic lead
carbonate. This, when pure, is a rather expensive
pigment, whereas, for certain purposes, the consumer
requires a product that can be obtained at a low price.
In order to satisfy this demand, the only course open
to the colour-maker is to mix the white lead with a
120 EARTH COLOURS
cheap white substance, which enables him to turn out
different grades of white lead, which, although low in
price, are far inferior to the pure article in covering
power. Pure white lead being itself a very heavy
substance, the only bodies suitable as adjuncts are
such as are also of high specific gravity; and of all
the cheap pigments known, heavy spar is the only
one endowed with this property. Consequently, this
substance is extensively used in making the cheaper
grades of white lead and the pale kinds of chrome
yellow.
The only cases in which the addition of heavy spar
to a colour can be regarded as an intentional fraud on
the consumer is when he is sold, as pure white lead,
chrome yellow, etc., a product really composed of a
mixture of such colour and barytes. Moreover, the
presence of barytes in white lead can be easily detected
by a simple examination, pure white lead readily
dissolving, with considerable effervescence, in strong
nitric or acetic acid, whereas barytes is insoluble in
all acids, and therefore remains, as a heavy white
powder, at the bottom of the vessel. In this way
both the presence and amount of barytes contained
in a sample of white lead or chrome yellow can easily
be ascertained.
The preparation of barytes for the purposes of the
colour-maker is entirely a mechanical operation. The
barytes, which though fairly hard is easily reduced,
is crushed with stamps, ground in a mill and finally
levigated, it being impossible to obtain a sufficiently
fine powder even by repeated grinding.
Native barytes must not be confounded with the
artificial barium sulphate sold as permanent white
or blanc fixe, which is an extremely finely divided
WHITE EARTH COLOURS 121
barium sulphate obtained by precipitating a solution of
a barium salt with sulphuric acid or a soluble sulphate,
and is a painters' colour that is highly prized for certain
purposes. Both the native sulphate and the artificial
variety have the property of remaining completely
unaltered by exposure to air, and they can therefore
be mixed with any kind of pigment without fear of
the colour deteriorating.
As a rule, barytes is first roughly crushed in edge-
runner mills or stamps, and then ground to the extreme
degree of fineness obtainable in ordinary mills. Even
with the greatest care, however, it is impossible by this
means to obtain sufficient fineness of division for mixing
with fine colours, the only way in which this can be
accomplished being by levigation.
Given a fairly pure white barytes to begin with,
levigation furnishes a handsome white pigment that
can be mixed with colours of any kind; but when
used by itself in association with oil or varnish, its .
covering power is very low and the colour never
perfectly white. Native barytes is therefore unsuitable,
as such, for paint.
Varieties that are not pure white are sometimes
corrected with ultramarine, added in the grinding-
mill. If the yellow tinge is due to iron compounds,
this can often be remedied by treating the finely ground
material with hydrochloric acid, which dissolves
them out, this treatment being followed by a thorough
washing with pure water.
As already mentioned, white lead is most frequently
mixed with barytes, this being usually added when
the white lead is being ground, by feeding the two
materials to the mill and grinding them together.
The crudeness of mechanical methods of reduction
122 EARTH COLOURS
is clearly exemplified by comparing the most carefully
ground and levigated barytes with that obtained
by artificial means. The permanent white largely
used in the production of wall-paper, and quite unalter-
able in air, is, chemically speaking, identical with
native barytes, viz. barium sulphate. The two also
seem to be identical in crystalline habit, as is usual
in the case of one and the same mineral, whether native
or prepared by artificial means. Artificial barytes
is obtained by treating a soluble salt of barium with
sulphuric acid, or a solution of sodium sulphate (Glauber
salt), so long as a precipitate continues to form.
This precipitate is barium sulphate, which subsides
completely on account of its extreme insolubility, this
being greater than that of any other salt known. The
rapid rate of deposition results in the formation of
extremely small crystals, which, being colourless and
reflecting the light completely, appear to be perfectly
white. Even when permanent white is applied in
very thin layers to any surface, its covering power is
very considerable, by reason of the extremely fine sub-
division of the material.
This behaviour of artificial barytes in comparison
with that of the natural product, affords an important
hint in connection with the preparation of earth colours,
namely, that in order to obtain products of specially
good quality, the endeavour should be to reduce the
raw materials to the finest condition possible. This
result is accomplished most securely by bestowing
the greatest care on grinding and levigation ; and it
is therefore highly important that the manufacturer
should select, from the various apparatus used in
reducing the materials, those that are best adapted
for the purpose.
WHITE EARTH COLOURS 123
CARBONATE OF MAGNESIA
Although carbonate of magnesia is seldom used
alone as a pigment, it can be advantageously employed
as such when circumstances permit. It is met with
not infrequently, in Nature, in a crystalline form, as
magnesite or bitter spar, the latter name arising from
the fact that the soluble salts of magnesia have a bitter
taste. Still more frequently, magnesia occurs in
association with calcium carbonate, in the mineral
dolomite, which contains up to 20% of magnesia.
A less abundant native mineral is hydromagnesite,
which consists of basic magnesium hydrocarbonate.
Hydromagnesite is a very light, chalk- white mass,
with a non-greasy feel, which, when reduced to a
soft powder, forms an excellent material for paint.
It is highly inert, in a chemical sense, and can therefore
be mixed with the most delicate colours, having no
other effect thereon than to render them lighter in
shade.
This product can also be prepared artificially, by
treating a dissolved magnesium salt with a solution
of carbonate of soda, the result being the formation
of a pure white precipitate, which is very brilliant
when dry, and is characterised by unusually low specific
gravity. In some places, conditions are such that
this preparation can be made on a large scale at very
low cost. For instance, there is a spring at Bilin, in
Bohemia, the water of which contains large quantities
of alkali carbonates in solution ; whilst in the vicinity
of Saidschlitz is a spring fairly rich in magnesia salts.
The waters from these two springs are concentrated
by evaporation, and mixed in large tanks ; and when a
sufficient deposit of the resulting basic carbonate of
124 EARTH COLOURS
magnesia has accumulated, it is taken out of the tanks,
placed on linen niters and washed with water. The
residue is dried slowly, without the employment of a
high temperature, and then forms a white powder,
which is very light and can be used for a number of
purposes, chiefly medicinal, though it is also well
adapted as a material for paint.
For this latter purpose it is, however, far too expen-
sive; but since the conditions obtaining at Bilin are
certain to occur elsewhere, we have included carbonate
of magnesia among the earth colours.
On account of its specific lightness, carbonate of
magnesia is specially adapted for making pale shades
of certain delicate lake colours, which, if toned with
even perfectly pure chalk, would undergo alteration in
course of time. Carmine, for instance, can be graded,
by the addition of carbonate of magnesia, into every
possible variety of shades between the pure red of
carmine itself and the palest pink; and the resulting
colours are quite permanent whether mixed with gum
solution or any other vehicle.
TALC
Although this mineral is not used as a pigment by
itself, it must be mentioned here because it is not
infrequently employed for mixing with other colours,
and is also used in the wall-paper industry. It also
serves to distribute certain pigments in a state of fine
division, the " rouge vegetal " of the perfumer, for
example, usually consisting of talc and a small quantity
of very fine carmine.
In commerce the name talc is sometimes applied
to two separate minerals, true talc and steatite or soap-
WHITE EARTH COLOURS 125
stone. The former is rarely met with native as well-
defined crystals, mostly occurring as scaly masses in
primitive rocks. Thin pieces exhibit a certain degree
of flexibility. The hardness of this mineral is so small
that it can be scratched with the finger-nail; and its
sp. gr. is 2'9-2-8. Talc is easily scraped, and the
powder remains sticking to the knife, a property which
renders the substance difficult to reduce to powder,
because it balls together and takes a very long time to
convert into a fine flour. The process is facilitated by
calcining the talc and quenching it in cold water, this
treatment increasing the hardness and at the same time
making it more brittle, and thus more easy to pulverise.
A characteristic feature of all the talc minerals is
their peculiar greasy appearance and feel. The colour
varies, white pieces alone being of any use to the colour
manufacturer. The yellow- or green-tinged varieties
owe their shade to the presence of ferric and ferrous
oxides. In chemical composition, talc consists of a
combination of magnesium sillicate with hydrated
silica, the supposed formula being : 4MgO . SiO2 +
H2O . SiO2, and the percentage composition : silica,
62-6% ; magnesia, 32-9% ; water, 4-9%.
STEATITE OR SOAPSTONE
Steatite so closely resembles talc in most of its
properties, that the two minerals were long regarded
as identical. Whereas, however, talc is scarcely acted
upon at all by the strongest acids, steatite is completely
decomposed by prolonged boiling therewith, although
both minerals have exactly the same composition.
As a pigment, steatite is far more important than
talc, and, as French chalk, is largely used for drawing
126 EARTH COLOURS
or writing. To prepare it for this purpose pure white
steatite requires no preliminary treatment, beyond
cutting the large lumps up into quadrangular prisms,
which are mounted in wood, like lead pencil, and used
for writing on the blackboard. The powder produced
in the cutting process is made up into pastel crayons.
With this object, the powder is mixed with a sufficient
quantity of some mineral pigment to produce a mass
of the desired shade, and is kneaded to a stiff paste with
water containing an adhesive such as gum, glue or
tragacanth mucilage. The mass is shaped into prisms,
which, when dry, are cut into pencils and mounted
in wood. Steatite being like talc, without action on
even the most delicate colours, can be used as a diluent
in the preparation of light shades.
CHAPTER V
YELLOW EARTH COLOURS
ALL the yellow earth colours, without exception,
have ferric oxide as their colouring principle, the
differences in shade being entirely due to the varying
proportion in which that oxide is present. The various
names under which they are known date back to a
period when the chemical nature of these colours was
still unknown, and have been mostly derived from the
locality of origin.
'file yellow earths can therefore be divided into two
groups, according to their chemical character. The
first group, in which the ferric oxide is present as
hydroxide, comprises all the ochres, Siena earth, and
a number of others which are obtained from native
ochre by special treatment. In the colours of the
second group, ferric oxide is still the colouring principle,
but is combined with other substances in place of water.
It is, as a matter of fact, incorrect to rank the ochres
in general as yellow earths, because they can be made
to yield nearly every variety of colour from the palest
yellow to the deepest red, brown and violet. These
colours merit the particular attention of the colour-
maker and the painter, being distinguished by very
low cost of production, unusual permanence and beauty
of tone. In the interests of that highly important
matter to the artist, namely the production of colours
127
128 EARTH COLOURS
of unlimited permanence, it is desirable that colour
manufacturers should bestow greater care on the
manufacture of these colours than has hitherto been
the case. An extremely favourable point about nearly
all these pigments is that they can be very cheaply
prepared by artificial means, so that the manufacturer
is in a position to turn out a large number of the hand-
somest and most durable colours with a small amount
of expense and labour.
THE OCHRES
Ochres are found in many localities, most frequently
in stratified rock and rubble. The deposits are rarely
extensive, mostly occurring in pockets or beds. Where-
ever found, ochre may be termed a secondary product,
that is to say, one that has been formed through the
destruction of other minerals. The analysis of ochres
from different deposits shows great divergence in
composition ; and some consist almost entirely of pure
ferric hydroxide, that has already undergone natural
levigation and can be used as a pigment as soon as dug.
Such a form is, however, rare, and most ochres are
intermixed with smaller or larger amounts of extraneous
minerals, the contamination being sometimes so great
as to preclude the use of the ochre as pigment by reason
of the high outlay required for extracting the colouring
constituents.
Occasionally, the ferric hydroxide is associated with
a certain proportion of clay, and as this increases, the
ochre passes over into ferruginous clay. This class
can also be used as pigment, in certain circumstances,
that is to say when it is sufficiently rich in ferric oxide
to furnish a deep red mass on calcination. When,
YELLOW EARTH COLOURS
129
however, the proportion of ferric oxide is low, its
pigmentary power is no longer sufficient, and the clay
has not the requisite beauty of colour. The ordinary
earth used for making tiles is an example of this class,
its colour in the raw state being an ugly brownish-
yellow, but turning a dull " brick " red when fired.
In some deposits the ferric oxide is accompanied by
lime. Unless the latter exceeds a certain proportion,
such ochres, too, are suitable as pigments, the lime
being easily removed by simple levigation ; but when
the amount of lime is high, it is difficult to obtain
certain highly coloured shades of ochre from such
material. These shades entail the calcination of the
ochre, and the temperature required is oftentimes
insufficient to transform the lime into the caustic state.
Moreover, the presence of caustic lime would be a
drawback in some cases, it being then impossible to
mix the ochre with other colours without endangering
the shade through the action of the lime on these latter.
The following analyses will show the percentage
composition of ochres from various deposits :
Ochre from —
Comoal
(Savoy).
Vierzeii.
St. Georges.
Ferric oxide
19
23-5
25
Lime ....
2
Alumina ....
20)
Magnesia ....
Silica ....
4!
69-5
70
Water . .
7
7
5
In the majority of cases the mineralogical character-
istics of an ochre enable conclusions to be formed as
to its suitability as pigment. Good ochre is more or
9
130 EARTH COLOURS
less yellow to dark brown in colour, and can easily be
crushed between the fingers to a soft, fine powder
which feels like powdered steatite and does not produce
a sensation of grittiness, this latter indicating the
presence of fine grains of sand in the ferric oxide. The
behaviour of the ochre in presence of water is specially
important. If it adheres firmly to the tongue, and
forms a fairly plastic paste when mixed with a little
water, the mineral contains a large percentage of ferric
oxide, and as a rule will yield ochre of good colour.
In general it may be said that the value of an ochre
varies directly with its content of ferric hydroxide or
oxide, because when this is large the ochre will furnish
a wide range of colours under suitable treatment.
A simple test for quality consists in weighing out
an exact small quantity (10 grms.), and heating it to
a temperature not exceeding 110° C., until the weight
remains constant. A simple calculation then gives the
amount of uncombined water in the sample. Since
the proportion of such water varies in different parts
of one and the same deposit, the test must be repeated,
in order to obtain accurate results, on samples taken
from different points, or, preferably, on a properly
prepared average sample.
Even drying changes the colour of ochre considerably.
To ascertain the behaviour of an ochre on calcination,
a large sample is dried at 110° C. until the weight is
constant, and divided up into a number of small samples
weighing, say, 10 grms. each. The samples are then
heated to different temperatures, one to the melting-
point of lead, another to that of zinc, and so on.
The higher the temperature employed, the more will
the colour of the ochre approximate to red ; and
specimens very rich in ferric oxide will give bright red
YELLOW EARTH COLOURS 131
colours. Beyond this range, a further increase in
temperature will give violet shades, varying with
the temperature and the duration of heating. After
this preliminary test, it is desirable to make another
on a larger scale, with quantities up to about i Ib.
For this test, the different kinds of ochre frequently
found in the same deposit should be mixed together,
in order to obtain an idea of what the mean product,
obtained in working on the large scale, will be like.
On the whole, the results of this second test will be
the same as in the first series, the only object of the
second test being to gain information which may be
particularly valuable in practical work. The bottles
in which the calcined samples are stored should be
marked with the temperature and length of heating, so
that, when it is subsequently desired to obtain an ochre
corresponding to a particular sample, all that is
necessary will be to heat it to the same degree from
the same length of time. The performance of this
simple test will be of great assistance in standardising
the work with a minimum loss of time.
When it is desired to ascertain the composition of
an ochre superficially its behaviour towards hydro-
chloric acid maybe noted. A weighed quantity of the
freshly dug (undried) ochre is treated with pure acid,
free from iron, which will dissolve out the ferric oxide
and lime, leaving clay and quartz sand behind. The
presence of lime is indicated by effervescence on contact
with the acid ; and if there is no effervescence, lime is
absent. At the end of several hours the acid is care-
fully decanted from the undissolved residue which is
then stirred up with water, left to subside, and weighed
when dry. This method will give the amount of
substances, other than ferric oxide and lime, in the
132 EARTH COLOURS
sample. These substances usually consist of clay or
sand.
For a quantitative determination, a small quantity
—usually I grm. — is weighed out, treated with a
corresponding amount of hydrochloric acid, and the
solution filtered into a glass. The residue on the filter
is washed with distilled water, the washings being
united to the acid solution.
This solution is treated with ammonia so long as a
precipitate of ferric hydroxide continues to form, this
being collected on a tared filter and dried at 110° C.
The precipitate may be regarded as pure ferric
hydroxide, and its weight will indicate the proportion
of hydroxide in the ochre with sufficient accuracy
for technical purposes.
In reality, however, it is not pure ferric hydroxide,
but contains in addition all the oxides that are precipi-
table by ammonia, lime being always carried down as
well. It is therefore desirable to dissolve the precipitate
with a little hydrochloric acid, and reprecipitate with
ammonia.
CALCINING (BURNING) OCHRE
In many places ochre is only put through a very
simple mechanical preparation before being sold for
pigment, namely left to dry in the air so that most of
the uncombined water evaporates. No matter how
this drying process is protracted, however, it is impos-
sible to get rid of all the water in this way, a certain
proportion being retained by the hygroscopic action
of the ferric hydroxide, and to expel this the mass
must be heated to above 100° C. Drying is usually
succeeded by pulverising and sifting the loose earthy
mass, which is then ready for sale.
YELLOW EARTH COLOURS 133
When the ochre contains sand or stones, this treat-
ment is not sufficient, and levigation is necessary. No
particular trouble is involved, the mineral being fairly
heavy as the result of its content of ferric hydroxide.
A simple method of treatment suffices to improve the
value of the ochre considerably, and enables a grade
that is not particularly bright-coloured in its natural
condition to be converted into products of very hand-
some tone and various shades. This treatment con-
sists in heating the raw ochre to a definite temperature,
during which process the colour changes progressively,
and any desired tone can be obtained by suddenly
cooling the hot mass.
The reason for this phenomenon is that the higher
the temperature, the larger the amount of water driven
off from the ferric hydroxide, until finally, when a very
high temperature has been reached, the whole of the
water is expelled, and the ferric hydroxide is trans-
formed into ferric oxide. The hydroxide is brown,
whereas the oxide, provided the temperature has not
been raised too high, exhibits the characteristic colour
known as " iron red."
Consequently, the colour of moderately calcined
ochre ranges through a whole scale frcm brown to red ;
and the higher the temperature employed, the redder
the tone. If the heating be protracted after all the
hydroxide has become oxide, the latter undergoes
molecular change, increasing considerably in density
and altering in colour; and after very prolonged
heating, the colour finally becomes violet.
The calcination, or burning, of ochre is ordinarily
performed in a very crude manner. The mineral is
crushed to the size of peas, and spread out on an iron
plate which is made red-hot. As soon as the ochre
134 EARTH COLOURS
has reached the desired shade of colour, it is dropped
into a tub of water and then crushed to powder. The
calcination requires great experience on the part of
the operator, because so long as the product is hot, it
has quite a different colour from that assumed on
complete cooling. Since only comparatively small
quantities of ochre can be treated in this way, and the
operation unnecessarily increases the cost of the
product, owing to the large consumption of fuel, it is
highly desirable to employ a simple calcining apparatus
capable of treating large quantities.
Such an apparatus may consist of an iron drum,
mounted with a gentle slope inside a furnace, from
which it projects at both ends. A shaft carrying a
sheet metal worm is rotated inside the drum; and the
whole apparatus is very similar to an Archimedean
screw.
When the iron drum is raised to a strong red heat,
and small quantities of ochre are fed continuously into
the upper end of the drum, the rotation of the worm
will push the material forward, and contact with the
glowing sides of the drum will produce the necessary
calcination, the degree of which can be modified by
altering the speed at which the worm is turned. The
calcined product is discharged at the lower end of the
drum, either into a vessel of water, or, if only moderate
heating has been applied, direct into a collector.
Fig. 28 represents an apparatus designed by Halliday
for the dry distillation of wood waste ; but, with slight
structural modifications, it can also be used for calcining
ochre. The material to be heated is introduced, in
small pieces, into the feed hopper B, and is carried
downward, by the worm C, into the red-hot drum A,
through which it is propelled by the worm D until it
YELLOW EARTH COLOURS
135
drops out, at F, into the tank G. The length of time
the material is subjected to calcination depends on the
speed at which the worm D is run. The pipe E carries
off the water vapour expelled from the charge.
In order to obtain a uniform product when ochre is
calcined in an apparatus constructed on this principle,
it is necessary that the material introduced should be
FIG. 28.
fairly regular in size, a condition which is easily fulfilled
by squeezing the freshly dug ochre between fluted
rollers, and then passing it over a series of screens, each
grade being then calcined separately.
Moreover, the apparatus is only suitable for calcining
at medium temperatures; and when highly calcined
products are in question, the operation is best performed
in fire-clay cylinders, or in thick cast-iron drums, similar
to gas retorts, built into a furnace.
136 EARTH COLOURS
Other devices for calcining ochre will be described
later.
OCHRES FROM VARIOUS DEPOSITS
As previously stated, ochres are frequently met with
in! Nature, both in the immediate vicinity of iron ore,
and also at considerable distances from such deposits.
In the latter case, the ochre must be assumed to be
the decomposition products of ferruginous minerals
and to have been carried off by water until the latter
became stagnant and allowed the ochre to settle down.
In their method of deposition these ochres are therefore
analogous to clay, and they, too, often contain large
quantities of extraneous minerals, which have given
rise to the diversified substances grouped under the
name of ochre.
Although ochres are so widespread in Nature, only
certain kinds, found in certain localities, have acquired
a high reputation. For the most part, these ochres
are such as have already been prepared in a high
degree, by Nature, for the purpose for which they are
employed.
Thus, we find that all the ochres which have acquired
a high repute among painters for particular beauty
of tone and permanence, are distinguished by two
properties : a high content of ferric hydroxide and great
purity.
The former of these properties imparts brightness
of colour; and such products will furnish, on calcina-
tion, a wide range of colour shades. When, as is the
case with the finer qualities of ochre, the mineral
contains only a very small proportion of impurities,
there is no difficulty in bringing it, by simple grinding
YELLOW EARTH COLOURS 137
or levigation, into a condition in which it is at once
fit for use as a pigment.
The Italian ochres have, for long ages, enjoyed a
high reputation for their beauty of colour and per-
manence. This category includes, for example, the
renowned Siena earth, Roman earth, Italian umber,
and other ochre colours. This high renown is probably
due less to the inherent properties of the mineral than
to the circumstance that the art of painting attained
a high state of development at an early period, and
that the artists paid special attention to the use of
bright and permanent colours for their work.
Although, at present, many deposits of ochre are
known that are quite able to compete, on the score of
beauty, with the best Italian products, the good name
of these latter has nevertheless been maintained. It
is true that the name of Italian ochre is often merely
borrowed, for application to a product originating in
some other country, varieties of terra di Siena, for
instance, being put on the market that have actually
been derived from deposits in Germany.
As a result of this custom, certain names, such as
terra di Siena, umbra di Roma, have become generic
terms, and their use denotes, not an intention to suggest
that the earth colours in question really come from
Siena or the vicinity of Rome, but that the properties
of the article are equal to those of the old-established
colours of Siena or Rome.
It would occupy too much space to go into an
exhaustive description of all the native varieties of
ochre, and would inevitably lead to a good deal of
repetition. It will therefore be sufficient, for our
purpose, to deal with only a few of them.
The best -known ochres are those of Rome and Siena,
138 EARTH COLOURS
the latter being frequently called, in commerce, by its
Italian name, terra di Siena.
Roman ochre forms yellowish-brown masses, of
fairly fine texture and composed of ferric hydroxide
and clay. They are put on the market both in the
raw and calcined state. On calcination, the colour
soon changes to red, and if carefully performed, the
resulting colours have a very warm, fiery tone.
Closely approaching Roman earth is the English
ochre, which is worked more particularly in Surrey,
and is not infrequently sold as Roman. In many
deposits this English ochre occurs in such a high state
of purity that the best pieces are picked out and sold
without being even crushed or ground. The pieces
of lower quality are very carefully ground and levigated,
for the purpose of being calcined for the production of
different shades, and then furnish highly prized colours.
In point of chemical composition, the ochre family
also includes terra di Siena, bole, umber and Cassel
brown. These minerals, however, are not yellow like
ochre, but brown, and will therefore be dealt with
along with the brown earth colours.
ARTIFICIAL OCHRES
Products very similar, both in chemical composition
and colour, to the native ochres can also be very simply
and cheaply made by artificial means. Their prepara-
tion may be particularly recommended to colour-
makers who desire to turn out a wider range of iron
pigments, but are not in a position to obtain natural
ochres at a low price.
In the manufacture of artificial ochre, an endeavour
is made to imitate the natural processes which have
YELLOW EARTH COLOURS 139
led to the formation of ochre, and, of course, to avoid
anything likely to hinder the production of a suitable
colour earth, for example the presence of sand or a
considerable admixture of extraneous minerals.
As already mentioned, the chief impurities in natural
ochres are clay and sand, both of which can be easily
excluded during the manufacture of artificial ochre,
or their amount controlled in such a manner that paler
or darker products can be obtained at will, and the
tone varied, in any desired manner, by calcination, as
in the case of the native article.
The raw material for artificial ochre is always a
ferrous salt, which can be purchased in large quantities
and at very low prices, namely green vitriol, which,
in the pure state, consists of ferrous sulphate, FeSO4
-f 7H2O. This substance forms sea-green crystals,
which are readily soluble in water and impart an
objectionable inky flavour thereto. On exposure to
the air, green vitriol turns an ugly brown colour, and
is no longer completely soluble in water, passing gradu-
ally into the condition of basic ferrous sulphate. This
is because ferrous oxide is a highly unstable substance,
which attracts oxidation and changes into ferric oxide.
This latter, however, requires for the production of
soluble salts a larger quantity of acids than does ferrous
oxide, and therefore the oxidation of ferrous sulphate
in the air leads only to the formation of salts that are
imperfectly saturated with acid, namely basic salts.
When a solution of green vitriol is left exposed to
the air, basic ferric sulphate is also formed, which
settles down to the bottom of the vessel as a rusty
powder. If, however, a corresponding quantity of
sulphuric acid be added to the solution at the outset,
the resulting ferric sulphate remains in solution.
140 EARTH COLOURS
On treating the green vitriol solution with one of
caustic potash, caustic soda or quick lime, the ferrous
oxide is thrown down as the corresponding hydroxide,
forming a voluminous greyish-green precipitate. This
hydroxide still possesses a great affinity for oxygen,
and when the precipitate is brought into contact with
air, its colour rapidly changes to a rusty red, through
the transformation of the ferrous hydroxide into ferric
oxide. The ferrous hydroxide can also be precipitated
by alkali carbonates, the deposits behaving in exactly
the same manner as that thrown down by the caustic
alkalis.
Various methods can be adopted in the preparation
of artificial ochre, the selection depending on the
properties desired in the finished product. To obtain
an ochre with particularly good covering power, the
method must be different from that employed to furnish
a cheap product, in which low price is more important
than covering power.
In the former case, the ferrous hydroxide is mixed
with substances which, in themselves, possess fairly
high covering power, such as chalk or white clay; in
the second, gypsum, which is of low covering power,
is used.
The preparation of the cheapest kinds of artificial
ochre will be described first, followed by that of the
higher grades which belong to the most valued artists'
colours.
For cheap artificial ochres, the ferrous hydroxide
is thrown down by caustic lime from a solution of
green vitriol. According as a lighter or darker shade is
required, two to three parts of ferrous sulphate are
dissolved in water, care being taken to select crystals
of a pure green colour, since those that have a rusty
YELLOW EARTH COLOURS 141
look are only imperfectly soluble, because they contain
basic ferric sulphate.
The solution will always be cloudy, owing to the
partial precipitation of the hydroxide by the lime in
the water; but this is immaterial. For the precipita-
tion, a milk of lime is prepared by slaking one to two
parts of quicklime (according to the quantity of ferrous
sulphate to be treated) in water, and stirring this up
in enough water to make a thin milk. Care must be
taken to exclude any large particles of lime, since these
would find their way into the finished product and
make the colour uneven. On this account, the milk
of lime should be carefully strained through a loosely
woven cloth or fine sieve, into the precipitation
vessel.
The ferrous sulphate solution is then poured in, the
mixture being kept stirred, and an ugly, grey-green
precipitate is produced, consisting of a mixture of
ferrous hydroxide and calcium sulphate, the reaction
being explained by the equation :—
FeSO4 + Ca(OH)2 - Fe(OH)2 + CaSO4.
The larger the amount of ferrous sulphate solution
added to the milk of lime, the darker the resulting
ochre. As soon as all the ferrous sulphate is in, the
stirring is suspended, and the liquid is left until quite
clear. The water is drawn oif through tapholes in
the side of the vessel, care being taken not to disturb
the fine precipitate, and fresh water is added, in which
the deposit is stirred up and again left to settle down.
This operation, which is once or twice repeated, is to
wash the precipitate.
When this object has been sufficiently accomplished,
the mass is shovelled out of the vessel and spread
142 EARTH COLOURS
thinly on boards, where it is left until the desired shade
of colour has been attained, the colour changing quickly
on exposure to air, owing to the oxidation of the ferrous
hydroxide into ferric hydroxide. To ascertain whether
oxidation is complete, a large lump of the mass is
broken across ; and if it is of a uniform yellow-brown
colour throughout, without being darker on the outside
than in the middle, all the ferrous hydroxide will have
been transformed into the ferric state. The product
can now be dried at once, and when ground will be
ready for sale.
To obtain different varieties from the product, it
is carefully heated (in a finely powdered condition) in
shallow pans; but the operation needs caution, or the
water in the gypsum present will be expelled, giving
rise to drawbacks that are manifested when the colour
is used.
For instance, in mixing such a colour with water,
the gypsum would again absorb water and cause the
whole mass to set as a useless solid lump. Since
gypsum parts with its water at a comparatively low
temperature, it is better not to heat these cheap ochres
at all, but to obtain the various shades by modifying
the proportion of ferrous sulphate employed.
Another defect of the ochres prepared by this method
resides in the excess of lime present, it being impracti-
cable to measure out the quantity of lime used with
such accuracy that only just enough is taken to pre-
cipitate the ferrous hydroxide, there being always a
slight excess. This lime is transformed into calcium
carbonate on the mass being exposed to the air, just
as in the preparation of Vienna white; but as the
saturation with carbon dioxide takes a considerable
time, some of the lime remains in the caustic state
YELLOW EARTH COLOURS 143
and is liable to affect other colours that may be mixed
with the ochre.
An artificial ochre uniting in itself all the qualities
of the natural product, and also capable of being shaded
by burning, can be prepared in the following manner.
An accurately weighed quantity of pure crystallised
ferrous sulphate is dissolved in a definite amount of
water, and the solution is treated with successive small
portions of crude nitric acid, until all the ferrous oxide
has been changed into the ferric state. The change
can be detected by a very decisive test. If a liquid
containing ferric oxide in solution is brought into
contact with a solution of red prussiate of potash
(potassium f erri cyanide) , no precipitate is formed in
the absence of ferrous oxide, but only a brown colora-
tion; whereas, if ferrous oxide is present, a beautiful
blue precipitate is formed at once, the colour of which
is so intense that very small quantities of ferrous oxide
can be detected by this means.
For the purpose now under consideration, the
presence of small amounts of ferrous oxide in the
solution is immaterial, because they are soon changed
into ferric oxide on exposure to the air. It might,
therefore, be asked, why take the trouble to oxidise
the ferrous oxide by means of an agent involving
expense, which could be saved by allowing the oxidation
to take place in the air?
The advantage, however, of the direct employment
of a solution of ferric oxide is that it gives at once a
colour that can be dried straight away; wrhilst at the
same time the colour undergoes no change in drying,
whereas it does when ferrous oxide solution is used.
The method of producing ochres from this ferric
solution varies according as the product is to be used
144 EARTH COLOURS
without any further treatment than drying, or is to be
modified by firing.
In the former event, caustic lime is again used as
the precipitant, but in only just sufficient quantity to
throw down all the ferric oxide in the solution. This
amount can be calculated exactly, 36-84 parts by weight
of pure burnt lime being required for every 100 parts
of pure ferrous sulphate taken. The actual quantity,
whether larger or smaller, will depend on the relative
purity of the sulphate and lime ; and this can readily
be ascertained by a simple trial.
The lime is used in the form of milk of lime, as
already described. If lime alone is employed, the
precipitate will consist of pure ferric hydroxide and the
calcium sulphate thrown down at the same time. The
resulting colour, when dried, will be an intensely brown
mass, which can be used in place of the very dark
natural ochres.
In order to obviate entirely the disadvantages
resulting from the presence of a large amount of caustic
lime in the precipitate, fine levigated chalk or white
clay is added in the preparation of the lighter shades
of ochre, the addition being made as soon as the two
ingredients have been brought into contact; and the
mixture is thoroughly stirred, to ensure uniform
admixture with the ferric hydroxide. The colour of
the settled deposit will be lighter or darker in propor-
tion to the amount of chalk or clay employed; and in
this way the whole range of shades from pale yellow
to bright brown can be obtained without the application
of heat.
Ochre that has been made with chalk is unsuitable
for toning by heat, because this treatment would
causticise the lime, and the ochre could not be mixed
YELLOW EARTH COLOURS 145
with other colours, since these would be affected by that
substance. On the other hand, when white clay is
used in preparing the ochre, the latter can be more
easily toned by firing, provided care be exercised in the
process. The ochre must be dried completely in the
air, and either spread out in thin layers on iron plates,
for the burning process, or else put into a drum, of the
kind already described, in which the mass is moved
onward by a worm.
The clay remains unaltered in firing, but the gypsum
parts with its water of crystallisation. In order to
restore the latter, the ochre issuing from the drum is
discharged direct into a vessel of water, in which it
can be kept in constant motion by a stirrer. The
water is soon warmed by the heat of the mass, and
absorption by the gypsum proceeds at a rapid rate.
When the whole charge has been fired and collected in
the vessel of water, the stirrer is stopped and the
precipitate dried, being then ready for use.
In certain circumstances, ochre can be made by other
methods. In large towns, ammonium salts are some-
times obtainable at a moderate price, being manu-
factured in large quantities as a by-product in gasworks.
For our purpose, crude gas liquor might be used, since
it contains ammonia for the precipitation of the
ferric hydroxide. In most cases, however, this gas
liquor contains only very small quantities of ammonia,
and, therefore, in a works of any size, very large vessels
would be needed for the production of a comparatively
small quantity of ochre. On this account, preference
is given to crude carbonate of ammonia, which is also
obtainable at low prices.
On bringing a solution of this salt into contact with
one of ferric oxide, ferric hydroxide is precipitated,
10
146 EARTH COLOURS
and the sulphate of ammonia resulting from the
reaction remains in solution. By stirring white clay
into the liquid at the same time, the ochre can be
correspondingly lightened in shade.
The precipitates obtained in this way can be dried
at once, and converted into any shade obtainable
with natural ochre, from brown to red, by strong
firing. The sulphate of ammonia still remaining in the
air-dried product is completely volatilised by the heat,
and the resulting ochres are even superior to the
natural varieties in beauty and permanence.
OCHRES AS BY-PRODUCTS
In the manufacture of certain chemicals, substances
of divergent composition are obtained which are sold
under the name of ochre and are used as painters'
colours. Whereas ochre, properly so-called, consists
of either ferric hydroxide or ferric oxide in association
with clay, lime, etc., the products now under considera-
tion are basic ferric salts composed of varying quantities
of ferric oxide in combination with certain proportions
of sulphuric acid.
These ochres are obtained as by-products in the
manufacture of green vitriol from pyrites, and in alum
manufacture ; and, according to their origin, they are
classed as vitriol ochre, so-called alum sludge, and pit
ochre. All the basic ferric sulphates of which they are
composed form fairly large crystals, and, therefore, in
most cases, the covering power is small. On this
account the products are of low grade and are put
on the market at low prices, for which reason they are
largely used in making cheap paints.
Vitriol Ochre. — Commercial green vitriol is, for the
YELLOW EARTH COLOURS 147
most part, manufactured from native sulphides of
iron. When many of these sulphides are piled in
heaps and left to the action of the air, oxygen is gradu-
ally absorbed and green vitriol is formed which is
dissolved out by rain and is collected in large clarifying
tanks
In the case of pyrites, however, the mineral must
first be roasted in a current of air, since otherwise its
conversion into green vitriol would only proceed in a
very sluggish manner. In any event, the aqueous
solution of ferrous sulphate has to be concentrated,
by evaporation, to the point at which the green vitriol
crystallises out.
Both in the clarifying-tanks and — still more so — in
the evaporating-pans, a rusty-looking sediment forms
at the bottom, consisting of basic ferric sulphate.
This originates in the partial oxidation of the ferrous
oxide (first formed) while the pyrites is exposed to the
air, and since the quantity of sulphuric acid present is
insufficient to saturate all the ferric oxide, basic salts
are produced.
The yellow-brown sludge deposited in the pans
during the concentration of crude green vitriol liquor,
constitutes the product termed vitriol ochre, which
contains varying amounts of ferric oxide, sulphuric
acid and water, according to the quantity of ferric
oxide resulting from the oxidation of the pyrites and
the character of the latter, e. g, :—
* Ferric oxide .... 65-70%
Sulphuric acid .... 14-16%
Water 13-16%
Although the colour of these ochres is not particularly
handsome, they can be transformed, by firing, into
colours of fairly good quality. As this subject will be
148 EARTH COLOURS
more thoroughly gone into when dealing with the
preparation of the red iron pigments, the applicability
of these ochres will only be casually referred to here.
During the burning process, these ochres, of course,
part with the whole of their contained water; and by
protracted, high calcination, the whole of the sulphuric
acid can also be expelled, so that finally nothing but
pure ferric oxide is left.
Alum Sludge. — Solutions of crude alum always
contain a certain amount of ferric oxide which settles
down at the bottom of the pans during concentration.
This sludge, too, consists of basic ferric sulphate, but
is inferior in covering power to vitriol ochre, the
crystals being of coarser grain. On the other hand,
the ochreous sediment from the alum concentrating-
pans has the valuable property of being readily
transformable into red-brown to pure red tones by
burning. For this reason, particular attention has
been devoted to this sludge in a number of alum works.
Since the products are only of value when burned,
and the shades thereby obtained are always red, they
will be dealt with more fully along with the red earth
colours.
Pit Ochre. — Springs containing small quantities of
ferrous sulphate and other salts are met with in many
iron mines, but, in most cases, the amounts are too
small for their recovery by artificial concentration to
be contemplated. If, however, the conditions allow
of the springs being easily diverted, they may often be
utilised for the preparation of low-grade ochre.
The chemical composition of these pit ochres varies
considerably, and depends on the geological character
of the locality. Water can only dissolve such minerals
as occur in the form of fairly readily soluble com-
YELLOW EARTH COLOURS 149
pounds; and for this reason pit waters are always
solutions of the metals which are found in the mine.
The variety of compounds that may be present in an
ochre can be seen from the subjoined analyses of
ochres deposited from pit waters at Rammelsberg.
As elsewhere, two distinct classes of ochre are met with,
having a conchoid and an earthy fracture respectively.
The latter usually contain rather more ferric oxide,
and, in particular, a higher content of foreign sub-
stances, the most important of which is quartz sand.
In the Table, the ochres with conchoid fracture are
marked A, and those with an earthy fracture, B.
A. B.
Ferric oxide . . . 68-75 63-85
Zinc oxide . ... 1-29 1-23
Copper oxide . . . 0-50 0-88
Sulphuric acid . . .9-80 J3'59
Water . ... 15*52 18*45
Clay and Quartz . . . 4-14 2-00
The preparation of the ochre is a simple matter,
consisting in collecting the mass and sorting out the
loose, earthy portions of a pure yellow colour from the
denser and darker parts. The former are dealt with
separately, usually by a simple process of levigation,
for the sole purpose of getting rid of the earthy matter,
quartz sand in particular.
The denser varieties require much more work, but
yield a far superior product, which, by suitable treat-
ment, can be converted into the finest grades of ochre.
The first operation consists in a very careful crushing,
and as the pieces are often very hard, they are treated
in ordinary or stamp-mills, edge-runners being also
employed with advantage.
The product reduced by any of these means is passed
through a number of sieves, to separate the fine
150 EARTH COLOURS
particles from the coarse; and the finest dust is burnt.
This last treatment causes a considerable loss in weight,
both the accompanying water and most of the sulphuric
acid being volatilised. However, since, as already
stated, all varieties of ochre can be obtained, the
process is consequently very remunerative notwith-
standing the loss in weight it involves.
Yellow Earth. — From the particulars given in the
general description of the earth colours, yellow earth
may also be regarded, to some extent, as an ochre,
but one containing a large proportion of foreign sub-
stances. It might, however, be more accurately
termed a clay contaminated with a considerable amount
of quartz sand and a certain proportion of ferric oxide.
The method of preparation is on the same lines as for
ochre, but burning is never practised, nor is the treat-
ment so careful as for the better grades of ochre, the
low price of the colour making this unremunerative.
CHAPTER VI
RED EARTH COLOURS
THE number of minerals that can be directly used
as red earth pigments is comparatively small, and by
far the greater proportion consist of ferruginous colours,
a few of which are obtained by the mechanical treat-
ment of native iron ores or clays coloured red by ferric
oxide, the majority, however, being formed by burning
certain materials of another colour. To these belong
nearly all the materials mentioned in connection with
the ochres and the brown iron colours, together with
a few by-products of the chemical industry.
In addition to the foregoing, which have ferric oxide
for their pigmentary principle, is the native mercury
sulphide, occurring, as scarlet, crystalline masses, under
the name of cinnabar (vermilion). The only reason
for including natural vermilion with the earth colours
is to make the list complete, the largest proportion
of this pigment being prepared by artificial methods.
The product sold as " Chinese " vermilion may, in
former times, have really been introduced from China
into Europe, and prepared there by grinding and
levigating the best-coloured lumps of the natural
cinnabar; but, at the present time, all the vermilion
made — in Euro peat least — is from sulphur and mercury,
by artificial processes, and the name Chinese vermilion
is merely retained to designate a particularly fine grade.
152 EARTH COLOURS
On the basis of occurrence and chemical properties,
the red earths can be classified into several groups.
The first comprises natural products requiring only
mechanical preparation, such as the minerals known as
hematite, micaceous iron ore, Elbaite, etc., and the
special modification of red ironstone termed raddle.
All these minerals consist almost entirely of ferric
oxide in a pure state. The mineral, bole (red chalk,
terra sigillata, Lemnos earth), is chemically allied to the
ochres, being, like them, composed of alumina, fre-
quently accompanied by lime and small quantities of
magnesia, but differing in that ferric oxide is always
present in bole, whereas the ochres always contain
ferric hydroxide.
The second group consists of the artificial reds
obtained by burning or calcining raw materials, whose
ferric hydroxide is more or less transformed by heat
into ferric oxide, such as( vitriol ochre, pit ochre and
alum sludge.
Of late years the artificial earth colours have attained
a high degree of importance. They are obtained in
large quantities in the manufacture of sulphuric acid
from green vitriol. Formerly, it is true, they were
also used as pigments under the name of caput mortuum
or colcothar, but were not held in much esteem; and
it is only within recent times that it has been discovered
that these inferior by-products can be converted into
very handsome and brilliant colours, which now form
important articles of commerce.
BOLE
Bole, Lemnos earth, terra sigillata, etc., is, for the
most part, a product of the decomposition of highly
RED EARTH COLOURS 153
ferruginous minerals, and occurs, in the form of lumps,
having a conchoidal fracture, in pockets or detritus.
The lumps have a sp. gr. of 2-2-2'5, are Isabella brown
to dark brown in colour, and give a slightly greasy-
looking streak. There are two distinct varieties of
bole : the one adhering firmly to the tongue, whilst
the other lacks this property and, when placed in water,
crumbles down to powder in emitting a peculiar noise.
The composition of the boles varies, but all of them
may be regarded as alumino ferric silicates combined
with water. Most of the specimens examined from
different deposits contain 24-25% of water, 41-42%
of silica, and 20-25% of alumina, the remainder con-
sisting of ferric oxide with small traces of manganese
oxide.
Some varieties, however, are exceptional and contain
only 30-31% of silica and 17-21% of water, e. g. those
from Orawitza and Sinope. Lemnos earth, the true
terra sigillata, is mostly silica (66%) with 8% of water,
and contains a smaller percentage of ferric oxide than
the others. It is also of a distinct colour, lighter than
the true boles and having a greyish or yellowish tinge.
The behaviour of the different kinds on burning is
just as diverse as their chemical composition. Whilst
some kinds are infusible at even the highest tempera-
tures, and merely change into hard, red masses; others,
again, fuse at a moderate heat. This difference is due
to chemical composition, those high in silica being
generally less refractory than those in which alumina
preponderates.
In order to render the boles suitable for painting,
they are put through a somewhat different treatment
than the other earth colours. The freshly dug material
is first sorted, the uniformly coloured lumps of fine
154 EARTH COLOURS
texture being set apart and suffused with water, with
which they form a pasty mass of low plasticity, which
is kneaded by hand to make it homogeneous, and is
then stirred up with more water. When the lumps
have distributed in the water, the latter is drawn off
into a second tub, and the residue is stirred up with
fresh water, the treatment being repeated until the
effluent no longer shows any signs of colour.
The liquid in which the finely divided bole is sus-
pended is left to settle, and the bole subsides as a fine
powder, which is dried to the condition of paste,
pressed into moulds and dried completely.
Owing to its low content of ferric oxide, the colour
of bole is not particularly bright, but is very permanent
— a property equally shared by all the other ferric
oxide pigments.
NATIVE FERRIC OXIDE AS A PIGMENT
In nature, ferric oxide forms extensive deposits,
which, by reason of the light red colour characteristic
of certain varieties of ferric oxide, are largely employed
in painting. These colours may be classed among the
oldest known to mankind, ferric oxide pigments having
been used frequently in the most ancient paintings.
The most important varieties of ferric oxide for our
purpose are : iron glance, with its modifications,
micaceous iron ore and frothy hematite ; red hematite,
and raddle.
IRON GLANCE
This substance forms handsome black crystals of
very high lustre, which, when small and scaly, con-
RED EARTH COLOURS 155
stitute micaceous iron ore. Both, when rubbed down,
furnish a dark red powder of no particular beauty.
Micaceous iron ore forms the transition stage into
frothy hematite, or iron cream, the sole difference
being that the crystals of the latter are much smaller,
and the scales finer, the iron-black colour passing
gradually into cherry red. At the same time, the
lustre, though still high, loses most of the metallic
sheen exhibited by micaceous iron ore.
HEMATITE
The variety known as hematite or bloodstone,
sometimes occurring as shiny nodules, is distinguished
by its handsome red colour. Some of the lumps are
composed of long, thin crystals grouped about a
common centre so as to form a globular mass. Despite
its bright colour, the hardness of hematite (between
3 and 5) prevents it from being used as a pigment,
the value of the product not being commensurate
with the cost of reduction.
RADDLE
There are numerous deposits of red ironstone, in
the state of fine earth, where the operations of grinding
and levigation have, to a considerable degree, already
been carried out by Nature. These deposits form the
mineral which, under the name of raddle, is often used
as a pigment for ordinary paints. It may be con-
sidered to have originated in the transformation of
red ironstone, by the natural forces that can every-
where be seen disintegrating rocks, namely water
and frost, into a fine powder, which has been trans-
156 EARTH COLOURS
ported, often over long distances, by water, and has
finally settled down.
In places where the process has been carried out in
this manner, the raddle will be in a condition, as regards
fineness of division and beauty of colour, that leaves
nothing to be desired, and the material itself is ready
for use as a very valuable pigment. Large deposits
of this kind, however, are of rare occurrence; but
there are plenty in which the ferric oxide is associated
with varying quantities of clay, sand, and sometimes
lime.
The conditions here are on all fours with those of
clay, which, too, has been formed in a similar way.
Pure clay, the so-called kaolin, is a highly valuable
material, whereas ordinary loam — highly contaminated
clay — is only of low value. In judging the quality of
raddle as a pigment, the presence of impurities is of
less account than their nature; and in some cases a
very highly contaminated raddle may be worth far-
more, as a pigment, than one containing only very
small admixtures of extraneous substances.
As stated above, the ordinary impurities in raddle
are clay, lime and quartz sand. An admixture of
clay, even if fairly large, is no great drawback, since
the material can be used in its natural state, and also
be toned by burning. Lime is less favourable, for
though a calcareous raddle can be used as it is, the
lime parts with its carbon dioxide on calcination,
becoming changed into caustic lime and imparting
to the product qualities which preclude its employment
for a number of purposes, especially for mixing with
delicate organic colours.
The presence of quartz sand is immaterial when the
raddle is to be burned, inasmuch as sand is unaltered
RED EARTH COLOURS 157
by calcination. But it constitutes a drawback because
it makes the fine raddle gritty and unsuitable for fine
paint work. The only way to eliminate this impurity
is by levigation — an expensive operation which should,
as far as possible, be avoided for these native ferric
oxides, because they must be sold very cheaply, and
have to compete with the large quantities of oxide
obtained as a by-product of the chemical industry.
The suitability of a given specimen of raddle for use
as a pigment may be easily ascertained by weighing
out exactly 100 grams and heating to about 120° C.
The loss of weight will give the amount of water in
mechanical retention. The residue is suffused with
strong vinegar, and left for several days, being stirred
at frequent intervals. The carbonates of lime and
magnesia present will dissolve in the acid, the ferric
oxide remaining untouched. The liquid is decanted,
and the residue washed several times writh water and
dried, the diminution in weight being a measure of
the carbonates in the sample. If the vinegar has
turned a yellow colour, the presence of ferric hydroxide
in the mineral is indicated, this hydroxide being readily
soluble in acetic acid. If the residue feels gritty, it
contains quartz sand, the amount of which can be
found with sufficient accuracy by levigating the mass
and weighing the sandy residue after drying.
Deposits occur, in many places, of a mineral similar
to raddle, but formed under peculiar conditions.
Thus, there are found, in the vicinity of brown-coal
deposits that are rich in pyrites, earthy masses which
are occasionally of a handsome red colour and consist
of a variety of minerals admixed with a considerable
proportion of ferric oxide.
These masses probably originated in fires in the coal
158 EARTH COLOURS
seams, whereby the pyrites became transformed into
ferric oxide and basic ferric sulphate; and where the
deposits are of sufficient size, they may be advan-
tageously utilised in the production of cheap reds.
In most cases, however, the minerals must be levigated,
owing to the frequency with which they contain large
proportions of extraneous minerals in a gritty condition.
BURNT FERRIC OXIDE AND OCHRES
It has already been stated, in dealing with the
yellow ochres, that these colours can be toned by
burning, part of the ferric hydroxide losing its water
and changing into red ferric oxide. The more severe
the burning, the larger the amount of ferric oxide
formed and the nearer the colour of the product
approximates to red. According, however, as the
original cchre was yellow or brown, the tone of the burnt
colour will lie between orange and brownish red. If
the heating be pushed so far as to transform all the
ferric hydroxide into oxide, the red will come more
and more into prominence in proportion to the amount
of hydroxide in the original material. If the product
consists entirely of ferric oxide, as is the case with that
obtained, as a by-product, in the manufacture of English
sulphuric acid, a pure red ferric oxide (caput mortuum,
colcothar, English red, etc.) will be obtained. If the
heating be increased above a certain point, the pure
ferric oxide will change colour, assuming a brown to
violet tone according to the temperature employed.
(a) Burning in the Muffle
Since, as a rule, the quantity of material treated
in the preparation of these brown, violet to black ferric
RED EARTH COLOURS
159
oxide pigments for the purposes of the painter on
porcelain is not large, the same kind of muffle furnace
(Fig. 29) as serves for making enamels can be used.
The fireclay muffle M is inserted in a reverberatory
furnace 0, with a good draught, and is raised to a
white heat. The finely powdered material to be burned
is spread out evenly on plates of sheet-iron or fire-clay,
FIG. 29.
and introduced into the white-hot muffle, where it is
left for a period corresponding to the colour desired.
To save time, the plates may be pre-heated in a second
muffle arranged above the first.
By this means a large range of tones can be obtained
from one and the same material, by heating it to
different temperatures; and the colours, so produced
are distinguished, not only by their warmth of tone,
but also by very high stability. In fact, they may be
160 EARTH COLOURS
regarded as permanent, because very strongly calcined
ferric oxide only passes very slowly into solution even
under prolonged boiling in the strongest acids. Owing
to this excellent property, which is equalled by very
few other pigments, and the low cost of preparation,
these colours deserve the most careful consideration
by all manufacturers who are in a position to obtain
suitable material in sufficient quantities.
(b) Caput Mortuum, Colcothar
Previous to the English method of making sulphuric
acid by the oxidation of sulphur dioxide with nitric
acid, this acid was manufactured by heating dehydrated
ferrous sulphate (green vitriol) ; and even now, fuming
sulphuric acid — oil of vitriol, or Nordhausen sulphuric
acid — is largely obtained by the same process.
When anhydrous ferrous sulphate, FeS04, is exposed
to a very high temperature— strong white heat — it is
decomposed into sulphur trioxide, SO3, sulphur dioxide,
SO2, and a residue, mainly composed of ferric oxide
and a little basic ferric sulphate, which remains behind
in the heating-pan. In fact, even at the highest
possible temperatures obtainable in the furnaces used
for the distillation of the green vitriol, it is impossible
to recover the whole of the sulphuric acid, a small
portion being tenaciously retained by the iron.
This red residue is sold under various names —
colcothar, caput mortuum, English red, Indian red,
etc. — and is used as a low-grade pigment, and also as a
polishing agent. The name caput mortuum is a
survival from the time of the alchemists, and was
probably applied to indicate a dead-burned product,
from which all the active ingredients had been removed.
Although, in former ages, this substance was held
RED EARTH COLOURS 161
in low estimation as a pigment, attempts have been
made in recent times to convert it, by suitable treat-
ment, into a more valuable product ; and these
attempts have been crowned with success, affording
another instance of how a high commercial value can
be imparted to a waste product by proper manipulation.
(c) Calcining Ferric Oxide
In order to obtain a series of tones of colcothar, it
is subjected to repeated calcination, but not by itself,
since it would require an extremely large quantity of
fuel to effect any change of tone in view of the very
high temperature the material has already been
exposed to in the sulphuric acid plant. If, however,
salt be added, then a variety of tones can be obtained
without recourse to any particularly high temperature.
It is frequently stated that the only effect of the
presence of salt is to keep the calcining temperature
uniform, inasmuch as the salt volatilises at a strong
red heat, and when that temperature is reached, the
whole mass cannot get any hotter until the whole of
the salt has passed off, all the heat applied being
consumed in transforming the salt into the state of
vapour.
As a rule, however, the amount of salt added does
not exceed 6% of the weight of the charge to be
calcined ; and this quantity does not seem to be
sufficient to keep the temperature at a uniform level
through the several hours required for the calcining
process. The author is therefore of opinion that the
salt also has a chemical action on the material during
the calcination.
As already mentioned, colcothar is by no means
pure ferric oxide, but always contains basic ferric
ii
162 EARTH COLOURS
sulphate. Now, it is feasible that some reaction may
take place between the basic sulphate and the sodium
chloride at calcination temperature, with the formation
of caustic soda, which, being a far more powerful base
than ferric oxide, deprives the latter of sulphuric acid,
sodium sulphate being formed. The chlorine of the
salt combines with the iron to form ferric chloride,
which volatilises at a glowing heat.
According to this hypothesis, therefore, the addition
of common salt in the calcination of colcothar is less
for the purpose of maintaining a uniform temperature
within certain limits than for decomposing the basic
ferric sulphate present and inducing the formation of
a product consisting entirely of pure ferric oxide.
The various tones obtained are due to the varying
length of exposure to the heat.
The following method is pursued in the conversion
of colcothar into iron pigments on a manufacturing
scale. The crude colcothar from the sulphuric acid
j)lant is ground, as finely as possible, in ordinary mills,
and the resulting soft powder is intimately mixed with
salt, 2, 4 or 6% being the usual proportions added.
The calcination is ordinarily continued for six hours in
the case of the mixture containing the largest amount
of salt ; but only two hours, or even one, for the other
mixtures.
The operation is carried on in earthenware pipes, a
large number of which (up to sixty) are built into a
furnace. The latter is fired very carefully, the tem-
perature being raised only very gradually, since ex-
perience has shown that much better coloured products
are obtained in this way than by raising the mass
quickly to a high temperature.
When incandescent ferric oxide is allowed to cool
RED EARTH COLOURS 163
down with unrestricted access of air, the colour is not
nearly so bright as when air is excluded during the
cooling. Since air has no action on ferric oxide, this
remarkable phenomenon cannot be due to the presence
of the air, but probably to the influence exerted by the
rapid change of temperature on the arrangement of
the finest particles of the oxide. Nevertheless, some
manufacturers hold that rapid cooling, with restricted
access of air, improves the colour.
To exclude air from the ferric oxide during calcina-
tion, the open ends of the pipes are flanged and covered
with close-fitting plates, which are luted with clay.
The expansion of the internal air as it grows hot would
burst the pipes unless a means of escape were provided,
which consists in leaving small vent holes in the cover
plates.
As previously mentioned, calcined ferric oxide is
very inert, chemically, so that, when the calcination
has been strong, prolonged boiling with the most power-
ful acids is needed to bring the oxide into solution.
If the heating has been continued up to the strongest
white heat, and the ferric oxide maintained in that
condition for several hours, even hot sulphuric acid
will have only a slight effect on the oxide, and the only
way to make it more readily soluble is by fusion with
potassium bisulphate.
Now indifference to chemical action is just the
property required of a pigment for fine work; and in
this respect, the ferric oxide colours are superior to all
others. The gradations of tone that can be obtained
from ferric oxide by varying the calcination are very
numerous, comprising all between iron red, red-brov/n
and pure violet.
The author has tried heating ferric oxide for a
164 EARTH COLOURS
considerable time at a very high temperature, equiva-
lent to the strongest white heat, and obtained a product
which was no longer pure violet, but had a decidedly
blackish colour. Perhaps, by greatly prolonging the
heating, it might be possible to get a pure black; but,
even if this were so, the matter would be of no special
interest, because black pigments for paints can be
prepared in a much cheaper manner. All that would
be accomplished would be the proof that ferric oxide
actually undergoes an extensive molecular modification
when heated.
FERRIC OXIDE PIGMENTS FROM ALUM SLUDGE
Alum is manufactured from alum shale and alum
earth, the former being a carbonaceous clay shale
interspersed with pyrites, and the latter a clay charged
with pyrites and bitumen. The raw materials are left
in heaps for several years, the pyrites being thereby
oxidised with formation of free sulphuric acid and
ferrous sulphate. This free acid reacts further on the
clay, which it transforms into sulphate of alumina;
and by leaching the heaps with water, a solution is
obtained which contains the sulphate of alumina and
the ferrous sulphate. On the liquor being concen-
trated, a basic ferric sulphate is deposited, which is
worked up into red pigment.
For this purpose it is first levigated in a special
manner, the sludge from the pans being placed in a
large vat, suffused with water, and kept in slow circula-
tion by stirrers, which distribute the particles in the
water, forming a turbid liquid. This liquid is con-
ducted into a. gently sloping shute, the sides of which
are perforated with openings at certain intervals, to
RED EARTH COLOURS
165
allow part of the liquid to run off into large collecting
vessels underneath.
The heaviest of the suspended particles settle down
first and are flushed out by the water escaping through
the first opening. The finer the particles, the longer
they remain in suspension, so that the liquid escaping
through the last holes carries off only a very fine
powder. The liquid collected in the different vessels
is allowed to subside and is then drawn off from the
FIG. 30.
firm deposit. The operation is repeated with fresh
quantities of sludge until sufficient sludge has been
collected for further treatment. The collecting vessel
furthest away from the intake of the shute contains
the finest levigated material, and this is used for making
the best ochres.
The levigated mass is dried in a very simple manner,
being usually spread out on boards, which are exposed
to the air in open sheds, covered with a roof to keep
out the rain. Here the sludge is left until it forms a
pasty or earthy mass, and is then calcined.
i66
EARTH COLOURS
The best calcining furnace is of the type used for
colcothar; but the pipes must be connected to an
exhaust pipe for carrying off the vapours disengaged
during calcination.
However, since alum manufacturers do not usually
go in for making the highest-grade pigments, simpler
calcining furnaces are used, consisting of reverberatory
furnaces in which the heating-gases are allowed to act
directly on the materials of the charge. A front
FIG. 31.
elevation and section of such a furnace are shown in
Figs. 30 and 31. The furnace is constructed with
several arches, one above another, marked c, k, d.
The charge is introduced through the openings b and
b'. The furnace chamber is at a, and the ashpit at g.
The gases of combustion flow over the charge on the
hearths of the several arches and escape, at the top,
into the stack, along with the acid vapours liberated
from the glowing mass.
The further the hot gases get away from the fire,
the cooler they become, and therefore the less strongly
heated the charge on the upper hearths. Consequently,
RED EARTH COLOURS 167
the resulting product (ferric oxide) from the different
stages of the furnace differs in colour; and a number
of gradations can be obtained by blending. The ferric
oxide pigments prepared in this way are not pure
oxide, but also contain small quantities of sulphuric
acid and metallic oxides which were present in the
original crude sludge. However, by reason of the
simple process of preparation employed, these pigments
are usually sojd at lower prices than those from colco-
thar; and for less fine work they are excellent.
CHAPTER VII
BROWN EARTH COLOURS
IN point of chemical composition, the majority of
the brown earth colours are closely allied to the reds,
both kinds containing ferric oxide. The main difference
consists in that, in the brown earths, the ferric oxide
is combined with water to form ferric hydroxide.
Many of the brown earth colours, however, are of
entirely different chemical composition, and either
consist mainly of organic matter derived from the
decomposition of plants — and therefore very similar
to brown-coal or peat — or else contain varying quanti-
ties of inorganic substances mixed with these dark-
coloured organic decomposition products.
The brown earth colours form a highly important
group, some of the members of which are used in the
finest paintings, and, for certain purposes, cannot be
replaced by other pigments. Those containing ferric
hydroxide are found — though not very frequently — in
natural deposits, the most celebrated being the terra
di. Siena, occurring in the vicinity of that city.
TERRA DI SIENA
This highly renowned pigmentary earth is found in
deposits, and, in the crude state, forms dark brown
masses which are devoid of lustre, crumble readily
between the fingers, have a smooth conchoidal fracture
BROWN EARTH COLOURS 169
and absorb water with avidity, in consequence of which
property they adhere to the tongue. Their chief
chemical constituent is ferric hydroxide, with which,
however, variable quantities of sand, clay and ferric
oxide are admixed. These admixtures cause a con-
siderable divergence in the colour of the earth, ranging
from pure brown to reddish -brown, and, in the case
of very impure lumps, to an unsightly yellow-brown.
Mineralogically, terra di Siena is often regarded as a
distinct species which, according to the results of
analysis, must be considered, not as ferric hydroxide,
but as ferric silicate combined with water. Sometimes,
a portion of the ferric oxide is replaced by alumina, so
that the percentage composition of the mineral becomes
approximately: ferric oxide, 66%; silica, n%;
alumina, 10% ; and water, 13%. The hardness of this
mineral is 2*5, and the sp. gr. 3-46.
The method of formation of terra di Siena was
probably on the same lines as that already described
in the case of ochre, namely by the breaking down of
minerals — in this case brown ironstone — and natural
levigation, the powder being deposited in places where
the water containing the ferric hydroxide in suspension
came to rest and allowed the solid particles to settle
down.
The best lumps of terra di Siena in point of purity
and colour can be used as pigments without any
preparation; but in most cases the earth is lightly
calcined, in order to improve the colour. This treat-
ment enables a whole series of tones, from pure brown
to the brightest red, to be obtained. The stronger the
heating, the more water expelled from the hydroxide,
and consequently the closer the approximation of the
colour to that of ferric oxide.
170 EARTH COLOURS
The pigments met with in commerce as terra di
Siena can also be prepared artificially, by making ferric
hydroxide and heating this, when dried, until the
requisite tone is attained. For this purpose, ferrous
oxide is precipitated from green vitriol and exposed to
the air, under which conditions it is rapidly transformed
into ferric oxide, and the greyish-green colour of the
mass changes to brown. Lighter tones can be obtained
by the addition of inert white substances; and, in
other respects, the method of preparation is the same
as that of artificial ochre.
These pigments are sold under various names, the
dark shades, between pure brown and red brown, being
usually called terra di Siena or mahogany brown,
whilst the paler sorts are sold as satinober — more
correctly satin ochre, golden ochre, etc. Other pig-
ments, chemically allied to the ferric oxide or ochre
pigments, are sometimes found on the market under
various and entirely arbitrary names.
It may be pointed out that the greatest confusion
exists in the nomenclature of pigments, to such an
extent that, in many cases, neither the chemist nor
the manufacturer knows precisely what pigment is
implied by a given name. The confusion is still further
increased by the use of names taken from different
languages.
TRUE UMBER
Umber, properly so called — also known as Turkish,
Cyprian or Sicilian umber, from the country of origin
• — derives its name, according to some authorities, from
the province of Umbria (Italy), where a brown earth
is found, though others ascribe it to the Latin " umbra "
BROWN EARTH COLOURS 171
(shade) because of the pigment being used for painting
shadows.
True umber is an earthy mass of fine texture and
liver-brown colour, merging into chestnut in some of
the lumps. Chemically, it consists of a double silicate
of iron and manganese combined with water, a portion
of these metals being usually replaced by alumina.
The greater hardness (1*5) and higher specific gravity
(2*2) of true umber in comparison with Cologne earth
(which is quite arbitrarily termed "umber-"), form a
ready means of differentiation between the two.
According to Viktor Merz, the umber found in
Cyprus consists of : ferric oxide, 52% ; manganese
oxide, 14*5%; and alumina, 3%; and is, possibly,
merely a mixture of clay with hydroxide of iron or
manganese. An umber examined by Klaproth con-
tained 13% of silica, 5% of alumina, 48% of ferric
oxide, 20% of manganese oxide and 14% of water.
The tone of umber can be modified, in the direction
of red, by calcination, but this process is seldom
employed, the dark brown shade of this colour being
the one most appreciated.
In some parts of northern German}7, Thuringia in
particular, the iron mines contain smaller or larger
pockets of ferric hydroxide, of a fine earthy texture,
from which umber is prepared, by levigation and
calcination. The product is sold under various names :
chestnut brown, wood brown, mahogany brown, bistre
flea brown, roe brown, according to the shade of the
calcined pigment.
A mineral (" siderosilicate," according to Von Walter-
hausen) composed of ferric silicate, and approximating
in this respect to terra di Siena, is found in the neighbour-
hood of Passaro (Sicily) in deposits of tuff. It forms
172 EARTH COLOURS
masses which are transparent at the edges and are
usually liver-brown to chestnut in colour. The hard-
ness of the mineral is 2*5, the sp. gr. 2*713, and the
average chemical composition: silica, 34%; ferric
oxide, 48-5% ; alumina, 7-5% ; and water, 10%.
The foregoing are only a few examples of brown or
red-brown earth colours. In all these minerals the
pigmentary principle is iron, in combination either with
oxygen alone (ferric oxide), with oxygen and water
(ferric hydroxide), or silica compounds (ferric silicate),
and always associated with certain quantities of other
metallic oxides, especially alumina and manganese
oxide. Although but few of these minerals have
gained any special reputation as pigments, there is
no doubt that similar minerals, which are certain to
occur in or near many deposits of iron , ores, could
equally well be used for that purpose. There is no need
to emphasise that the discovery of such a mineral
would be a very valuable find, and that the products
obtainable therefrom could be utilised to great
advantage.
The testing of a mineral for its suitability as pigment
is a very simple matter, all that is required being to
subject a small quantity to the same treatment that
is applied to the earth colours on a large scale. For
this purpose a few pounds of the mineral are levigated,
and the residue is dried. To ascertain the tones
obtainable by calcination, small samples — of about
100 grms. — are placed in crucibles, and gradually
heated in a furnace. When the masses have attained
a sufficient temperature, the samples are taken out of
the furnace, at intervals of ten minutes, and left to
cool. It will then not be difficult to decide whether
the mineral is at all suitable for the purposes of the
BROWN EARTH COLOURS 173
colour-maker; and if so, these tests afford at once an
indication of the temperature and time the mineral
must be heated in order to obtain pigments of definite
tones.
COLOGNE EARTH (COLOGNE UMBER)
The application of the term " umber " to this earth
can only have been based on a certain similarity in
colour to true umber. In chemical composition, how-
ever, the two are quite different, Cologne earth really
consisting of a mixture of humic substances. It is
well known that the rotted wood found in the interior
of decaying trees is often a handsome brown colour;
and all woody matter, after lying a very long time,
finally acquires this colour, owing to the transformation
of the wood into dark-coloured compounds richer in
carbon. This effect can be seen on the large scale, in
Nature, in the case of coal, brown coal and peat.
Now Cologne earth consists of a brown-coal mould,
dark brown in colour, of earthy character and of such
low cohesive power that it crumbles with ease. Owing
to this character, Cologne earth can be easily ignited
by the flame of a candle, and then burns with a strong,
smoky flame, leaving very little ash and disseminating
the peculiar bituminous smell given off when brown
coal is burned.
The geological characteristics of Cologne earth enable
one to conclude that, where similar conditions prevail,
materials of analogous nature may be discovered. This
earth is found embedded in a deposit of brown coal, in
which it forms pockets, and occasionally large bodies.
Now, brown-coal deposits of enormous extent occur in
very many localities, as for instance in Upper Austria
174 EARTH COLOURS
and in Bohemia; and many of these mines are sure
to contain pockets of brown-coal mould, which have
perhaps been overlooked, but might very well be
utilised in the preparation of colours of very similar
character to Cologne earth.
The preparation of this material is very simple.
The earth coming from the deposits is put through a
simple levigation treatment which leaves, as residue,
lumps of semi-decomposed wood, mineral admixtures,
sand, etc. The levigated earth is sold in the form of
cubes.
Cologne earth comes into the market under various
other names, such as : umber, Cassel brown. Spanish
brown, etc.
The fiery brown which was so greatly preferred by
the famous painter Van Dyck, and named Vandyke
brown after him, was of very similar composition to
Cologne earth, and is said to have been obtained from
a deep brown peat earth. The Vandyke brown of the
present day, however, is almost invariably a ferric
oxide pigment, toned to the proper shade by suitable
calcination.
ASPHALTUM BROWN (BITUMEN)
As a natural product, which can be used as a painters'
colour without any special preparation, asphaltum
(bistre, bitumen) may also be classed among the earth
colours. Chemically, it is composed of hydrocarbons
of various kinds, and is thus similar to tar; in fact,
asphaltum may also be regarded as a natural tar
resulting from the decomposition of various orgaric
substances. Many deposits of this mineral are known,
and two of them are particularly celebrated : those
BROWN EARTH COLOURS 175
of the Dead Sea, in Syria, and the Lake of Asphalt, in
Trinidad. Both deposits consist of craters filled with
water on which the asphaltum floats in large cakes.
Several kinds of asphaltum are met with in com-
merce, ranging in colour from brown to black. The
preparation of the material as a pigment is confined to
grinding the mass, which is always of a low degree of
hardness. Being readily soluble in oil of turpentine
and then furnishing the most beautiful brown tones
when laid on thinly, the pigment is usually sold in this
condition, although it is also ground in oil for the same
purpose.
Finally, it may be mentioned that various useless
materials can be transformed, by suitable treatment,
into brown pigments closely resembling Cologne earth
and applicable to the same uses. Such pigments can
be prepared from brown-coal slack (from inferior brown
coal) or bituminised wood — a variety of brown coal
looking like charred wood — by treating these materials
with a lye made from wood ashes and lime, and washing
and drying the residue.
CHAPTER VIII
GREEN EARTH COLOURS
ALTHOUGH the number of green-coloured minerals
is large, but few of them are suitable for painters'
colours, because they occur so rarely in Nature that
their employment for this purpose is out of the question,
more especially since a very large number of green
pigments can be obtained by artificial means. The
most important of the earth colours in this category
are Celadon green, or green earth, and malachite green
— the latter, however, less so, because the substance of
which it is composed can be prepared artificially.
GREEN EARTH, OR CELADON GREEN
This mineral is of a peculiar green colour, and the
name " Celadon green " has been universally adopted
in the nomenclature of colour shades. Green earth
occurs native in many places, being the decomposition
product of an extensively distributed mineral, augite,
crystals of which are found in many of .the deposits.
The green earth of Monte Valdo, on Lake Garda (Upper
Italy) has been used for a very long time as a pigment.
It is chiefly prepared in Verona for distribution in
commerce, and from this circumstance has acquired
the name " Verona green," or " Verona earth." The
earth is also found in Cyprus and Bohemia, where it
176
GREEN EARTH COLOURS
177
frequently occurs as the decomposition product of
basaltic tuff. However, whether obtained from Monte
Valdo or elsewhere, the product is always placed on
the market as Verona earth.
Native green earth is always tough, mostly occurring
in amygdalous lumps, but occasionally in the crystalline
form of augite. It has a fine-grained fracture, a hard-
ness between i and 2, and a sp. gr. between 2*8 and 2*9.
The colour is not always quite uniform, pure lumps
having the characteristic Celadon green appearance,
whilst impure lumps are olive green to blackish green.
In chemical composition it is chiefly ferrous silicate,
and this compound must be regarded as the actual
pigmentary principle of green earth. In addition, it
contains varying quantities of other compounds which
influence the depth of shade of the product.
Verona earth chiefly consists of ferrous oxide in
combination with silica; alumina, magnesia, potash,
soda and water being also present. Analysis shows it
to contain : ferrous oxide, 21% ; silica, 51% ; magnesia,
6%; potash, 6%; soda, 2%; and water, 7%.
The green earths from Gosen, Atschau and Mannels-
dorf , near Kaaden (Bohemia) and the Giant's Causeway
(Ireland) have the following composition :
•
^aden.1 ££^
Silica
4r 56-4
Alumina .
3 2-1
Ferrous oxide
23 5'i
Ferric oxide
14-1
Lime
8
Magnesia .
2 5'9
Potash
3 8'8
Carbon dioxide
19
Water
— 6-8%
12
178 EARTH COLOURS
On account of the large quantity of mechanically
associated water, freshly dug green earth is greasy in
character, like wet clay. In partial drying, most of
this water evaporates, the mass becoming earthy and
adherent to the tongue. Sometimes the colour is an
ugly brownish -green, owing to the presence of a con-
siderable amount of ferric oxide formed as the result
of changes set up by exposing the mineral to the air.
Ferrous oxide is a very unstable compound, having an
energetic tendency to combine with more oxygen and
thus undergo transformation into ferric oxide ; so that
when green earth is left in the air for a long time, a
considerable proportion of its ferrous oxide is oxidised
to ferric oxide, the mass thereby assuming the brown
tone in question.
Such an unsightly product can, however, be con-
verted, by simple treatment, into one of very bright
and handsome appearance ; and it is this possibility that
first enabled green earth to attain importance as a
painters' colour. Formerly it was only used as a
material for common work, being added to whitewash
or employed for indoor paints.
When the crude green earth is treated with very
diluted hydrochloric acid, the compound of ferrous
oxide and silica is left intact, but most of the extraneous
admixtures are removed. Ferric oxide, in particular,
passes into solution, and the calcium carbonate largely
present in some kinds of green earth is also dissolved.
After prolonged contact with the crude earth, the
acid liquor takes on a brownish coloration from the
dissolved ferric oxide. Since the presence of iron
salts has no influence on the purification of the green
earth, the most impure, highly ferruginous hydrochloric
acid can be used, and the liquor can afterwards be
GREEN EARTH COLOURS 179
employed in the preparation of artificial ochre by
leaving it in prolonged contact with any strongly
ferruginous mineral, such as brown ironstone, which
neutralises the surplus acid. This liquor is then
precipitated by lime, alkali, etc., the resulting deposit
consisting mainly of ferric hydroxide, the further treat-
ment of which is conducted exactly as described in
dealing with the preparation of artificial ochre.
The treatment of the crude earth is best carried on
in the same vessels as are to be used in the subsequent
levigation process. After the acid liquor has been
drawn off, the earth is brought into contact with water,
stirred up well, and the wrater run off, by opening tap-
holes in the side of the vessel, into sett ling- tanks, where
it is left until all the suspended matter has completely
subsided.
The colour of green earth can also be toned by the
addition of yellow ochre, thus producing a range of
greens with a yellowish tinge. These lighter shades,
however, are seldom met with in commerce, the trade
judging the quality of green earth more particularly
on the depth of colour.
Green earth is a valuable pigment for all kinds of
painting, on account of its extreme permanence. It
may be applied directly over lime without suffering
any change, whereas most of the cheap green colours
are destroyed in like circumstances. This behaviour
renders green earth specially valuable in fresco work,
although it is also largely used as an oil colour.
Augite is of frequent occurrence in volcanic districts ;
and in such localities, deposits of green earth are certain
to be found. The test for the suitability of a green
earth consists mainly in treatment with dilute hydro-
chloric acid. If the mineral assumes a handsome green
i8o EARTH COLOURS
tone, it will generally form a useful pigment. The
test may be supplemented by applying the colour to
a fresh coating of whitewash, under which conditions
it should remain unaltered.
ARTIFICIAL GREEN EARTH (GREEN OCHRE)
A product sometimes put on the market as green
earth or green ochre has nothing beyond its name in
common with green earth properly so called, except a
certain similarity in colour. This pigment is prepared
by mixing yellow ochre to a thin pulp with water and
adding about 2% (of the weight of ochre) of hydro-
chloric acid. After a few days, a solution of 2 parts of
yellow prussiate of potash is added, and if the liquor
still gives a precipitate when tested with ferrous
sulphate, this last-named salt is added so long as such
a precipitate continues to form.
The deposit is washed, and dried in the ordinary
way. When the right proportions have been taken, a
pigment is obtained that coincides fairly in point of
tone with true Verona earth. It is, however, inferior
in point of permanence, the Berlin blue present being
somewhat unstable and decomposing very quickly
when brought into contact with lime. The reaction
taking place in the production of so-called " artificial
Celadon green " is that the hydrochloric acid used
dissolves ferric oxide from the ochre, the addition of
the yellow prussiate of potash then forming a blue
precipitate of Berlin (Paris) blue which, in, conjunction
with the yellow of the ochre, gives a green-coloured
mixture.
GREEN EARTH COLOURS 181
MALACHITE GREEN
Although the pigment sold under this name is nearly
always an artificial product, it cannot be omitted from
a work dealing with the earth colours, because, in
former times, it was prepared exclusively from the
mineral malachite. Owing to the fact that artificial
malachite green is one of the cheapest of colours, the
troublesome work involved in the mechanical prepara-
tion of the native pigment has been almost entirely
abandoned, and the malachite itself is now utilised to
greater advantage as a source of copper.
Malachite green (or mountain green) is found in nearly
every case where copper ores exist, and is still — though
very rarely indeed — prepared, in a few places, from the
mineral, the dark-coloured lumps being picked out
because the lighter-coloured ones would furnish much
too pale a colour.
The treatment of malachite for the preparation of
pigment presents certain difficulties owing to the com-
parative hardness (3*5-4) of the mineral, which is also
rather heavy (sp. gr. 3*6-4-0). On the large scale, the
selected mineral is first put through a stamping-mill,
and then ground, very hard stones being required for
this purpose. The fine product from this (usually wet)
process is levigated and dried.
The pit water of some copper mines contains certain
quantities of blue vitriol (copper sulphate) in solution ;
and such pit water is generally treated for the recovery
of a very pure form of copper, the so-called cementation
copper. The liquor might also be worked up into
malachite green, by collecting it in large tanks and
precipitating the dissolved copper oxide with milk of
lime, the bluish-green deposit separating in association
182 EARTH COLOURS
with gypsum being transformed into a light malachite
green by washing and drying. A darker green, free
from gypsum, could be prepared by using a solution of
carbonate of soda as precipitant.
Neither the native nor the artificial malachite green
is particularly handsome in colour; and both possess,
in addition, the unpleasant property of gradually going
off colour in the air, all the copper compounds being
quite as sensitive to sulphuretted hydrogen as those of
lead, and finally turning quite black under the influence
of that gas.
CHAPTER IX
BLUE EARTH COLOURS
ONLY three minerals are known to be suitable as
pigments ; and indeed, at present, only two, the third,
lapis lazuli, being now of merely historical interest.
Nowadays, no one would think of using this rare and
expensive mineral as a pigment, since ultramarine,
which has the same pigmentary properties,, is extremely
cheap, whereas the pigment from lapis lazuli was
worth its weight in gold. The only two blue earth
colours of any interest at present are malachite (copper)
blue, and the blue iron earth Vivianite; and even
these, though by no means rare, are little used, since
artificial blues are now made which are far superior
in beauty and can be obtained so cheaply that the
natural pigments are put out of competition.
MALACHITE BLUE (LAZULITE)
Lazulite and malachite (mountain blue) are of
frequent occurrence in copper mines, and the former
is distinguished by its beautiful azure blue colour,
which, however, suffers considerably when the crystals
are reduced to powder. Both minerals are very
similar in chemical composition, and consist of cupric
carbonate. The formula of malachite is 2CuOCO2 +
H O, that of lazulite being 3CuO(CO2)2 + H2O, so that
183
184 EARTH COLOURS
the only difference between them is that of the relative
proportions of the substances in combination. Lazulite
is also rather hard (3- 5-4*0), but owing to the small
size and brittle character of the crystals it is not very
difficult to pulverise. In the air, malachite blue
behaves in much the same way as malachite green,
turning black in presence of sulphuretted hydrogen.
Malachite blue is chiefly used for indoor work, and
also as a water colour ; but it is always rather pale and
dull.
VlVIANITE
This mineral — also termed blue ochre — is a trans-
formation product of various iron ores, and occurs
native as fairly extensive deposits in some places,
especially in peat bogs. It forms ill-defined crystals,
which are of a low degree of hardness (2*0) and vary
in specific gravity between 2'6 and 2*7. The colour
of the freshly won mineral is whitish or pale blue, but
soon changes to a dark blue in the air, owing to the
oxidation of the ferrous phosphate, originally present,
into ferric phosphate.
Vivianite can be transformed into a pigment by a
simple process of crushing and levigation; but the
product is never very handsome, and, at best, is only
suitable for quite common paint work, though character-
ised by considerable stability.
CHAPTER X
BLACK EARTH COLOURS
ONLY two minerals are known that can be used as
black earth colours, namely black chalk or shale black,
and blacklead or graphite. Whereas the former of
these is of merely subordinate importance, most of
the black chalks being prepared artificially, graphite
is all the more so because it is employed, not only as
the sole material for lead pencils, but also for making
graphite crucibles, as blacklead stove polish, as a
lubricant, etc. One of its numerous applications is
in connection with the electro deposition of metals, its
high electrical conductivity causing it to be used for
coating the interior of the moulds in which this deposi-
tion is effected.
GRAPHITE
Graphite, also known as plumbago or blacklead,
consists of carbon. It is usually spoken of as pure
carbon, but from a very large number of carefully
conducted analyses, it would appear that native
graphite is never quite pure, even the finest grades of
the mineral containing 96-8% of carbon at the most.
The accompanying substances — which in some cases
185
186 EARTH COLOURS
form nearly 50% of the whole — are of divergent com-
position and consist of iron, silica, lime, magnesia and
alkalis. Even the combustible constituent of graphite
is not pure carbon, but always contains a certain—
though small — proportion of volatile substances. These
slight traces of volatile matter are of considerable
importance in connection with the hypothesis on the
origin of the mineral.
Contrary to the old idea, it is now almost universally
considered that, instead of being of volcanic origin,
graphite consists of the remains of long-dead organisms,
and in this respect is closely related to coal. This
hypothesis, however, fails to explain one point, namely
the crystalline nature of graphite ; for even anthracites,
which form the oldest coals known to have had their
origin in the decomposition of organic substances, do
not reveal the faintest traces of crystalline structure.
The upholders of the theory that graphite was formed
by the action of plutonic forces adduce, in support,
the fact that graphite can actually be produced, in
certain chemical processes, at high temperature.
Molten cast-iron in cooling causes the separation of
carbon in the form of graphite ; and the same substance
is also formed, in large quantities, in gas retorts,
through the decomposition of certain carbonaceous
compounds when brought into contact with the
glowing walls of the retorts. Recent investigations,
however, have shown that the temperature necessary
for the transformation of non-crystalline carbon into'
crystalline graphite is by no means so high as was
formerly supposed ; and it is now known that the change
takes place at as low as red heat. Possibly the two
theories could be reconciled by the assumption of a
very old coal — such as is found, for instance, as anthra-
BLACK EARTH COLOURS
187
cite in many parts of the world — being so strongly
heated, by plutonic action, as to change into
graphite.
Native graphite crystallises in the form of hexagons,
mostly tabular ; but really well -developed crystals are
of extremely rare occurrence, and by far the greatest
quantities of this mineral are found in the condition
of dense lumps, in which only the crystalline structure,
and not any decided crystals, can be discerned. The
hardness of the mineral fluctuates within fairly wide
limits, ranging from 0-5 to ro. The sp. gr. averages
r8oi8-r844, but, in the case of impure lumps may
increase to 1-9-2-2.
The following analyses will give some idea of the
considerable divergence existing between graphites
from different deposits :—
Carbon
Ash .
Water
SIBERIAN GRAPHITE
94-28
5-72
4°-55
56-56
2-80
PORTUGUESE GRAPHITE
Carbon .
Water (chemically combined)
Ash
42-69
3-96
53-35
BOHEMIAN GRAPHITE
Carbon
Alumina
Silica
Magnesia
Lime .
Ferric oxide
Potash
Water and volatiles
Sulphur
61-01
7-80
J7-34
1-03
2-56
5-54
0-87
3-24
0-51
6-86
14-18
o-53
0-80
4-00
0-91
2-89
0-62
i88 EARTH COLOURS
GRAPHITES FROM UPPER STYRIA
i
2
3
Carbon ....
85-00
87-16
82-21
Ash ....
14-89
12-66
I7-92
4
5
6
Carbon ....
82-40
81-10
55*5°
Silica ....
12-38
11-61
2I-OO
Alumina ....
3-90
5-60
14-56
Ferric oxide
°'53
Manganese protoperoxide .
Lime ....
0-62\
0-02/
2-OO
4-84
Alkalis
Trace
Trace
O-62
Sulphur ....
—
—
0-30
Loss on incineration .
. —
—
2'43
Of these Styrian specimens, Nos. 1-4 are crude
kinds, of sp. gr. 2*1443; No. 5 was levigated in the
laboratory, and No. 6 was levigated from an inferior
quality at the mine.
According to the character of the crystalline
structure, the colour of graphite varies, but is mostly
deep black. Very pure specimens, such as the beau-
tiful graphite blocks (from the renowned Alibert
graphite mines in Siberia) which, as a rule, are only
to be seen in exhibitions and mineralogical collections,
have the appearance of unpolished steel or white pig
iron (spiegeleisen) . The most important property of
native graphite is its low hardness and cohesion, in
consequence of which it leaves a streak when drawn
over the surface of paper.
Graphite seems to be of frequent occurrence all over
the world, though only few deposits are known which
yield a product that is suitable for all the purposes to
which graphite is applied.
In European countries, Austria is particularly rich
in graphite; and very large deposits of this mineral
are found in Bohemia. Considerable deposits also
occur in Bavaria, where they have long been worked.
BLACK EARTH COLOURS 189
English graphite is celebrated for its excellent quality.
All these European deposits, however, are surpassed,
both in extent and in the quality of their products, by
those discovered in Siberia, the largest being that
producing the aforesaid Alibert graphite and situated,
near the Chinese frontier, in eastern Siberia. At one
time, America imported all her blacklead pencils from
Europe, having, at that period, no known graphite
deposits furnishing a suitable product. At present,
however, deposits of this kind have been found in
California, and there can be little doubt but that many
others of this valuable mineral remain to be discovered
in that enormous continent, the geological investigation
of which is still far from being complete.
The graphite of some deposits is so highly con-
taminated by extraneous minerals that it cannot be
utilised, since the cost of purification would exceed the
value of the product. On the other hand, the purer
kinds, when suitably refined, yield a graphite that is
fully adapted to all requirements.
The refining process may be either chemical or
mechanical, the choice of methods depending entirely
on the character of the associated minerals. If these
mainly consist of coarse, stony fragments, preference
should be given to mechanical treatment ; but if they
are of such a character that they cannot be eliminated
in this way, chemical methods must be employed.
Sometimes the two systems are combined, by first
subjecting the graphite to a rough mechanical purifi-
cation, and then completing the operation with chemical
reagents.
The mechanical treatment consists in first removing
as many of the impurities as possible by hand-picking,
and grinding the remainder in edge -runner mills, along
igo EARTH COLOURS
with water. The turbid liquid, containing the powdered
graphite and extraneous minerals in suspension, is
led through long launders, the sides of which are
notched at intervals to allow the water to overflow into
large pits. The graphite settling in the first of these
pits contains numerous particles of the heavy associ-
ated minerals; but that remaining suspended in the
water and carried on to the further pits constitutes
the bulk. The water is left to clarify completely in
the pits, and is then drawn off, the pasty residue being
shaped into prisms, which are compressed under heavy
pressure, to increase their density, when partially d^.
Although levigation will remove most of the accom-
panying extraneous minerals, it cannot eliminate the
ash constituents of the graphite. Experiments made
in this direction have demonstrated that the ash
content of the levigated graphite is exactly the same
as that of the crude material. Whilst these ash con-
stituents do not affect the quality of graphite for cer-
tain of its uses, they nevertheless impair its beautiful
black colour to a considerable degree. The chemical
treatment necessary to eliminate these constituents is
attended with many difficulties, the chief of which
resides in the fact that the ferric oxide present is in a
form that is not readily accessible to the action of
chemicals. For this reason, attempts to purify graphite
with crude hydrochloric acid are hardly likely to prove
successful, since both the ferric oxide and the accom-
panying silicates obstinately resist the action of this
acid.
In order to obtain graphite of a high state of purity,
the attempt must be made to bring this ferric oxide
and the silicates into a soluble condition. This can
be accomplished in various ways, and the choice of
BLACK EARTH COLOURS 191
the method will depend on the purpose for which the
graphite is intended. For example, the operations may
either be confined to purification, or else include the
attainment of a maximum condition of subdivision.
When foliaceous graphite has to be treated — and this
kind of graphite cannot, in its original condition, be
used for making lead pencils — it is preferable to employ
a method which will produce both the above results.
The purification may consist in crushing the graphite
to powder, and fusing this with a mixture of sulphur
and carbonate of soda, whereby the silicates present
are converted into soluble compounds, and the ferric
oxide into ferric sulphide. On extracting the melt
with water, a portion of the contained salts pass
into solution and is carried off. The residue is then
treated with dilute hydrochloric acid, which dissolves
out the ferric sulphide, with liberation of sulphuretted
hydrogen, and leaves the graphite in a very pure
condition after wrashing.
In order to render foliaceous graphite suitable for
lead pencils, a different method is pursued, but should
only be employed in special circumstances, on account
of the expense entailed.
According to the process recommended by Brodie,
the graphite, ground to coarse powder, is mixed with
about one-fourteenth of its own weight of chlorate of
potash, this mixture being heated, with two parts by
weight of sulphuric to each part of graphite, in a water
bath so long as fumes of hypo chlorous acid continue
to be disengaged. The heating must be performed in
stoneware or porcelain vessels, those made of any
other materials being strongly corroded by the chlorine
compounds formed.
When the evolution of fumes ceases, the mass is
192 EARTH COLOURS
allowed to cool, and is carefully washed with a large
volume of water, the residue being then dried and heated
to redness. During this calcination the graphite under-
goes a peculiar change, increasing considerably in bulk
and forming an exceedingly soft powder which, after
another washing, consists almost entirely of chemically
pure carbon.
Graphite purified in this way can be used for any
purpose for which this material is employed, and may
be made into the finest lead pencils. However, as
already mentioned, this process is usually too expensive
for general application.
The use of graphite for writing is more ancient than
is usually supposed, having been tentatively employed
between 1540 and 1560. It was during this period
that the graphite mines in Cumberland were discovered ;
and the extremely pure graphite found there soon began
to be used as a writing material.
Up to the close of the eighteenth century, lead pencils
were made by selecting pure lumps of graphite and
sawing them into thin rods, which were then encased
in wooden sticks. Apart from their high price, these
pencils exhibited various defects, one of the chief being
that a stick of such pencil was seldom of uniforrh hard-
ness throughout its length, most of them being so soft
in parts as to make a deep black, smeary mark, whilst
other parts would hardly give any mark at all.
The defects inherent in native graphite are com-
pletely removed by the method now generally employed
in making lead pencils; and on this account the old
process of sawing the lumps has been abandoned.
Graphite with a fine earthy texture alone is suitable
for lead pencils, scaly varieties being useless for this
purpose, unless specially prepared, since they will not
BLACK EARTH COLOURS 193
give a solid black streak. By means of the Brodie
process, however, even the most highly crystalline
kinds can be rendered suitable for this purpose.
Siberian graphite is distinguished by extremely high
covering power, and is specially preferred for the
manufacture of pencils. Excellent varieties for this
purpose are also found in many parts of Europe ; and
indeed, a large proportion of all the lead pencils used
throughout the world are made from Bohemian, Styrian
and Bavarian graphite.
At present, all pencils are made from ground graphite,
the extremely finely ground and levigated material
being kneaded into a paste with clay. This operation
fulfils a twofold purpose, the plasticity of the clay
increasing the cohesion of the individual particles of
graphite, whilst the amount of clay used determines
the hardness of the pencil.
The larger the proportion of clay, the harder the
pencil when baked, and therefore the paler the mark
the pencil will make on paper. In the pencil factories,
the clay is incorporated in special machines; and the
operation requires extreme care, since only a perfectly
uniform mixture will give a composition of regular
character in all cases.
The intimately mixed material is formed into thin
rods, which are dried and then baked, the heat driving
out the water in the clay and transforming it into a
solid mass.
An addition to this main application of graphite,
the mineral is also used for making crucibles, chiefly
for melting the noble metals. Crucibles of this kind
are largely manufactured near Passau, Bavaria, and
similar crucibles are made in England from Ceylon
graphite.
13
I94 EARTH COLOURS
Another important use for graphite is as a coating
for iron articles to protect them from rust. For this
purpose, however, only the inferior kinds are employed ;
and these can also be made up into excellent cements
capable, in particular, of offering considerable resistance
to the action of heat and chemicals.
To complete the tale of the applications of graphite,
its employment as a lubricating agent for machinery,
especially for reducing friction in machines made of
wood, may be mentioned. Latterly also, the finest
levigated graphite has come into use, in admixture
with solid fats or mineral oils, for lubricating large
engines, for which purpose it is excellently adapted.
BLACK CHALK
Black chalk, slate black, Spanish chalk, crayon, etc.,
is not a chalk at all, in the mineralogical sense, but
consists of clay shale of varying colour. Some kinds
of this shale are pure black, almost velvet black, and
these are considered the best. Others have a more
greyish or bluish tinge and are of low value as
pigments.
The purer the black, the finer the grain of the
material, and therefore the greater its value to the
colour-maker. The variety obtained from Spain is
generally admitted to be the best, and for this reason
the name of Spanish chalk has been applied to all
similar minerals.
In all cases the black colour of Spanish chalk is due
to carbon; but the particular modification of carbon
present has not yet been accurately identified. Accord-
ing to some, it is chiefly graphite, whereas others ascribe
the colour to amorphous carbon. Apparently, the
BLACK EARTH COLOURS 195
material found in different deposits contains either one
or the other of these modifications of carbon.
Deposits of black chalk are fairly plentiful, but in
many of them the material is so contaminated with
extraneous minerals that a somewhat troublesome
method of preparation is needed to fit them for the
purpose of the draughtsman. With this object, the
native product must be ground extremely fine, and
the powder levigated; and owing to the expense of
these processes, they are now seldom used, it being
possible to obtain a good black chalk far more cheaply
than by levigating the natural material.
This artificial black chalk is prepared by mixing
ordinary white chalk, or white clay, with a black
colouring matter, shaping the mass into prisms, and
sawing these into suitable pieces when dry. The
white pigment may either be mixed with some very
deep black substance, such as lampblack, or stained
with an organic dyestuff , which is, in reality, not black,
but either very dark blue or green.
The usual colouring matter used with white chalk
is lampblack, mixed to a uniform paste with thin glue,
a suitable amount of clay or chalk being incorporated
with the mass. The production of a perfectly homo-
geneous mixture entails subjecting the paste to a
somewhat protracted mechanical treatment. When
the mass has become perfectly uniform throughout,
it is shaped into prisms, which are exposed to the air
to dry and are then cut up with a saw. Instead of
prisms, the mass can be shaped into thin sticks, which
dry more quickly.
A very handsome black chalk can be made, with
comparatively little trouble, by treating chalk with a
suitable quantity of logwood decoction previously
196 EARTH COLOURS
mixed with sufficient green vitriol solution to render
the liquid a deep black. This liquid is added to the
dry chalk, intimately mixed therewith, and the pasty
mass shaped into sticks. The colouring agent may be
replaced by a solution of logwood extract blackened
by the addition of a small quantity of chromate of
potash ; or black dyestuffs may be used.
CHAPTER XI
THE COMMERCIAL NOMENCLATURE OF THE
EARTH COLOURS
MENTION has already been made of the great con-
fusion prevailing in the nomenclature of pigments,
and that many of these are put on the market under
a variety of names taken from different languages.
Although the number of the earth colours is far
smaller than that of the artificial colouring matters,
the nomenclature is in a no less confused condition.
Most frequently, earth colours are named after the
localities where they are either discovered or prepared,
in combination with the word indicating the colour
of the product — for example : Cologne white, Vienna
white — or the term " earth " (Verona earth, Veronese
green, etc.). Whilst these names give, to some extent,
an indication of the nature of the pigment, others
have no reference to it at all; such as colcothar, bole,
umber, etc. Finally, a number of other names in
use are calculated to produce the impression that the
earth colours in question are of an entirely different
nature to their real one. As an example, we may
cite the name " French chalk," which is not a chalk
at all, but consists of the mineral talc. Black chalk,
again, is not chalk (calcium carbonate), but a black
shale; and graphite is often termed blacklead,
although it contains no lead at all, and the name is
197
198 EARTH COLOURS
merely a survival from the time when pencils of
metallic lead were used for drawing.
In order to bring some kind of order into the various
names which are applied to the earth colours, a list
of those in current use is appended. Many of these
names, it may be stated, have been selected in a purely
arbitrary manner, some manufacturers, for instance,
selling ordinary chalk under a variety of foreign
names, for the purpose of thereby obtaining higher
prices. These borrowed names would seem to be
superfluous, to say the least. Pure and properly
levigated chalk is the same article everywhere, whether
prepared from English, French or German limestone;
and in all cases the simple name, " chalk," with an
explanatory " single," " double," or " triple " levi-
gated, should be quite sufficient.
In the case of earth colours that are really obtained
of special quality in certain localities, such as terra di
Siena, green earth from Verona, or the like, the corre-
sponding name might be retained, even if the pigment
did not originate from the locality in question, as a
generic term for a pigment possessing certain properties
and of a certain composition.
In the following classification, the names of the earth
colours are given in accordance with their colour and
chemical composition.
WHITE EARTH COLOURS
Carbonate of Lime :
Chalk; levigated chalk; Vienna white; Spanish
white ; marble white ; artists' white ; Bougival white ;
Champagne chalk; Paris chalk; Cologne chalk;
Mountain chalk; craie; blanc mineral; Blanc de
COMMERCIAL NOMENCLATURE 199
Champagne; Blanc de Meudon; Blanc de Bougival;
Blanc de Troyes; Blanc d' Orleans; Blanc de Rouen;
Blanc de Briancon.
Basic Carbonate of Lime :
Vienna white ; Vienna lime ; pearl white ; whiting ;
Blanc de chaux ; Blanc de Vienne.
Note. — The calcareous marls, consisting of carbonate
of lime and clay, are also frequently sold under the
above names, the same being the case with gypsum.
Silicate of Alumina :
White earth; pipeclay; Dutch white; Cologne
earth ; terre d' Argile ; Argile blanc ; Terre blanche.
Silicate of Magnesia (mineralogically, talc and
soapstone) :
Talc; Venetian earth; French chalk; Venetian
white; glossy white; feather white; shale white;
face-powder white ; Blanc de Venise, Blanc d'Espagne ;
Blanc de fard.
Note. — Fine grades of white lead are also sold as
Venetian white, Spanish white and shale white; but
can easily be recognised by their weight. The term
"prepared" white, frequently applied to earth
colours in the trade, usually indicates that the material
in question has been either levigated, ground or burnt —
in short, put through some kind of preparatory treat-
ment— and is therefore in frequent use for all the
colours.
Barium sulphate :
Heavy spar; barytes; heavy earth; mineral white.
Precipitated colours :
Permanent white ; blanc fixe.
200 EARTH COLOURS
YELLOW EARTH COLOURS
Ferric hydroxide, with admixtures of ferric oxide,
clay, lime, ferric silicate, basic ferric sulphate, etc.
Ochre; iron ochre; golden ochre; satin ochre
(satinober) ; pit ochre ; vitriol ochre ; Mars yellow ;
Chinese yellow; Imperial yellow; permanent yellow;
terra di Siena ; umber ; Italian earth ; Roman earth ;
bronze ochre; oxide yellow, etc.
Yellow ochre; JaunedeMars; Terre d' Italic.
Ferric Silicate :
Yellow earth; Argile jaune; yellow wash.
RED EARTH COLOURS
Ferric oxide (with alumina and silica).
Bolus ; bole ; Terra sigillata ; Lemnos earth ; red
chalk; raddle; Striegau earth.
Ferric oxide :
Colcothar; English red; angel red; Pompeii red;
Persian red; Indian red; Berlin red; Naples red;
Nuremberg red; crocus; chemical red; Crocus
Mart is iron saffron ; caput mortuum ; raddle ; rouge
de fer; Rouge de Perse; Rouge des Indes; Rouge
de Mars; Rouge d'Angleterre.
. BROWN EARTH COLOURS
Ferric oxide :
Ferric hydroxide; Ferric silicate (conf. Yellow
Earth Colours, which are often sold under the same
COMMERCIAL NOMENCLATURE 201
names as the browns. The paler kinds are usually
called " pale " or " golden," such as pale ochre, golden
ochre, etc.). Terra di Siena; burnt Siena; satinober;
mahogany brown ; Vandyke brown.
Ferric silicate, Clay :
Umber; umber brown; Roman earth; Roman
umber; Turkish brown; Sicilian brown; Cyprus
earth ; chestnut brown ; burnt umber ; ombre ; Terre
d'ombre ; Ombre brulee.
Organic decomposition products :
Cologne umber; Cologne earth; Cassel brown;
Spanish brown; mahogany brown; Vandyke brown;
brown carmine; Terre brun de Cologne; Brun de
Cologne ; Brun d'Espagne ; Ombre de Cologne ; Brun
de Cassel; Terre d'Ombre; Cologne brown.
Asphaltum (mineral rosin) :
Asphaltum brown ; bistre ; earth brown ; bitumen ;
pitch brown ; Asphalte ; Brun de bitume ; Bitume.
GREEN EARTH COLOURS
Ferrous oxide with silica, alumina, lime, etc. :
Green earth ; Verona green ; Celadon green ; Verona
earth ; Italian green ; stone green ; Bohemian earth ;
Cyprus earth; Tyrol green; permanent green; green
ochre; Terre verte; Terre de Verone ; Vert d'ltalie.
Cupric carbonate :
Malachite green; mountain green; Hungarian
green ; copper green ; mineral green ; Tyrolese green ;
shale green; Vert de montagne; Vert d'Hongrie.
202 EARTH COLOURS
BLUE EARTH COLOURS
Cupric carbonate :
Malachite blue; mountain blue; lazulite blue;
azure blue; mineral blue; copper blue; Hamburg
blue; English blue; Cendres bleues; Bleu d'azure;
Bleu de cuivre ; Vert-de-gris bleu ; blue verditer.
GREY EARTH COLOURS
Grey clay shale :
Mineral grey ; silver grey ; stone grey ; slate grey.
BLACK EARTH COLOURS
Carbon :
Graphite ; blacklead ; plumbago ; iron black.
Clay shale :
Black chalk; slate black; Spanish black; Spanish
chalk; oil black; Schiste noir; Noir d'Espagne.
INDEX
Alabaster. See Gypsum.
Alumina, silicate of, 21, 22
Aluminium-potassium silicate,
21
Alum sludge, 32
— , artificial ochre from, 148
— , ferric oxide pigments from,
164-167
Ammonium salts, artificial ochre
from, 145, 146
Anhydrite, 19
Anthracolite, 13
Arragonitc, 13
Asphaltum, 37, 38. See. also
Bitumen.
— , brown, 174, 175
Augite, 179
Azurite, 33
Ball Mills, 55-56
Barium carbonate, 20
— sulphate. See Barytes.
Barytes, 19, 20, 119-122
— , artificial, 133
— , correcting colour of, 121
— , detecting, in white lead,
120
— , low covering power of, 121
Black chalk, 38
— earth colours, 185-196
— , trade names of, 202
— earths, 6, 38-39
— schist, 38
Bitumen, 174, 175
Blanc fixe, 19
Blue earth colours, 183-184
, trade names of, 202
— earths, 4, 33-36
Bole, 31, 32, 152-154
Bone breccia, 13
Brown coal, pigments from, 175
— earth colours, 168-175
— , trade names of, 200
— earths, 5, 36—38
Calcareous marl, no, in
— tuff, 12
Calcining, 81
— Ferric oxide, 161-164
— furnaces. See Furnaces.
— lime, 88-90
— ochre, 132-136
Calcite, n, 12, 14, 15
Calcium carbonate, 12, 14, 15, 16
— , action of acids on, 15
— hydroxide, 16
— sulphate. See Gypsum.
Calc sinter, 12
— spar. See Calcite.
Caledonian brown, 36
Cappagh brown, 36
Caput mortuum. See Colcothar.
Carbon brown, 37
in limestone, 16
Cassel brown, 37, 38, 174
Celadon green. See Green earth.
Chalk, 13
, black, 194-196
— , colour of, 103
, correcting colour of, 104,
105, 106
— , covering power of, 106
— , grinding, 101
— , impurities in, 103, 104
— , precipitated, 107-103
— , preparation and properties
of, 98-106
Classification of earth colours,
4-8
Clay, 21-23
203
204
INDEX
Clay, formation of, 113
— , impurities in, 114-119
— in ochre, 128
— , levigating, 114-117
Colcothar, 160, 161, 162
Cologne earth, 173, 174
Commercial nomenclature of
earth colours, 197-202
Crushers and Breakers, 43-45
Crushing, 77-80
— machinery, 43-60
Disintegrators, 58-60
Distemper, weatherproof, 94
Dolomite, 18
Draining and Drying, 66-77
Drying appliances, 73-77
Dyestuffs for improving earth
colours, 85
Edge runners, 48-55
English red, 160
Ferric hydroxide in ochre, 128-
132
oxide, artificial ochre from,
143-144
as by-product, 30
, burnt, 158-164
— , calcining, 161-164
— in lime, detection of,
— , native, as pigment,
- pigments from alum
sludge, 164-167
— , range of colours, 29
— shading, 28
, violet shades from,
164
Ferrous sulphate, artificial ochre
from, 139-143, 146-148
Filter- cloths, cleaning, 72
Filter-presses, 70-73
Furnaces, calcining, 158, 162,
163, 166
Granulator, 43
Graphite, 38, 39, 185-194
as a lubricant, 194
— as anti-corrosive, 194
Graphite in the manufacture of
lead pencils, 191-193
— for crucibles, 193
— , refining, 189-192
Green earth, 176-180
, artificial, 180
— , improving, 178
— colours, 176-184
— , trade names of, 201
— earths, 5
Grey earth colours, trade names
of, 202
Grey earths, 38
Gypsum, 18/19, in, 112
Heavy spar. See Barytes.
Hematite, 155
— , brown, 23, 30, 31
— , red, 28, 30
Hydro -extractor, 66-70
Improving earth colours, 84, 85
Indian red, 29, 160
Iron cream, 29
— glance, 154
in limestone, 17
ore, bog, 25, 31
— , micaceous, 28
Ironstone, brown, 23, 24, 25
— , clay, 24
, red, 28-30
Kaolin, 21, 22, 112-119
Lazulite, 183
Lemnos earth. See Bole.
Levigation, 60-65
Lime, absorption of carbon
dioxide by, 93
— , action of, on casein, 94
, , on colours, 93, 98
, calcining, 88-90
— . — , caustic, preparation of,
87-94
, double compound of oxide
and carbonate, 93
— from mussel shells, 98
— , impurities in, 91, 92
— in clay, 22
, eliminating, 117-
119
in ochre, 129
INDEX
205
Lime in the preparation of arti-
ficial ochre, 140-144
— , moulding, 96-98
, quick, 16
— , slaked, 16
Limestone, 11-18
, suitability of, for colour-
making, 92
Limonite, 25
Magnesia, carbonate of, 123, 124
— in lime, 91
— in limestone, 17
Magnesium silicate, 21
Malachite, 35
-blue, 183
Marble, n, 14, 15
Minerals, testing for suitability
as pigments, 172
Mine sludge, 32
Mixing earth colours, 81-84
Moulding, 85, 86
Mountain chalk, 12
milk, 12
Muffle, burning ochre in the,
158-160
Muriacite, 19
Muschelkalk, 13
Ochre, 24, 25, 26
, blue. See Vivianite.
, calcining, 132-136
— , English, 138
, green, 180
— , pit, 148-150
— , Roman, 137, 138
— , Siena, 137, 138
—.testing, 130-132
— , toning with chalk, 144
— , toning with clay, 144
— , vitriol, 146-148
Ochres, 128-150
— , artificial, 138-146
— as by-products, 146-150
, burnt, 158-164
— from various deposits, 136-
138
, Italian, 137, 138
Oolithic limestone, 13
Organic matter in lime, 91
Pastel crayons, 126
Pearl white, 94
Permanent white, 19, 122
Pipeclay. See Kaolin.
Preparation of colour earths, 40-
86
Pulverisers, 56-58
Raddle, 29, 155-158
, impurities in, 156
.testing, 157
Raw materials for earth colours,
8-39
Red earth colours, 151-167
, trade names of, 200
Red earths, 4, 27-33
Sampling raw earths, 9
Selenite, 18
Shading pigments with perma-
nent white, 19
Siena, Terra di, 25, 26, 27, 168-
170
, . See also Italian
ochre.
Siderosilicate, 171
Sifting, 77-80
Soapstone, 20, 21. See also
Steatite.
Spanish brown, 174
Sprudelstein, 15
Steatite, 20, 21, 125, 126
Stamps, 45-48
Talc, 20,21, 124, 125
Terra sigillata. See Bole.
Testing purity of raw earths,
10
Trade names of earth colours,
197-202
Ultramarine, 33
Umber, 36, 170-174
, Cologne, 173, 174
, true, 170-173
Vandyke Brown, 38, 174
Vermilion, 151
Verona earth. See Green earth.
Vienna white, 95-98
Vivianite, 33, 34, 184
White earth colours, 87-126
206 INDEX
White earth, trade names of, Working earth colour deposits, 9
in, 198, 199
White earths, 4 Yellow earth colours, 127-150
White raw materials and pig- , trade names of, 100
mentary earths, 11-23 Yellow earth, 150
Witherite, 20 Yellow earths, 4, 23-27
PRI.NTP.D IN GREAT BRITAIN BY RICHARD CI.AV & SONS, LIMITED,
PARIS GARDEN, STAMFORD ST..S.E. , AND 13UNGAV, SLMOLK.. „
The Manufacture of Paint
THE PRACTICAL HANDBOOK FOR PAINT
MANUFACTURERS, MERCHANTS AND PAINTERS
BY
J. GRUIGKSHANK SMITH, B.Sc.
Second Edition— Revised and Enlarged
DEMY 8vo. 73 ILLUSTRATIONS. 300 PAGES
CONTENTS
PART I
I. — SCOPE OF SUBJECT AND DEFINITION OF TERMS.
II. — STORING AND HANDLING RAW MATERIAL.
III. — TESTING AND VALUATION OF RAW MATERIAL.
PART II
IV. — PLANT AND MACHINERY.
PART III
V. — THE GRINDING OF WHITE PIGMENTS.
VI. — THE GRINDING OF EARTH PIGMENTS.
VII.— THE OXIDE OF IRON PIGMENTS.
VIII.— THE GRINDING OF BLACK PAINTS.
IX. — THE GRINDING OF CHEMICAL COLOURS.
X. — -THE GRINDING OF PIGMENTS IN WATER.
XI. — THE GRINDING OF COLOURS IN TURPENTINE, GOLD
SIZE, AND SPECIAL MEDIUMS.
PART IV
XII.— MIXED OR PREPARED PAINTS.
XIII. — ENAMELS AND ENAMEL PAINTS.
PART V
XIV. — MODERN CONDITIONS WHICH AFFECT THE SELECTION
AND APPLICATION OF PAINT.
XV. — THE DESIGNING, TESTING AND MATCHING OF PAINTS.
XVI. — Cost Charges — Cost of Handling — Carriage and
Delivery of Goods — Cost of Materials — Machinery
as affecting Manufacturing Cost — Electricity as
Motive Power — Manufacturing Oncost — Prices —
The Future of the Industry.
Price 12s. 6d. net (Post Free, 13s. 3d. Home and Abroad).
PUBLISHED BY
SCOTT, GREENWOOD & SON,
8 Broadway, Ludgate, London, E.G. 4
The Chemistry of Pigments
BY ERNEST J. PARRY, B.Sc., (LOND.), F.I.C., F.C.S.
AND
JOHN H. COSTE, F.I.C., F.C.S.
Demy 8vo. 5 Illustrations. 280 Pages
CONTENTS
Chapter I. — Introductory
Light— White Light— The Spectrum—The Invisible Spectrum
—Normal Spectrum — Simple Nature of Pure Spectral Colour
— The Recomposition of White Light — Primary and Comple-
mentary Colours — Coloured Bodies — Absorption Spectra.
Chapter II.— The Application of Pigments
Uses of Pigments : Artistic, Decorative, Protective —
Methods of Application of Pigments : Pastels and Crayons,
Water Colour, Tempera Painting, Fresco, Encaustic Painting,
Oil-Colour Painting, Ceramic Art, Enamel, Stained and
Painted Glass, Mosaic.
Chapter III. — Inorganic Pigments
White Lead— Zinc White— Enamel White— Whitening— Red
Lead — Litharge — Vermilion — Royal Scarlet — The Chromium
Greens — Chromates of Lead, Zinc, Silver and Mercury —
Brunswick Green — The Ochres — Indian Red — Venetian Red —
Siennas and Umbers — Light Red — Cappagh Brown — Red
Oxides — Mars Colours — Terre Verte — Prussian Brown —
Cobalt Colours — Coaruleum — Smalt — Copper Pigments —
Malachite — Bremen Green — Scheele's Green — Emerald Green
— Verdigris — Brunswick Green — Non-arsenical Greens —
Copper Blues — Ultramarine — Carbon Pigments — Ivory Black
— Lamp Black — Bistre-— Naples Yellow— Arsenic Sulphides :
Orpiment, Realgar — Cadmium Yellow — Vandyck Brown.
Chapter IV.— Organic Pigments
Prussian Blue — Natural Lakes — Cochineal — Carmine — Crim-
son— Lac Dye — Scarlet — Madder — Alizarin — Campeachy —
Quercitron — Rhamnus — Brazil Wood — Alkanet — Santal Wood
— Archil — Coal-tar Lakes — Red Lakes — Alizarin Compounds —
Orange and Yellow Lakes — Green and Blue Lakes — Indigo-
Dragon's Blood — Gamboge — Sepia — Indian Yellow, Puree
— Bitumen, Asphaltum, Mummy — Index.
Price 12s, 6d. net (Post Free, 13s. 3d. Home and Abroad).
PUBLISHED BY
SCOTT, GREENWOOD & SON,
8 Broadway, Ludgate, London, E.G. 4
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