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Price One Shilling.
AN ESSAY
ON
THE ART
OF
PAINTING ON GLASS.
BY
EMANUEL OTTO FROMBERG.
LONDON: JOHN WBALE, 1851.
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AN ESSAY
THE ART
PAINTING ON GLASS.
FROM THE GERMAN OF
EMANUEL OTTO FROMBERG.
EotOron:
JOHN WEALE,
ARCHITECTURAL L.1 BRAKY i - 59, HIGH HOLBORN
MDCCCLI.
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HUGHBS AND CO., PBINTBBS,
king's BXAD court, gouge SaUAKB.
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CONTENTS.
Introduction p, 1 — 13
CHAPTER I.
On the Quality and Composition of the Pigments — Preparation of the
Pigments — Fusion of the Fluxes — General Reflections concerning the
Colouring Materials 13 — 28
CHAPTER II.
On the Pigments in particular 28^—78
CHAPTER III.
The Mechanical Part of Glass-Painting 78—101
CHAPTER IV.
The Work of the Glazier 101—109
CHAPTER y.
On the Ingredients for Coloured Glasses 109—116
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ON THE ART OF PAINTING ON GLASS.
INTRODUCTION.
Painted windows are allowed to be the richest and most
magnificent application of art to the interior decoration of
monumental buildings. The brilliancy and liveUness of the
colours of which glass is susceptible on account of its trans-
parency will always secure to this kind of painting a distin-
guished rank among the arts. The dazzHng combination of
colours^ which fills the observer with astonishment on account
of their brilliancy, as well as on account of their variety, gives
a peculiar character to painted windows which prevents them
from falling into oblivion, even should they be neglected for a
time. If we go back to the elementary condition of the art,
the symmetrical arrangement of pieces of glass of various
forms and colours, combined with more or less skill, they are
by no means entitled to engage our attention in a higher
degree than the lustre of gold, and the most Kvely colours
of the richest decoration. Yet with what magical charm must
they not attract our observation and excite our admiration,
when painting invests form with these beautiful colours, and
breathes life and soul into them !
Painting on glass seems to have made a fresh start within
the last few years. The favour with which the productions of
this art have been universally received, promises it soon a new
and brilliant career. But how has it happened that this
species of painting, whose magical effects have been so justly
appreciated, has been neglected for so long a time, after having
for several centuries exercised the genius of artists, and been ^
cultivated by them to the exclusion of almost every other?
Painting on glass has, in fact, experienced a remarkable
fate. In the periods of barbarism, when war and devastation
had smothered the taste for the arts, it came into existence,
and was never more flourishing than in those centuries of
ignorance. At the period of the regeneration of science and
the arts, when good taste and the love of what is beautiful
and true to nature had begun to spread through Europe, its
splendour seemed on the wane, and in process of time it
became almost entirely extinct.
A
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Z ESSAY ON THE ART OF
What, then, are the causes of so remarkable a decay ? In
the twelfth century, this art, which had been for a long time
still in its infancy, seemed encouraged by the piety of the
monks (among whom alone the wrecks of the sciences and of
the secret arts were yet to be found) to make a vigorous
attempt to release itself from the state of oppression into which
it had fallen. But while the spirit was active and ready to
soar, the means were wanting, and much time was spent in the
choice of the proper mode of carrying it into practice. Many-
kinds of painting would have to be tried ; and, at a time when
taste had not yet been refined by long practice, and especially
by the study of classical models, the preference would naturally
be given to that kind of painting which was most capable of
seducing the eye by the brilliancy of its colours. The com-
bination of colours was the sole charm that painting could lay
claim to ; the merit of a beautiful outline, on the contrary, one
which exhibited grace, purity, and correctness of form, was
utterly neglected.
Hence the splendour of painted glass, and the almost
universal admiration paid to it by those who felt themselves
compelled to do so by the guardian genius of the art.
But in the succeeding centuries taste was cultivated. Art
had less of the material, and found other faciUties and other
means of producing effect. She strove after effects of a much
higher kind ; and as genius endures with impatience the bridle
of a difficult execution, the obstacles which the painting on
glass presents to the expansion of thought would considerably
diminish the favour that it had enjoyed in consequence of the
liveliness of its colours. Oil-painting was soon called into
existence by Jean de Bruges. This new form of the art, the
imitation of nature, gained the victory over the one which had
preceded it, and painting on glass was driven from its throne.
The consequent decline of the art was not, however, sudden
and precipitate. The revolution which in the sixteenth century
took place in the arts, was also not without its influence upon
the progress of painting on glass. Its most beautiful works
were produced at that time. The most celebrated men did not
disdam to support it by their talent, and never at any time
was it more deserving of honour. It had, however, attained
the summit of its greatness, and was soon to become a spectacle
of the most complete decay.
Many causes undoubtedly conspired against its further pros-
perity. To these, among others, belong the reHgious dis-
turbances and the wars wluch followed the reign of Francis I.,
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PAINTING ON GLASS. 3
and the calamities that were brought on by subsequent dis-
cords. But, above all, we must take into consideration the
very strong prepossession in favour of oil-painting, awakened
by the brilliant successes of its productions ; and, moreover,
the favour with which the art of engraving on copper, as well
as all those arts which have drawing for their basis, were
received. These arts shared among themselves the consider-
ation that had been formerly given exclusively to painting on
glass. Finally, we must bear in mind the singular patronage
bestowed on oil-painting by those in power.
The various phases of the art of glass-painting clearly ascer-
tain the mighty influences to which it must have yielded. The
study of them offers to the inquirer matter for the most serious
consideration. In the twelfth and thirteenth centuries, in the
shadow of barbarism, if I may be allowed the expression, we
see it making considerable progress. In the fourteenth and
fifteenth centuries it bears unUmited sway over the arts. In
the sixteenth century it becomes a rival to oil-painting, and,
after an obstinate struggle, sinks into oblivion. All the old
painted windows have something characteristic of the period
to which they belong. The modifications which successively
appeared in them offer to the archaeologist points of the
deepest interest for observation and comparison, while they
are at the same time the seals of their respective eras. These
distinctive marks exist not merely in the painting, but also in
the general conception of the entire work. We have only to
cast our eyes upon a window of the twelfth and thirteenth cen-
turies, and we shall discover in a truly remarkable manner the
whole spirit of the Gothic monuments of that period. This
kind of architecture, so rich in its ever alternating details, yet
without any confusion, and regular in its monotony, succes-
sively presents that remarkable feature which we recognize
alike in the entire structure and in each of its parts ; and the
reason of this lies in the fact, that the observer, before he can
comprehend the numerous details, gradually perceives that
they form an essentially harmonious whole. This peculiarity
prevails in Gothic church windows to an extraordinary degree.
When viewed at a certain distance, they present a magnificent
decoration of the most lively colours, distributed in a manner
worthy of the most skilful workers in mosaic. It is a rich
carpet whose simple but graceful pattern is illuminated in
alternate gradations of colours admirably combined. Upon a
closer inspection, we obtain a complete view of the forms of
the various ornaments which contribute to the general effect.
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4 ESSAY ON THE ART OF
the naiVet^ of which makes us feel less regret at the imperfec-
tion of the execution.
Gothic painting horders on the mosaic, and we may go so
far as to say that to this it owes its origin. Like the mosaic,
its art was primarily limited alinost exclusively to the sym-
metrical arrangement of pieces of glass of various colours.
The art of drawing played here only a subordinate part ; but,
like the latter, it afterwards borrowed its forms from nature.
In proportion as the taste for correct drawing was carried to
perfection, the simple arrangement of glass lost its importance,
and was finally eclipsed by the art of painting:
In the sixteenth and seventeenth centuries correct deline-*
ation completely gained the ascendancy in painted windows,
and we find in these neither the effects of symmetry nor of the
mosaic. It underwent a change in every particular to an his-
torical kind of painting, of a much higher and more elaborate
style, but which was no longer so rich and so brilliant : the
sentiments of the soul gained at the expense of the impressions
of the senses.
In the intervening centuries a transition took place from the
one extreme to the other. At first we see the ground diver-*
sified with a thousand brilliant colours, on which, however,
larger figures, drawn with greater skill, and after; a pattern
more or less understood, are occasionally to be met with, and
occur as exceptions. In process of time these figures are sur-.
rounded with splendid borders or friezes, which are finally
obliged to give way to architectiu-al backgrounds and the
imitation of the antique. These are the various forms under
which the art of painting on glass was cultivated during a
period of more than six hundred years. In the eighteenth
century it seemed to be completely lost.
But is it destined to lie neglected because the reign of oil-
painting will endure ? Painting on glass cannot for a moment
contend for the approbation of artists as the rival of the latter.
The difficulty which attends the practice of it, and the ne-
cessity for the co-operation of chemistry in the productions of
this kind of painting, leave its resources ever insufficient to
enable it to vie with oil-painting, to which, on that account, it
must unquestionably yield the pre-eminence. But if the artist
who devotes himself to glass-painting is content to avail him-
self only of the legitimate resources of his art, if he does not
strive aifter eflTect nor endeavour to pursue the path of oil-
painting, we have not the sUghtest hesitation in asserting that
painting on glass, considered as a monumental style of painting.
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PAINTING ON GLASS. 5
constitutes an art, inimitable in itself, capable of being executed
with great spirit, and worthy of engaging the most distinguished
talents.
But then the painter on glass must refrain from attempting
to imitate oil-painting. On the contrary, he must acquire the
conviction that, although these two arts have unquestionably a
point of contact, they nevertheless possess sides extremely
dissimilar. To these belong, in the first place, the proper
modes of practising them respectively ; secondly, the different
conditions under which their effects are produced. Thus, for
example, painting on glass, on account of the distance at which
the picture is placed from the spectator, requires to be treated
in a perfectly distinct manner. It excludes detail, which, on
an opaque surface, is susceptible of great effect, but which,
through the transparency of the glass, is lost, even should not
a defect in the burning have done injustice to the talent of the
painter. But if, after all, the artist be bent upon giving to his
performance all the harmony of an oil-painting, he must sacri-
fice the transparency and the liveliness of the colours, which
constitute the most beautiful feature of this kind of painting :
besides, the presence of the lead-work and the iron bars, which
unite the various portions of a painted window, and which it is
in vain to attempt to conceal entirely in the shadows of a pic-
ture, must ever prove the stumbling-block on which the claim
of the artist to imitate oil-painting is sure to founder.
There is a fact which observation itself has demonstrated to
an inexperienced artist, and which must lead us to deliberate
upon the method of proceeding in practising the art of painting
on glass. It is this, viz. that in the colouring of a very elabo-
rate cartoon the half-tints, which have been diversified in gra-
dations of colours delicately and harmoniously blended together,
always run together into one colour, producing the same effect
in every part as soon as they are viewed at a distance. It is
not our intention to account for this remarkable phenomenon,
but we content ourselves with merely calling the attention of
the glass-painter to the fact, in order that he may join with
us in drawing the following conclusion from it.
A painted window of very elaborate execution, which is
designed to adorn a building dedicated to the worship of God,
and consequently must be viewed at a certain distance, not
only loses the fineness of the details, but is also obscured by
the blending of the half-tints, and therefore becomes heavy,
dry, and hard.
The art of painting on glass, for the purposes of decoration.
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b ESSAY ON THE ART OF
is founded solely and entirely upon the observation of the
above-mentioned fact. This species of painting, in fact, owes
its origin to the necessity which the artist is under of con-
forming his work to the conditions imposed upon him by the
distance at which the spectator must view the painting. What
would be said of an artist who thought of treating decorative
painting exactly as he would miniature-painting ? If we insist
strongly upon this truth, it is for the purpose of applying it to
painting on glass, which is indisputably a decorative kind of
painting, and must be cultivated in this acceptation, except
perhaps in the case of small cabinet windows, similar to those
which are known under the name of Swiss-painted glass. If
these rare exceptions be not taken into account, the painted
glass in church- windows is at such a distance from the spec-
tator, that it would be more prejudicial than useless to attempt
perfection and elaborateness of detail, as it is practised in the
case of an oil-painting. A pure and correct style of drawing,
united to a simple and vigorous copying, are the qualities
which the painter on glass must^ before all things, endeavour
to attain.
Unfortunately the assumption of superiority which has im-
pelled mankind in all ages to outdo their predecessors has also
misled those who, in modern times, have attempted to revive
the art of painting on glass. It is seldom that we avail our-
selves of the experience of the former, and we attribute to their
impotency that which is in reaUty the result of mature con-
sideration, which, however, we are utterly incapable of appre-
ciating. Thus, too, it has been taken for granted that the last
painters of the sixteenth and seventeenth centuries were not in
a condition to make further advances in their art. Their per-
formances were criticized at the same time that they were com-
pared with the contemporaneous productions of oil-painting.
The recognition of this principle would, in our opinion, have
been more modest as well as more reasonable, viz. that the
glass-painters of former times preserved their art free from all
imitation ; that a long experience taught them to be content
with borrowing spirited ideas from oil-painting, and not to aim
at producing the game effects by means differing so widely
from one another, so that in the end they made painting on
glass an isolated art, unlike any other in its effects as well as in
its means, and one which is subject to peculiar conditions.
But, on the other hand, it has been said that the ancients did
not understand the art of painting on glass, and under this
impression an attempt was made to execute what they had
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PAINTING ON GLA8S. 7
ueyer been able to perform. This kind of emulation is un-
doubtedly noble and honourable ; but it is to be regretted that
such praiseworthy exertions have been misdirected, for, instead
of continuing the progress begun by the ancients, when it was
so easy to do so, artists consume their emulation in impotent
attempts, until a personal experience induces them to resume
the labours of the ancients at the point where the latter have
left off. We must, however, do justice to him who rescued
from obHvion an art that had been so long neglected. For
this our thanks are due to the learned superintendent of the
royal porcelain manufactory at Sevres, Herr Brongniart, who
contributed much to its revival.
We believe we have now satisfactorily defined what we
understand by the art of painting on glass. In our opinion
there is still a future for this art, if its votaries do but follow
the paths marked out for them. With respect to the kind of
composition best adapted to it, we shall content ourselves with
merely remarking that the good taste of the artist alone must
preside over his conceptions. As, however, some have given
their opinion in favour of the Gothic style exclusively, others
in favour of the style at the period of the revival of the art,
according to the several directions which their studies have
taken, we cannot forbear expressing our opinion upon this
subject. We have no thought of enUsting followers for either
party, but without the smallest intention of drawing a com-
parison in this controversy, we may be permitted merely to
hmt that the artist should not allow himself to be taken up
with these discussions, and that in the conditions of his art
alone he has to seek for the spirit that must direct the
suggestions of his imagioation. He should avail himself of all
the resources afforded him by the nature of the things on
which he has to exercise his talents. Ought he, for example,
to forego the powerful effects which a skilful arrangement
of colours enables him to produce? Should he disdain to
derive advantage from the lively colouring of the glass because
it is the most goodly dowry of the Gothic style of the art ?
We think not ; but we are far from advising him, on the other
hand, to sacrifice to such endeavours all the resources of
painting which the revival style is capable of affording him,
provided that he uses all these means with moderation, and,
we repeat it, keeps within the conditions of his art. We
submit this opinion to the intelligent artist who does not
allow his ardent spirit to be carried away by his ideas; but
we have no hope of convincing those who, in the wanderings
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8 ESSAY ON THE ART OF
of their fancy, as well as in the right path, have hecome the
slaves of their imagination.
A very natural question presents itself to the mind, with
regard to the erroneous belief which universally prevails, that
the secrets of the art which were known to the ancients
are lost. Are we capable, if not of surpassing, at least of
resuming and continuing, the labours of the ancients ? When
we compare the glass of the old church -windows, of any
period whatever, with the glass of our manufactories, we
cannot for a moment doubt that our system of manufacturing
it produces far more perfect results, certainly as far as regards
its transparency, whiteness, and clearness, and generally with
respect to all those qualities which are peculiar to glass. Be-
sides, it is allowed that the methods of working have been
considerably improved. And if, on the other hand, we
compare fragments of old painted glass with that which we
manufacture at the present day, it will appear in the most
convincing manner, that our painted glass is not in the sHghtest
degree inferior in point of colour to that of the ancients. There
was a time when the manufacture of coloured glass was dis-
continued, because, in consequence of the decline of the art of
painting on glass, this article was of no further use ; but none
of the secrets of the colouring were lost. Persons who were
little aware of these circumstances, and mistook the effect for
the cause, maintained that the reason why the art yielded
nothing more was, that the painters on glass no longer under-
stood how to produce the ruby of the ancients. But this
assertion was very soon shown to be false ; for as soon as the
determination to restore painted windows manifested itself,
the glass-house of Choisy in France, among others, proved by
the most successful results, that the art of manufacturing
•coloured glass was in no way lost, but was only asleep. In
fact, we possess a multitude of receipts of the ancients* accord-
ing to which coloured glass was produced in former times.
Moreover, in pigments we are much richer than the ancients ;
our pigments, too, are much better, more adhesively enamelled,
in consequence of the improvements which have been intro-
duced into the system of burning in the colours in modern
times.
From what has been premised, we may conclude that our
artists are in no respect in want of the material elements ; on
the contrary, they are far better aided by resources and means
which await their disposal than the ancients were; and if their
works attain no remarkable superiority, the reason for it must
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PAINTING ON GLASS. 9
be sought for in the fact, that the best means and modes of
execution in the arts are unfruitful when they are not employed
with sufficient taste and spirit.
Glass-painting has, in our time, risen into hfe again, sur-
rounded by the fairest hopes; and what we know of the
artists who are already practising it, leaves us not the sHghtest
reason to doubt that it will soon be cultivated with a success
worthy of the present age. We would gladly contribute in
some degree to draw it forth from the state of oblivion in
which it has been for a long time buried. For this purpose
we shall now communicate a number of practical observations
which lie in the sphere of glass-painting. We believe that we
cannot contribute more eflFectually to the diffusion of the art,
than by clearly laying down the methods employed in its
practice. We shall advert to those both of the ancients and
of the moderns, and compare them with one another, in order
to show the improvements that have taken place in our time.
At the present day, when artists as well as amateurs are most
zealously engaged in glass-painting, we flatter ourselves that
this information will not be uninteresting to many. Little has
yet been written about it, and the majority of the publications
that have appeared on the subject treat more of the history
than of the practice of the art.
The art of painting on glass by no means consists in the
mere application of the colouring materials to the surface
of the glass, by methods similar to those employed in oil-
painting. The colours used are of a peculiar kind, and pos-
sess the power of vitrifying at a high temperature, and of
fixing themselves unchangeably upon the glass : consequently
the glass, after the paint has been applied, must be exposed
to a certain heat in a furnace adapted to this purpose. Ap-
propriate means must also be employed in the application of
vitrifiable colours.
A painting on glass— as, for example, a church-window, —
always consists of a great number of pieces of coloured glass,
whose various hues illuminate an ornamental pattern or an
historical subject. These pieces of glass are either symmetri-
cal or irregular, so as to agree with the sentiment exhibited
in the composition itself. After they have been arranged in
their proper places, they are encased in lead, and united so as
to form one complete piece. These pieces are united by an
iron frame-work, called the arming.
After the brief explanation we have just given of the condi-
tions to which glass-painting is subject^ we have to determine
A 5
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10 ESSAY ON THE ART OP
its connection with other arts. In the first place, it is evident
that the science of the chemist must be united to the talent of
the piUnter, and that the glazier himself must lend his assist-
ance. We have therefore divided this little work into several
sections, in which the various branches of the art will be suc-
cessively treated of. We have circumstantially discussed the
following particulars :
1. The quality of the pigments, their composition, their
preparation, and lastly, all the chemical operations which are
necessary previous to the painting; — 2. The means used in
laying on the colours ; the various methods employed for this
purpose, and everything which has reference to the proper art
of glass-painting; — 3. The manner in which the vitrifiable
colours are burnt in ; — and 4. That part of the glazier's art
that is concerned in the putting together painted windows in
churches. Finally, we have given an account of the various
mixtures with which the glass is coloured en masse.
Glass-painting, as it is practised at the present day, has
scarcely anything in common with that of the ancients, as far
as regards the colours. When this art, which had been
entirely neglected for a whole century, was rescued from ob-
livion, the improvements in thie manufacture of glass had so
materially changed the quality of this substance, that the
ancient methods employed in the painting were no longer
applicable. Towards the middle of the eighteenth century,
glass was still composed almost exclusively of flint and potash
or soda. This simple silicate was deficient in fusibiUty, and
preserved an extraordinary tenacity even at the highest tem-
perature; it was difficult to purify, occasioned an enormous
expense in fuel, and was not capable of being worked well* In
the year 1760, Bosc d* Antic tried a mixture of lime in the
form of carbonate of lime, which Kunckel had before proposed.
A striking improvement in glass was thus obtained. When
combined with siUca and with soda or potash^ it forms a
bisilicate, which is much more fusible than the simple siUcates.
The proportions of these substances were, however, for a long
time badly determined, and it was not till lately that the
makers were enabled to impart that fusibility to glass which
renders it so easy to work and so cheap to manufacture.
It is obvious that the pigments of the ancients, which were
prepared for a hard kind of glass, could not have fusibiUty
enough for the glass which is now manufactured. Other ^
ingredients had to be sought for. But if the ancient methods
^^f)ainting were no longer in accosdance vdth the quality of
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FAINTING ON GLASS. 11
the glads, still less were we acquainted with them. Moreover,
the knowledge of enamel colours for metals, as well as for
various kinds of earthenware, afforded easily applicable prin-
ciples, according to which a series of properly fusible colours
could be composed. The vast conquests of chemistry within
the last fifty years, promised besides, to this manufacture,
a very certain success. Modern glass-painting is thus almost
entirely of late origin ; and, with the exception of the process
of burning in, has experienced modifications in every par-
ticular in a very remarkable manner. If we believe the testi-
mony of the authors as far back as the period at which Levieil
wrote, the glass-painters at that time burnt in their colours in
iron boxes, in which the glass was arranged in layers with
calcined and pulverized lime strewed between the strata of
glass. But towards the year 1758, an English artist made
known a new method of burning in, which he employed him-
self, and which, with some trifling modifications, has remained
in use since that time. We shall describe this method more
particularly in its proper place. The superiority of this new
method of burning in the colour over the ancient method, and
especially over that of Levieil, is incontestable. According to
the method of the latter, the melted pigments were placed in
contact with powdered lime : a portion of this powder adhered
to the colours, and injured the transparency. If this was not
always the case, the reason was, that the pigments, on account
of their slight fusibility, merely adhered to the surface of the
glass, and then again its transparency was impaired. The
painted windows of the Levieil family furnish a proof of this,
and in particular that in the chapel at Versailles, the blues of
which are so obscured that they appear black ; and among
others, Pierre Levieil himself, in his work, admits the fact.
What we have said about the process of burning in, as far
back as the time of Levieil, is founded upon the testimony of
the writers who have left us some verjr interesting notices of
the state of glass-painting at that period. To these, among
others, belong Kunckel, Haudicquer de Blancourt, Levieil, &c.
We have not, however, received their opinions upon this matter
without due reflection. For how could it be supposed, that in
the sixteenth century, when both glass^painting and enamel-
painting were so generally honoured and cultivated, it should
never have occurred to any of the artists, who were frequently
skilled in both arts, to subject the process of burning in to those
conditions which are inmspensable in enamelhng, — namely,
a contrivance for heating the plates of glass, isolated and
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12 ESSAY ON THE ART OF
entirely remoTcd from contact with any other hody which calf
adhere to the colours when in a state of fusion, soil their sur-
face, and deprive them of their transparency ? This fact may
easily be explained with regard to the former centuries, iii
which the Gothic style only was cultivated in glass-painting.
Because, as this kind of painting is limited to a mere outline
upon a ground of glass, coloured en masse, little depended
upon the smoothness and brilliancy of the colours which were
applied in the painting ; on the contrary, a complete opacity
was indispensable. But in the age of a Pinaigrier or a Jeaa
Cousin, when the use of pigments for painting almost entirely
superseded that of coloured glass, it is hardly to be supposed
that the glass-painters were unacquainted with a method of
burning in, similar to that which is now employed. And this
is the less credible, inasmuch as the works of this period
prove that pigments could be prepared of great clearness, free
from all impurity, and just as good as the enamel-painters
could produce. It may well be supposed that the traditions
of the Levieil family have not informed us what the process
was before their time. The progenitor of that family lived
somewhere about the end of the seventeenth century. Glass-
painting, which was gradually declining, was at that time only
cultivated by a few artists. The Pinaigriers, and those of
their school, had carried all the secrets of their art with them
into the grave; and this was the case at that time with all
who practised an art which was enveloped in mystery. Even
the very writers who furnished the public with information
upon the arts, always reserved that which was most useful for
themselves. Cassius did so, according to his own confession,
and Levieil made it a subject of complaint against Kunckel
and Taunai. Before Guillaume Levieil, but one artist, Jacques
de Parol, had written about glass -painting, and from this
common source several authors who followed him, as well as
the painters who in the most modem times devoted themselves
to glass-painting, seem to have drawn. To the former belong
Felibien, Florent le Comte, and Haudicquer de Blancourt ;
and to the latter the Levieils and the brothers RecoUet. This
is proved by their receipts, which they have transmitted to us,
which are ror the most part like one another, and are many of
them completely identicEd.
It may be easily supposed that the Levieil family were un-
acquainted with the method of their ancestors, who kept it a
secret, when it is certain that Pierre Levieil himself, in spite of
his profound erudition^ was ignorant of what was anterior to
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PAINTING ON GLASS. 13
his time, although it had been made known. Fifteen years
after the appearance of the Enghsh work already mentioned,
and actually at the time when he was writing, he still continued
to adhere to the manifestly antiquated traditions of his family.
However it may be in other respects, we have certainly, in the
present state of our knowledge, no reason to envy the ancient
glass-painters in regard tq their methods of operation. Conse-
quently, we have nothing to hope for from pretended discoveries
of the secrets of the ancient artists, which were at times so
pompously announced, because, as we have before stated, the
improvements which took place in the arts in consequence of
the progress of science, have placed us in an entirely new situ-
ation, which makes the methods of operation that were in vogue
at a period far distant from the present time appear utterly
CHAPTER I.
OF THE aUALlTY AND COMPOSITION OF THE PIGMENTS.
By the pigments necessary for painting on glass are understood
vitrified or vitrifiable substances of various colours, which are
applied to the surface of the glass, and fixed by being exposed
to a temperature which brings them into a state of fusion.
Several qualities are indispensable to the pigments : 1 . fusi-
bility at a given temperature; — 2. the power of adhering
firmly to the glass and completely uniting with it; — 3. a
peculiar transparency, or an opacity; — 4. a glassy appearance
after fusion; — 5. a sufficient hardness to resist entirely the
friction of solid bodies; — 6. insolubility in water; — 7. the
being imchanged by the action of the air, moisture, and the
gases, which are ordinarily diffused through the atmosphere ; —
lastly, 8. an expansibility equal to that of the pieces of glass
that are to be painted with them.
The fusibiUty of the pigments must always be greater than
that of the glass. As the latter becomes soft at a red heat of
some intensity, it is necessary that the pigment should be in a
state of fusion and become fixed to the glass before it reaches
the temperature at which it would be spoilt by bending, from
being at the point of fusion.
The pigments are almost always more or less transparent,
and only a few must be opaque. In contradistinction to the
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14 ESSAY ON THE ART OF
other enamel-paintings, which convey to the eye mertly re-
flected rays of light, a painting on glass receives its colour
from transmitted rays. We can understand, therefore, that
transparency is a quality very frequently necessary to the pig-
ments. It is not always indispensable that this transparency
should be perfect and possess the clearness of glass ; on the
contrary, it is often an advantage when the objects which are
behind the window cannot be distinguished. A half-trans-
parency is usually sufficient, provided that it admits of a rich
and magnificent colouring; but there are cases in which the
painting requires perfectly opaque pigments.
The hardness of the pigments varies according to their com-
position. They must always possess a degree of hardness
sufficient to enable them to resist easily the friction of hard
bodies ; but since the causes which operate mechanically upon
painted windows, to the destruction of the pigments that He
on the surface of the glass, are exceedingly rare, the artist
need not always exclude those pigments that are even of
moderate hardness.
The resistance of the pigments to the chemical action of
bodies must be such that they cannot be affected by any of
those agents to the influence of which they are ordinarily ex-
posed, e. g. the action of the air, water, sulphuretted hydrogen,*
and other gases difl\ised in the atmosphere; but it matters
little whether the pigments are capable of being acted upon by
bodies with which they only accidentally come in contact, or
not.
The unchangeableness of the pigments is as conditional as
that of the glass, and is usually in proportion to their hard-
ness.
Expansibility is one of the principal qualities of which the
pigments must possess a precise and accurate amount. In the
frequent changes of temperature which the painted plates of
glass undergo, during and after the burning, the expansibihty
of the pigment must be in exact proportion to that of the glass.
Were it otherwise, the expansion and contraction taking place
irregularly in both bodies, would produce movements in the
glass in opposite directions, which must occasion numerous
fractures. These are in fact the accidents produced by pig-
ments whose expansibihty is ill suited to the glass. Pigments
of this kind crack and spHt, and soon peel ofP the suiface of
the glass in the form of scales, while the glass itself, which on
* Or, hydrosulphoric acid.
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FAINTING ON GLASS. 15
account of its thickness possesses firmness and a greater power
of resistance, remains uninjured.
The pigments are composed: — 1. of colouring materials,
which in most cases helong to the class of metallic oxides ;
2. of fluxes or vehicles for colour, which are vitreous or vitri-
fiahle compounds, through the medium of which the colouring
matter is fixed upon the glass. These fluxes are generally
silicic, horacic, or borosilicic salts, in which the acids are com-
bined with the bases in certain proportions, and whose state of
neutralization varies according to the several indications of
which we shall afterwards have occasion to speak.
In order to colour the pigments, the colour which a sub-
stance in its uncombined state affords is sometimes employed,
sometimes that afforded by its combination with another sub-
stance which usually forms a part of the flux. In either case
the colouring matter is always mixed with the pigments.
This observation admits of & very nice distinction between
them, so that we have divided them into two classes.
The first class comprehends those pigments in which the
colouring matter is uncombined with the flux, and is in a state
of simple mixture, as, e. g., in the case of oil-painting the
colour is mixed with the oil. We shall call them pigments
coloured by mixture.
The second class comprehends these whose colouring matter
is in combination with the flux, has become a constituent part
of it, and forms with it an entire vitrified mass, possessing all
the properties of glass itself. We shall call them pigments
coloured by combination.
This classification of the pigments has not been invented
merely for the purpose of systematical arrangement, but is
grounded rather upon practical considerations of the greatest
importance.
The composition of the fluxes is not arbitrary. Inde-
pendently of the peculiar qualities which they must possess in
order to be really unchangeable, it is also necessary, since they
are the medium through which the union between the vitreous
and the colouring matter is effected, that they should be
adapted to the nature of the former, to insure their adhesion
to it for a long period, and that they should also accord with
the qualities of the colouring materials which they have to
unite with the glass. The necessity there is for the fluxes
being accommodated to all the requirements of the colouring
materials, is the principal reason why a much larger quantity
of this vehicle must be employed, as we shall show hereafter.
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16 BSSAY ON THE ART OF
We shall first consider the composition of the fluxes in the
several relations they bear to the colouring matter.
In the pigments of the first class it is necessary that the flux
be of such a quality that it will preserve the colouring matter
in the isolated state upon which the obtaining the requisite
colour depends, and that it contain nothing that can effect a
change in the properties of the colouring matter. In the fused
colours of the second class, on the contrary, it is indispensable
that the flux should exhibit a powerful action upon the colour-
ing matter, by which action the combination from which the
colour is to be obtained is efi^cted. We shall now make a few
observations, from which we shall deduce the principles of the
composition of the fluxes, considered in that point of view in
which we exhibited them above.
The fixed acids combine with bases in all proportions ; but
each of these combinations has a certain point of saturation at
which, when in a liquid state, it possesses just as Uttle aflinity
for a greater quantity of base as for a greater quantity of acid.
This neutral state takes place in the most easily fusible combi-
nation, and the reason is as follows :
If among the combinations of a fixed acid with a base but
little or not at all fusible, that be chosen which possesses the
greatest fusibihty, and an attempt be made to unite with it
successively fresh quantities of base, it will be observed that
the temperature must be raised in proportion to the increased
amount of base brought into combination. This, for example,
is the case with the siUcates of lime, iron, cobalt, copper, &c.
If, on the contrary, we wish to add successively to the combi-
nation of a fixed infusible acid with a base fresh quantities of
acid, it is a well-known fact that the temperature must like-
wise be raised in proportion to the quantity of acid which has
entered into combination. Therefore it may be asserted that
in the combinations which consist of a fixed acid and a base,
beginning at the most fusible combination, an increase of base
or acid requires a proportionate increase of temperature, pro-
vided that the substance which is to be added is not very easily
fusible, and its combination in consequence independent of the
temperature.
The principle which we have just established is certainly
subject to modifications, sometimes in favour of the bases,
sometimes in favour of the acids, according as they are more or
less fusible. In the silicates of lead the same degree of tem-
perature is not necessary for the combination of a quantity of
base, as is requisite for the combination of a larger quantity of
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PAINTING ON GLASS. 17
acid, because the ready fusibility of the former makes its com-
bination with the silica independent of the temperature. But
we have in the borates of iron, cobalt, and copper, an example
of the contrary, and here the rule is modified in favour of the
bases, because the fusibility of boracic acid renders an increase
in the temperature unnecessary. But exceptions like these are
not to be found in the combinations we have just been speaking
of, if, in the place of the fusible element, an infusible one be
substituted in the mixture. This is the case, for instance,
when oxide of iron is added to silicate of lead, or silicic acid to
borate of lead.
From what has been premised, we infer that when we begin
at the neutral state, the temperature which is requisite to unite
an oxide with a flux affords a rule for determining the disposi-
tion of this flux to become still more saturated. The more
it is saturated, the greater difliculty it has in combining with
a larger quantity of base, provided that the latter is not fusible-
The proportions of the base which have to be brought into
combination depend upon the temperature; the amount of
base is determined according to a given temperature, the
above-mentioned cases of easy fusibihty excepted. If, there-
fore, we were to add a fresh quantity of base under the same
conditions, it would not enter into chemical combination. This
circumstance has now been taken advantage of in the composi-
tion of the fluxes of the pigments of the first class.
When the temperature at which the pigments pass into the
liquid state is determined, the proper point of saturation for
them is at the same time that which is proper for the flux,
because we are assured that the colouring matter which has to
be combined with it will remain uninjured. If, then, we take
the melting point of the pigments at a cherry-red heat, expe-
rience teaches us that the triple silicic and the double sub-
boracic salts of lead, soda, and potash, which are then com-
pletely fused, can be saturated no further. If, therefore, we
wish to colour a pigment with an oxide which shall only remain
with its flux in the state of mechanical mixture, we must add
to the latter the triple siUcic and the double sub-boracic salts,
of which we have just been speaking.
But if we wish to obtain a colour by means of an oxide
which is to enter into chemical combination with the flux, the
degree of saturation at which this combination is effected is
fixed with as little precision as that of the temperature. If in
this case it is judicious to employ a less saturated flux, this
may only be done within the limits in which the pigment pre-
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18 ESSAY ON THE ART OF
serres its indispensable physical properties. As the tempera-
ture lends its aid, the combination of the oxide is always
obtained. Hence it follows, that however the composition of
the fluxes of the pigments of the first class is subject to certain
strict conditions, this is not the case with the pigments of the
second class. But even in these, as we shall soon see, direc-
tions of great importance must be given.
In the composition of fluxes for pigments of the first class
we have taken a moderate red heat as the point of saturation,
for the following reasons : first, the glass which is painted is
capable of bearing only a slight degree of heat, and the standard
is its point of fusion, consequently the temperature must not
be raised to this Hmit. Besides, the degree of saturation which
we have recommended is at the same time that at which the
flux accommodates itself best to the expansibility of the glass,
without our being thereby obliged to sacrifice the other
desirable quaUties of the pigments.
The saturation of the flux, and the temperature which it
has to undergo, are, however, not the only things which must
be attended to in the composition of the pigments. There are
other secondary conditions, which are Hkewise of importance,
partly to prevent the combination of the oxides vnth the pig-
ments of the first class, partly to favour their combination
with those of the second class. The exact degree of heat is
not always easily obtained ; and if it should happen to be ex-
ceeded in the pigments of the first class, the flux immediately
regains its power over the colouring oxide. The change in this
substance is in proportion to the quantity of flux. Hence
we have a reason for prescribing as Uttle flux as possible in
pigments of this kind.
An opposite principle directs us to use as much flux as pos-
sible in the pigments of the second class. Besides, it is knovni
that a greater saturation of the oxide is still more favourable
to its combination. In order that the pigments of the first
class may not be exposed to injurious alternations of tempera-
ture, the fluxes are not fused together with the oxides before
they are required ; while, on the other hand, no use is made
of the pigments of the second class until a previous fusion has
shown a perfect combination of the colouring matter.
We have already observed that the saturation of the triple
silicic and double sub-boracic salts has been selected because it
fulfils the requirements of the colouring matter and of the glass,
without endangering the intrinsic qualities of the pigment. In
fact, we are strictly obUged to confine ourselves to these Umits
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if we wish to avoid the unpleasant results that have been men-
tioned above.
When a salt of silicic acid and a metallic oxide is combined
with a silicic salt, having an alkaline base, by the agency of
heat, the one is dissolved in the other. Does this result from
an act of combination, or from simple mixture? The learned
observations of Dumas upon the fortxiitous crystallization of
glass have proved that the diiferent kinds of glass are com-
posed of certain definite silicates, and we have reason to beheve
that they are in a state of combination with one another. But
even independently of the quality of these silicates, their
various states of saturation produce numerous modifications
in the properties of these compositions. The most important
fact, however, which has been noticed, is the following : Mr.
Faraday has observed, that if only a slight addition be made to
the quantity of oxide of lead which the common flint glass
contains, this glass, which before was quite proof against
moisture, then acquires the property of a hygrometer in a
remarkable degree, and in damp air soon loses its transparency.
Several others have ascertained the truth of this fact from their
ovm experiments. Flint glass is a silicic compound, of which
the acids contain eight of oxygen to one of base. Whenever
glass in general contains a greater quantity of base, it is much
more easily affected by water. This may be said of window-
glass, looking-glasses, &c., especially when they have been
polished. All these combinations yield an alkaline sihcate,
which is soluble in boiling water, and an insoluble silicic earth
is precipitated. This takes place at the various degrees of
saturation which lie between the octosilicate and the bisilicate.
But it is a very remarkable fact, and one which has been
especially observed with regard to those glasses which contain
lead, that if flint glass, containing a soluble alkali, be reduced
to a bisilicate in such a way as to be combined with a greater
quantity of lead, this flint glass when pulverized gives up
almost all its alkaUne silicate in cold water, and that, too,
almost immediately.
Hence it is that the combination of a silicic salt of lead with
an alkaline silicate, which is very easily decomposed in mass,
in proportion as we descend from the octosilicate, possesses no
stability at all when we reach the bisilicate; for then the latter
lias become soluble in cold water, and is immediately dissolved
in it. It is, however, probable that this is not the case with
all bisilicic compounds ; for basic silicates are combined with
one another in those kinds of glass which contain lead, whilst
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20 ESSAY ON THE ART OF
in bottle-glass, for example, silicates of any kind are combined
with basic silicates, and combinations like these have more
stabiHty. But our business here is only with the silicates
which contain lead; for the pigments that are usually em-
ployed almost always contain lead. The reason of this is, that
the sihcic salts of lead are extremely useful in modifying the
expansibiUty of the pigments. By increasing or diminishing
the quantity of oxide of lead, we almost always succeed in im-
parting a degree of expansibility to the pigments equal to that
of the glass. The same result cannot be obtained from an
alkaline sihcate. This shows us the reason why potash is
avoided in the composition of the pigments. The necessary
fusibihty and expansibihty require that the fluxes should be
brought into that state of saturation in which they have very
little stability, and are very liable to decomposition. At a hign
temperature the potash is decomposed, and evaporates ; when
cold, the pigments are easily affected by moisture. This dis-
agreeable circumstance is avoided by substituting for the
potash borate of soda : the latter is much more fusible than
the sihcate of potash, and consequently enables us to obtain a
proper fusibihty without lowering too much the degree of
saturation. And so less colouring, less liability to change,
and greater hardness, are simultaneously obtained.
The whole matter may be shortly summed up thus :
1 . In the pigments coloured by mixture, those silicates only
may be used whose acids contain at most three times as much
oxygen as the bases.
2. In the pigments coloured by combination a greater quan-
tity of oxygen in the acids can only be of advantage when all
other conditions have been complied with.
3. No pigment containing lead is to be prepared which con-
tains the silicate of an alkali in a state of saturation beyond
that of the trisiUcate ; that is to say, which contains a smaller
portion of acid, or a greater quantity of base.
4. In every case, the indispensable conditions of fusibihty,
hardness, and expansibility must be satisfied.
In the composition of fluxes, silicic and boracic salts of
various metals are usually combined, because the salts formed
by these combinations possess greater fusibility, and because
among the simple silicates and borates, which might, perhaps,
be sufficiently fusible, they would not have the requisite white-
ness if they were employed alone. For instance, the silicic
and boracic salts, which contain a great quantity of base,
" ' ^ Dossess sufficient fusibihty, but they have a yellow
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coloar, which is more dbtinct in proportion as they are satu-
rated. It is therefore necessary to combine them with a cer-
tain quantity of alkaline silicates or borates^ in order to render
this colour less conspicuous.
It would be better if the silicic or boracic salts which are
used in the pigments were all insoluble, Hke those of lime, alumi-
num, lead, &c. But the necessity for obtaining a great degree of
fusibihty requires the use of alkaline siUcates and borates,
which, within certain limits, obtain a sufficient stability from
their combination.
From the principles which haTC been laid down above, it
might seem that two kinds of fluxes are sufficient for the two
classes of pigments. This would certainly be the case if
nothing more than the proper colour were attended to in the
preparation of the pigments. But these pigments, which are
made on purpose to be laid on the glass, must possess the
same expansibility as the latter. Now the physical properties
of the pigments are modified in a remarkable degree by the
yarious substances employed in colouring, coUeotively and
separately, in different ways. Consequently, it is only by
changing the nature of the flux that we are capable of impart-
ing the requisite expansibility to the pigments. Hence, also,
arises the necessity for the existence of a great vrriety among
the fluxes. When we come to treat of the pigments parti-
cularly, we shall also specify the fluxes proper for each. We
shall, however, mention a few here, which may be adduced as
an illustration of the rules we have laid down.
FLUXES FOR PIGMENTS OF THE FIRST CLASS.
No. 1. No. 2. No. 3.
Silica 1 pt. 3 pts. 2 pts.
Oxide of lead 3 ,, 8 „ 6 „
Calcined borax „ 1 „ 1 „
Haudicquer de Blancourt, who has described the prepara-
tion of the flux No. 1, in his Art de la Ferrerie, calls it rocaille,
and it was formerly used as a glaze for common pottery-ware.
This flux, however, whose state of saturation is admirably
adapted to the preparation of pigments of the first class, can-
not be advantageously employed in every case. It frequently
happens that a colouring oxide, when mixed with it, contri-
butes to its decomposition, since it favours the separation of
its elements. The pigment then undergoes a change on expo-
sure to the air, the surface loses its brightness, and crumbles
to powder. We are^ however, unable to specify the nature of
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22 ESSAY ON THE ART OF
the action of the colouring substance. Perhaps it is only
mechanical, and proceeds from the great distribution of parts,
and from the porosity itself, which a powder in the state of
simple mixture imparts to the pigment; perhaps, too, the
oxide of lead has less affinity for the silica than the new sub-
stance which tries to supplant the first.
The rocaille flux is, aiter aU, only employed with advantage
in the pigments of the first class when they have previously to
be melted. The more intimate mixture of the flux with the
colouring matter then imparts a greater density to the pig-
ment, which defends it against the action of the air. We pre-
fer this explanation. When the pigment does not require to
be melted first, it is advisable to substitute No. 2 or No. 3
for No. 1 ; for they are only a modification of the latter, and
possess greater stabihty.
FLUXES FOR PIGMENTS OF THE SECOND CLASS.
No. 1. No. 2. No. 3. No. 4.
Silica 3 pts. 1 pt. 3 pts. 3 pts.
Minium , . . . . 8 », 8 „ 6 ,, 6 „
Borax 3 „ 2 „ 3 „ 2 „
Saltpetre „ „ 1 „ „
Every pigment might be prepared according to the above
directions, if nothing but the good quaHty of these vitreous
compounds were had in view. But those pigments chiefly
that are to be produced by combination are so changed in
regard to their expansibility by certain oxides, e, g. those of
copper and manganese, that, in order to destroy the efl'ect of
the latter, it is necessary to reduce the fluxes to a state of
saturation, which cannot be done by the use of alkaline sili-
cates, for they must be employed in such small quantities that
they may be enveloped, as it were, by the other siUcates, and
thus protected from the action of the water.
In this case the fluxes are very much saturated and less
fitted to dissolve the oxides. But even then, methods may be
employed to facihtate their combinations, which we shall de-
scribe when we come to speak of the pigments in particular.
OF THE PREPARATION OF THE PIGMENTS IN GENERAL.
The preparation of the pigments, which embraces a number
of particulars with regard to each, may, however, be reduced
to two general methods of operation, according as they are
coloured by mixture or by combination.
In the first case, as we have already said, a flux in which
base preponderates is chosen; and with this object in view.
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PAINTING ON GLASS. 23
care is taken, at the same time, that the colouring oxide shall
remain as short a time as possible in contact with the liquefied
flux : for this purpose they are only mixed together by means
of the runner upon the mill-stone,* and the mixture is not
heated until it is ready to be laid upon the glass, and exactly
as much flux is used as is necessary to give body, smoothness,
and brilliancy, after the burning in, to the pigment.
In the second case — 1. A pigment must be selected in
which the acids predominate as much as possible; — 2. It is
also necessary to fuse them together in a strong heat, in order
to facilitate the reaction; — 3. The flux must likewise be pre-
sent in as large a quantity as possible without injuring the
richness of the colouring; — 4. The oxide must be perfectly
free from combination, which might impede its imion with
the flux.
These are the most important varieties of the pigments,
with regard to their composition and preparation. We subjoin
some fnrther considerations concerning the pigments collec-
tively, that is to say, concerning the means of modifying their
properties according to circumstances.
With respect to transparency, the pigments coloured by
simple mixture are remarkably dissimilar to those which are
coloured by chemical combination. It will be readily under-
stood, that an opaque colouring matter diffused through a
glass vessel diminishes the transparency of the latter, so that
the enamel which is produced by it will be less permeable to
light than another which has been coloured by a substance
dissolved in a flux. It is also just as evident, that in the for-
mer case the opaque colouring matter diminishes the transpa-
rency of the flux in proportion to the quantity in which it is
added to it. Thus the transparency of the pigments is in-
creased according as the quantity of colouring matter is dimi-
nished. But this can only be done at the cost of the colouring,
and in such circumstances, where intensity of colour is not
required, it will be better even to impair its stability ; for the
more flux there is, the greater the action known to take place
upon the colouring metallic oxides. With regard to the pig-
ments of the second class, their transparency can only be
diminished by the mixture of substances which impart opacity
to them.
The hardness of the pigments under circumstances in other
respects the same, increases in proportion to the quantity of
* Described under the head * MUl for grincUiig the pigments.'
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24 ESSAY ON THE ART OF
the silica. This is just the case with regard to their resistance
to the action of chemical agents. Consequently the opposite
efPect is produced as soon as the hase is made to predomi-
nate.
We now come to their expansibility. It is of great import-
ance to make this property of the pigments accord with that
of the glass : it may, however, be very easily modified in the
case of the former. With regard to this, we believe we have
observed, that in the borate, silicate, and borosilicate of lead,
the base usually produces the opposite effect to that of the
acid; but we are unable to state whether the one diminishes
or the other increases the expansibility. It is enough for us
to know, that if a pigment becomes fiiU of cracks, the proper
degree of expansibihty may easily be imparted to it either bv
increasing or diminishing the quantity of oxide of lead. We
would recommend here the former method as the proper one
in almost all cases.
FUSION OF THE FLUXES.
After the requisite quantities of the various substances that
are to be used for a flux have been levigated and accurately
weighed, nothing more remains than to fuse them. The pow-
der is first well rubbed and mixed in a mortar. After careful
mixture, it is put into a covered crucible, which is placed in
the furnace; a gentle heat is first applied, which is gra-
dually increased, until the whole is brought into an undis-
turbed state of fusion, and all bubbles have ceased to form.
The crucible is then taken out of the furnace, and the contents
are poured into a vessel of cold water : they are then collected
and dried upon paper. The action of the cold water splits the
mass into small fragments, which can afterwards be more
easily levigated. Without this precaution, the flux would
concrete into a vitreous mass, difficult to pulverize. This is
exactly the mode of operation when we have to combine a flux
with a colouring metallic oxide for a pigment of the second
class, as is the case with all vitrifications of fluxes or pig-
ments.
If we would have our colours bright and pure, it is of essen-
tial importance that the pigments should be prepared from
none but the purest substances. It is consequently necessary
that we should be well acquainted with the properties of the
substances to be employed. We therefore think it advisable to
make a few preliminary remarks concerning some of them,
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FAINTING ON GLASS. 25
and to give a full account of them afterwards in their proper
places.
Silica. — Silica is procured from flints, and white ones, or
those of a beautiful black colour, are selected for this purpose.
The yellow flints are less pure, and contain a great quantity of
iron. For this purpose they are heated red-hot, and then
thrown into cold water to quench them. If the action of the
cold water fails in cracking them in such a manner as to allow
of their being easily crushed by the fingers into a coarse sand,
the operation is repeated; they are then reduced to powder,
and rubbed through a silken sieve. This powder must then
be washed, and the supernatant water poured off, as long as a
fine powder continues to float on the surface, which gives the
silica a yellow colour. This powder appears to consist of im-
purities, which originate partly from the furnace, partly from
the cast-iron mortar which is employed in levigating the flint:
they seem to be united with very fine silica, which could
better be spared than retained in company with these foreign
bodies. The particles of iron, being very fine, are carried off
by the water in which the powder is washed, and separated
before the process of heating the siHca (of which we shall pre-
sently speak), in which those particles of iron would otherwise
be further oxidized, and might not be so easily affected by
acids.
After the silica has been washed in the manner above de-
scribed, it is strongly heated and thrown a second time into
cold water. The Httle grains of which it is composed undergo
a fresh division, which faciUtates the action of the acids with
which they come in contact, and also the fusion of the metals.
They are now treated with hydrochloric acid, washed and dried.
Silica might also be obtained in the same way from white
granular quartz ; but even the purest granular quartz contains
a greater quantity of iron than flint does, on which account the
preference is usually given to the latter.
Borax, — The borax which is used in the preparation of the
pigments is ftiaed borax, which must not be confounded with
calcined borax. The latter contains still a greater quantity of
water. It is indispensably necessanr to make use of the fused
and perfectly vitrified borax, not only to enable us to calculate
the quantities exactly, but also to avoid the swelling which
would take place if borax in any other form were employed.
We shall hereafter enlarge more minutely upon the maimer
in which this operation is performed. When the borax is in
a state of fusion^ and appears perfectly clear, it is poured upon
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26
ESSAY ON THE ART OF
a smooth stone : it then presents the appearance of a heautiful
white and entirely colourless glass. It must now be kept in a
bottle well corked.
Minium, — The orange-coloured minium^ as it is called, is
most usually employed : it is the purest, and can be used with-
out requiring a particukr preparation. We shall only obs^e,
however, that the minium must part with 2*23 per cent, of
oxygen, in order that it may be reduced to the state of prot-
oxide.
Of the other bodies which are used for fluxes we shall have
occasion to speak elsewhere. Now that we have finished
treating of the pigments in particular, we intend to subjoin a
few general reflections, which we recommend to the considera-
tion of the reader.
GENERAL REFLECTIONS CONCERNING THE COLOURING
MATERIALS.
The chief colouring substances of the pigments are metallic
oxides. Sometimes they are simply mixed with the whole
body of the glass ; at other times they are combined with the
sihca, and probably form double salts with the siHcates of the
fluxes. The analogy which prevails between the pigments
and the other kinds of glass supports this hypothesis. It is
known that in these compositions such alkaline siHcates as are
soluble in water when uncombined become almost insoluble as
soon as they are combined with other silicates, e, g, with sili-
cate of lime, silicate of lead, silicate of aluminum, &c.
Now the combination of these bodies is the sole means by
which their properties can be modified.
In the pigments coloured by mixture, the colouring oxides
are not always used separately; sometimes several are em-
ployed, after they have been previously combined with one
another. But the conditions to which they are mutually sub-
ject are always independent of the flux.
The oxides which are combined in a pigment of this descrip-
tion do not give it the colour which the mixture of their
respective colours would produce, but they ^ve it peculiar
shades of colour, which are determined by their state of com-
bination.
Several oxides are frequently employed also in the pigments
of the second class, but are not combined with one another,
and the colour which is obtained is only the result of the mix-
ture of thfe colours which each oxide produces of itself.
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PAINTING ON GLASS. 27
. The combination of the oxides with one another is a valu-
able auxiliary in the preparation of the pigments. Sometimes
these combinations impart to the oxides a greater power of
resisting the action of the flux; at other times^ on the con-
trary, they facilitate their solution in the latter. It is evident
at opce that the former are used for the pigments of the first
class; the latter, on the other hand, for those of the second
class. In the former case, we should combine oxide of iron,
for instance, with oxide of zinc; because the former, by virtue
of this combination, which possesses great stability, would
prove much more powerful in resisting the action of the flux,
and thus the real colour of the oxide could be given to the
pigment.
In the second case, on the contrary, we should combine
oxide of cobalt with oxide of lead, so that their less stable
combination may bring the former of these bodies into a state
of fine division favourable to the action of the flux. This
direction may be complied with in a very simple manner, if,
instead of preparing the flux beforehand in order to combine it
with the colouring matter by a second fusion, the colouring
oxide be heated when mixed with the ingredients of the flux ;
for the oxide of lead, which forms one of these ingredients,
will dissolve the colouring oxide, and by that means dispose it
to combine more easily with the silica. We have therefore no
hesitation in prescribing this method universally for the pre-
paration of the pigments of the second class. We know no
reason of suflicient importance to oblige us to use fluxes pre-
viously vitrified, as is the case with the pigments coloured by
mixture.
The combination of two oxides forms a real salt, in which
the one appears in the character of the base, and the other in
that of the acid. We shall now give a list of these bodies, in
which those that act as base and those that act as acid are
classed according to the energy they exhibit :
ACID OXIDES.
Antimonic acid.
Antimonious add.
Stannic acid.
INDIFFERENT OXIDES.
Protoxide of tin.
Oxide of antimony.
Oxide of chromium.
Sesquioxide of manganese.
Oxide of iron.
Oxide of aluminum.
Oxide of zinc.
BASIC OXIDES.
Protoxide of iron.
Protoxide of manganese.
Protoxide of lead.
Oxide of silver.
Oxide of bismuth.
Protoxide of cobalt.
Oxide of copper.
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28 ESSAY ON THE ART OF
We shall now give a few examples of the ahoye-mentioned
compositions, as they are frequently employed :
Antimonite of lead Yellow.
cobalt Bark green.
— ^— — copper Pistachio green.
Perantimonite of iron Bees'-wax yellow.
zinc Yellow.
Zincate of iron Yellow ochre colour.
It is evident that a great numher of combinations similar to
those we have mentioned might be formed. A variety of
mixed gradations of colour are obtained from these compounds,
which in painting are called broken ttnts, and which are of
great service to the artist. Among these compounds are :
Ferrate of manganese,
chrome,
cobalt,
— — ^ copper,
Manganate of cobalt,
copper;
-^— chrome,
Cuprate of silTcr, &c.
We shall treat of the preparation of these colouring sub-
stances hereafter.
CHAPTER II.
OF THE FIGMENTS IN PARTICULAR.
Red for the flesh-tints. — This pigment is coloured with
oxide of iron, as it is obtained by calcining the green vitriol of
commerce; but it must first be purified, principally in order to
rid it of the sulphate of copper, which it almost always con-
tains, and whicn, when the pigment is used, turns the red
black.
Purification of the vitriol, — Dissolve the vitriol in twice its
weight of cold water, and throw into the solution iron turnings,
iron filings, or iron cuttings of any kind. The sulphate of
copper will be decomposed, and the copper precipitated in the
metallic state, in the form of a reddish powder. The solution
should be stirred from time to time, and after the precipitation
is completely at an end, the liquor poured off and filtered.
To know whether all the copper has been precipitated, dip
into the solution a bright blade of iron, and see whether it is
covered with a surface of red copper.
The filtered solution of sulphate of iron must be put into an
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PAINTING ON GLASS. 29
iron or leaden vessel, which is placed in a common oven,
in order that it may become so concentrated by boiling as
to have lost fths of the water which has been taken into solu-
tion. Its specific gravity will then be 40^ of Beaum^s areo-
meter, and the solution will begin to grow turbid ; it is then
left to crystallize in a wooden vessel covered with wax ; twelve
hours afterwards the mother liquor is poured off, and the
crystals are collected, strained, and dried.
Drying of the salt. ^-In order to dry the salt, and to
perform the subsequent operation, two pounds of salt at the
most should be treated at a time, for the sake of greater
facility in manipuktion.
Two pounds of the purified crystals are accordingly put into
an iron vessel, which is placed in a furnace, and a moderate
•fire is applied, in order to melt them in their water of crystal-
lization. When this has taken place the liquid mass begins to
boil, and soon assumes the consistency of thin paste, and the
colour of clay softened by moisture. After it has become con-
centrated the temperature is lowered, that the melted mass
•may not boil over. In proportion as the water evaporates and
the contents of the vessel are drying they are continually
stirred, and during this process the bottom of the vessel espe-
cially must be scraped with an iron ladle having a long handle,
until the salt is at length converted into a more or less coarse
powder. It is now allowed to cool, after which it is pounded
in an iron mortar, and finally rubbed through a silken sieve.
.In this state it is fit for the preparation of the red pigment.
Preparation of the red pifftnent. — For this operation a very
thick cast-iron vessel is required, and it is not a matter of
indifference whether merely a common iron vessel of moderate
thickness be employed for this purpose or not; for there
would be a danger of its being peHbrated with holes and
destroyed before the conclusion of the operation : cast iron is
more durable than wrought iron when applied to this use.
An earthen cylindrical furnace must likewise be employed,
with a bottom of the same substance, and open at the top. A
hole is bored at the bottom, to receive the muzzle of a small
smith's bellows. The dimensions of this furnace must be six
inches in diameter, and the same in height. In default of such
a furnace, a common one might be used, but the temperature
can be regulated much better in the one we have just de-
scribed ; for as soon as the operator ceases to blow, the fuel,
receiving no air from any quarter, begins to go out. The
vessel must be of the same width as the furnace.
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30 ESSAY ON THE ART OF
These proportions for the furnace and the iron vessel must
be adapted to the quantity of the salt that can be used at
once, in order to perform the operation with due precaution.
The sulphate of iron, prepared according to the above direc-
tions, is heated until it assumes a dark red colour. Mean-
while it is continually stirred with an iron scraping instrument
until the operation is finished, in order that fresh surfaces
may be continually exposed and the whole may be uniformly
heated. The powder at first becomes yellow, then brown, and
at last assumes a greenish brown appearance, which after cool-
ing changes to red. An acid and pungent gas is now dis-
engaged. The operation is continued until the powder is
reduced to about two-thirds of its volume. It is now taken
from the fire and allowed to cool.
After a little practice, we may know by the colour of the
substance when the operation nnust be brought to a conclusion.
Should the operator, however, want experience, let him take
portions of the substance at different periods of the operation,
and he will thus be sure of obtaining the requisite tint. In
every case the operation must be concluded before the gas has
ceased to be given off; for if it were prolonged to this point, it
would be all to no purpose, and the result could only then be
used as a dark brown or iron violet colour.
The red which has been obtained is put into a vessel, and
boiling water poured upon it to dissolve the sulphate of iron
that has not yet been decomposed. It is frequently stirred,
allowed to subside, and the supernatant water is poured off.
The red pigment is then cleansed from a few impurities re-
maining in it by being stirred in a vessel of fresh water, which
is quickly poured off as soon as these impurities have subsided.
If this operation be repeated as often as is necessary, the
impurities will be completely removed. They are generally of
a greenish grey colour. The powder of oxide of iron is now
washed in a filter with cold water imtil the latter exudes from
the filter perfectly tasteless. The result, when dried, is now
ready to be mixed with the flux.
Theory, — ^What takes place in this operation? In the first
place a portion of the acid of the salt is decomposed into sul-
phurous acid which is given off, and oxygen, which converts
the rest of the salt into persulphate of iron, which is mixed
with the oxide of iron that has been liberated. This sulphate
is now decomposed again; sulphuric acid is disengaged, and
oxide of iron, mixed with undecomposed persulphate, remains.
It is well known that the red oxide of iron changes its colour
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PAINTING ON GLASS. 31
in proportion as the temperature to which it is exposed is
increased. At first it possesses a yellowisli red colour, which
afterwards passes into a darker red, and finally into violet. If,
therefore, it is required to produce a delicate red for the flesh-
tints hy calcining sulphate of iron, the latter must he. acted
upon hy a heat sufficient to decompose it, hut still not so great
as to deepen the red which has already formed. Hence is
evident the importance of always keeping it at a dull red heat,
and of stirring it inc^santly, so that the parts which he at the
hottom may not be too much heated. With respect to this
circumstance we subjoin the following practical remark, viz.
that in order to keep the powder at a duU red heat, while it is
stirred, the bottom of the vessel must be at a cherry-red heat.
The operation must be concluded before the sulphate, is all
decomposed (aniT this is of the greatest importance), so that
the red which is formed may be mixed with a certain quantity
of this salt. When the latter has been dissolved by the water
in which it is washed, the oxide that remains is in a state of
finer division, and possesses a lively red colour.
If the operation be continued too long, it seems to be pre-
judicial to the beauty of the red, even if the temperature be
not very^high. We may infer from this, that the oxide con-
denses not only in cou sequence of the high temperature to
which it has been exposed, but also on account of the length
of time during which it has been heated. And this is the reason
why only a small quantity of the salt should be operated upon
at a time.
Red of a beautiful tone from iron may be obtained with
much greater certainty by a method which is based upon the
theory we have just explained. A mixture of sulphate of iron
and sulphate of potash is calcined, as in the foregoing opera-
tion, these salts haviug been previously combined in the state
of solution, and then evaporated and dried by heat. The
mixture may be heated merely in a crucible, provided that
care is taken to increase the temperature slowly up to the dull
red heat, and to keep it so until the operation is finished.
But this latter method is less certain. In every case the un-
decompo^ed sulphate of iron, as well as the sulphate of potash,
is separated by repeated washings with hot water.
There is still another method which furnishes a red of great
richness of colour, particularly for the flesh-tints. It consists
in grinding sienna in 9, solution of sulphate of potash, drying
it by beat in an iron vessel or even in a crucible merely, and
calcining it the proper length of time at an incipient red heat.
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32 ESSAY ON THJC ART OF
in order to developct the colour of the oxide of iron. The pro-
duct is then washed with boiling water^ to separate the sulphate
of potash.
A similar result is obtained by calcining a mixture of equal
parts of green vitriol and alum, previously united in solution,
and proceeding in other respects according to the method we
have just described for obtaining the red from sulphate of iron
and sulphate of potash.
The preparation of the red from iron Requires much more
deUcate manipulation than would be si^posed. Whatever
method be employed, the calcining must be performed with
the greatest care, and we would particularly caution the reader
against imagining that the details we have given are super-
fluous. It is difficult to determine which n^thod of prepara-
tion deserves the preference, because, on a Xmparison of the
various methods, we cannot always be sure of being subject to
the same conditions, and we often lay to the charge of the
method that which ought properly to be attributed to the
• manipulation. If we were to recommend one method more
than another, it would be the second, that which we have most
frequently employed.
It now remains for us to explain why sulphate of gotash is
used in the preparation of the red obtained from iron. This
salt is unchangeable at a red heat, and at this temperature has
no chemical action upon the component parts of the sulphate
of iron. Its action here is entirely mechanical. It supports,
as it were, the complete decomposition of the sulphate of iron.
But, although it remains undecomposed itself, its presence is
by no mean^ a matter of indifference. When the sulphate of
iron is dried in contact with the sulphate of potash, it is kept
by the latter salt in a state of fine division, analogous to that
which it possesses in the state of solution ; for every minute
particle of iron is surrounded by numerous minute particles of
sulphate of potash, so that, when the oxidie of iron is separated,
it must preserve the extremely fine division of the salt by which
the oxide of iron was produced. It thus escapes that conden-
sation and conglomeration which oxide of iron always under-
goes when it is heated by itself. The unchangeableness of
sulphate of potash at a red heat is the only reason why this
salt has been selected for the operation in question. It is like-
wise employed for the same purpose in similar cases, where, as
in the present instance, the object is to obtain oxides free from
water in a state of the most minute division. There are various
methods of applying it to this purpose.
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PAINTING ON GLASS. 33
1 . Sulphate of potash is calcined at a red heat, together
with the metallic sulphate whose oxide is to he obtained, pro-
vided that this sulphate is decomposable. The process is the
same as has been described in the case of the red obtained from
iron.
2. If the salt is not decomposable, the oxide in question is
precipitated with potash; the solution is then evaporated to
dryness, and the product is submitted to a red heat in a
erudble. The sulphate of potash is next removed by boiling
water.
3. If the oxide of another salt, not a sulphate, is to be ob-.
tained,* we proceed as follows : after the oxide has been preci-
pitated with potash, and the precipitate washed, it' is mixed
with a saturated solution of sulphate of potash, next evaporated,
and the rest of the process is the same as we have described
above.
M. de Montami treated certain oxides, probably with the
Bame object, with chloride of sodium in a similar manner, but
his efforts were misdirected. He triturated red oxide of iron
and chloride of sodium dry in a mortar together, and calcined
the mixture in a bright red heat. In this way he calcined
oxides, free from water and already condensed by the action of
the fire, with the chloride. The oxides could not penetrate so ,
deeply into the latter combination, which was applied in a
solid state, as if it had been in the state of a red-hot liquid ;
and besides, they had undergone a very great change from the
heat before it was introduced.
The red pigment is composed of
Red oxide of iron 1 part.
Flux No. 1, or No. S, of the 1st class .... 3 parts.
The flux must be pulverized before it is weighed, because a
certain quantity is always lost by pulverizing it in an iron
mortar. It is then mixed with the oxide of iron, and the
mixture levigated; at the same time a sufficient quantity of
water is added to obtain a liquid paste, which is afterwards
dried on plates.
' This pigment must contain as much flux as will give it
brilliancy when it is exposed to a cherry-red heat ; a larger
qufintity of flux must, however, be avoided, because the flux
would re-act upon the colouring oxide, and contribute to the
production of a green silicate of iron which would be pre-
judicial to the purity of the red. We shall afterwards
explain how this pigment is to be prepared for use, and
b5
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34 ESSAY ON THE ART OF
under what conditions it must be heated after it has been
laid on the glass.
If a greater degree of freshness and brilliancy is required in
the red pigment which is used for the flesh-tints, a certain
quantity of chloride of silver must be added. The yellow tint
which this combination produces imparts a greater lireliness to
the red colour of the iron. The taste of the artist can alone
determine the proportion of the substance to be added. The
chloride of silver must previously be fused with the flux. The
oxide of iron is added sdterwards.
Purple {purple-red) pigment, — The preparation of the
purple is a very delicate operation, the success of which is
extremely uncertain. The reason of this is, that the method
which is employed in the majority of cases for the preparation
of a solution of tin furnishes an exceedingly variable compound,
although the process is on eveiy occasion the same. The pre-
cipitate which by this method is apparently obtained under
exactly the same conditions, frequently varies from a more or
less lively purple to a violet more or less dark, and even of a
blackish colour ; and even the purple of the most beautiful
tone is not always proof against the drying, and is turned black
by the separation of the gold.
The method employed in most cases is as follows : Take aqua
regia of 8 parts nitric acid and 1 part sal-ammoniac, diluted
with twice its weight of distilled water. Set the apparatus in'
a cold place, and introduce small pieces of tin one after another
in proportion as they are dissolved. The action of the acid
must be slow, and without a considerable quantity of heat
being evolved. When the liquor has assumed a yellow colour,
not very intense, it is fit for Use: Oh the other hand, dissolve
pure gold in aqua regia containing 1 part nitric acid to 2 parts
hydrochloric acid. The gold hi^ now to be precipitated by
means of a solution of tin. For this purpose, a few drops of
the solution of gold are poured into a glass, and at least a
thousand times their volume of water is added. Into this
liquid some of the solution of tin is introduced, a drop at a
time, until the water is coloured red. This liquor is kept in a
vessel by itself, and the same process is repeated until at
length tne requisite quantity of purple is obtained. In a few
moments the purple collects in red flakes, which sink to the
bottom. As soon as all the purple is precipitated the super*
natant liquor is poured away. It is then washed several times
with distilled water, which is likewise poured away. Next it is
filtered, and whilst it is still moist it is triturated with its flux^
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PAINTING ON GLASS. 3$
The success of the operation depends upon the manner in
which the salt of tin is prepared. In order that the purple
maj he of the proper quality, the action of the acid upon the
tin must he neither too strong nor too weak; if it is too weak,
too much protochloride of tin is obtained^ and if it is too
strong, nothing hut perchloride of tin is produced. It is ne-
cessary to know how to keep the acid in exact equilihrium, so
as to obtain a mixture of the two salts combined in as nearly
as possible the proper proportions. The difficulties which ac-
company the regulating the effect of the acid in such a way as
to obtain every time the same result, although the operation is
not performed under the same conchtions of temperature, are
obvious to every one. Sometimes the protochloride, at other
times the perchloride, is in excess : this explains the different
appearances which this composition exhibits when used.
If the products of the operation are uncertain, variable in
their colour, and disposed to decomposition during the process
of drying, there is every reason to believe that the salt of tin is
of a bad quahty. The difficulty of imparting the requisite
properties to this composition induced M. Reboulleau, among
others, to seek for a simple and easy method of procuring with
certainty a solution of tm adapted to the preparation of the
purple. It is composed in a way that Dumas also considers
the best, viz. of one atom of protochloride and one atom of
perchloride of tin.
The method employed by M. Reboulleau for this purpose is
as follows : Protochloride of tin is first prepared by introducing
grains of pure tin into a leaden vessel which can be closed by
a hd of the same metal. A small quantity of concentrated
hydrochloric acid is then poured upon the grains. The appa-
ratus is gently warmed in a sand-bath, and small quantities
of add are gradually introduced until the tin is dissolved.
The liquor is evaporated to 40^ of Beaum^s areometer, and
allowed to crystallize. We have then merely to convert a
definite portion of the protochloride of tin into perchloride.
The crystallized protochloride is dissolved in a sufficient quan-
tity of water ; the solution is divided into two parts, one of
which is set apart, while chlorine gas is infused into the other
until the protochloride is completely changed into perchloride,
which is removed for fear it shomd precipitate gold. This
solution is added to the one which was set apart, and thus a
solution of tin is obtained, in which the two chlorides are pre-
sent in accurately determined proportions. The success which
this method insures in the preparation of the purple is so per-
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36 ESSAY ON THE ART OF
fectly certain, that it may be safely recommended to glass-
painters and other artists.
The solution of tin serves only for precipitation from chlo-
ride of gold which is prepared by dissolving gold in aqua regia
composed of 1 part nitric acid and 4 parts hydrochloric acid,
evaporating it until it is dry, in order to drive off the excess of
acid, and adding a quantity of distilled water sufficient to dis-
solve the salt which has been obtained.
The precipitation of the purple is a part of its preparation
that requires very delicate manipulation. The manner in
which the mixture of the two salts is effected is not a matter
of indifference. Two methods may be employed : the solution
of gold may be poured into the solution of tin, or pice versd;
but these two methods do not promise equal success. The
following remarks will iJhow why one method is to be preferred
to the other.
When the precipitation of the purple is effected by the mix-
ture of the salts of tin and gold, one of these three things will
take place : viz., either the salts will be in proper proportions,
or the salt of tin will predominate, or the salt of gold will be
in excess.
If the salts are in proper proportions, the purple precipitate
follows, accompanied by certain indications with which it is
necessary to be acquamted. The liquor assumes an intensely
red colour, similar to that of wine. The precipitate does not
follow immediately, but the purple remains a longer or shorter
time, as the case may be, in the solution, and frequently it is
several hours before the separation is complete. When the
precipitation takes place too quickly, it is always a proof that
the purple is of a bad quality and contains an excess of gold.
If the salt of gold is in excess, a precipitate is formed which
varies from a pale rose colour to a more or less lively red, and
the separation takes place immediately. In this case also the
purple is imperfect.
If, on the contrary, the salt of tin predominates, no forma-
tion of purple takes place ; the liquor assumes a yellowish or
rose-coloured appearance, without affording any precipitate.
From what we have just stated, any one would be inclined
to think that if the quantities of the two salts are previously
determined, it would be sufficient to pour the solution of the
one into that of the other, and to stir them together. But
this expectation is not easily realized ; and it is thought more
advisable to obtain a proper mixture by adding successive
drops of the solution as long as it shall be found necessary;
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PAINTING ON Q1.ASS. 37
nor even then is it immaterial whether the solution of tin he
poured into the solution of gold, or the solution of gold into
the solution of tin.
Is the sotution of tin to he poured into the solution of gold?
As the addition is made hy drops, the gold remains in excess
in the liquid until the whole of the solution of tin has heen
introduced. Consequently an imperfect precipitate may be
formed if the proper quantity of the solution of tin be not
introduced with tolerable rapidity ; and this is very frequently
the result. If, on the contrary, too much solution of tin be
added, no precipitation takes place, and it becomes necessary
to reverse the operation, and to add the solution of gold.
This method is therefore very uncertain.
Is the solution of the salt of gold to he poured into that of
the salt of tin? So long as the salt of tin is in excess no
precipitation takes place ; but if we continue to add gold the
purple soon makes its appearance, and we can always stop at
the right time, because we are not obliged to hurry ourselves
in the least degree. Only an inexpert operator runs the risk
of introducing too much solution of gold. In such a case, a
precipitate of bad quality would be produced, and the experi-
ment would entirely fail.
It is evident that, of these two methods, that according to
which the solution of gold is poured into the solution of tin
has the best prospect of success, and is the least liable to
accidents. It may be asserted that it is the only one which
affords invariable results : it admits, too, of the treatment of
any quantity we please, while the other method is only appli-
cable when the solution has to he treated with a few deca-
grammes'^ at a time.
But whatever method be pursued, the solutiitn of tin must
at all events he diluted with a thousand times its weight of
water, in order that the precipitate may be so much the more
finely divided and more gelatinous. After the precipitation of
the purple powder has subsided, it is put into a filter and
washed with distilled water : it is advisable then to dissolve it
in ammonia, and keep it in a well-stopped bottle : the capa-
bility of being dissolved in ammonia is the test of its good
quality; — if this property is wanting, we may be certain that it
is of no use, for it will not possess durability.
In order to unite the purple with its flux, the latter is pul-
* The decagramme is equivalent to 154*42 English grains avoirdupois
(the gramme being 15*442 grains), according to the new metrical system
of weights and measures.
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38 ESSAY ON THE ART OF
verized, moistened with the ammoniacal solution of the purple
powder, and the mixture is effected by levigating them to-
gether upon a plate of glass. The relative quantities of the
purple and the flux depend upon the richness 6f the colour
which is intended to be given to the pigment. One-tenth of
the purple in a dry state gives an intense colour. If we know
the composition of the purple, the quantity of gold that is
used, the quantity of ammonia in which the purple has been
dissolved, it is an easy problem to determine the proportions
of this solution, which must be added to the flux, in order to
have used the equivalent of one-tenth of dry purple.
The purple colour which is produced in fused pigments by
the above-mentioned composition proceeds from metallic gold
in a state of exceedingly fine division. The same colour is
likewise obtained from pure chloride of gold, sulphuret of gold,
and fulminating gold, in similar circumstances, and these com-
binations play the same part as the purple powder. When the
latter is mixed with its flux, and strongly heated, the tin is
separated from the gold; but the latter, reduced to the metallic
state, remains in a state of the most minute division, because
the flux, being melted, is present in a liquid form. But as
soon as the gold can collect together in particles of a larger
size, there is a transition from red to violet and blue. This
phenomenon is produced by several circumstances, which arise
from the following causes :
The flux for the purple must contain not much lead, but, on
the contrary, a great quantity of acid, and must at the same
time possess great fusibility. Stannic acid has in fact a great
aflinity for oxide of lead. When it is combined with gold in
the form of the purple powder, and is brought into contact
with a flux containing a large amount of base, and at the same
time lead, it leaves the gold in order to combine with the lead
before the pigment is completely melted. This premature
separation of the tin from the gold faciUtates the agglome-
ration of the particles of the latter, which then assume the violet
or blue colour we have been speaking of.
A large proportion of acid gives more stability to the silicate
and borate of lead, so that it is more capable of resisting the
action of the stannic acid.
The colour of the purple is, on the other hand, destroyed
when the flux with which it is mixed wants fusibility. The
temperature necessary to fuse the mixture produces at the
same time the decomposition of the purple before it can be
l^rought into its incipient state of division. So also a purple
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PAINTING ON GLASS. 39
pigment minted with its flux, if too strongly heated, would
undergo the same change from a contrary action, because the
too great fusibility of the flux is favourable to the condensation
of the gold. Consequently it is necessary that the purple, at
the moment when it is being decomposed, should be held in
solution in a dense mass of liquid glass, in which every one of
its particles may, as it were, remain isolated, in the same way
as an oleaginous body is suspended in a shmy fluid.
The flux which seems best adapted to the purple is the
following :
Calcined borax 7 parts.
Silica 3 ,.
Minium 1 part.
This flux possesses sufficient fusibility, and at the same time
great stability.
Another still more fusible flux :*
Calcined borax 7 parts.
Silica 3 „
Minium 1 part.
These substances are fused together.
Another still more fusible flux :
Calcined borax 7 parts.
Silica 1 part.
Minium I tt
These substances are likewise fused together.
For the purpose of obtaining a still more fusible flux, it is
not unusual to increase the quantity of borax very considerably,
without employing the other substances in larger quantities.
Thus, for example, the following flux is used :
Calcined borax . . . .•..•.•.•.. . 12 parts.
Sand ' 1 part.'
Minium 1 >/
A flux like this is, indeed, far more fusible than the fore-
going, and more conveniently used for glass-painting, because
only a moderate heat is required for it ; but such a compo-
sition is liable to imperfections, and is not only more easily
afl^ected by moisture, but is also in the habit of coming off in
scales. The purple colours generally possess these imper-
fections. It is owing to the great quantity of acid which tney
contain that their expansibility does not accord with that of
* There is evidently some mistake in the original, this and the fore-
going flux being perfectly identical.
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40 EBSAY ON THE ART OF
certain kinds of glass. When this kind of painting is looked
at with a magnifying glass, after the homing in, several cracks
and fissures may be observed ; and after a certain time, espe-
cially when the painting has been exposed to moistare and a
variable temperature, it scales off, so that at last the glass
becomes perfectly bare. It is of importance to examine in
every case whether the pigments agree well with the glass in
regard to expansibility, and it is frequently necessary either to
vary the composition .of the flux or to choose another kind of
glass.
A carmine tint is given to the purple by adding to it a small
quantity of chloride of silver, previously fused with ten times
its weight of the flux which is used for the purple.
As the purple from gold is prepared in different manufac-
tories in a great variety of ways, and the preparation is attended
with various results, we shall conclude by stating several
methods, for which we are indebted to men whose reputation
is great in the scientific world.
According to Bastenaire Daudenart, the purple is prepared
in the following manner : Nitric acid and gold are taken in the
proportion of 8 parts, by weight, of the former to 1 of the
latter. In order, however, that the operation may be per-
formed with greater certainty, it is better, in case the acid
should be more concentrated at one time than at another, and
in order that similar results may be always bbtained, to intro-
duce gold into a retort until the acid is completely saturated.
£special care must be taken that the solution of gold is in a
perfectly neutral state.
The nitro-muriatic acid,* for the solution of gold, is com-
posed of
Nitric acid . . • . . ...... . 4 parts.
Muriatic acid • . . . .....'. . 1 part.
The former acid is poured into a retort/which is placed in a
heated sand-bath, the muriatic acid is then introduced, and the
liquid is at the same time stirred; a few minutes after, the
gold is thrown into it, a small piece at a time. Gold riband
should be preferred, which may be bought at the goldbeater's,
or else the gold, if it iis at idl thick, should be beaten very
thin. When the acid is observed to be incapable of dissolving
any more gold^ the retort is taken out of the sand-bath and
placed upon a layer of rushes of a concave form, to receive the
globular part of the retort. The Uquid is left for some hours
* This is erroneously called ' Salpetersaiire/ nitric acid, in the original.
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PAINTING ON GLASS. 41
by itself, after which it assumes a beautiful dark yellow
appearance.
The solution of tin is likewise made with nitro-muriatic acid,
but instead of 4 parts of nitric acid 6 are used, and the fol-
lowing compound is obtained :
Nitric acid 6 parts.
Muriatic acid 1 part.
Distilled water 4 parts.
The solution of the tin requires infinitely greater care than
that of the gold ; it must take place without any effervescence,
and very slowly. Tin leaf, such as is used for covering
mirrors, is selected for this operation. The solution takes
place without heat, and only a very small quantity at a time is
introduced into the acid which has been diluted with water ;
e.g, pieces of tin leaf eighteen millimetres'^ square, one after
another, for the space of twelve hours, until the acid is com-
pletely saturated. When both the solutions have been pre-
pared in this way, the one is precipitated with the other ; and
this is the moment when it is the most difficult to obtain a
beautiful purple. Many persons have been quite discouraged,
and have given up even attempting to prepare this colour,
because they have undertaken the operation too carelessly,
when, in fact, it requires great nicety, or because they have
not clearly understood the theory of the phenomena which
take place in the mutual decomposition of the two soluble
salts. Care must first be taken that both the solutions are
very considerably diluted with water, for without this precau-
tion gold is precipitated by the tin in a metallic state, which
causes such a cohesion of the minute particles of the metal,
that the desired results cannot be obtained in the precipitate.
To avoid this disagreeable occurrence, which renders the whole
operation fruitless, a large glass vessel, three parts full of dis-
tilled or at all events very pure water, must be taken, and
into this a certain number of drops of the solution of gold
must be poured. The number of drops must be in proportion
to the size of the vessel. Suppose the vessel is capable of con-
staining one litre'\ of water, as much as eighteen drops of the
solution of gold may be dropped into it. This done, the
Hquid is stirred with a piece of the tube of a barometer, and it
must then be of a pale yellow colour, but very clear ; eight,
ten, or twelve drops of the solution of tin are then dropped
* The irtetreSA 39*37079 English inches; the milUmetret heing the
1000th part of the metre, is therefore equivalent to '03937079 inches,
t The litre, or cubic decimetre, is -22009687 of an imperial gallon.
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42 ESSAY ON THE ART OF
into the vessel, and care is taken to stir the mixture well, just
wlule the solution is being dropped. As soon as the Hquid is
observed to have assumed a colour like that of red wine, no
more solution of tin is added, (we should suppose that this
would take place at the sixth drop,) because if the process be
continued after the beautiM red colour has appeared, only a
purple is obtained, which approaches too near to violet.
There must be a large vessel of porcelain, or well-glazed
Delf ware, ready to receive all the red liquid which is trans-
ferred from the glass vessel into which the solution of tin was
dropped. Accordingly, when the liquid in the latter vessel is
well saturated with the purple colour, it is introduced into the
large vessel of Delf ware, and a fresh quantity of water is poured
into the glass vessel, and then eighteen drops of the solution of
gold. Whilst the solution of tin is being dropped, the liquid is
stirred with a glass rod : the drops of the solution of tin must
never amount to more than two-tlurds of those of the solution of
gold, especially if the purple is to be of a beautiful rose colour.
When as much gold and tin as is considered necessary has
been precipitated, idl the water which has been coloured red is
poured together, and then left undisturbed. In the course of
twenty-four hours a reddish brown precipitate settles at the
bottom of the Delf ware vessel. To hasten this subsiding,
one or two pinches of kitchen salt may be thrown into the red
liquor. Some writers have also recommended the addition of
a certain quantity of fresh urine ; but some solution of phos-
phorus may be more advantageously substituted for the latter.
In other respects, it is far better that the subsiding at the
bottom of the vessel should take place slowly and of itself, and
in that case the supernatant liquor must be perfectly clear.
This is poured off, and the remainder edulcorated several
times with plenty of water : the precipitate is collected upon a
piece of white paper, and dried in the shade. In the course
of a few days it is easily removed from the paper, whereupon
it is put away in a wide-mouthed bottle with a greased stopper,
and kept from the light.
The formation of the purple proceeds : — 1. from the elective
affinity of gold and tin ; 2. fi*om the high state of oxidation of
the tin ; and 3. from the circumstance that the solutions are
diluted with a great quantity of water, to weaken the affinity
of the acid for the oxides as bases ; for so long as the metals
(eold and tin) are intimately combined with the acids in which
they are dissolved, there is no chance of obtaining a purple of
a beautiful colour.
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PAINTING ON 6LA88. 43
Moreover, all the precipitates which are obtained bj means
of gold and tin differ from one another in a variety of circum-
stances. The quantity of water poured into the first vessel,
namely, in that into which the solution of tin is dropped to
mix with the solution of gold ; the number of drops of the
solution of tin in proportion to the number of those of the
solution of gold ; the purity of the water in which the precipi-
tation is effected : all these circumstances might produce a
great variety of tints. However, in general the purple is the
more disposed to assume a violet colour when the precipitate
contains a greater quantity of tin in proportion to the gold; on
the other hand, the purple is more beautiftil or more rose-
coloured, the more solution of gold the precipitate contains.
It is therefore quite at the option of the experimenter to pre-
pare a more or less beautiful purple, if he only act upon this
intimation with proper care. Proust and Oberkampf * have
analyzed several red and violet-purple precipitates : they found
in a beautiful rose-purple — 20*58 oxide of tin, and 7 9 '42 gold;
and in a violet-purple precipitate, dO* 18 oxide of tin, and 39*82
gold.
According to Berzelius, the purple contained — 2835 oxide
of gold, — 65*00 oxide of tin, — 7*65 water.
According to Buisson, the purple precipitate contains —
28-50 gold,— 65-00 oxide of tin,— 5*20 chlorine,
Cassola, Professor of Chemistry at Naples, gives the fol-
lowing receipt for the preparation of the purple: Hydro-
chlorate of gold is prepared in the usual way : at the same
time, tin filings are digested in vinegar for two or three days,
and the solution is filtered. The solution of gold is then
diluted with four or five times its weight of water, and the
solution of protacetate of tin is generally poured into it until
the red precipitate has formed; it is then washed and
bottled up.
Cassola asserts that he obtained the same results by using a
solution of protonitrate of tin, procured from tin filings, Re-
solved in concentrated nitric acid which had been diluted with
fifteen parts of water. According to what he says, this solu-
tion of protonitrate of tin must not be used until it has been
for two days in contact with the tin. Both solutions — that of
the tin and that of the gold— must be mixed cold, in which
case a purple precipitate, without a tinge of black, is imme-
diately produced. Cassola, however, prefers the process with
* Annales de Chimie, t. Ixxx. ct Ixxxvii.— (iVb/c qfthe Author,)
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44 ESSAY ON THE ART OF
acetate of tin. He also employed protosulpbate of tin, diluted
largely with water, for the precipitation from the solution
of gold, and obtained the same favourable results. The same
phenomenon, however, with regard to the colour of the pre-
cipitate, takes place in precipitating with sulphate of tin, if it
is used in excess, as if muriate of tin is employed in too great
a quantity.
A beautiful purple may also be obtained by the following
method : One part thin gold leaf is dissolved in aqua regia,
the solution is poured into a glass, and diluted with 15 parts
rain water ; a solution of H part clean tin filings in muriatic
acid, which has been allowed to cool, is added, and at the same
time the mixture is continually stirred. After it has remained
a quarter of an hour undisturbed, one-half part clean urine is
poured into it, and the whole is well stirred. About two
hours afterwards the liquor is poured away from the purple,
which has now subsided, and the latter is completely edul-
corated.
According to Stegers, a very beautiful purple is prepared in
the following manner : Fine gold is dissolved in aqua regia.
If the gold has been alloyed with silver, the solution is poured
away from the precipitated chloride of silver; the latter is
washed with distilled water and added to the solution, which,
without being filtered, is evaporated with a moderate heat until
a thick incrustation of crystals is formed, and only a Uttle of
the red solution bubbles up from under the incrusted surffice
when the vessel is inclined on its side. The whole is now left
to cool, whereby it gradually solidifies throughout, and is dis-
solved without delay in ten times its weight of water; to avoid
the attraction of moisture, the solution is filtered, and a small
quantity of metallic gold is left behind. In order to wash out
the filter, a portion is reserved out of the water, which has
been accurately weighed, and this is afterwards added to the
solution. The crystallized salt of tin of commerce is quite
^ood enough for the purpose ; if it is moist it should be dried
between printing-paper. One part of this salt is dissolved in
four parts of distilled water ; the solution is filtered and used
immediately, because in time it becomes turbid, owing to its
attracting oxygen from the air, and submuriate of tin is depo-
sited in the form of a white powder. Next, 1 part of ^m-
arabic is dissolved in 3 parts of hot distilled water, and the
solution is filtered through gray blotting-paper, because print-
ing-paper, on account of its greater closeness, impedes the
passage of the glutinous fluid. When the three liquids have
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PAINTING ON GLASS. 45
been prepared in the foregoing manner, 20 grains of the
solution of gum are mixed with 3 ounces of distilled water,
and, after careful stirring, 14 grains of the solution of tin are
introduced. The vessel in which the latter was weighed out
is rinsed with a little water, the whole is then mixed with 23
grains of the solution of gold, and the vessel which contained
it is likewise rinsed, not with water, but with the mixture
itself. The colour which arises from the above-mentioned
proportions of these ingredients is a fiery reddish brown ; it is
only in the fire, when this preparation is used for glass-paint-
ing, that it developes a purple unequalled in beauty. The
colour may be slightly affected by the action of the acid which
has been disengaged by the formation of the purple in the
liquid; but tins is obviated by diluting the solution with
twice Hts weight of water, dissolving 10 grains of bicarbonate
of potash in it, and mixing with it, according to the above
direction, some of the solution of tin containing gum.
To separate the purple, whose precipitation is at present
retarded by the gum, alcohol is added to the mixture until it
grows very turbid; for this purpose about twice its weight of
7^ per cent, spirit is required, that is to say, if bicarbonate of
potash has been used, otherwise three times is necessary. In
the course of an hour, if the solution has been occasionally
stirred during this time, the purple is precipitated in reddish
brown flakes, and the supernatant Hquor remains clear, with
only a slight tinge of colour. This is decanted, and the preci-
pitate is then washed with some more spirits of wine; it is
strained through a filter of printing-paper, gradually deprived
of its moisture by being squeezed with the latter through blot-
ting-paper, removed to a rubbing saucer, and then ground with
weak 50 per cent, spirits of wine to a thin paste, which is
boiled for three minutes in a vessel proper for the purpose^
and then poured into a cylindrical glass. As soon as it has
subsided in this vessel, the liquor is poured away, and is re-
placed by twice as much water. This operation is repeated,
by which means the gum is removed, all except a slight resi-
due, which can do no harm. Should the purple subside very
slowly from the last liquor, and form a dense, almost transpa-
rent, red stratum over the sediment (a fact which proves a
disposition to be dissolved), the water must be poured away,
ana a small portion of strong alcohol must again be added to
the remainder, in order that the purple may coagulate rather
more densely, and the last addition of Hquid may be filtered.
In either case, tilie precipitate from which the liquor has been
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46 ESSAY ON THE ART OF
strained is pressed, as before, with the filter, between blotting-
paper, and, while moist, is scraped off with a blunt knife, and
dried in a porcelain saucer, whereby it diminishes considerably
in bulk, and becomes of a perfectly dingy colour.
According to Buisson, the purple is prepared with the
greatest certainty in the followmg manner : 1 gramme of the
purest tin is dissolved in a sufficient quantity of muriatic acid.
The solution must be neutral. Further, 2 grammes of tin are
mixed with aqua regia, composed of 3 parts of nitric acid and
1 part of muriatic acid, so that the solution contains no proto-
chloride of tin. Lastly, 7 grammes of fine gold are dissolved
in a mixture of 1 part of nitric acid and 6 parts of muriatic
acid, and this solution must Ukewise be neutral. The latter is
diluted with 5|- litres of water, the solution of perchloride of
tin is added to it, and that of the protochloride is intr^uced
a drop at a time, until the precipitate that is being formed
possesses the requisite colour. This precipitate is edulcorated
as quickly as possible.
It is of importance to know that the perchloride and the
protochloride must not be produced separately, but both the
chlorides at the same time, to enable us to obtain the purple
from chloride of gold; and that it is further requisite that the
three chlorides should be prepared for use perfectly free from
acid.
Herr G. Creuzburg,* a practical chemist, particularly re-
commends, in the preparation of the purple, the use of a solu-
tion of tin chemically pure ; he therefore advises that the
broken tin ore should be first treated with nitric acid, which
dissolves all other foreign metals, and converts the tin into an
oxide. After the blue solution which contains the foreign
metals, and among others copper, has been poured away from
the oxide, which is in the form of a white powder, and the
latter has been washed, dissolved in muriatic acid, and inspis-
sated, chloride of tin, chemically pure and firee from acid, is
obtained. ' It now only remains to observe, that perchloride of
tin, digested with metallic tin, gives protochloride of tin.
When the two solutions of tin have been prepared in this man-
ner, the remainder of the operation is performed according to
Buisson's method. After a few slight preliminary experiments
have shown about the maximum of perchloride of tin that
the solution of gold will bear, in order to give the purple with
the protochloride, the requisite quantity of perchloride is
* Journal fur praktische Chemie, herausgegeben vom Prof. Erdmaan
und Prof. Schweigger-Seidel. Bd. ix. Heft. 6.— (iVb/e qfthe Auihor.)
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PAINTING ON GLASS. 47
poured all at once into the very dilute solution of gold, which
is not thereby rendered turbid, and a slow precipitation is pro-
duced by the introduction of the protochloride, very much
diluted, until the purple tint appears.
It was now a still more difficult task to find a suitable flux
and other ingredients, in order to obtain a beautiful enamel
colour. At last it was discovered that antimony and white
enamel were the substances which, when mixed with the flux,
were capable of lightening the tint of fine purple, so that the
rose colour was particularly beautiful. The addition of metallic
silver and chloride of silver, by which the purple colour used
formerly to be brought out more strongly, was not applicable
to this purple, and gave a useless colour, mingled with a homy-
looking yellow, even on the addition of a very minute quantity
of these substances.
Porcelain, of diflerent kinds of glazing, gave various tints
with one and the same purple. Herr Creuzburg observes,
that antimony not only brings out the purple tint more finely
than any thing else, but also gives considerably more body to
the colour, so that it bears more flux, and consequently a given
quantity goes further.
Persons who were experienced in the preparation of a
beautiful purple, and with whom he was personally ac-
quainted, assured him that they never used the precipitate of
Cassius in the preparation of their finest purple, unless they
had obtained it accidentally. According to their assertion, the
precipitate, which is of a grey colour approaching .to violet,
should give the most beautiful purple ; the dirtier the colour
of the precipitate, the more brilliant the purple which it gives
when fused. This dirty precipitate should contain more gold,
and urine should be employed in its preparation.
Frick, the privy-counsellor of the mines, gives the following
process for the preparation of the purple : Tin is dissolved in
very dilute aqua regia, without the application of heat, until
the Hquid begins to assume slightly the appearance of opal ;
the tin is then taken out and weighed, the Hquid is diluted
with a very great quantity of water, and some dilute solution
of gold and dilute sulphuric acid, in certain given weights, are
simultaneously poured into it whilst the mixture is being
stirred. The quantity of the solution of gold to be poured
into the solution of tin must be such that the weight of the
gold will be to that of the tin as 36 to 10.
We are indebted to Professor Fuchs, of Munich, for a re-
markably simple method of preparing the purple. A solution
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48 ESSAY ON THE ART OF
of permuriate of iron is added to a solution of protochloride of
tin until the former loses its colour and assumes a greenish
tinge. The mixture is then diluted with water, and some of
it is poured into the solution of gold which has heen properly
diluted. The most heautiful purple is immediately produced*
while the protoxide of iron remains in the solution without
having any injurious effect. The precipitate becomes lighter-
coloured in drying, and appears as a dirty brown powder.
Herr C. F. Capaun considers the method of Professor Fuchs
the best, judging from his own experience, but recommends
the following process in the preparation of the purple powder:
Let a solution of perchloride of iron be diluted with three parts
of water, to which let a solution of protochloride of tin, pre-
pared from one part of protochloride of tin dissolved in six
parts of distilled water by means of a few drops of muriatic
acid, be added, until the mixture has assumed a greenish
colour. Let this mixture be further diluted with six parts of
distilled water, and kept ready for use. If both the solutions
were to be diluted at once with the whole quantity of water,
the transition of the brown into the greenish colour would not
be so clearly perceived. In the mean time, let pure muriatic
acid be poured upon so much gold as is required for the ope-
ration : let the whole be heated to boiling, and pure nitric acid
be introduced, in small quantities at a time, until all the gold
is dissolved : an excess of acid, especially nitric acid, should
be avoided. To this solution let a portion of distilled water
(360 times that of the gold to be used) be added, and some of
the solution of iron and tin be poured into the mixture, whilst
it is being stirred, as long as precipitation takes place. The
precipitate will be of a beautiful purple, when dry will appear
browner, but will dissolve in ammonia and fused pigments
with an intense purple colour.
Dr. Bolley, however, could not always obtain exactly the
same preparation according to the above method ; he therefore
attempted to procure a solution of the sesquioxide of tin in
another way. The combination of chloride of tin and chloride
of ammonium {pinhalt, as it is called, a salt consisting of an
equal number of atoms of chloride of tin and chloride of am-
monium) seemed very well adapted to this purpose. This salt
is anhydrous, and not affected by the air, so that when exposed
to dry it undergoes no changes which would be likjsly to induce
an unscientific chemist to adopt wrong methods of proceeding.
It contains ah accurately balanced quantity of chloride of tin
not liable to change, and this very circumstance renders it
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PAINTING ON GLASS. 49
adapted to tlie preparation of the oxidized matter whicli lies
half-way between the protoxide and the peroxide.
Dr. Bolley does not recommend the production of this sesqui-'
salt of tin by the use of a prescribed weight of protochloride of
tin dissolved in water containing muriatic acid, because any
directions for doing this would be uncertain on account of the
various quantities of water which the salt of tin contains, and
partly on account of the increased oxidation of the latter : ex-
perience alone teaches us that perchloride of tin boiled with
tin can be converted into the protochloride, and consequently,
if the right quantity of tin be used, into the sesquichloride.
According to Dr. Bolley's experiment, perchloride of tin com-
bined with sal-ammoniac is the same in this respect as per-
chloride by itself. The pinksalt contains 70*8 per cent, per-
chloride, of which 32 '3 per cent, is tin : if this amount of tin
be increased by one-third, the quantity of chlorine remaining
the same, sesquichloride will be produced from the perchloride.
Consequently 100 pinksalt requires 10*7 metallic tin. Dr.
Bolley subjected pinksalt and tin in the proportion we have
specified, together with water, to the action of heat until the
tin was dissolved : he then used the solution for the precipita-
tion of the purple.
We now proceed to give a more detailed account of the ex-
periment: 1*34 grammes of gold were dissolved in nitro-
muriatic acid, an excess of acid being carefiilly avoided, and
the solution was diluted with 480 grammes of water, the pro-
portion specified by Capaun. To 10 grammes of dry pinksalt
lie added 1*07 of tin filings; 180 grammes of water were
weighed out, and of these about 40 grammes were immediately
brought in contact with the tin and pinksalt, and heat was
applied until the tin was dissolved. The solution was now
mixed with the remaining 140 grammes of water, and some of
it was gradually introduced into the sUghtly warmed solution
of gold until all precipitation ceased : the precipitate soon sub-
sided, was taken out of the filter, washed and dried at the
temperature of 100° C: it weighed 4*92 grammes, and had
become dark brown. The strained liquor was yet only of a
pale red colour. The precipitate dissolved when digested in
strong ammonia. The gold which it contained (calculating
from the quantity of gold consumed) amounted to 21*4 per
cent., a result which is most in accordance with Fuchs' analysis
of the purple : he found in it 1 9 per cent, of gold. In either
case, every one who is engaged in the preparation of the purple
for the purpose of applying it to the arts, will find in what we
c
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50 ESSAY ON THE ART OF
have stated an infallible means of obtaining the most effieadoua
proportions of the peroxide and the protoxide of tin that have
yet been discovered.
Of the Blue, — ^The blue pigment receives its colour from
oxide of cobalt, and belongs to the class of fused pigments
which are coloured by chemical combination. The oxide of
cobalt combined with silica and boracic acid acts as base.
This pigment is also one of those which require to be iiised
before they are employed. As the peroxide is the most easily
prepared of all the oxides of cobalt, it is usually mixed with
the flux, for we know that it passes into protoxide at a high
temperature. This change takes place much nlore rapidly and
more effectually when assisted by the action of a fixed acid.
The presence of the flux satisfies the latter condition. The
peroxide of cobalt is reduced in contact with sihca and boracic
acid, and readily unites with them in the state of protoxide.
The ease with which the reduction and combination take place
is naturally in proportion to the abundance of sihca or boracic
acid in the flux. But if, on the contrary, the flux contains too
great a quantity of base, the cobalt, upon which the action of
the acids is now less powerful, is reduced with difficulty to the
state of protoxide, and requires an intense heat to effect a
perfect combination; and even then the colour is seldom pure,
and a blackish blue tint is usually obtained. Another circum-
stance, viz. impurity, contributes to impede the solution of
the cobalt in the flux. There are even anterior combinations
of cobalt with certain oxides, which, whenever they happen,
render it extremely insusceptible of the action of the flux. In
this case it very frequently assumes a greenish tint. It is
therefore evident that everything which contributes to resist
the conversion of the peroxide into the protoxide opposes the
combination of cobalt with the acids of the flux. The whole
resistance hes in this ; for if the protoxide is for a moment
produced, it is rapidly absorbed again, as will appear from the
following remarks.
Certain necessary conditions of expansion frequently require
that the vitrification of the cobalt, contrary to the directions
we have given, should be effected in very saturated fluxes. In
such a case its combination is assisted in a remarkable manner
by the following process. If a certain quantity of oxide of
antimony be added to the peroxide of cobalt at the moment of
its mixture with the flux, by virtue of its great affinity for the
oxygen, it rapidly and completely effects the reduction of the
peroxide of cobalt. The antimonious add which is thereupon
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PAINTING ON GLASS. 51
produced does not injure the purity of the colour in the
slightest degree, and does not perceptibly impair the transpa-
rency, provided that too much oxide of antimony is not used.
It is extremely probable that the protoxide of tin produces the
same effect.
A mixture of oxide of zinc and peroxide of cobalt produces
the same effect in another way. The remarkable endeavour of
the oxide of zinc to enter into various combinations with the
protoxide of cobalt affects the peroxide in the same way as the
silica itself.
Phosphoric and arsenic acid likewise favour the solution of
protoxide of cobalt in the fused pigment, whether they are
added in an isolated state or in combination with the cobalt
itself, that is to say, in the state of phosphates and arseniates.
In the former case, they act by increasing the quantity of the
acids ; in the latter, because they contain cobsit in the state
of protoxide. It is then only necessary to mix them with the
flux.
We have said that the peroxide of cobalt is generally
employed in order to obtain silicate of cobalt, which colours
the fused pigment blue. The principal reason why this is
chosen is, that if protoxide were used, it would be converted
into peroxide before the combination is effected, because it
bums at a red heat. The carbonate would furnish the same
result. But a combination of oxide of cobalt with oxide of
zinc, which resists the action of heat better, may be employed
vrith. advantage. This combination is obtained, by dissolving
one part of sulphate of cobalt and two parts of sulphate of
zinc m a sufficient quantity of water. Into this liquid a solu-
tion of carbonate of potash is poured until no more precipitate
is produced. This precipitate is placed in the filter, washed,
and dried, and is then the composition which was required.
Not only does the oxide of cobalt produce the richest colour ,
of all the oxides, but also a very small quantity only is neces-
sary to impart a very deep colour to the iused pigment. The
blue pigment is composed as follows: Peroxide of cobalt 1
part, or zincate of cobalt 3 parts, flux from 6 to 9 parts.
The flux which is used for the blue pigment is one of the
three which we have specified among the fluxes for pigments
of the second class. The flux and the oxide are levigated to-
gether, the mixture is put into a crucible, which is kept at a
red heat until the contents are perfectly fused and all ebullition
has ceased ; the pigment is then poured into cold water, dried,
and levigated.
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52 ESSAY ON THE ART OF
The ojdde of eobalt is employed in various proportions
according to the strength or weakness of the colouring. The
taste of the artist is easily satisfied in this respect.
As far as regards the blue pigments of the ancients, they
are the best of all their compositions. The receipts of Feli-
bien and Haudicquei: de Blancourt, which we possess, afford a
very excellent coloured glass. The blue pigments that Levieil
and the brothers RecoUet used to employ seem to have been
drawn from the same source, although the imitation is not
very correct. This pigment was composed in the following
manner:
Mimam 1 part.
Oxide of cobalt 1 „
Silica 4 parts.
Nitrate of potash 3 „
It is a quadrisilicate, and reminds us of M. Guinant's com-
position of flint glass. For 2 atoms of oxide of lead, 2 atoms
of protoxide of cobalt are substituted here.
This enamel, like the majority of the ancient ones, is not
capable of being used as a pigment, on account of its slight
fusibility. However well it might have been adapted to the
window-glass of the ancients, it is certainly unfit for the glftss
of the present day, which is much more fusible.
The saturatmg point of this pigment was applied by the
ancients to other colours, particularly to green, on account of
which we have dwelt longer upon it than we should otherwise
have done.
Of the Yellow. — ^The fused pigments may be coloured yellow
by a ^eat number of substances.
A hvely and briUiant colour is obtained by means of metallic
silver. The oxide of uranium by itself, when dissolved in a
flux, furnishes also a beautiful yellow ; but, in mo^t cases, those
oxides which are used as colouring materials are combined with
one another by twos, and often in greater numbers. Thus,
e. g, we combine protoxide of lead with antimonic acid ; pro-
toxide of lead with oxide of iron ; oxide of zinc with oxide of
iron ; oxide of iron with antimonic acid. Other compounds
likewise furnish useful yellow pigments. Among these are
chloride of silver, chromate of lead, &c.
Each of these colouring substances produces the colour
which belongs to it.
SDver gives a yellow varying from siskin yellow to purple.
The oxides of lead and antimony also furmsh a siskin yellow,
bat it is opaque.
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PAINTING ON GLASS. 53
The oxides of zinc and iron give an octre-colouTed yellow.
Ghromate of lead, too, yields a very lively yellow tint, &c.
Among all these colounng substances, the chloride of silver,
sdncate of iron, and antimoniate of lead, are considered the
best. The three tints which are produced by these colouring
materials satisfy all the requirements of glass-painting.
Yellow from tilver. — This colour is obtained without the
intervention of a flux. The process consists in covering those
parts of the glass that are to be stained with a paste composed
of chloride of silver and calcined yellow ochre, both levigated
t(^ether with water. After the glass has been heated to a red
heat in the muffle, the layer of ochre which adheres to the sur-
face is removed by means of a spatula, and the glass is found to
be stained. The yellow obtained in this way varies ^m siskin
yellow to purplish yellow. It is not always, however, at the
option of the artist to obtain any of these tints whatsoever, for
some kinds of glass are stained by this process only bright light
yellow, while others are capable of receiving a deep orange
colour. The orange colour is fi^equently only to be obtained by
repeating the process once or twice.
That kind of glass which, when plastered over with clay, is
most disposed to lose its glaze, and which partly or entirely
gives up the potash which it contains, seems to receive the best
colour.
Dumas is of opinion that the white kinds of glass, those
which contain a great quantity of aluminum, are the best for
staining with silver, and they are the very kinds which, accord-
ing to this chemist, can be the most easily deprived of their
glaze. In the act of parting with their glaze, which is effected
bv the assistance of a paste or cement, a formation of certain
silicates which crystallize, and a separation of a part of the
bases, take place. Those of them which are volatile, e.p. the
alkalies, are disengaged, and the fixed oxides, e.p, those of iron
and manganese, pass into the state of sesquioxide.
This is the case with resard to the separation of a portion
of soda or potash, upon which the colouring of the glass by
means of silver depends. When the glass is covered with the
paste of clay with which chloride of silver is mixed, and exposed
to a red heat, the chloride is volatilized, its vapour saturates
the glass, and as soon as it comes in contact with the potash
which has been liberated, the silver is reduced to the metallic
state; chloride of soda or chloride of potash is produced and
volatilizes, while the metallic silver is fixed upon the surface,
and often even penetrates a considerable depth mto the body of
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54 ESSAY ON THE ART OF
the glass. If the quantity of reduced silver is small^ the colour
is siskin yellow. If it is more considerable, the yellow becomes
deeper, and passes over into a more or less intense red.
The latter colour may be obtained in less time,* and with
greater certainty, by using glass with which a certain quantity
of chloride of silver has been mixed at the moment of its manu-
facture. For this purpose the glass must be well refined, and
contain no excess of uncombined alkah by which the chloride
of silver would be reduced too soon. The colouring is theu
effected in the manner we have already described.
That it is the reduced metallic silver which produces the
colour in the glass has been demonstrated by very decisive
experiments.
If we take glass with which 777th of its weight 'of chloride
of silver has been mixed, heat it red-hot, and in this state
cause a stream of hydrogen gas to pass over its surface, the
glass immediately receives an intensely red colour, which result
is Ukewise obtained by covering the glass with pure clay. In
the former case, the action of the hydrogen gas upon the chlo-
ride of silver is manifestly the principle upon which the colour-
ing of the glass depends. Now the action of this gas consists
in reducing the silver to the metallic state ; for if the quantity
of chloride of silver be increased, the surface, when acted upon
by the hydrogen gas, is covered with a stratum of silver pos-
sessing a metallic lustre.
The same colour is also produced, as we have stated, by the
surface being in contact with a layer of clay, and we beheve
that the nature of the process by which it is produced is exactly
the same. It does not appear, however, as if the clay effected
this by an immediate action, that is to say, by the reduction of
the chloride of silver. We attribute this action to one of the
phenomena which accompany the depriving the glass of its
glaze, namely, to the volatilization of the potash.
Even up to the present day, the staining the glass yellow has
been attributed to the oxide of silver ; but experience incontro-
vertibly proves that this effect is produced by metallic silver.
In this there is a striking analogy between the manner of
colouring of the latter and that of gold. The yellow or red
colour proceeds from the silver in a state of extremely fine divi-
sion, and the purple from gold in the same condition.
Wlien the silver is in such a state of division as to be
capable of colouring the glass, it only continues so under cer-
tain conditions. Accordingly, as soon as the yellow is pro-
duced, care must be taken not to let the glass pass into the
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l^AINTING ON GLASS., 55
^tate of fusion, for in the disturbance produced by the motion
of the liquid mass the particles of silver assemble in larger
g;roups, from whence arises a new colour in place of the former.
The silver in this state of less minute division assumes a very
intense blue colour. Hence we arrive at this practical conclu-
sion, viz. that silver is only capable of colouring glass at that
moment of its formation at which it has been brought to the
proper density, and that for the colouring of fused pigments it
can only be used within certain limitations of fusibility at a heat
which the glass shall be able to bear.
We have one or two more words to say upon the preparation
of the paste for the yellow. Ferruginous clay is used, com-
monly known by the name of yellow ochre. This clay must
be calcined, otherwise the paste which is laid on the glass
would be fidl of cracks, in consequence of the diminution in
bulk which clay undergoes when exposed to a strong heat.
Those parts of the glass which correspond to these cracks would
then receive no colour. Consequently, the yellow ochre must
be raised to at least as high a temperature as the glass in the
muffle.
The relative quantities of chloride of silver and burnt ochre
are as follows :
Chloride of silver 1 part.
Ochre from 6 to 12 parts.
A greater proportion of chloride of silver would cause the
paste to adhere too strongly to the glass. The chloride of sil-
ver and the clay are carefidly rubbed upon a glass plate with a
sufficient quantity of water to form a paste, which is applied in
a thick layer to the surface of the glass by means of a paint-
brush.
This yellow is frequently employed to give brilliancy and
liveliness to the red obtained from iron. In this case it is ap-
plied to the side opposite to the painting.
Chloride of silver, by itself, is also capable of staining the
fused pigments yellow ; but then it imparts to them a colour
of its own, if it mixes with them without being decomposed.
Here metallic silver is not the cause of the colour. The chlo-
ride of silver must be fused with its flux in the proportions of
from 1 to 2 parts of the former to 10 of the latter. When
nnited to one of the fluxes for pigments of the first class, it is
used as a mixture to the flesh-red to give it brilliancy. With
the purple it produces a carmine tint, and only a very small
quantity requires to be mixed with it.
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56 ESSAY ON TBt ART OF
The yellow pigment which chloride of silver furnishes can
aIso be employed in an isolated state.
Orange colour from silver. — ^This orange colour is only a
shade of the yellow from silver which we have been speaking
of, and is likewise prepared in the same way, except that for
chloride sulphuret of silver in the same proportions is sub-
stituted. Similar phenomena may be observed during the
process, and metallic silver is always the element of the colour.
Orange yellow is obtained with much greater certainty from
sulphuret of silver, while on the other hand chloride of sUver
answers much better for a light yellow, although it is not
always in our power to produce any given colour.
Bed from silver, — A red colour is also infallibly obtained
by using sulphuret of silver. In this case the preparation
which was employed for the orange yellow is slightly modified
by adding a few drops of sulphuric acid at the moment when
it is being used. If no red be obtained at the first laying on
of the pigment, it must be laid on a second time.
The sulphuret gives an intense colour with much greater
certainty than the chloride of silver, for at a red heat it
decomposes of itself, and can therefore produce its effect
without the aid of the unglazing. The latter, however, is
not prejudicial to it, and is only to facilitate the saturation of
the glass by metallic vapours. The mixture of sulphuric acid
has, in our opinion, no other object than this. Some sulphate
or other is undoubtedly formed by the agency of this acid,
which is decomposed in the red heat by t^ silica of the clay
and occasions the internal motion of the glass, whilst it affeets
its surface. This is what actually takes place in the unglaring
of the glass, which unglazing we are convinced is effected by
means of a mixture of sand and sulphate of Ume.
The unglazing which is capable of facilitating the colouring
of ^he glass by silver is not so great a change in the glass as
to deprive it of all its transparency, but merely a slight
modification by which its properties are uninjured. It is
worth observing, however, that a piece of glass stained red
in this manner does not possess the purity of colour which
belongs to one coloured by copper. When the tone of colour
is the same for both» that which was coloured by silver
appears the darker.
Sulphuret of silver is prepared in the following manner x
Pure silver is melted in a crucible, half its weight of sulphur
is then introduced. The sulphuret of silver which is generally
used is prepared by heating together 2 parts of silver and 1
^^ ""Iphuret of antimony.
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^AiNTtNG Oi^ feLASS* 57
Oehre Telltno. — ^The ochre yelW pigment is one of those
^sed pigments which are coloured oy mixture. It receives
its colour from the mixture of oxide of zind and oxide of iron.
This fused colour is a zincate of iron in which each atom of
the one oxide is comhined with an atom of the other oxide.
This salt is ohtained from sulphate of iron and sulphate of
zinc precipitated with potash, soda, or their carbonates.
The persulphate of iron is prepared by dissolving 96 parts
of protosulphate of iron in 100 parts of water, and adding half
as much sulphuric add as the salt already contains. The
solution is heated to boiling, when small portions of nitric acid
are introduced, until all nitrous acid has ceased to be dis-^
engaged. This operation may be explained in the following
way:
The protoxide of iron contains
1 atom of iron « 4 * . 339*21
1 }) » oxygen 100-
1 „ ,, add 50M6
1 „ „ protosulphate of iron . . « . 940*37
The persulphate of iron consists of
2 atoms of iron 678*43
3 H M oxygen 300*
3 „ „ acid 1503*48
1 „ ,, persulphate of iron .... 2481-91
It is therefore necessary for the obtaining 1 atom of per^'
sulphate of iron to add to 2 atoms of protosulphate of iron
1 atom of sulphuric acid, that is to say, half as much as the
salt already contains, and 1 atom of oxygen besides, which
is supplied by the nitric acid.
The protosulphate of iron, when Crystallized, contains
44 to 45 per cent, water, and the sulphuric acid of commerce
contains only 81*68 per cent, pure acid. Consequently 18
parts of the sulphuric add of commerce must be added to 100
of crystallized protosulphate of iron.
In order to form zincate of iron, so that each atom of the
one oxide shall be combined with an atom of the other oxide,
a solution of persulphate of iron and another of sulphate of
zinc, both having the same weight on the areometer, must be
prepared separate^, and so much in volume must be taken
from each, that the salt of zinc may be to that of iroii as 2
to 5 ; or a solution of the salt of zinCi weighing 10^ JB., and
c 5
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58 ESSAY ON THE ART OF
another of the salt of iron, weighmg 25° B., may be mixed
together in equal quantities. The precipitate which is then
obtained by means of potash, or its carbonates, will contain in
its composition the proper proportion ; for sulphate of zine
consists of
1 atom of oxide of zinc ....... 503*32
1 „ „ acid 501-16
1 „ ,, sulphate of zinc 1004*48
The quantities of the sulphate of zinc and the sulphate of
iron, each of which contains 1 atom of oxide, are in the
proportion of 2 to 5.
When the oxides are precipitated by means of a fixed alkali
any excess of the precipitating substance must be avoided, to
prevent the oxide of zinc from being dissolved again. Just so
much, however, is added as will produce a complete precipi-
tation. The product is filtered, washed, and dried. Lastly,
it is submitted to a red heat, in order to drive off the water
which it contains. If the solutions of zinc and iron, after
they have been poured together, are treated with an alkaline
carbonatei the precipitate must in this case be heated to drive
off the carbonic acid. The calcining is intended moreover to
give stabihty to the combination of the two oxides. If it is
required to colour a fused pigment by chemical combination
or simple mixture, the object must be to obtain the colouring
oxide in a state of the finest possible division. By this means,
fn the former case the combination takes place more easily,
and in the latter the mixture is more intimate. Now calcining
is always opposed to the direction we have given, for oxides
condense by being calcined. This is also the case with the
kind of pigments we have just been speaking of. On this
account it answers to use the means which we recommended
for obtaining the flesh -red; for they not only admit of com-
pliance with the conditions which require calcining, but of
the production of the colouring matter in a state of the finest
division. We will now speak of the mixture of sulphate of
potash.
The precipitate from the two oxides contained in the carbo-
nates of zinc and iron is mixed with a concentrated solution of
this salt ; the liquor is evaporated, and the precipitate, when
dry, is calcined at a dull red heat. It is then softened with
water, placed in a filter which separates the zincate of iron
from the solution of sulphate of potash : this precipitate is
carefully washed and then dried.
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l^AINtlNG ON GLASSi 59
This we consider to be the best process for the preparation
of zincate of iron. The colouring matter which is obtained
must now be united with the flux which serves as a vehicle for
it. The best kind of flux for ochre yellow is No. 3 of the
first class. They are mixed in the following proportions t
Zincate of iron 1 part.
Flux 4 parts.
As this fused pigment belongs to that class which is coloured
by mixture, it ought not to be melted before it is used ; we
have, however, fomid it useM to frit it, i, e. to keep it at a dull
red heat for some time ; it is then levigated for use. In this
way the colour becomes more beautiful and more transparent.
This method should be employed principally in that kind of
painting which the French glass-painters call peinture par
erUevage.
Yellow from antinumite of lead, — ^The fused pigment which
is coloured with antimonite of lead belongs to the foregoing
class, inasmuch as it contains the colouring matter in a state
of simple mixture. It is prepared by fusing in a crucible
1 part of antimonic acid and 3 parts of minium, levigating the
product and mixing with it twice its weight of rocaille-flux,
and then fusing the mixture again. This pigment has the
fault of not being transparent. Therefore, although it is one of
those which are coloured by mixture, it is advisable to melt it,
in order to impart to it the greatest possible transparency.
The quantity of flux must also be very small, for the oxide of
lead being attracted by the flux is easily Hberated from the
antimonious acid, and the latter then gives only a feeble,
opaque, white colour to the glass. The rocaille-flux is there-
fore preferred in this case also, because it is the most saturated,
and consequently the least disposed to act upon the colouring
matter.
The tint produced by antimonite of lead may be modified
by adding some oxide of zincate of iron.
In order that it may be better understood what takes place
in the process by which this colour is prepared, we may men-
tion that at a red heat the antimonates are converted into
antimonites.
Yellow from antimonite of iron. — ^The yellow obtained from
this compound belongs to the same class as the two preceding,
like them it wants transparency, but it possesses a very ridi
colour, so that it can be used in many cases for mixed tints,
and for the shading of green and brown» It is also of great
use when employed alone.
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60 ESSAY. ON THE ART OF
Antunonlte of iron is obtained throi^h double decomposition,
that is to say, by precipitating persulphate of iron with anti-
monate of potash. The product is washed, dried and calcined.
Antimonite of iron is likewise prepared by direct mii^ture pf
antimonic acid with oxide of iron in the proportion of 4 to 1 .
This latter method admits of the relative quantities of the two
compounds being varied at pleasure. The antimonite of iron,
like the foregoing, is used with the rocaille-flux or with some
flux of the first class. In the former case the mixture must
be brought into a state of j^sion, according to the treatment
of the rocaille-flux in all other circumstances. In the latter,
case it is only levigated : 3 parts of flux to 1 of colouring
matter are required.
We think it superfluous to speak of the pigments that can
be prepared with the aid of the other yellow colouring materials
which have been mentioned ; they are Httle studied, and more-
over are not in general used. It may be brieflv stated, how-
ever, that in modern times the metal uranium has been used
in Germany for the preparation of a yellow pigment. The
process is as follows : Uranium ore is broken in pieces and
roasted, dissolved in nitric acid, the solution filtered and the
lead which is present in it precipitated by the addition of some
drops of sulphuric acid. The clear green solution is then
evaporated to dryness, and the salt is heated until it is con<
verted into a yellow mass : 1 part of the substance so oh*
tained is ground with 3 parts of a flux composed of 4 parts
of minium and 1 part of powdered silica, fused together and
levigated.
Of the Green, — The green pigment generally receives its
colour from oxide of copper, oxide of chromium, and protoxide
of iron, either separately, or by the combination of two or more
with one another. The ingredients of this pigment are in
combination with the flux. It is difficult to make its expan-
sibihty the same as that of the glass on which it is laid. The
oxide of copper, even in a very slight quantity, alters this
property in a great degree, and disposes the pigment to shell
off the surface of the glass. The oxide of chromium does not
partake of this (j^uaUty in so remarkable a manner, but it re-
quires a very fusible flux to dissolve it. Now this fusibihty
can only be obtained by mixing with the flux substances which
modify its expansibility in the same way as oxide of copper
does, and borate of soda in particular is one of these substances.
The colour which protoxide of iron gives by itself is not rich
enough^ consequently this protoxide can never be employed
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PAINTING ON GLASS. 61
alone. In most cases it is used in combination with oxide of
copper.
To colour a i^ed pigment green> the oxide of copper must
be in combination with one or more acids of the flux, that is
to say, it must be in the state of borate, silicate, or borosihcate.
One of the fluxes which we have specified for the pigments of
the second class seems best able to effect its solution, but these
fluxes receive a degree of expansibility from the copper, which
ill accords with that of the glass. Those fluxes therefore must
be avoided which contain an abundance of acid, and those
should be preferred which contain a large amount of base,
especially of oxide of lead, although they possess less capability
of being dissolved. Flint glass * (fondant rocaille) seems best
adapted for this purpose, and yet no large proportion of oxide
of copper can be mixed with it without producing the un-
pleasant result just stated. The green pigment is therefore
best obtained from the following composition :
Miniom 12 parts.
Silica ... * 4 „
Oxide of copper 1 ,,
Red oxide of iron i pert.
This pigment may be prepared in two ways :
1. The colouring oxides are melted in a crucible with the
flux which has been previously prepared and ground together
with them.
2. The oxides are ftised together with the substances proper
for the composition of the flux, after which the whole is well
mixed and ground together.
It IS not immaterial which of the two methods is employed ;
we have always preferred the latter. In this method, when
the mixture begins to be red-hot, the oxide of lead be^s to
melt, and dissolves the oxide of copper, while at the same time
the latter is acted upon by the silica. This previous union of
the two oxides is intended to separate the particles of the
copper so as to dispose it to combine with the silica as soon as
ever it is acted upon by it. In the former case, on the con-
trary, the oxide of copper is merely acted upon l^ the silica of
the flux.
The foregoing receipt for the green pigments affords a colour
of no great intensity, and we cannot expect to obtain a richer
green by merely increasing the quantity of copper, for this, as
^ Bleiglas, lit. lead glass. This flint glass, the composition of which is
given under the head No. 1 of the fluxes for pigments of the first class,
will be ibuid to contain more oxide of lead then the English flint glass.
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62 ESSAY ON THE ART OF
we liave just shown, could not be done with safety. AcCord-
ingly, in order to increase the respective proportions of the
colouring oxides without risk, the composition of the flux must
be modified at the same time by an addition to the quantity of
oxide of lead.
Darker Green.
Silica * . , . » 1 part.
Minium 4—7 parts.
Oxide of Copper 1 part.
In these receipts we have no intention of laying down ab-
solute proportions, for we well know that there are certain
kinds of glass which will bear a greater quantity of oxide of
copper without the colour shelling off. But we have endea-
voured to impart such qualities to our pigments as will insure
continual success, and consequently we have perhaps carried
our caution to an extreme^ rather than render the success
doubtful.
Oxide of chromium does not answer so well for the green ^^
the fused pigments as oxide of copper ; for it seldom obt
so pure and bright a colour as the latter. Even if it is com-
bined with very fusible fluxes it generally wants transparency^
and yet the quantity of the substances which impart greater
Nihility to the flux cannot be safely increased.
Oxide of chromium is mixed with one of the fluxes for pig-
ments produced by combination, in the proportion of 1 part of
oxide to 9 parts of flux, the same proportion that we specified
for oxide of copper j and we are of opinion that it is better to
heat the oxide of chromium with the elements of the flux than
with the flux itself.
Notwithstanding what we have said concerning the pro*
perties of protoxide of chromium, it should not be neglected in
the preparation of the pigments. As glass-painting frequently
bears a certain degree of opacity very well, oxide of chromium
may be employed with advantage, especidly if it is only used
for the colouring of a pigment by mixture. The green colour
peculiar to it, which it possesses before it is united with a flux,
makes it very well suited to this purpose, and the only problem
now is to preserve it in a state of sufliciently fine division.
There are other substances which may be used for green
pigments, but they give an opaque colour. Among these are
zincate of cobalt, Rinmann's green, and antimonite of cobalt.
Of course these compounds are not acted upon by the flux, and
merely form pigments of the first class.
With respect to this matter we will describe a method
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PAINTING ON GLASS. 63
Sifhich» in our opinion, seems to promise manifold advantages.
When it is required to mix oxide of chromium with a fused
jpigment containing le^, the neutral chromate of lead may he
used, which furnishes at the same time oxide of chromium and
oxide of lead, or at least a part of the latter. When this salt
is submitted to a red heat it parts with oxygen and leaves
behind a combination of the two oxides in the following pro-
portions :
1 atom of oxide of lead 1394*6
1 „ „ chromic add 650*7
1 „ „ neutral chromate 2045*3
And this gives after calcining,
1 atom of oxide of lead 1394*6
^ ,t „ ,, chromium 502*
1 ,, „ plumbate of chromium .... 1896*6
It is evident that the oxide of chromium, which is thus held
in combination with oxide of lead, must be in a state of most
minute division, whereby its union with the siUcates or boro-
silicates of the flux is facilitated to a remarkable degree. This
pigment may be composed in the following way :
Silica 2 parts.
Minium . ^h n
Calcined borax 3 „
Calcined chromate of lead 2 „
These ingredients are melted and poured out.
The old glass-painters frequently availed themselves of the
combined effects of yellow and blue, in order to obtain a green.
Thus, for example, they gave to glass which had been coloured
blue in the melting pot a yellow stain with the help of sulphuret
of silver and the paste. This method is not in general use at
the present day.
Of the Violet, — ^There are two modes of preparing violet
pigments. The first consists in employing a colouring sub-
stance which is capable of producing a violet of itself ; accord-
ing to the second, the requisite violet is obtained by mixing a
blue with a red pigment. The substances which are capable
of bringing out a violet without the assistance of another
colour are the purple precipitate of Cassius and oxide of man^
ganese.
We have already explained, in speaking of the red pigment^
under what circumstances stannate of gold affords a violet
colour, and have observed, that the purple of Gassius, whenever
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64 E88AY ON THE ART OF
it is decomposed before the fiised pigment can dissolve its
particles, produces a violet and even a blue^ according as the
change which takes place in it is more or less complete ; and
this happens in the case we are now speaking of. If 1 part of
the purple of Cassius be mixed with 9 parts of one of the fluxes
coloured by combination, without fusion, a violet pigment is
obtained. This product is the effect of two causes which
operate simultaneously, that is to say, the want of fusibihty in
the pigment, and the too great abundance of oxide of lead.
Should the violet which is obtained in the manner we have just
specified be not brought out with sufficient clearness, it may be
still better developed by adding to the fluxes of the first section
a little of flux No. 1.
Oxide of manganese gives an exceedingly deep violet colour,
but, as we have elsewhere observed, it imparts to the pigments
an extraordinary disposition to crack and shell off. This
unfortunate property is easily overcome, however, by adding a
large quantity of minium to its composition.
The composition should be as follows :
Silica 1 part.
Minium 6—8 parts.
Peroxide of manganese i part.
When this oxide of manganese is heated in contact with
oxide of lead, it rapidly gives up oxygen, and is converted into
sesquioxide.
This pigment is distinguished from the rest by this remark-
able peci:dU[arity, namely, that it can only be employed on
condition that it is not painted on with any liquid, which,
after the burning in, leaves a carbonaceous substance in its
mass. This will be more apparent, when we remember that
peroxide of manganese is used for whitening glass, on account
of its property of extracting the carbon, which would impair
its clearness. Here the carbon acts upon the manganese,
deprives it of a portion of its oxygen, and converts it into a
colourless protoxide, while it is liberated itself in the state of
carbonic add.
Now this would be the case if the pigment, at the moment
of its liquefaction, contained carbon which was produced by
the vehicle with which it was laid on the glass before the
burning in. We would therefore recommend for this purpose
a solution of borate of soda, instead of the gum or sugar
water, and. the thickened essence of turpentine, which are
generally used.
The violety which is obtained by a mixture of blue and red.
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PAINTING ON 6I.A88. 65
is prepared with comm(m smalt and the purple of Caasius, the
proportions varying at the discretion of the artist.
There is another violet, which is produced hy the red oxide
of iron^ which is exposed to a white heat for some time ; hut
it is almost entirely opaque, and produces on the glass only a
kind of opaque and dull violet-grej. The pigment which
is thus obtained is, nevertheless, of great use in glass-painting,
and is prepared in the following way :
Violet oxide of iron 1 part.
Flux 3 parts.
No. 3 of the pigments of the first class is used for the
flux.
Of the Broum. — The brown is an undefined colour, which
may be prepared in a variety of ways, according to the require-
ments of the art, and the taste of the artist. We will describe
the principal browns.
Zincate of iron, which contams one atom of zinc, and two of
iron, ^ves a yellowish brown, which is frequently used. It is
prepared in the same way as ochre yellow, except that the
quantity of tlie salt of iron is doubled. For the composition we
are now speaking of, the salt of zinc must be to the salt of iron
as 2 to 10. * The flux proper for this is the same as that which
is used for the ochre yellow, and is also employed in the same
proportions.
Zincate of iron, ochre yellow, as it is called, when mixed
with either red oxide of iron or burnt sienna, gives a reddish
brown, whose shade of colour varies with the proportions.
The same zincate of iron (ochre yellow) forms, with burnt
umbre, a much darker brown than the preceding, and one
which has no affinity to red.
Oxide of iron alone, when properly prepared, affords a
brown of considerable utility. It is prepared in the moist way
as follows : A solution of persulphate of iron is treated with
an alkaline or carbonized oxide (with potash, soda, ammonia,
or their carbonates) ; the precipitate which is obtained is either
an hydrated oxide or a carbonate. It is placed in a filter,
washed and dried, and then subjected to a red heat, in order
to drive off the water or the carbonic acid. After this prepa-
ration the oxide of iron presents a brown colour, whose shade
may be varied by heating it more or less. This oxide of iron
bears no resemblance to that which is obtained in the dry way,
although the calcining develops in it a more or less distinct
red tone.
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66 ESSAY ON THE ART OF
Burnt umber, when mixed with a flux, also affords a brown
pigment.
A mixture of peroxide of manganese and oxide of iron like-
wise serves as a brown pigment of some utility. In order to
prepare all these pigments, it is merely necessary to know that
each of the colouring substances we have mentioned must be
mixed with thrice its weight of flux No. 3 of the pigments of
the first class.
No previous fusion is required. The colouring materials
are merely brought into a state of the most intimate mixture
by levigation together. The brown pigments are all pigments
of the first class, and possess great opacity, but by virtue of
this property they are exactly adapted to the use to which
they are applied.
Of the Black, — ^The completely opaque pigments, whatever
their proper colours may be, when laid upon glass, appear
black in a transmitted light. The reason of it is this, viz.
that a substance which allows no transmitted ray to reach the
eye must cause a shadow, and consequently produce a black
colour. Therefore either a black or an opaque colour may be
used for the black Unes of a drawiag upon ^ass, or the parts
that are in shadow. There is, however, a difference between
the effect produced by a really black colour and that of any
other colour not transparent.
When we look at a large transparent surface, e. g. a church
window, besides the refracted rays transmitted through the
glass, the eye receives also a few reflected rays, especially if
the external light is not particularly intense. In this way the
pigments, which only produce a black colour by virtue of their
opacity, are slightly tinged with the colour which properly
belongs to them. The black which proceeds from them thus
loses its hardness, and falls more agreeably upon the eye. On
this account pigments of broken tones, as they are cidled, are
used for the black lines and shadows of a picture, and of these
the brown pigments exhibit a great variety.
A pigment which shall appear black in a reflected as well as
in a refracted Ught, may be prepared in two ways :
1. Opaque substances of a black colour may be mixed with
a flux, e, g. oxide of iron, peroxide of manganese, peroxide of
cobalt, and oxide of copper, in such considerable quantities as
to prevent their vitrification taking place during the time of the
burning in. A black pigment, by mixture, is thus obtained,
which is composed of —
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PAINTING ON GLASS. 67
Oxide of copper 1 part.
Peroxide of manganese 1 m
Peroxide of cobalt 1 »>
Oxide of iron 1 »»
Flint glass 8— 12 parts.
2. A black pigment by combination may also be prepared,
and, after fusion, a certain quantity of oxide may be united
with it, merely in the state of mixture, in order to render its
opacity complete, as well as to impart to it the property of not
splitting off in scales. This is therefore a combination of the
two kinds of pigments. For the preparation of this compound,
the following ingredients are melted in a crucible, in the follow-
ing proportions :
Oxide of iron 2 parts.
Oxide of copper 2 ,,
Peroxide of manganese 4 1 part.
Flint glass 5 parts.
Fused borax ^ part.
As soon as the whole is melted, the contents of the crucible
are poured out and levigated with
Peroxide of manganese *. 1 part.
Oxide of copper 2 parts.
The theory of the composition of this pigment may be ex-
plained in the following way : By fusing together oxide of iron,
oxide of copper, and oxide of manganese, with the flux of flint
glass and the borax, borosilicates are formed. The two
former are green, and the latter are violet-red. Now green
and violet- red, in proper proportions, give a black. The
oxides which are afterwards added, as we have already stated,
only render the opacity complete, and give the pigment a pro-
per expansibility. The advantage of this latter circumstance
is evident, when the effect of the oxides of copper and man-
ganese upon expansibihty is considered. If, on the other hand,
it is required to prepare a pigment which shall only appear
black in a reflected light, let the following ingredients be mixed
together without being fused :
Black oxide of iron 1 part,
or even
Red or bright violet oxide of iron . . 1 part.
Flint glass or flux 2 — ^3 parts.
When this colour is used, it appears black by a transmitted
light, but by reflected light it assumes a reddish tint approach-
ing to violet, less hard and more agreeable than black.
0/ the White. — The white pigment is almost useless in
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68 EBSAY ON THE ART OF
glass-painting. We shall, howeyer, treat of it more fully than
we should otherwise have done, in order to complete the series
of the piem^ts of which we have here given a short account.
The white pigment has been used in modem times for orna-
menting panes of glass with a kind of drawing whose whole
effect depends upon the combination of the dull and opaque
white of the pigment with the transparent glass, without the
aid of any other colour. This branch of art, which borders a
little upon the one we are treating of, seems, however, to have
received no particular cultivation. The white pigment is a
vitreous compound of more or less opacity, and this opacity is
owing to the presence of zincic or antimonic add. This pig-
ment is prepared according to the following process : An alloy
is first made of 20 parts of tin to 80 parts of lead. The metal
is melted in an iron ladle or a vessel of a similar kind, and the
oxide which covers its surface is removed as fast as it is formed.
When the entire mass has been converted into oxide, the
calcining is continued a little while longer, while the oxide is
stirred incessantly until it assumes a perfectly homogeneous
appearance. It is then poured into a vessel full of water and
well stirred, in order to effect the separation of the unoxidized
grains of the alloy, which are mixed with the mass. These
grains collect at the bottom of the vessel. The oxide is now
dried and is fit for use. When this substance is mixed with
potash, silica, and borax, in the proportions which we are about
to specify, and then vitrified at a high temperature, a white
pigment is x>btained.
The white pigment of commerce that is used in enamel-
painting may also be employed for the purposes of glass-
painting, although it is rather deficient in fusibility. It is
usually composed of the following ingredients :
Potash 1 part.
Oxide of lead 8 parts.
Silica 6 „
Stannic acid 2 „
Calcined borax 2 „
This enamel is also prepared by fusing together in a crucible
the following ingredients :
Carbonate of potash 2 parts.
The oxides of tin and lead calcined ... 5 „
Silica 3 ,,
Borax 1 part.
As soon as the enamel has ceased to bubble, and appears in
the state of a clear liquid, it is poured out. It must be melted
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PAINTING ON OLA8S. 69
several times after being leyigated, or at least pounded in a
mortar. It is thus freed, from the excess of potash which it
contains, and at the same time rendered whiter.
The enamel of M. Clouet, as a specimen of an enamel
coloured by antimonious acid, may be contrasted with the
above. It is prepared from
White glass 12 parts*
Uncalcined borax 4 „
Saltpetre 1 part.
Purified on/iMontiiiii dkphoretieum • . 4 parts.
This is a quadruple borosilicate of lime, potash, soda, and
alumina.
It is less adapted to painting on glass than the one we have
mentioned before, and more £sposed to scale off, since it is
deficient in oxide of lead.
We recommend the following enamel, which seems to contain
the qualities requisite for painting on glass* Its composition
is as follows :
Silica 3 parts.
Caicine* 7* „
Calcined borax 2 ,,
The ingredients are melted, and then poured out.
The old glass-painters frequently used white pigments
similar in composition to those which we call pigments of the
first class. Levieil prepared a white pigment by mixing sul-
phate of lime (gypsum) with rocaiUe-flux (flint glass).
Felibien, Haudicquer de Blancourt, and the Abbot of Marsy
propose the same substance, but with a different enamel.
FHnt, pulverized rock-crystal, and the powder of calcined
bones, nave also been used for this purpose; a better result
would have been obtained with stannic add employed in the
same way.
Calculation of the atomic weights of the pigments. — Now
that we have shown that the various lands of glass are com-
posed of several silicates in definite proportions, probably in
combination with one another, the study of their atomic weights
must on this accoimt be the more interesting. Moreover, if
we would employ ourselves in making satisfactory inquiries into
the art of composing the pigments, this cannot be done with
any success unless we are acquainted with the calculation of
the atomic weights which are made use of in these compositions.
We will therefore communicate an easy method of deteimining
* By calcine is meant the calcined oxide of an aUoy of 20 parts of tin
and 80 of lead.— (^t</Aor'« note.)
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ESSAY ON THE ART OF
the composition of a pigment, or any other kind of yitreous
substance, according to the atomic weights, as follows :
Silicic acid . . . .
192-6
Sesqoioxide of manga-
Boradc acid. . . .
435*98
nese . ... . .
1011*5
Oxide of aluminam .
643-33
Peroxide of manganese
555*7
Oxide of calcium . .
35603
Stannic acid . . .
935*29
Oxide of potassium .
587-91
Antimonious acid . .
1006*4
Nitrate of potash . -.
1264-93
Oxide of zinc . . .
503*32
Oxide of sodium . .
390-92
Oxide of cobalt . . .
469-
Subcarbonate of soda .
666-25
Peroxide of cobalt . .
1038-
Borate of soda . . .
1262*88
Oxide of chromium .
1003-
Oxide of manganese .
1467*
Oxide of copper . .
495*6
Oxide of iron . . .
978-
Protoxide of lead . .
1394*5
"When it is required to determine the number of atoms of
each of the bodies which are used for a pigment whose com-
position is known, each of the expressions of the formula must
be multiplied by one and the same number, e.g, 100, 1000, or
10,000, just as it may happen to be necessary, in order to
enable us to divide all these expressions by the atomic weight
of the substance which they represent.
Let a pigment, for example, be composed of
Silica 45 parts.
Potash 6 „
Oxide of lead 70 „
If only the two first expressions of the formula were con-
sidered, it would be sufficient to multiply them by 100 ; for
the products would be divisible by the atomic weight of the
silica (192-6) and that of the potash (587-9). But it is
evident, that they must be multiplied by 1000 in this case on
account of the third expression. Accordingly we get
Silica 45,000 divisible by 192*6
Potash 6,000 „ „ 587*9
Protoxide of lead . . 70,000 „ „ 1394*5
If the division is performed, the following result is ob-
tained:
Silica 240 atoms or 24
Potash 10 „ „ 1
Protoxide of lead 50 „ „ 5
Consequently, if every atom of these bodies contains 100
parts of oxygen, the quantities of this gas will be to one
another as the atoms themselves, namely :
Silica 2,400 or 24
Potash 100 „ 1
Protoxide of lead 500 „ 5
The oxygen of the acid is four times as much as that of the
A double quater-silicate thus appears. The oxygen
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PAINTING ON GLASS.
71
of the potash is moreover to that of the oxide of lead as 1 to
5, and consequently we have 1 atom of the qoater-silicate of
potash and 5 atoms of the quater- silicate of lead.
To compose an enamel, then, which shall contain 5 atoms
of silicate of lead and 1 atom of silicate of potash, we must
take
5 atoms of oxide of lead » 1394 x 5 «= 6970
1 atom of potash » bS7 x I => 587
Further, if it he required that these bases should form with
the silica quater-silicates, we must take as many atoms of silica
as are necessary, in order that the latter may contain four
times as much oxygen as the bases. Now if the oxygen of the
bases amounts to 6, we get for the silica 4 x 6 = 24.
24 atoms of silica =^ 192 x 24 » 4608.
By simphfying the expressions we obtain at length
Protoxide of lead 70 parts.
Potash • • • 6 „
Silica 45 „
In order to obviate the necessity of making calculations, we
have drawn up Tables, by the assistance of which the reader
may discover at the first glance the quantity of any body which
corresponds to a given number of atoms.
SiUeie Acid.
Oaide qfPotamum.
Atoms.
Ptfti.
Atoms.
Parts.
5
1
1 . .
... 0-6
10
2
2 . .
. . . 1-2
15
3
3 . .
. . . 1-8
20
4
4 . .
. . . 2-3
25
5
5 . .
... 2-9
30
6
6 . .
. . . 3-5
35
7
7 . .
...41
40
8
8 . .
. . . 4-7
45
9
9 . .
... 5-3
50
..... 10
10 . .
... 5-9
•
Subcarbonate of Potash.
Baracic Acid,
Parts of the s
Atomi.
Puru.
Atoms of Potash.
carbonate.
1
0-4
1 . .
... 0-9
2
..... 0-9
2 . .
. . . 1-7
3
1-3
3 . .
. . . 2-6
4
1-7
4 . .
. . . 3-4
5
2-2
5 . .
. . . 4-3
6
2-6
6 . .
. . . 5-2
7
30
7 . .
. . . 6-
8
3-5
8 . .
. . . 6-9
9
. ^ ... 3-9
9 . .
. . . 7-8
10
40.
10 . .
. . . 8-6
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ESSAY ON THE ART OT
Nitrate (fPoiaah,
Atom* of Parts of
Potash. the Nitrate.
1 1-3
a 2-5
3 3-8
4 5-
5 6-3
6 7-6
7 8-8
8 101
9 11-4
10 12-6
Oxide of Sodium,
Atoms. FarU.
1 0-4
2 0-8
3 ..... 1-2
4 1-6
5 1-9
6 2-3
7 2-7
8 3a
9 .• ... 3-5
10 3'9
Borate of Soda,
Atoms d FarU
Boiacic Atoms of of the
Add. Soda. Borate.
2 . . 1 . . 1-3
4
. 2 . . 2-5
6
. 3 . . 3-8
8
. 4 . . 5-
10
. 5 . . 6-3
12
. 6 . . 7-6
L4
. 7 . . 8-8
16
. 8 . . 10-
18
. 9 . . 11-3
20
. 10 . . 12-6
Sabcarbonate of Soda,
nm. Parts.
1 0-7
2 1-3
3 2-
4 2-7
5 3-3
6 4-
7 4-7
8 5-3
9 6-
10 6^
Oaide of Caieiim,
Atoms. Parta.
1 0-3
2 0-7
3 11
4 1-4
5 1-8
e ..... 21
7 2-5
8 2-8
9 3-2
10 3-6
03Bide ofAbiimkmnu
Atoms. Parta.
1 0-6
2 1-3
3 1-9
4 2-6
5 3-2
6 3-8
7 4-5
8 51
9 5-8
10 6-4
Protoatide uf Lead,
Atoms. Parts.
1 1-4
2 2-8
3 4-3
4 ..... 5-6
5 7-
6 8-4
7 9-7
8 IM
9 12-5
10 14-
Jntftticf}!.
Atoms of Parts of
Protoxide. Minium.
1 1-4
2 2-8
3 4-3
4 5-7
5 71
6 8-5
7 10-
8 11-4
9 12-8
10 14-3
The difference between minium
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PAINTINO ON GI.A88.
73
and protoiide of lead is so con-
siderablCi that it may be peroeiyed
at once in any given number of
atoms, as will appear from a com-
parison of the two Tables.
Protoxide qfManganeae.
Atomi. Parts.
1 1
2 2
3 3
4 4
5 5
6 61
7 71
8 81
9 9-1
10 101
Deutoxide qf Manganese.
Atomi. Parti.
1 1-5
2 2-9
3 4-4
4 5-9
5 7-3
6 • • • • • 8*8
7 10-3
8 . . . . . 11-7
9 ..... 13-2
10 14-7
Peroxide qfManganeee,
Atoms of Parta of
Protoxide. Peroxide.
1 11
2 2-2
3 3-3
4 4-4
5 5-5
6 6-7
7 7-8
8 8-9
9 . «... 10
10 Ill
Peroxide qf Manganese*
Atoms of Parts of
Deutoxide. Peroxide.
1 1-7
2 2-3
3 . . « f f 5
4 6-7
5 8-3
Atoms of Parts of
Deatoxide. Peroxide.
6 10
7 11-7
8 13-3
9 ..... 15
10 16-7
These two latter Tables show the
quantities of peroxide of manganese
which are necessary to. produce by
their decomposition a given number
of atoms of protoxide or deatoxide.
Oxide
Atoms,
1 . .
of Zinc,
Parts
0-5
2 . .
i
3 . .
V5
4 . .
?-0
5 . .
?•.•>
6 . .
7 . .
3
3*5
8 . .
4
9 . .
4*5
10 . .
5
Oxide
Atoms.
1 . .
qflron.
Pwrts.
1
2 . .
|.q
3 . ,
?r9
4 . •
3*9
6 . .
5
6 . .
5-9
7 . .
6-9
8 . .
7-8
9 . .
8'8
10 . .
9-8
Protoxide of CgbaU,
Atoms, Parts.
1 0-5
2 , .... 1-3
.3 1-4
4 1-9
5 ..... 2-3
6 2-8
7 3-3
8 .• ... 3-7
9 4-2
10 4-7
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74
E8SAY PN THB AKT OF
Atoiqs of Ptrtt of
Plrotoiddei Peroxide.
1 ... . . Of 5
2 ..... 1
a ..... 1-5
4 2-1
5 ., ... 2-6
6' . . . . • 3'I
7 3<6
8 ..... 4-1
» .• ... 4-7
10 ^ . • . . 6-2
The Table of the peroxide of
cobalt has been compMed on the
same.pnnciple aa that of the per-
oxide of manganese.
AMMi.
1
2
3^
4a
&
6
7
8
(knde-qfChr^imum^
10>
Parti.
1
2
a
&
6
7
8
9
10
Aibi&s,
r
2
3
4
5
Cfxide qjf Copper,
Parts.
0-5
1
1-5
2
2-5
6 ..... 3
7 3-5
8 4
9 4-5
10 5
Antittumkmi Acid*
Atoms, Patta.
1 1
2 2
Anthmmie Acid,
Atoms of Parts of
Imimoniow A&timonSe
Acid. Acid.
1 1
2 ..... 2»1
a .• ... 3*2
4 41
5 ft-3
6 6*3
7 7*4
8 8^
9 ..... 9-5
10 ..... 10*5
Sfatmie Acid.
Atoms. Puts.
1 ..... 0*9
2 ..... 1*^
a M
4 3-7
a 4-7
e ..... 5-6
7 6r5
8 ..... T'b
9 8'4
10 ..... 9-3
In order that we may make no mistake in laying down the
proportions of die oxygen of the base to that of the acids, we
must remember that these compositions contain different
quantities of oxygen to one atom, namely *.
Parts.
1. Boradcacid .... 300
2. Alumina . 300
3. Setquioxide of manganese 400
4. Protoxide of manganese . 300
5. Peroxide of manganese « 200
6. Oxideofin>n, . i. . ., 300
7. Stannic acid . .
8. Antimonic acid .
9. Antimonious acid .
10. Oxide of chromium
11. Tha other bodies .
Parts.
200
500
200
300
. 100
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PAIIITING ON QJJLm4 75
B^bre we conclnde ifae chemical part of onr sulsject, we
wish to direct the attention of those who are engaged in the
preparation of pigments for glass-painting to a series of comr
positions which are capable of being used with very great
adyantage in the art. We wish to speak of certain omnrana*
tions of the colouring oxides with one another, about which we
have at present said very Httle. Further back, where we were
speaking of the colouring substances in general, we expressed
our opinion concerning these kinds of compositions, which we
likewise admitted into the class of salts. We have already
had occasion to consider a certain number of these combinations
which ave very much used m the rawufacture of the pigments^
and we now intend to impart what information it is in our
power to give upon a few other matters less known, but which
seem capable of affording valiuible resources to the art of
painting on glass ; and lastly, we intend to point out those
particuhrs which, in onr (pinion, deserve to be studied. We
wish to open to fresh observers a fruiti^ field, full of objects
both usefol and interesting, and we only regret that it was not
our lot to explore it ourselves.
The most stable amongst the combinations which the indif-
ferent oxides are capable of entering into with the basic oxides,
are the following :
Protoxide of manganese and oxide of iron ;
Protoxide of manganese and peroxide of cobalt ;
Protoxide of manganese and oxide of copper ;
Protoxide of manganese and oxide of chromiam ;
I^toxide of manganese and oxide of zinc ;
Oxide of iron and peroxide of cobalt ;
Oxide of iron and oxide of copper ;
Oxide of iron and oxide of chromium.
Amongst all the other possible combinations of the colouring
metallic oxides with one another, there are a few more of great
stability,, although certain of the oxides out of which th^ ar^'
formed are remarkably deficient in this stabilitr. Thus, among*
other combinations, there are a cuprate and a plumbate of silver,
of which the oxide of silver is reaucible at less than a red heat,
and further, a manganate of silver, which appears to possess
the same property,
Protomde of mafiffcmue and oxide of inm combined in
equal atoms » — ^First process: Two solutions, both of eqsal'
weight, according to Beaum^s areometer, one of protosulphate
of iron, and the other of protosulphate of manganese, are
mixed together ; they are then raised to the boiling poin^ and
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76
E86AY ON THE ART OF
treated with an excess of carbonate of soda. The precipitate,
after it has been washed and dried, is moistened with nitric
acid> and exposed to a red heat in a crucible, until it is per-
fectly decomposed. This operation explains itself, and we
need merely remark, that these sulphates contain equal quan-
tities of protoxide, and that both the prqtoxides are conyerted
into sesquioxides, and give an equal number of atoms.
Protosulphate of Iron.
•2 at. protoxide ... 878*42
2 at. acid ..... 1002-32
2 at. protosulph. iron . 1880*74
SesqutQjnde or Peroxide of Iron.
2 at. protoxide .... 878*42
1 at. oxygen ] 00*00
1 at. peroxido
978*42
Protosulphate qf Manganese.
2 at. protoxide . . . . 911*4
2 at. acid ..... 1002*32
2 at. protosulph. mang. • 1913*72
Sesquiojnde (f Manganese,
2 at. protoxide . . . 911*4
1 at. oxygen .... 10000
1 at. sesquioxide
. 1011*4
Second process ; A stream of chlorine gas is caused to pass
through water with which carbonate of manganese, or the oxide
obtained by precipitating sulphate of manganese with potash,
has been mixed. The oxide which is obtained in the state of
hydrate is mixed in proper proportions with hydrated oxide of
iron, and exposed to a red heat.
Hydrated Peroxide of Manganese.
2 at. manganese • . . 711*5
4 at. oxygen .... 400
Water 150
Hydrated Peroxide of Iron.
2 at. iron 678*45
3 at. oxygen .... 300
Water 168*70
1 at. peroxide .... 1147*15 1 at. peroxide .... 1261-5
The following result is obtained by heating the mixture :
Anhydrous SegiUoxide of Man-
Anhydrous Peroxide of Iron.
2 at. iron 678*45
3 at. oxygen .... 300
978-45
ganese.
2 at. manganese .
3 at. oxygen . .
711*5
300
1011*5
These hydrates, it is evident, must be mixed together in
equal parts.
Protoxide of manffanese and peroxide of cobalt, — ^This com-
bination is obtained by precipitating with an alkali, as in the
case of iron, from a solution of sulphate of manganese and
sulphate of cobalt, in equal quantities and of equal density,
and calcining the precipitate merely at a red heat. The atoms
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PAINTlNd ON GLASS. 11
of the acids ftre here likewise comhined in ^qual proportions.
We have assumed these proportions for all analogous com-
hinations.
Protoxide of manganese and oxide of copper, — ^The com-
position is prepared in the same way as the foregoing ; but it
IS better to effect the precipitation from the sulphate of copper
and sulphate of manganese with an alkaline carbonate, because
the oxide of copper is very likely to be re-dissolved if an alkali
is used. Moreover, the proportions of the solutions must be
equal. We may proceed in the same way in the calcining ; it
is as well, however, to add a little nitric acid to the mixture,
to render the oxidation of the manganese complete.
Protoxide of manganese and oxid^ of chromium.-^Tlaa com-
bination arises from pouring chromate of potash into a solution
of sulphate of manganese. The protoxide of manganese is
oxidized at the expense of the chromic acid, which is thus
transformed into oxide of chromium.
Protoxide of manganese and oxide of sitter. — ^This com^
bination arises from heating a mixture of peroxide of manga-
nese and metallic silver, which has been precipitated with
copper from a solution of nitrate of silver.
Oxide of iron and peroxide of cobalt, — ^This is obtained
from a mixture of solutions of sulphate of iron and sulphate of
cobalt, which are treated with an alkaline subcarbonate, in
order that the precipitate which is thus obtained may be
calcined.
Oxide of iron and oxide of coppw.-*-The same process as
before.
Oxide of iron and protoxide of copper. — ^A mixture of oxide
of copper and finely triturated iron filings is subjected to a
red heat. The copper returns into the state of protoxide, and
gives up its oxygen for the iron to form an oxide with it.
Probably it would be necessary, in using this compound as
colouring matter, to keep it careMly from the air as soon as
it passes into a Hquid state. We may avail ourselves of this
composition for colouring glass red, but it is probable that
in this case there is no combination between the two oxides.
Oxide of iron and oxide of chromium. — ^This combination
takes place when a solution of protosulphate of iron is treated
with chromate of potash. The protoxide of iron is brought
into a higher state of oxidation by means of the oxygen which
the chromic acid gives up, and the chromic add is thereby
reduced to the state of oxide of chromium.
Protoxide of copper and oxide of silver, "-^Thia combination
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78 SWBAY ON THE ART OF
.is obtained hj heating in a cradble oidde cf copper ymHh
-sietallic silTer in tiie form of powder. The same phenomena
may be observed here as in the case of iron and copper.
Plumhates. — ^We hare akeady mentioned the fact, that
oxide of lead to a great degree possesses the property of
eombining with the other metallic oxides. The combinations
kito which it enters with them are particniariy Talnable,
although for an entirely different reason from that for which
the nmikr compounds of which we were just now [^leaking
are yaluable : they are Taluablci because the oxide of lead
imparts a yery sl^ht stability to them. But while it com-
bines with the oxides, it mssolves them, separates their
partides, and facilitates in a remaricabie manner the ntrifi-
cadon of those pigments which are coloured by combinatkMi.
The most important of the plumbates are those of cobalt,
copper, and manganese, which are prepared by snbyectnag
minium with the oxides of one or other of these metak to
a red heat.
CHAPTBR IIL
THE MECHANICAL PARt OF GLASS-PAI^tlN^.
FreparatUm of the eolaurs far tue, — ^We shall now dis-
tinguish the colouring materials, whra combined wi& their
fluxesi by the epithet colour,
Hie colours must first be i^ednced to an extretnehf fine
powder, by Icmg-continued levigation upon porphyry, or in a
miH of poroelaui biscuit, constructud expressly for this )mr-
pose, which we shall describe hexaafter. In order to do this
It is necessary to mix with it a saficient quantity of pure
Ifater ; it is then dried and put away in well-stopped bottfes.
The levigated oolonrs, at the time when they am used, are
mited with liqmds of yarious kinds, but all for the pnrposey*— •
1. df giting them sufjicient stickiness and consistency, so that
they may not run on the surface df the glass ;— »2< of causii^
them to adhere sufficiently, when dir^ until the time when
they are fixed by the firing ;— =-3* m adapting them to the
various kinds of paint-brushes which are generally used fai
ghuw-pamting.
These liquids are usually — water, oil of turpentine, and oil
''f lavender, somewhat thickened by the substances which jtfe
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PAINTING ON GLASS. 79
to fix tlis eoloiurs, after they are dry» upw the glaas. Thu4«
for exaaiple, SBgar-candy or borax are g;eneraUy loixecl with
water ; thiokeiied oil of turpentine and oil of l^eoder to the
oils of turpentine and lavender. Thia inspiasation is produced
by a longer aotion of the air upon these essential oila or
eaaenoes. Among the prepanudona which are brought under
the name of thickened essence of turpentine, &€*> we must
distingoisli between those which have been really thickened by
the action of the air, and those which are notbing more than
tb« residue after the distillation of the ordinary essences of
turpeaiine and lavender. A residue Uke thu will never
answer our purpose well, and we might just as well use
turpentine, or even varnish. But it will be explained here-
after why liquids of this kind must be rejected,
Thdfait or thiekened essence, as it is called, is obtained by
expoaiiig to the air and light one of the above-mentioned
essential oils in a glass vessd with a wide mouth, whiob must
be covered with a woven tacture that will allow the air to
pass throng with(mt difficulty. The contents are stirred
from time to time, and the essential oil has attained to the
proper decree of inspissation when it is of the consisteney of a
thick syrup.
The water colours and the essential oil colours may both be
advantageously employed, and they are frequently used alter-
nately in one and the same painting.
The implements with which the coioura are laid on g^bss are
Taxbns sorts of paint-brushes ; viz.
1. Marten' S'hair pencils, brushes which run to a fine
pomt, and are generally used for water-colour painting.
2. FUehpeneUs, — These are not finely pomted, but cut off
abruptly at the lower part, so that they present a fiat surface
perpendicularly to their length.
3. Broad badger' M-hair peneiU.—Th&u^ are broad and flat
brushes for softening the colours.
4. Stoekfiah-taUa, of soft hog's bristles, of the same form as
the foregoing.
5. Hardpeneiht of the same form as the fitch pencils, only
prepared from hog's bristles.
6. Hog* s^bristU pencils, such as are used for oil-painting.
Use of these pencils. — In order to \slj an uniform coating of
colour upon the glass, a marten's-hair pencil must be used
when the space which has to be covered is small; if, on
the contrary, it is large, either a stockfish-tail, or a cut badger' s-
hair pencil, must be used. By the help of theae implement^i
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So KS8AY ON tUE ART OF
tbe dolour id laid on the glass and spread orer its sur&cd
bj being rubbed in one direction^ and then in the opposite^
until the whole coating is uniform. Painting of this kind has
generally furrows or channels which are caused hj the hairs of
the pencil, but these may be removed when it is considered
necessary, by means of a fitch pencil, or, still better, by meaiia
of a cut badger* s-hair pencil.
The fitch pencil, as we haye stated^ is terminated by a flat
surface, and it is used for touching the glass with the ends of
its hairs, but not for drawing over the surface, as is the case
with the other pencils. If this is done skilfully, and continued
for a sufficient length of time, not only will the streaks thereby
disappear, but the colour will also be diffused uniformly over
the whole of the surface which is to be covered. The pencil
must, however, be used until the colour is almost dry, and a
very short time before this takes place the ciolour will be
distributed with perfect uniformity.
The fitch pencil answers very well for laying on thin coat**
ings of very liquid colour, when they are required. A little
colour is taken with the pencil from the palette, and lightly
dabbed on those parts which are to be covered with the cdour.
The marten' s-hair pencil, on the contrary, is used for laying
on those colours which are required to be fused, for marfang
the lines, &c.
The hard pencil, of the same form as the fitch pendl, ig
used for removing certain water-colours which were previously
laid on the glass, in order to execute in this way a species oif
painting of which we shall presently speak more fully.
For mixing up the colours properly, certain precautionanr
measures are necessary. When a colour is to be mixed with
an essential oil, for example, a httle of this colour, which, as
we have before stated, must have been previously well levigated
and dried, is put upon a palette and mixed, by means of a
spatula, with a sufficient quantity of thickened essence to
allow of its being worked to a stiff paste. It is then diluted
with more or less essential oil of turpentine, according as the
colour which is to be laid on is intended to be of a more or less
rich tone; it is then ground a little more in the mill, and
used in the manner above described. These precautions are
especially important towards insuring a successful use of the
fitch pencil.
The process is the same with r^ard to the water-colours^
except that instead of a thickened essence a thick syrup is
used, and instead of essential oil of turpentine, water. The
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IPAtNTING ON 6LA88r 8^
object of this method of operation is to determine the propor-
tion of sugar or thickened essence without difficulty. For the
borax colours, and even for those in which pure essence of
turpentine is used, we are limited to the employment of as
much liquid as will yield a thick paste. As a general rule^
the enamel colours should not be much moistened*
It is not immaterial whether a water-colour or an oil-
colour, and whether essence of turpentine or essence of
lavender, be used. The water-colours are, generally speak-
ing, more liquid than the oil-colours, under circumstances in
other respects the same. They have less consistency, and
consequently do not admit of being softened so well with the
paint-brush. On a large surface of paint, the oil-colours are
much better adapted to forming an uniform layer of colouv^
and the streaks and furrows which the stockfish-tail leaves
behind may be more easily removed by means of the cut
badger' s-hair pencil. The use of the fitch pencil also insures
a far more satisfactory result with oiUcolours, and cou'-
Seqnently these colours are preferred in all cases which are
analogous to the above.
In comparing the essence of lavender with the essence of
turpentine, we find that the latter dries much sooner than the
former ; the foiirter, therefore, is employed whenever a liquid
which is long in drying is required. It is not used alone in
the preparation of the colours, but as much of the essence of
turpentine is mixed with it as is necessary to cause it to dry at
the proper rate. These means are employed when the fitch
pencil is to be used for softening, which always requires time.
In the water-colours we cannot advantageously substitute for
sugar other viscid substances, e. ^. gum, &c. ; for the former
has the advantage of being much softer, and at the same time
is not so liable to peel off. .4
Two styles of execution essentially different from each other
are adopted in glass-painting. That which in former times was
chiefly employed, namely, the Gothic, consisted in using no
enamel for colouring the glass, but in merely marking the out^*
lines and shadows with a brown opaque pigment upon glass
which had been coloured in the melting-pot. This style of
painting, or rather of drawing, notwithstanding its simplicity^
is susceptible of the most beautiful effects which the decora-
tion of panes of glass is capable of producing. To it we are
indebted for the beautiful Gothic windows which adorn our old
churches. This style of glass-painting is moreover the easiest
of axeeution^ for the artbt need only be a skilful dimughtsmani
D 5
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1^ kliAT off TBk Ai£r W
and has oocaaon lor notloDg else, except one ahigje piglBl^
whieh is easilj prepared, and not at all dtfieult to use.
The aecond style bekngs to the period of the iviiYal of
ciass-paintin^ ; it supposes Hkevnse the use fai coloured g^ai%
but at the same tune requires the use of nmnerons |iigmeiits»
by means of idiieh the artist exeeotes a teal pamting on iht
^ass, which^ in regard to its effects, may ht cxwipared with oil&
painting. The artist is therefore mider the necessity of bar*
mg a jmlette coveted with Titrifiable cidonrs, whidi is of the
same nse to him as a palette supplied with oil«cokH»s is to
the oil^painter. The exeention of this style is litiffienlt^ and
requires a partienlar study.
Grlass^paindng^ taking it altc^ether, presents numerous aad
considerable dlffienltiesi which proceed from the ntttife of the
cokrars and of the glass itself, either in relatioiti to the mechA-
nical application of the pigments, or in relation to the reahsi^
tion of the results which the artist endeavours to obtain. We
trill mention the kinds of manipulation whi<^ are necessary in
the trarious cases*
The smooth and polished surface of glass is not rery ft^
iROurable to the laying on the colours. When we are liyifig
airy colour on the glass, a second stroke of the brush frequently
takes otf what the first had deposited; and thence arises the
Impossibility of hesitating in this work. This difficuky has
disposed some artists to paint only on glass whose smooth sur^^
face has been reibored by sflndstone or emery.
Bttt the greatest obstacle to the painter arises from the niu
ttire of the vehiclei which does not easily adlint of the sirreral
coatings of colottr being kid one oter the others which, tta
account of the substances that are ttsed, do not even adhere
aftet the first costing Is thoroughly dry. As the tehiele Ibr
fixing On the first coating retains its solubility, it is capable of
being softened and removed from the surfi»e by the sueceed-
hig coatings. To meet this difficulty it was hapfiily suggested
that tiie vehicle should be changed in etety coating of coloor
ivhieh was laid on over another. Thus^ for example^ we paint
trith water'cokmt upon an cnl-colouri and iHee tersd, tnth oil-
colour upon a water-colotn*. The fixing substances ^ the one
eolour being insoluble in the liquid of the other, there is no dan-
ger o£ removsng^ by a second application of the colour, that
which has been produced by a first
The inoOitteniencei however, which accompanies the employ-
ment of colours prepared in different wi^s, gave lise to the
,diseotery of a metihod by whu^ we mtey paint with the same
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PUHfivim on GU9i. Bt
srdwdiei. 1^ ivktoYcryym arrived at bj obsenring^be follower
ing fact. Ad oil-eoloor, rendered vudd with thicl^ned essen*
tial iA of tdrpentiiie^ or Veiuce turpentine, retains ito aolubilitj
ia tbia liquid, if it has been dried ov expoBure to the air ; but
if the painted pieces of glass haye been submitted to a mode*
ivitef tempAttture in a drjing-ovcin* the colour loses its easj
BolubiUtyi bears the friction of the pencil yerj well^ and the
fir iMi coatings vbaj be laid on Without difficulty.
In prooe^Uis^ aeoordine to the method which we have just
desdribedj the painter wiU sometimes have occasion to inter*
tv^t his worki m order to dry the pamted pieces of glass in
the oven i and this drcumstance was so disagreeable to many
artistsi that they considered how they might avoid the neces-
sity of doing so, and at length th^ succeeded in painting on
glass with water or oil colour^ as is the practice in p^jn^jng on
canva«» without being obHged to dry the first painting. We
jnust lobservei however, that this is to be attributed more to
their skill and dexterityi thin to the method itself to whidi
they asdribe this invaluable superiority in praetbing their art.
tn Order to explain their nlethod of proceedings we will consi-
der a painting with water-colours. The artist first of all makes
tip his colour, and mixcis with it no more sugar than is neces^
sary to cause it to adhere. This colour laid on, he paints upon
the fli^st coating with colours whi<^h contain mote dissolved
SttgaTi without being thereby of a thicker oonsistenqy^ The
increasing solidification o£ the tehicle prevents its solvent action
Upon the colour which was originally laid on.
If this is dexterously performed, it admits of the painting
being impaste, to use a technical expression. Not^g but
.kmg expmence lUid perfeet dexterity will — we repeat it-^ena-<
ble us to employ this method with advantage.
F^or the execution of paintings on glassy particularlt Ootbic^
a method as simjde as it is ingenious is frequently haa recourse
to, which enables the artist to work with the greatest certainty
of success. This mode of painting is designated by the French
term peintute pat enle^agUi and consists in drawing the out^
lines with an oil'^x^our, and painting over the whole with a
water-colour, whichi after it is perfecdy dry^ is to be removed
with a hard paint-btush. By a dexterous use of this paint-
.brush» portions of colour of different degrees of thickness are
removed, according as shadows^ half-tints^ or lights are
required. Wherever the ^ss is left bare by a compile remo^
tbI of the colour, we get the lights; the places which the
.brush has spared give the shadows^ and the half-tints are jiro'
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S4 EMAt oft TttlE AUt OF
duoed by iin imperfect remoTal of the colonr. In this way tke
process of hijing on the ground-colour of the picture is per-
formed, and after the hitter has undergone the firing, it is again
painted orer, aiid retouched with a colour which has been
made up with sugar or thickened essence.
The colour prope'r for the peinture par ertleva^e is an entt-
mel colour, with which a little borax is mixed, and which n
then levigated with enough water to grre a thick pastes The
colour is laid on bv means of a stockfish-tail, and softened
uniformly with a cut badger^ s-hair pencil, with the assistance
of which the colour is easily spread orer the glass. The small
quantity of borax which has been mixed with the colour
causes a sufficient adhesion df the latter to the glass, so that
the hard pencil may be fearlessly applied to the removal df the
water-colour.
, The majority of the pigments of the second class are weD
adapted to the employment of this method. There are, how-
ever, among the pigments of the first class, some whose colour-
ing matter is acted upon by botax^ so that a oombinatioti
arises which imparts to the colour which has been laid on too
much adhesiveness to admit of its being removed with the
hard pencil. Among these colours are ochre. ydlow and all
those pigments which contain oxide of zinc.
This unpleasant circumstance may, however, be certainly
obviated by fritting these pigments before they are levigated
for mixing with borax.
To frit a pigment is to heat it to that point at which it
coagulates, and, by reason of an incipient fusion, forms a mam
of the consistency of dough.
Pigments of the second class are seldom used for the pein^
ture par enlevage. As this kind of painting only has in view
a simple sketch upon a previously coloured ground, it requires
in most cases merely an opaque pigment coloured by mixture.
That which is best adapted to this purpose consists of a flnx
of fiint glass and of brown or violet oxide of iron in the usiiid
proportions, with a mixture of calcined borax, whose weight is
equal to the eighth part of that of the flux. The proportions
of the borax and the flux are calculated for the preparation* tif
flux No. 3 of the ])igments of the first class.
The method which we have just described is capable of an
important modification with respect to the preparation of the
colour proper for peintur^ par enleifage. Instead of dissolved
K/sr»^. merely the oil of turpentine of commerce is used. In
\ the drawing is first sketched with a pen in water*
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t»AINtlKd ON OhknUi 85
colour, and a eoating of oil-colour is laid on oyer it. This mo^
dification is based upon the property which essence of turpen^
tine possesses of giving sufficient firmness for the enlevage pen-
cil to be used. The essence of turpentine owes this property
to a small quantity of resin which it contains, but which is not
to be found in it after it has been rectified by a new distillation^
Hence it follqws that this method is the opposite to the former.
Certain precautionary measures, however, ought not to be neg*
lected, to obtain a successful result. The water-colour must
accordingly be properly mixed with sugar, so that the oil-colour
may be incapable of softening it. Moreover, after the oil-
colour is laid on, it must be submitted in the drying-oven to a
temperature sufficient to produce the requisite firmness. It i»
also necessary to avoid adding essence of turpentine frequently
to the same colour, because the proportion of resin is likely to
be increased by that means, and the consequence would be too
strong an adhesion of the pigment to the glass, so that it could
not then be easily removed with the enlevage pencil.
Circumstances arise in which this method is rather compli*
cated, and this is the case when it is required to paint upon
glass which has not been coloured. In order to paint a head,
iox example, a light tint is laid on for the colouring of the
lights, and upon this a coating of colour suited to the enlevage,
in order to form the shadows and half- tints. Thus, upon a
drawing prepared withwater-colour, the first coating must be
laid on with a colour mixed with thickened essential oil, and
the second with a colour mixed with borax ; but if the draw-'
ing has been executed ill colour mixed with thickened essential
oil, the first coating of paint must be laid on with colour mixed
with sugar-'Water, and the second with colour moistened with
pure essential oil, in conformity vdth the directions we have
given above.
The peinture par enlevage is not only simple, but is also
executed in a very short time. It is peculiarly adapted to
decoration, especially to the execution of such designs as stand
out in light colours from a dark ground, and to the represen-*
tation of the embroidered parts of drapery. In this case the
enlevages are executed in a different way. To make the
drawing clean and correct, wooden scrapers of various forms
must be substituted for the hard pencils. The colour which
is here operated upon is of no particular kind, but may be amy
pigment whatever that has been levigated vdth thickened
essential oil.
Finally, glass-^peinting justifies every possible artifice the*
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86 ISSSAY ON THB AET OF
4>bject of wbidi is to lessen difficuUm, or to ensUe ^ to
obtain results which are not to be obtained in any other wftjr.
Thus, for example^ it is not uncommon to paint both sides of
the gkss, in order to obviate the unpleasant results of lajing
on one colour over the other* The same method is adopted
jn the case of yeUow from silyer, because the substances bj
which it is produced ought not to be brought into immediste
joontact with the colouring enamels. In this way a green is
obtained* that is to say, by giving a yellow stain to the back of
•fk piece of g^ass which has been painted Uue.
. The same method is also followed in order to impart bril-
liancy and a remarkable freshness to the brownish reia colours
Jrom iron ; that is to say, a yellow is applied to the opposite
«ide of the brown.
Whatever method be adopted, the erajdoym^nt of the dolouns
is always, however, subject to certain restrictions. Care must
be taken to use as few mixtures as possible. The edoors are
by means of the latter mutually destroyed at the temperature
^ which they are burnt in, while at the same time new Combi-
nations take place. By avoiding a mixture of the pigmcSntB
we obtain purer^ less perishable, and much moie certain
colours. It is better to lay one ccdour over the other, indt^
of mixing them. In order, therefore, to produce an effect
similar to that which may be obtained with odire ydlloiir audited
from iron, the yellow is first hiid on and then tempered wiUi red.
It is also of consequence to lay on the pigments as thin as
posrible, and there are sevend reasons for so doing. Thus if
the colours are laid on too thick, the adhesive substances that
are in them form a mass which possesses too little softn&» and
flexibility^ The motions causea by expansion, which the glass
undergoes in the changes of temperature^ unless they are
shared by &e painting, at length overcome the adhesion of
the colours and loosen them from the surface of the glass.
Tfads always takes place when water-colours and oil-colours are
laid cm in alternate coatings, for the heterogeneous qualities of
the substances by which Uiose colours are fixed are still more
conducive to this result. The mere drying of itself is often
sufficient to produce this unpleasant result m consequence of
the unequal contraction of these substances.
On the other hand the water-colours certainly part with the
water they contain* in dryii^ on the glass, but then tjiey
retain the sugar wUch was mixed with them ; the oil-cdours
give up the volatile part of their vehicle, whilst die <Uck^Md
essence which is in the latter has lost the power of being eon-
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»ATNTIVO ON GLASS. 87
^fUA into Tapmir^ and consequently remains upon the glass.
Henoe it §tM(m$i tllat thi^se substances must be oonsamed in
the faingi and leaVe i olirtain <]uantity of carbon lying between
the partides of thcf pi^^eirt. The hodj redacts upon oertain
eombinataonsi, ^. f, the oxides of iron and lead, which it turns
black by tMung thenl ef their oxygen, which it does so much
the more, as iha <!arbiin bekg pt«itected by a thick coating of
colour resists combusdoli ihe longer. This orcumstanee is at
least of impdHatlce with regard to the flesh-^eolouiing. This
eoHtretempit ithldi We hat« just pointed out would not take
place if a firm sabstance^ and one perfeetfy volatile at a oertain
temperatorei were intaxiably used as a means of fixing on the
colour, as, fat example^ camphoi', or borax^ which contains no
carbon at alL But the former substance is yet to be tried, and
the latter ootdd not always be ccmveniently used. Neverthe-
less, it follows froin what wii hare afarea^ stated^ that, of the
substances which are adapted to fixing the painting upon the
ig^ass, die preference shomd be giten to those whose combus*
tion leaves the least carbon behind^ and which at the same
time preserve n emak ductility^ as a security ligamst (bracking
and nillcdog. Perhaps wax would answer veiy well with
tegard to tl^se two particulars, for a portion of it Volatilises
when it is exj[k>Sed to heat^ and it possesses moi^ver great
dacttiity. For thk pnipose we have only to dissolve it in
essential oil of turpentine.
tt woald be a tiraste of time to descfribe the various mampu-
litions used herci which are purely artistical ; and #e would
father therefore confine oiirs^lves to some fbw partteular cases.
Painting on glass is always done by <!Opying oUrpaintings, or
original designs expressly prepared^ whicn are called tartoons.
The original design is traced by means of transpajcnt paper,
and this is used for transferring ^e drawing to the gkss. For
this purpose the piece of glass is laid upon the tracing, which
has been first sjAread out upon a table. The tram^arency of
the glass makes it eSsy to fofichv the outlines and shadows of
the tracing ; but if the glass is very much coloured and desti-
tute of transparency, it bcicomes necessary t6 use a perforated
pattern for finding Out the dtawing through the g^ass. This
is a kind of tracing, the outlines of which are drawn through
numerous oontigoous ponetures of the size of a needle's point.
The paper whi(£ lies on the glass is then struck with a bag of
powdered charcoal, and liie outlines appear marked with suffi-
cient distinctness. It is best to begm by drawing in Indian
>ink the object to be painted^ when it is in our power .to correct
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8S E8SAY ON tHE ART OF
and modify the sketch, just as we think proper, and then the
painting may he executed according to it with the greatest
certainty. If corrections are made after the pigments have
been laid on, a want of uniformity and completeness will
always be observed in the work. Besides, correcting is then
much more difficult. In other respects the same rules and
precautions are applicable to glass-painting as to painting on
canvas. The easel and the Maulstick are used equally in
both. The easel is constructed in a particular manner, which
we shall describe hereafter. The pieces of glass that are
placed upon it must be fastened with a soft adhesive wax pre-
pared solely for this purpose. In order to take advantage of
the transparency of the glass, the painter places his picture
between himself and the light, and can at once judge of the
effect of his work, as he proceeds.
A paint-brush or a goose-quill is used for drawing on glass
with a pigment. A water-colour is best adapted to the goose*
quill. oS-colours, which are preferred for painting upon a flat
surface, because they admit of being diffused more uniformly
with the pencil^ are not suited so well to the quill and do not
adhere so well to the glass ; nevertheless they are frequently
used. A large quantity of sugar faciUtates the drawing ; still
it is necessary to define the amount which should be mixed
with the colour* Six grains of sugar to one drachm of colour
produce a sufficiently strong adhesion, but for the quill at least
seven grains of sugar must be mixed with the colour. A
mixture of twelve grains would cause the colour to peel off.
It is a very good precaution, when it is intended to pamt with
a quill, to wash the glass beforehand with essential oil of
turpentine. Drawing m Indian ink is Ukewise more easily
executed if this plan be adopted.
Should it devolve upon a glass-painter to paint a chnrch
window of large dimensions, he must begin by dividing it into
more or fewer compartments which are surrounded by iron
bars properly disposed and firmly united, so that all parts may
be equally firm. He first determines the direction which is to
be given to the iron bars and lead casing which are intended to
unite the different parts of the picture. In doing this he must
study to make these parts follow the outlines as mnch as
possible, and to hide them in the shadows in such a way that
they may not injure the eflFect of the picture. He ought ndt
to be afraid of giving them a proper thickness, and increasing
their number when necessary for the sake of greater firmness |
this is of the greatest importance for the duration and preser-
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PAINTING ON 6LASS# 89
▼ation of a work of art which is always exposed to the riolenc^
of the winds. It is a necessary condition which fetters the
author of the original design himself in his composition. He
must always take care that the ex^ution of the painting will
not require pieces of glass which are too large to admit of being
durably fixed by means of iron bars and leading.
If the artists of the present day understand not only how to
insure firmness to a painting on glass, but also at the same
time how to satisfy the demands of the art as far as is neces-
sary, — this, whatever may be said of it, is an improvement
which we owe to the modem style of paintings and to which
the manufactory at Sevres has greatly contributed. As for the
rest, both these conditions may be satisfied at once without
much difficulty. The presence of the iron bars and of the
leading does not disturb the harmony of the painting nearly so
much as would be supposed. Can this be a magical result of
this kind of human creation ? The human mind has the power
of dismissing these obstructions, and the largest of them always
appear Hke objects which are outside the picture. Now,
whether the ancients sacrificed everything to solidity, or were
deficient in mechanical skill, their church windows afford
frequent instances in which the painting is far from harmonizing
with the materials which support it. Thick iron bars are fre-
quently carried across the figures or abruptly cut the masses
in the finest centre of hght. Every one, therefore, will cer-
tainly coincide in our opinion, namely, that this is granting too
much to the demand of soUdity and too Uttle to the effect of
the painting. But did not a very simple principle of economy
frequently predominate in this matter ? This hypothesis seems
at least reasonable, the construction of the arming being one of
the most expensive parts of the construction of painted church
windows. "When we come to work the iron bars into compli-
cated forms, this, as every one must see, can only be done at
a great expense. It is a consideration of some importance,
why the ancients thought themselves obhged to restrict the
claims of the art to limits marked out by economy.
After we have explained the various manipulations of the
proper art of glass-painting, we must not omit to speak of a
branch of industry which has something in common with the
art with which we are at present engaged. We mean the
manufacture of the motisseline glass, which consists in covering
panes of ordinary window-glass with a coating of opaque white
enamel, upon which a transparent sketch is drawn by enlevage,
which seems to be executed upon a faiutly-poUshed ground.
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90 BSSAY OK THE ABT OF
To obtain ^Ioob result, a fitdi pencil must be used, whose fonn
-we have already described. This is dipped into thickened oil
of layender* and the surface of the glass is toudied perpei^
dicularlj with it until the whole is coyered with an unifona
jcoating. As soon as this lajer has acquired a proper con-
sistencj, a powder composed of one part of oxide of tin and
three parts of a flux of the second class is sifted over it. The
thickened essence retains a proper quantity of this enamel.
This coating is then dried for six or eight tkown, and after it
has become sufficiently hard, the excess of enamel powder is
remoyed by means of a badger's-hair pencil.
Now, in order to draw upon a plate of glass like this lines
which shall possess the polish and transparency of glass, we
la^ under the glass a pattern whose lines are sufficiently con-
spicuous eyen throu^ the layer of powder. The pattern
haying been previously well secured, the artist removes hr
means of an etching-tool the colour firom those places whiim
are indicated by the drawing. This painting is now exposed
to a proper degree of heat, that it may adhere or become burnt
in. There is Uttle in this operation that admits of the
application of the proper art of glass^painting, the use of
the wooden etching-tool alone being accidentally borrovred
from this art. It is an exceedingly convenient tool for eir
ecuting transparent drawings in imitation of embroidery ; it is
also much better adapted to entevagea than the steel scraper.
The method which is employed in producing white enamel is
preferred on account of its succeeding in a very short time,
but is better. adapted to white enamel than to the other
colours. Laying on by means of the fitch pencil insures
greater uniformity, and enables us to obtain the requisite tone
of colour with certainty.
The huming in of the painting. — ^The glass, after it has
been painted, must be exposed to a temperature at which the
pigments liquefy, and are thereby united to the class. For
this purpose the pieces of glass are arranged in a kind of box
of fire-clay, called the muffle^ which is placed in a furnace and
there heated until the colours which have been laid on axe
liquefied.
Descriptian of the furnace. — ^The furnace consists of four
walls formed by bricks standing on their ends ; these bricks
enclose a space which is again subdivided into three parts.
The upper compartment contains the muffle, which is open in
front for the purpose of putting the glass in and taking it
•out. This aperture must be walled up, after the mufEe has
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PAINTING ON GLA88. 91
t)een filled. Tlie intermediate space oontaina tiie fire-plaee ;
the lower space contains the ash-pit. A cast-iron grate
separates the fire-pot from the ash-pit ; it is composed of
single cast hars^ in order that it may be more easily P^Mured.
These hars are merely laid by the side of one another. TVo
or three arches span l^e fire-pot transversely, and are placed
at equal distances from each omer ; they serve for the support
of the muffle. An arch of burnt day is perforated with several
holes to allow the escape of the flame and the products of
eombustion. The muffle has a wide aperture in front for the
purpose of putting in and taking out the glass, and another
one of a small nze under the arch for the escape of the gases
which are evolved in the interior of the muffle. The door
whicfa closes the large aperture is usually provided with a sort
of pipe in the middle, through which the experimental pieces
of glass or the pyrometer are introduced into the muffle. By
means of the latter we are enabled to observe the temperature.
It is better, however, to have two apertures of the same kkid,
as we shall presently see on a closer examination.
Management of the painted pieees of glass in the muffle^r^
As tite colours which have been laid on are to be melted, the
plates of riass should not be placed in immediate contact with
me another, or they would necessarily bake into one mass.
They are therefore laid apart from one another by means
of fire-proof earthenware shelves, (in France, by means of
alabs of lava from Auvergne,) or plates of cast iron, which are
ranged horizontally in paraUel layers, so that a small space is
always left between them. These shelves are kept apart by
the ^isertion of small parallelopipeda of baked clay in the four
comers. The shelves are also laid upon cross-bars of iron, of
which two are used for every shelf, and are inserted at both
ends of the muffle into notched bars; the former method,
however, is preferred. Before the glass is put into the muffle,
the shelves must be covered with a stratum of Spanish chalk
tnixed up with water, so that the glass, if it should happen to
become soft when heated, may not adhere to the surface of the
shelves. This coating of the shelves must be carefully dried,
and then the pieces of glass must be laid side by side, but
vfithout touching one another ; moreover, a clear space must
be left at each comer of the shelves for the little supports
which we have already mentioned. After the muffle has been
filled, the aperture is hermetically sealed by being plastered
over with clay which is proof i^inst fire, and the front of the
fiirnace is wdled up with bricks. Thus shut up^ the glass
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92 ESSAY ON THE ART OF
may be heated without the colours being exposed eitb^f to the
action of the flames or the vapours which arise from the fire-
pot.
According to Dri M. A. Gesserti to whom we are indebted
for an exceUent history of glass-paintings the introduction of
the painted pieces of glass into the muffle is effected in the
following way in Germany i— »-Well-Galcined lime is first of all
slaked with water, and when it has crumbled to powder^ is
properly dried over the fire. The bottom of the muffle^ to the
thickness of an inch, is covered with this powder, as it falls
through a coarse hair sieve; this substratum is carefully
levelled, otherwise the pieces of glass might become distorted
in the firing ; these pieces of glass are then laid side by side,
but in such a manner that they may not be in contact either
with one another or with the sides of the muffle. A thin layer
of lime is then sifted over them, o^ther painted pieces of glass
are disposed in a second series, and the process is continued,
until we reach the centre of the muffle, at least the aperture
for drawing out the tests, which consist of strips of glass from
6 to 7 inches in length and 1 inch in breadth, painted Over
with patterns of the colours which are to be burnt in. These,
like the painted pieces of glass, are laid upon a stratum of
lime, and sprinkled over with a coating of the same, but are so
arranged that one end of them reaches to the oentre of the
muffle, while the other projects half an inch out of the test
aperture, to admit of their being taken hold of and drawn out
by the pincers. The artist then continues introducing the
other pieces of glass, and strewing lime, until all have been
arranged in their places in the manner above described, or the
muffle is full. If only one has to undergo the bunting in, the
muffle is filled with ordinary pieces of glass instead of painted
glass, and the only piece which is to be fused is introduced
into one of the interior layers of the muffle. The latter is then
covered up.
The furnace must be heated with wood, inasmuch as no
other kind of fuel will answer the purpose, at least for the spe-
cies of furnace of which we are now speaking, because the
heating of the muffle is effected less by radiation from the
fire- pot than by contact with the flames. Consequently that
kind of wood should be selected which gives a long and
bright flame, e.ff, the wood of the aspen, birch, or poplar,
which must previously be seasoned as thoroughly as possible.
The fire must be made to bum up slowly, in order that the
muffle may acquire the same degree of temperature in all
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PAINTING ON GLASS. 9S
its parts. The ignited matter must therefore haye time to pro-
pagate itself. Were we to proceed otherwise, the glass which
lay next to the sides of the maffle would have already endured
a degree of heat which would not only affect the pigments hut
would even hring the glass itself into a state of fusion, whilst
the plates of elass which were situated in the centre of the
muffle had omj obtained the proper degree of temperature.
The better to esci^e so unpleasant an occurrence, we must
proceed in the following manner :
The fire must be constantly kept at a moderate temperature,
until the muffle has attained a Uvely red heat, when the tem-
perature must be lowered. The heat penetrates into the inte*
rior, and the muffle at last becomes of a dull red heat ; the
fire is now stirred, and as soon as the muffle is again at a lively
red heat, is lowered a second time. This is continued until
the centre of the muffle is at the requisite temperature. Ac«
cording to this method it can never happen that the exterior
of the muffle has already attained a high temperature before
the interior is properly heated.
The temperature which it is intended to arrive at is generally
a moderate cherry-red heat; this is determined by looking into
the various apertures of the muffle. But these observations
are very liable to error, because we are not always in the same
condition with regard to light. Suppose, for instance, that the
room in which the fiimace stands is very well lighted, then the
red-hot muffle is not nearly of so bright a colour. In a more
subdued light, the red appears much more intense ; so that one
day's experience is frequently fallacious. It is therefore neces«
sary to hang curtains before the windows, in order to obtain a
tolerably uniform light for this examination.
The temperature is also determined by painted pieces of glass
for testing, which are laid in the apertures of the muffle ; but
such tests as these are not conclusive, inasmuch as they only
indicate a local temperature. An experiment made in one part
of the muffle affords no clue towards determining the state of
the other parts. But if after all the artist should be disposed
to use this test, he must endeavour to keep always below the
requisite temperature, and to lower the fire before the test*
glass has actually arrived at the proper degree of fusion ; for
he may be sure that the temperature will increase towards the
centre. Carmine is generally employed in experiments of this
kind. The changes of colour which it undergoes in mass,
when it is raised to a higher temperature, render it peculiarly
adapted to thb purpose. When not much heated it has a dirty
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94 ESSAY ON THE AKT OF
Tiofet eoloQf , it then fMtsset over into p«i^ eanaine ; and when
at length it is heated too much, it appears ydlow in reflected
lig^t» and yiolet in refi*acted H^t> that is to say^ upcm an
opaque hodj^ «.^. a piece of porcelain, it w3l appear jeHow ;
upon a |nece of glass^ on the eontrarj> in tsaunnitted li^bt^ it
Wiil appear idolet.
Finally, the temperature may he determined hy means of
particcdar instruments contrived on purpose to measure the
neat of the iiimace. Among these are the pyrameten of
Wedseweod and Brogniart^ The former is constructed on the
piino]^ that clay eontraets in proportion to the heat to which
it is exposed. It consists of a plate of copper, on whidi two
or three rulers, likewise of copper, are fixed. Between tlwse
is inserted a cylinder of cky hedged in the fire,, which has heeor
exposed to the degree of heat which is to be determined. The
more it diminished in bcdk, the greater was the d^ree of
heat which was applied. The instrument is drnded into 24(>
pasts* The aero answers to 580^*55 Centigrade; every d^ree
is equal to 72^*22 Centigrade. This p^yrometer is therefore
adap^ to tiie measurement of very hi^ temperatoBes» but
does net answer wdl in the ease hefiNre us^ in whi<di the other
is pr^erahle. This latter is eonstrueted on the piinoopfe of the
eispansihility of the metak by heat» and oondsts ef an a]^Ma»»
tu» of baked day, along winch a groove runs loi^tudmally,
but stops short of one of the ends. A hest of metal and mt*
oth«r of baked day lie in this groove in soeh a way that their
ends toiudi one another. The other end is fumKhed with a
dia^>late, in the centre of which a finger moves^ one end otf
wltidi is in contact vrith the earthenware bar in the groove^
That part of the in^rumooit which contains the bar of metal
is inttodaeed into the muffle m such a way that the bar shall
be completely in the mf^e^ As it expands by the heat, it
must necessarily, since it ties at the end of the groove, push
fi)rward the earthenware bar ; the letter again eomraunicates
the impulse to the finger, whkh then mdicateS upon the dia^
plate the expannon wUeh has taken place. The metaUie bov
must be made of a metal whose point of fusion is fiir beyond
the temperature necessary for burning in the painting; it must
not even become too soft at this degree of heat. Iron and
silver may be employed for this purpose, but silver is generally
preferred; because it becomes less oxidized. If a metal still
more difficult of fusion were reqiured^ a bar of platinum m%ht
beussd^
ia Older thai we may use this instnment cSectnally, the
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PAIMVINO ON 6UMM* 95
foOonving conditioiift irnist be complied with: the dcxfr of tha
muffle must be furnished with two apertures, one of which
must be made in the lower part, and the other in the muddle,
or rather about the upper tlurd of its heiaht. A pyrooieter ia
prepaiied for eaeh aperture. When the lower one indicates a
didl red heat, the fire is not aUowed to increase, but is stirred
in pnqpoition as the heat has been reduced below that tem«
p«sture* The artist proceeds generally according to the diree*
tions aboTe gi^en, until the upper pyrpmeter indicates a proper
tmnperatore.
jk beginning to lig^t the fire^ it is important to cause a
feeble current of i^r to pass through the muffle, in otder to
allow the gases» which are formed in the laiUst hj the eombi^
natkm of the substances which are used for fixmg the painilsng
with those which come out of the fire*pot, and peneteate
thvoi^h the sides of the muffle, to escape with n^idity. For
this purpose one of the apartarea ia front, as well as that of
the upper part^ is left half-^pen.. Upon the latter is also
placed a stove chimney, a few decimetres in length, which
when heated draws the air out of the interior of Uie muffle,
and acc^rates its upward motion. It is only when the muffle
is being first heatea that the most injurious of the external
gasea penetrate in larger quantities. After it has arrived at a
red heat» they become m<Hre completely dissipated on the
extoior. These gases always exerdse a pendcioua infltteoce
upon the pamting, which it ia necessary to guard agaiitst.
The creating ue current of air of which we have just been
spezldng is not less useful as a means of obviating an un«
pieaisant occurrence, which, without this precaution, frequently
takes place, viz. the cracking of the painting. This may Ite
explained as follows : When the muffle is bemg first heated; a
large quantity of smoke often penetmtes through the hcde aC
the top. The glass is still cold, whilst the sides of the muffle
are hot; consequently the aqueous vapour of the smoke eon^
denses in small drops upon its surfiice. As soon as the glasa
begias to grow hot, the water which lies upon it begins to boil
b^re it is converted into steam, and at last affecta the painting
vnth which it is in contact. The consequence of this is, that
a peculiar kind of cracking takes place, similar to that which
varnish undergoes when it is laid on an oil-painting just finished*
The possibility of this aocid^t is diminished by creating a
draught of air at the commencement of the heating ; but wm
draught should not be so strong as to chilly and consequentlv'
to cniok^ the objects wiUi which it eomifA in contact. If tha
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96 ESSAY ON THE ART OF
be avoided, it seems to be attended with no other prejadieial
results.
There is also another source of moisture^ viz. in the clay
with which the door of the muffle is plastered over. It is
customary, therefore, to heat for a short time the muffle, with
its contents, before plastering on the clay, and to close the
furnace. The muffle being already hot at the time the clay is
laid on, its contents are not so hkely to condense the vapour.
That kind of glass-painting which is executed with coloured
fused pigments is not finished after the first firing. The
fusion of the colours gives in general too feeble a tone of colour.
The pigments of the first class especially lose some of the
intensity of their colour when they are exposed to too high a
temperature. It becomes necessary after the first firing to
give expression to those parts which are deficient in this par-
ticular, and generally to impart the requisite harmony to the
whole. It is even sometimes useful to re-touch the work a£ter
the second burning in, and in this case a third burning in
becomes necessary ; but the painting is not capable of bearing
more than this, for a large portion of the pigments would be
considerably damaged and even entirely destroyed by the
repeated action of heat. When a painting is to be burnt in a
second time, and it becomes necessary to restore harmony to
the various parts of which it consists, the artist generally
lays them together in the position which they are afterwards
intended to occupy, in order that he may be enabled to
jn^e of their combined effect. For this purpose he fixes
them in their respective places upon the glass plate of the
easel, or upon a plate of glass of a proper size, which is
placed in a frame.
There are two ways of holding the pieces of glass together.
They may be fastened upon the glass plate with wax, which i&
spread out between the fingers and rolled upon a fiat surface.
The wax takes the place of the leading, and fills up all inter-
stices. The pieces of glass may also be encased in lead.
Fastening with wax has the advantage of being materially
cheaper than leading, only it is deficient in solidity. When it
is warm, the wax becomes so soft that the pieces do not hold
together well, and numerous accidents are the result. A work
which has occupied much time may be ruined in a single
moment. It is also difficult to free the glass entirely from the
wax which adheres to it without a considerable loss of time,
and if any wax remains behind it is injurious to the painting.
^idesj the idea that leading is more expensive is merely
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PAINTING ON 0LAS8. 97
illusory, when it is the painter himself who fastens the pieces
with wax; for the cost of the leading is more than compensated
for hy the value of his time, hecause the former is done hy a
glazier, who only solders together the ends of the strips of lead.
Fastening i^ith wax is only practised at Sfevres; at Munich
leading is preferred.
OF ENGRAVING UPON GLASS V^ITH BYDROFLtJORIC ACID.
This style of decoration, which has nothing in common with
the proper art of glass-painting, is nevertheless frequently and
advantageously employed as an assistant to this art. It is
generally used for overlaid glass, as it is called (white glass,
upon which a coating of coloured glass is fixed in the blowing).
It consists of a kind of white drawing upon a coloured ground,
and it is obtained by removing the coating of coloured glass in
all those places where it is intended to lay bare the white
stratum, according to the form of the drawing. The process
of engraving is as follows : The glass is first covered with a
coating of oilv copal varnish, or, what is better, with merely
linseed oil which has been boiled with litharge, in order to
preserve from the action of the acid the parts which are not to
be etched. This layer is dried in the drying-oven, and the
varnish is then removed by means of a graver or needle, and a
scraping instrument, from those parts where the glass is to be
acted upon by the acid. As soon as this operation is finished,
the plate of glass is laid horizontally upon a table, and a raised
border of wax is carried round the edge which is capable of
containing the acid, which is then poured upon the glass.
The acid is allowed to remain upon the glass as long as is
necessary to destroy the coloured stratum wherever it is ex-
posed. After this, the piece of glass is washed and freed from
the border of wax and the wax ground. The engraving is clean
and fine in proportion to the thinness of the coating of glass,
and the diluted state of the acid. If the acid is very much
concentrated, its action extends over the etching ground, and
the lines seem undermined. ,This kind of drawing is frequently
employed for exhibiting silver embroideries upon coloured
dresses. It is white at first, but may be coloured yellow, so
as thus to imitate gold ornaments.
Glass may be etched by means of hydrofluoric acid, because
the latter has the property of dissolving silica. But on account
of the exceedingly pernicious eflFect of this acid upon the health
of those who employ it, it has been hitherto almost impossible
to use it. In order, therefore, to apply the art of etching on
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98 ESSAY ON THE ART OF
glass to the arts and sciences, it became necessary to find out
another means of etching, perfectly harmless, and a suitable
etching ground. Dr. Bromeis, of Hanau, a clever young
chemist, and after him the celebrated professor. Dr. Bottger,
of Frankfort-on-the-Maine, each however by himself, and with*
out being acquainted with the researches of the other, have
now discovered such a method of etching, and by means of it
hav« etched plates of elass of any thickness, and have pre-
pared them for giving mipressions in the presses which have
been hitherto generally employed.
Professor Bottger gave a full account of this invention, which
is called hyaiography by the inventors, in a lecture delivered at
a meeting of the Physical Society of Frankfort. At the same
time he pointed out the advantages which were associated with
this discovery, of which we shall only particularize the fol*
lowing :
1 . The substance used for etching is perfectly harmless, and
no vapours or gases are evolved, which, as is the case in copper*-
plate engraving, are in the habit of producing an injurious
effect even upon the plate; moreover, it does not lose its
strength, the same quantity can be used for etching several
hundreds of lines in glass.
2. Glass admits of an exceedingly delicate treatment, and
the lines are, more perfectly developed than in engravings on
steel or copper.
3. All kinds of porcelain and fiint-glass may be very easily
supplied with colouring and glittering ornaments, by rubbing
in the drawings that are etched upon them with vitreous
porcelain colour, which is not difficult of fusion, and then
burning it in the muffle.
The inventors intend to make known their method, which,
independently of hyalotypy, promises to be of great interest,
for a very moderate remuneration, as soon as at least fifty
persons in the district of the Zollverein will combine to pur*
chase it.
The easel proper for glass-painting, — The easel consists of
an oblong wooden frame, whose greatest dimension is its height.
Its interior edge is furnished with grooves for the reception of
a plate of glass. This frame is placed in a larger frame, and
may be raised or lowered in grooves at pleasure. We have an
exact representation of it in the old sash-windows. The exterior
frame has a series of holes in the direction of its height, on
both sides, and the interior frame can thus be supported at
~v given height by means of pegs which txe inserted inta
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PAINTING ON GLASS. 99
these holes on either side. This apparatus is placed obliquely
o» a table, and supported in this position by two props at the
back, which are joined to the upper part of the large frame by
means of two hinges. These two pieces are joined together by
a cross-bar, and held at a proper distance from the frame by
means of moveable iron hooks ; a contrivance precisely similar
to what we find in step*ladders, for, like them, when it is not
wanted to be used, it can be shut up, so as to occupy less
room.
Mill for grinding the pigments. — The mill is a circular vessel
of porcelain, wbich receives the runner and the substances to
be ground. The bottom of it, towards the centre, rises in the
form of a conical or rounded projection, which forms with the
lower part of the side a wide groove, in which the runner
moves. The runner consists of a thick crown of porcelain,
formed out of the segment of a cylinder. It is placed per-
pendicularly in the vessel, projecting above it. Its lower edge
is situated in the groove. As it is supported by the bottom of
the vessel, it is capable of revolving on its axis in a horizontal
direction. The levigation of the pigments is therefore effected
by the rubbing of the lower edge of the runner upon the surface
of the groove. This groove is intended to retain the runner
in its position, and to cause the substances which are to be
ground to collect at the points where the grinding takes place.
The upper edge of the runner is level. It has two holes situated
opposite to one another, and bored perpendicularly from the
surface : they serve to fasten on the piece which is destined to
communicate motion to the runner.
A tin plate, of the, width of the runner, closes the opening
at the top. It has two pegs, which are inserted into the above-
mentioned holes. By this contrivance the runner is made to
follow every motion which is communicated to the plate.
There are two ways of causing the motive power to act by
the help of the plate of metal. We may act upon a point in
the circumference by means of a revolving handle, which is
let into a wooden plate that serves as a cover. This is the
simplest method: but as in this case the runner moves at
liberty in its receptacle, as soon as it meets with any check
in its circular motion it suddenly quits its path, and the moving
it then becomes very troublesome. This might be entirely
obviated by means of a pin passing through the centre of the
runner and of the outer vessel ; but it is better to produce
motion by means of a crooked handle, which is made to work
in the following manner: The upper surface of the metallic pla^e
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100 ESSAY ON THE ART OF
has in its centre a circular iron bar, well secured. This bar or
spindle serves as an axis, and turns in a hole which is bored in
a horizontal cross-piece, or rather in the centre of the lid of a
box which covers the runner. The handle is fastened on just
where the axis projects out of the box. By this means the
centre of the runner is always retained in the same place. It
assists the action of the handle, which thus distributes the
impulse it receives to all parts at the same time ; consequently
the runner cannot suffer any concussion, as in the foregoing
case.
When the pigments are being ground, it is often advisable
to increase the weight of the runner at the commencement of
the operation. This is done by placing upon the tin plate
another plate of lead of the same diameter, whose weight may
be determined at pleasure. It is moveable, and can be easily
laid on the plate, and at the same time as easily removed, for
which purpose it is merely necessary to take the handle off.
The drying-oven, — ^The drying-oven is constructed of baked
bricks, and at the bottom of it is a cast-iron plate. This cast*
iron plate lies over the fire, from which the necessary tempera-
ture IS conveyed to the interior of the oven. The interior is
provided with wire sieves, fixed in frames lying horizontally at
short distances one above the other, which admit of a free
circulation of air. In front of the oven there is a door, which
is closed as soon as the pieces to be dried have been laid in
their places. The heating apparatus is similar to that of a
common oven which is heated with charcoal or coal.
Furnace, — ^The furnace consists of a wall of baked bricks, —
a fire-pot, the interior of which is lined with fire-bricks, as
well as with a coating of clay which is proof against fire, — the
grating, — the ash-pit, — a pipe through which the air is con-
teyed from a pair of bellows into the ash-pit; and grating per-
forated with several holes, in order to diistribute the wind of
the bellows uniformly throughout the interior. A good smith's
bellows should be used for this furnace.
This kind of furnace is preferred to the common draught-
furnace for the preparation of the pigments, because it is capable
of producing a very high temperature, and by that means
shortening the operation.
Wax for fastening the plates of glass upon the easel, — ^This
wax is similar to modelling-wax, and consists of bees'-was,
4 parts, — Burgundy-pitch, 1 part.
It owes its ductiUty to the Burgundy-pitch, and its adhesive
ny principally to the greasy matter which the bees' •wax
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PAINTING ON GLASS. 101
of eommerce almost always contains. If this is found to be
wanting, the deficiency should be supplied by mixing with it
tallow in small quantities.
CHAPTER IV.
THE WORK OF THE GLAZIER.
The work of the glazier consists — 1. in cutting out the various
pieces of glass which are to be painted, and in giving them
exactly the form which the drawing requires ; — 2. in encasing
the glass in lead when the painting is finished, and forming it
into the panels of which the whole picture is composed; —
3> lastly, in arranging it permanently in the arming.
In the two first operations the glazier is guided by a cartoon
prepared expressly for this purpose. Upon this the outlines
of the pieces of glass are indicated by a mere line, which at
the same time determines the arrangement of the strips of
lead. The irons by which the glass is fixed in the vdndow
are likewise drawn here in their natural dimensions.
Of the cutting the glass. — The cartoon having been placed
on a table, the glazier lays upon the drawing a plate of glass
whose colour has been determined to a nicety by the painter.
He draws the outline of the piece with a hnish containing
white paint, and then cuts it with the diamond, after which he
brings it to the exact form by means of the riesel-iron.
Among other things he takes care to leave a space between
each piece of glass, which is determined by the thickness of
the interior of the strip of lead by whose edges the pieces of
glass are afterwards to be united. The colour with which the
glazier draws the outline of the pieces which are to be cut out
is composed merely of Spanish white and weak gum-water ;
the paint-brush which he employs for this purpose consists of
long, flexible hairs attached to a thin cylindrical stick. The
painter on porcelain also uses this pencil, and it has the ad-
vantage of yielding a line of uniform width.
Although the (hamond and its use are well known, we shall,
however^ mention a few interesting particulars in relation to
the properties of this instrument. The diamond which cuts
with its natural edge is preferable to that which comes from
the hands of the lapidary ; for such a diamond only cuts by
means of an angle artificially produced. The latter may be very
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102 ESSAY ON THE ART OF
ea^y seen, when looked at with a magnifying-glass, to possess
level sides, and to form a rectilineal edge where these planes
intersect. That of the natural diamond is not perfectly recti-
lineal, hut always convex ; and this is just the form which is
best adapted to cutting glass. When the diamond acts upon
the glass by means of one of its edges, it either cuts or furrows
it. But in this there is an essential difference, for a piece oi
glass which has been merely furrowed can never be so well
divided. The diamond leaves a scarcely visible trace upon
well-cut glass; on furrowed glass, on the contrary, a very
distinctly fringed groove may be observed. When the diamond
is cutting well, a low, monotonous sound may be heard ; if it
is furrowing, the sound which it produces is very percepitible
to the ear and even to the hand of the glazier. When a line
has been drawn with the diamond, and the endeavour which is
made to break off the piece of glass is fruitless, the attempt
must not be obstinately continued, for it is only necessary to
strike gently with the handle of the diamond or some other
hard body upon the side opposite to the cut, in order to cause
it to begin to separate; only a httle more trouble is then
required to carry on the separation along its whole length. In
this way we avoid breaking the glass. When it is necessary to
cut a piece of glass of such a form that it would be difficult to
cut it with the diamond, and the attempt would probably fail^
recourse may be had to the action of fire.
After the outline of the piece which is to be cut off has
been drawn, a little incision is made with the diamond in
any part of the line, the glass is then heated with a hot coal
at those points through which the separation is to be continued,
the heating beginning at the point where the diamond has
commenced the separation. If the coal is carried in this way
slowly over the glass, in proportion as the separation takes
place, the whole piece is at last disengaged. The glazier would
do well, however, to draw the piece rather larger than it ought
properly to be, for fear that any deviations which might take
place as the crack is being continued should spoil the whole
piece. The riesel-iron is used as a finishing tool to give the
requisite form to the glass.
To prevent the necessity of blowing incessantly upon the
co<\l, in order to keep it burning and sufiidently hot, pieces of
white wood saturated with subacetate of lead are used, which
possess the property of continuing to bum without further
assistance, as soon as it has once been ignited at one of its
ends. These pieces of wood are generally cut from the willow
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PAINTING ON GLASS. 103
or poplar, and are then steeped for several days in a solution
erf the ahove-mentioned salt, after which they are dried. They
are very convenient for hot coals.
The glazier must be supplied with rectilineal and also with
corviHnear rulers of various kinds, for cutting the glass.
Suppose he has to cut a piece of glass of complicated form —
he removes, first of all, from the plate of glass by a rectilineal
cut, the glass which Hes outside the proper outline, and then,
by several cuts in succession, he penetrates into the corners,
so that at last it is only necessary to use the rieaeUiron,
The rieael'iron. — This tool consists of a blade of soft steel
H ^ne "^ in thickness, and about 5 inches 5 lines in length.
At each end is a shallow notch in the direction of its thick-
ness; and by means of these notches a number of small
fragments are chipped off the glass one after another, so that
those parts which are outside the outline are removed with
tolerable rapidity. The glass to be operated upon is held in
one hand, and the riesel-iron in the other ; the edge of the
glass is then inserted without any difficulty into a notch of the
riesel-irony and by gentle pressure downwards and upwards,
the part which lies in the notch is broken off. This action is
rapidly and Continually repeated, but the use of this instru-
ment requires some skill. The riesel-iron must be made of
soft steel, and thus possess suffident flexibility to hold fast
the corner of the glass, and at the same time must be hard
enough to last a long time for this continuous work. Iron
would be too soft.
A riesel'iran of a somewhat different construction is also
used, which is suited to glass of any strength, and which can
be easily repaired. It consists of two perfectly level rulers,
5 inches 5 lines in length, at one end of each of which there is
a rectangular projection a little more than a line in length.
These rulers fit one upon the other in such a way that that
end of the one which is without a projection is received by the
angle of the other, so as to form the notch of the riesel-iron.
The two rulers slide upon one another, and are joined together
by a band, by the binding screw of which they can be fixed in
any position. By means of this apparatus, the proper width
may be given to the notch, and the instrument may be easily
repaired by being taken to pieces.
Of the leading, — ^After the various parts of a painting on
glass have been burnt in, the parts themselves must be united,
* Tlie line is -^ of an inch, according to French measure.
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104 ESSAY ON THE ART OF
80 as to form a whole. This operation is performed by means
of strips of lead in the following manner :
These strips of lead consist of two narrow ribbons joined
together lengthwise, by one narrow slip of the same metal
mnniug along the centre of both. The arrangement of these
three pieces produces a groove in each side of the strip of lead,
for the purpose of receiving the edges of the two pieces of
glass, which border on one another, and which are thus
separated by the middle slip, and covered by the overlapping
parts of the leading. The length of the strips of lead i»
determined at pleasure, and the breadth should be from two
to six linesy so that the overlapping part may be from two to
four lines in breadth. We shall afterwards explain how the
strips of lead receive their proper form.
The cartoon, according to which the pieces of glass have
been cut out, is also used for putting them together and lead-
ing them. The glazier begins as nearly as possible in the
centre of any of the panels which are to be made up. As
soon as the ^rst piece which is to be fixed has been laid in its
proper place, it is fastened in several places by pegs which are
driven into the table. These pegs or nails without heads
accomplish this purpose by the help of small pieces of miUed
lead, which are laid between the pegs and the glass. One of
the sides of the piece of glass is then enclosed in a piece of
lead. After this piece has followed the whole outline of the
piece of glass, during which pressure has been applied by
means of the lead-jack, the superfluous breadth is cut oflF with
the lead-knife. A second piece of glass is then fitted on,
which, like the first, is fixed with pegs, until a strip of lead
has been fastened on. The edges of th^ lead are pressed
down with the lead-jack, and the operation is continued until
the panel is finished.
The glazier then proceeds to the soldering, which consists
in applying the solder to the joinings, thus uniting the various
pieces of lead and imparting a greater firmness to the net which
they form.
The solder is an alloy of lead and tin, less flexible than the
pieces of lead themselves, — consequently it imparts to them a
greater power of resistance. In this alloy the tin and the lead
are united in equal quantities. Before the solder is used, it
must be previously prepared in such a way that it may be con-
veniently applied. Lead is melted in an iron basin or pot, and
as soon as it has become liquid, an equal quantity of tin is
added. This done, it is kept at a moderate temperature, and
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PAINTING ON GLASS. 105
a small quantity of resin or grease is thrown into it, which de-
oxidizes the metal, and thus restores it to a perfectly liquid
State. The oxide and the foreign hodies which float upon the
surface are then removed, and the metal is poured out. It is
best to cast it in thin ribbon, which on account of its form and
its pliability can be much more easily handled. This ribbon
may be obtained by pouring a small quantity of solder upon a
grooved plate, and inclining the latter a little in the act of
pouring. The inclination of this mould must be such that the
ribbons or strips which are thus obtained shall be suflOlciently
pliable without being too thin. As soon as the glazier has
provided himself with a sufficient quantity of solder, he sets to
work. The solder is applied by means of the soldering-iron :
this implement consists of a piece of copper in the form of a
cone, the base of which is prolonged and serves as a handle :
the latter is held in the hand by means of two concave and
moveable pieces of wood which completely surround the hand
in their junction. The soldering-iron might also be made of
iron, and then it would probably be more durable ; but iron
does not seem so well adapted to receive the solder as copper.
The soldering-iron must be previously tinned at its point. For
this purpose, it is necessary to have a tin plate rather concave
in form. After the soldering-iron has been heated in a proper
furnace, it is rubbed over a piece of sal-ammoniac, in order to
clean it, and then over the before-mentioned tin plate, upon
which a little resin has been sprinkled for the purpose of de-
oxidizing the tin. The soldering-iron then receives aportion of
tin from the tin plate, and becomes thereby tinned. It is ad-
visable to clean the tinned end of it, whenever it is taken from
the fire, by passing it over the tin plate, the metal of which
must be renewed as soon as it becomes necessary.
The soldering-iron having been properly heated and tinned,
is brought near to the lead, upon which a small quantity of
resin has been previously sprinkled. In this position, the sol-
der, which is applied with the other hand, is melted : it is then
diffused uniformly and regularly, by the point of the soldering-
iron being made to pass over the whole surface of the lead.
The soldering-iron must be heated to the proper temperature ;
if it is too hot, it will melt the lead ; if it is too cold, it gives
the soldering a wrinkled appearance, which proceeds from the
imperfect fusion of the alloy. The lead is soldered on one or
both sides, according to the degree of strength which is to be
fi;iven to the leading. As soon as the soldering is finished, the
lead is cleansed from the resin that remains by being rubbed
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106 ESSAY ON TB£ ART OF
with a piece of liaen which has been dipped in cdl of turpcsi'
tine.
The ftirnace for heating the soldering-iron is nothing more
than a circular tin box, with neither grate nor dranght. This box
is generally placed on three feet, sufficiently high to allow ol
its being conveniently used. The coals are made to bom up^
merely by a pair of hand-bellows.
Preparation of the glazier* s lead,*^The bars of lead are
cast in a mould, and when in a rough state bear some resem-
blance in shape to the strips we have already desmbed, for ihe
production of which these bars are used. The mould is a sort
of frame consisting of two parts, each of which is from 1 inch
6 lines to 1 inch 10 lines in breadth, from 5 to 7 lines in thicks
ness, and from 1 foot to 1 foot 6 inches in length. These two
pieces have each three longitudinal grooves, which, when they
are joined with those of the opposite side, form the cavities in
which the bars are cast. The parts of the mould are united
at one end by a hinge which admits of the grooves of the frame
being shut and opened at pleasure. At the other end of one
of these parts is a bifurcated handle which moves on a hinge,
and which is capable of clasping the opposite piece with its
fork and holding it fast. The grooves of the frame are termi*
nated by a transverse groove near the handle, into which the
metal is poured. The frame being shut, is held perpendicularly
by the handle with one hand, wlule the melted lead is poured
in with the other. As soon as the lead has solidified, it is
taken out of the frame, in order to make room for other
castings.
After the casting is finished, the bars are smoothed, that
they may be subjected to the milling machine, whereby they
are converted into what is called ' glazier's lead.'
The mUling machine, — The milling machine consists, first,
of two vertical cheeks, which are parallel to one another, and
are held together by strong cross-pieces, by means of screws
and burrs. Each cheek is furnished with two wide holes be-
tween the cross-pieces, in which there are as many revolving
cyHnders, which we shall presently describe more fully. The
corresponding holes in each cheek are at the same height from
the bottom of the machine. At the bottom of each of the
cheeks also there are two horizontal projections chamfered off»
and furnished with holes to receive the iron bolts with which
they are fastened to a bench. Two iron cylinders pass hori-
asontally through the intermediate space between the cheeks^
'^^^ their ends are let into the above-mentioned corresponding
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PAINTING ON GLASS^ 107
holes at each side. The upper cylinder projects beyond the
hinder cheek, and at the end of it is a tooth- wheel of 12 teeth,
which is held in its place by a burr. The other end of the
cylinder only extends to the outer surface of the cheek. The
lower cyUnder projects beyond the cheeks at both ends. At
one end there is a tooth-wheel similar to the one we have
already mentioned, into which it fits ; the other end is squared
to receive a handle. On each of the cylinders, at an eqiud dis-
tance from either of the cheeks, there is a wheel, or disk,
whidi is capable of being disengaged from or fastened to the
cylinder. These two disks are thus situated opposite to one
• another, without, however, coming in contact. The bar of lead
which is to be drawn out by the revolving motion of the disks
in opposite directions, when the cyhuders are set in motion by
means of the handle, must pass between these two disks.
Their thickness determines the breadth of the groove, and the
distance at which they are apart the thickness of the interior
strip. On the circumference of these disks there are a few
hnes engraved, like the scratches of a file, and these are for
the purpose of holding the lead more securely.
To each of the cheeks between the cylinders there is attached
a heavy piece, of comphcated form, which is called the die.
The die, which is let into a mortised hole in its corresponding
cheek, presents an angular surface on its opposite side, which
is bounded at the top and the bottom by a rectilineal part
called the nageleinsatz.
The two planes of the angular surface bear the name of
engwgeures in French, the broader one being the engorgeur
iT entree, the other the engorgeur de sortie* At the top and
at the bottom of each die there is a semicircular groove corre-
sponding to the adjacent cylinder. It is now evident that the
dies, in their mutual relations to one another, in conjunction
with the disks, complete the mill which is to give the proper
shape to tbe lead. They serve to form, the outer surfaces and
the sides of the lead, just as the disks form the core and the
grooves. In order ,to draw the lead, the end of the bar is
placed between the two disks in the intermediate space between
the dies ; the handle being now turned, the bar must pass
through the rolling-press, and receive the form and dimensions
of the latter. In this operation the lead is considerably
lengthened : a bar one foot long will give a strip of lead of
more than four feet in length. It is impossible to obtain
milled lead of various dimensions without having several pairs
of wheels and dies of proper sizes ; consequently, if the disks
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108 £8SAY ON THE ART OF
are fixed to the cylinders^ it is necessary to hare just as many
cylinders as disks.
The bench of the machine consists of a simple piece of wood,
the ends of which are supported by three legs fixed in the
ground.
Before the lead is placed in the machine, it must be smoothed
and then condensed. The latter operation is performed in the
following manner : One end of a bar of lead is bent into the
form of a right angle, which is placed under the foot, while
the other end is wound upon a piece of wood, by which means
the lead is forcibly drawn out : it is thus considerably length-
ened, and becomes more rigid. When it is to be used, the lead-
jack is passed between the overlapping edges to separate them
from one another, so that the glass may be more easily pressed
into the groove.
Of the arming, — ^The chief oWect of the arming is to impart
sufficient firmness and power of resisting external violence to
church windows. The net of lead which holds the pieces of
glass together is by no means capable of doing so for any
length of time, unless it is of very small dimensions or sup-
ported at certain distances by iron bars. But this iron-work
contributes sometimes even to the decoration of the window :
Gothic church windows afford an example of this, which con-
sist of numerous panels containing forms of various kinds
artistically disposed, so that the whole presents a very pleasing
appearance. Here the arming pays its tribute to the art by
afibrding the requisite strength to the window : it isolates tl)e
panels, and renders their agreeable outlines conspicuous, while
at the same time by its complicated pattern it forms a drawing
so much the more powerful and effective, as it appears black
upon a transparent groimd. In those windows, on the contrary,
where the arming is evidently of no use to the painting, but
is merely for the sake of rendering the glass secure, its presence
cannot be otherwise than prejudicial to the effect; therefore, in
that case, it must be our object to conceal it as much as pos-
sible in the shadows, unless we prefer presenting it to the eye
as a piece of trellis-work, independent of the picture, just as is
generally the case in church windows. The ancients did this
very frequently, from a principle of economy.
There are several other methods of constructing the iron
armings ; sometimes, for example, as in the case of Gothic
windows, simple iron bars, which have been bent into the form
of the outhne of the panels, are used, and the latter are
fastened to the bars by means of p^ns disposed at equal dis-
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PAINTING ON GLASS. 109
tances throughout the whole extent of the former; sometimeg
there are ir(Mi hars which in certain modern church windows
receive the panels in grooves in which they are fixed by nails
running transversely through them. Sometimes there are
merely thin iron bars disposed at the back of the paintings
which, by means of bands of the same metal laid on the lead
on one side, carried round it and twisted together on the other^
render the leading compact.
Amdngs which are furnished with grooves are difficult of
construction : the bars are composed of two plates, which are
connected longitudinally at right angles, one in the centre of
the other, by means of clenched rivets ; or they may be made
of a single strip of tin bent at right angles, and then bent back
again, so as to form two right angles, which constitute the
groove. This kind of arming is exceedingly expensive, and
will always increase the cost of those church windows to which
it is appHed. The Gothic arming, although simpler, is never*
theless always expensive, on account of the great strength of
the iron and the splints which belong to this kind of arming.
The arming which is constructed with thin bars and with
ribbons of lead is much more easily prepared, but is less
durable, and presents no pleasing appearance.
After the glazier has arranged the panels in the arming
vrkh splints, nails, or ribbons of lead, he cements all the places
through which the rain is capable of penetrating, and thus
finishes his wprk.
CHAPTER V,
ON THE INGREDIENTS FOR COLOURED GLASSES.
The preparation of the glass which is coloured in mass is,
strictly speaking, no part of glass painting, but belongs
properly to the art of making glass. Since, however, such
kinds of glass are frequently used by glass-painters, we shall
here briefly state the ingredients proper for the various kinds
of coloured glass.
If we mix with glass, at the time of its manufacture, certain
metallic compounds, we impart various colours to it, the shades
of which we are capable of varying to any extent by following
the general rule, viz. that according as it is required to increase
or diminish the intensity of the colour, a small quantity of the
colouring metallic oxide must be either added to or taken from
the mixture. The colour thus imparted is not superficial, as
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110 ESSAY ON THE ART OF
in the case of porcelain and enamel painting, but is diffosed
throughout the whole mass, the colouring matter having
become a real constituent part of it. In other respects, the
same substances are applied to this purpose as are used for
the colouring of the pigments in general.
Of the purple glass, — Gold is employed to give a purple
colour to glass, similar to that of ruby, for such glass is the
most perfect imitation of the ruby in colour, and resembles it
almost in brilliancy and Uveliness. There is no other sub-
stance which is capable of imparting a red of so beautiful a
tone to the pot-metal ; but extremely deticate manipulation is
required in the management of the gold, and a variety of pre-
cautionary measures are necessary to insure success. The
precipitate of Cassius is the preparation of gold which is most
frequently employed for colouring glass purple. We have
alreiady described the methods of obtaining it, and we shall
now state the reason why it is generally preferred to the rest.
The precipitate of Cassius is one of those combinations con-
taining gold which possesses the greatest stability. It resists
a high temperature, and if the reduction of the gold, as we
beUeve it to be, takes place after it has united with the glass,
this will only happen at the moment when it can be held by
the whole mass in the state of division in which it exists when
united with the tin. This combination is less disposed than
any other preparation of gold to pass over into violet or blue.
The purple of Cassius absolutely requires, in order to be used,
to be levigated in a gelatinous state with pulverized glass, borax,
jt any other substance which is capable of entering into com-
bination with glass, and this with a view to prevent a con-
glomeration of the particles of the gold. Its power of colouring
is such, that one part of the purple of Cassius is capable of
colouring 1000 parts of glass. In using this purple it is cus-
tomary to mix with it the sixth part of its weight of the white
oxide of antimony, in order to give the glass a faint tinge of
yellow, which reduces the red that slightly approximates, to
violet to a lively purple. In other respects, all that we have
said about the pigments coloured by means of the purple of
Cassius applies just as well to the colouring of glass in the
melting-pot.
Fulminating gold is also used for colouring glass purple.
It is one of those combinations which are obtained by precipi-
tation with ammonia from chloride of gold. Its property of
exploding at a very low temperature renders it unsafe to use.
Tn nrder to deprive it of this property, it is customary to mix
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PAINTING ON GLASS. Ill
it with silica, lime, &c. The same result is obtained with a
fixed alkali, the mixture being exposed to a gentle heat. It is
also sufficient for the purpose to levigate the mixture with
es^ntial oil of turpentine. But the precautionary measures
which we are under the necessity of adopting, in order to
prevent an explosion, are at the same time indispensable to
success in colouring, and that too on the same principle which
guides us in the preparation of the purple of Cassius. I mean
that the fulminating gold, whilfe moist, must be mixed with a
substance which will deprive it of its explosive property, and
at the same time hold it in a state of the most minute
division.
The purple colour of glass may also be produced by using
chloride of gold, sulphuret of gold, and even from aurates,
if the operator proceeds according to the method which pre-
vails in the other preparations of gold.
Of the red glass. — The red glass receives its colour from
copper, if not in the metallic state, at least at the degree of
strongest oxidation. To make red glass, a mixture of sul-
phuret of copper and oxide of iron is added to the melted mass
of glass ; or, after a green has been first produced in the mass
by means of oxide of copper, the metal is deoxidized by the
mixture of a substance containing carbon, and the colour is
thus made to pass over into red ; this deoxidizing substance is
usually bitartrate of potash. It has been found that any
other combination which contains carbon would produce the
same effect.
Carmine is obtained from copper only ; a darker red, from a
mixture of iron and copper, in which the former is to the
latter as three to one. The quantity of iron is diminished, if
it is intended that the tone of the colour shall approach
nearer to that of carmine. The glass must be worked as
soon as the proper red colour makes its appearance, othenvise
it would speedily disappear.
The red which is obtained from copper is so intense that it
is necessary to conduct the operation with the greatest care, in
order that the glass may not lose its transparency, for it is
often in danger of becoming perfectly opaque. Our object
must therefore be, to find out a method by which this risk
may be obviated. Now this method consists in overlaying
the white glass with an extremely thin coating of red glass
{flashed glass) ; it is only in this way that transparency can
be combined with a beautiful colour.
The ancients only mixed crude tartar, soot, or other de-
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112 ESSAY ON THS ART OF
.oxidizing bodies, mth the mass which had been coloured by
copper, for the sake of obtaining the red. But it is best
to use protoxide of tin, in order to obtain copper in the state
of protoxide. The action of protoxide of tin is not so transient
as that of the vegetable substances, whose action naturallj
terminates with their combustion, and in using which we are
liable to a double disappointment; for if they are< not suf-
ficiently consumed, the glass will not be so clear, nor the
colour so bright, and as soon as the colour has come out
properly we must proceed to work up the glass a9 quickly as
possible, because the colour i$ exceedingly liable to (&sappear.
But all this is avoided by using protoxide of tin. According
to Dr. Engelhardt's experiments, the red continued equally
fine throughout the whole process, and he never found himself
obliged to add deoxidizing bodies. He also met with oxide of
tin in all the ancient kinds of glass which he examined, and
this oxide was for the most part present in greater quantities
than the protoxide of copper itselif-
Now, since the colour imparted by protoxide of copper is
too intense to admit of its being worked up alone, and the
plate which has been coloured throughout would be opaque
<and appear dark brown, and since it would be necessary to
blow it exceedingly thin, in order that the red may become
transparent, we can only obtain manageable red plates of glass
by flashing a white plate with a very thin coating of red glass.
There is, moreover, this advantage in flashed glass, viz. that
the red coating may be ground off in different places ; thus
we obtain white sketches, or, by melting other colours on the
surface, sketches of various colours.
That this was the method employed by the ancients is ap-
parent from all the painted windows of the middle ages.
In order to make flashed glass two crucibles are necessary,
one containing the red and the other the white glass; the
workman first dips his pipe into the red mass and brings up a
small globule at the end of it ; he then covers this with a
proper quantity of white glass. The cylinder which is pro-
duced by blowing will exlubit a flashed glass of a beautiful
red colour. It is necessary for the purpose of thoroughly
uniting the red coating to the plate of white glass, and pre«
venting it from peeling off in the cooling, as happened in
Dr. Engelhardt's first attempts, that the composition of white
glass should be similar to, if not the same as, that of the red
glass ; but it is best to make the red glass a little more fusible
than the white.
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PAINTING ON GLASS. 113
Moreover, the ingredients of the red glass must contain
no oxidising substances. Dr. Engelhardt, who has satisfied
himself upon this subject at the glass-houses, proceeds as
follows: He places between the large crucibles for the red
glass a small crucible, and into this he introduces 4oz. of
protoxide of copper and 4 oz. of protoxide of tin, in addition to
tbe usual ingredients, for every 5 lbs. of the latter, if they
happened to contain minium. But if they do not contain
minium, he takes 3 oz. of protoxide of copper, and 3 oz. of
protoxide of tin for every 2 lbs. of salt which the ingredients
contained. If the protoxide of copper is not immediately
added to the frit, but is introduced into it afterwards, when
the latter is beginning to become clear, a much smaller
quantity must be added. For scarlet he uses for eveir 25 ibs.
of ftit ^Ib. of protoxide of tin and l^oz. of finely levigated
protoxide of iron : these are added just at the beginning of
the operation.
As soon as the glass has become clear, he mixes with it
1-^oz. of protoxide of copper, and the whole is well stirred up
together. Especial care must be taken to avoid bubbles,
which are extremely liable to be formed, and specks of sand,
as also that the white and the red mass be ready at the same
time to admit of their being worked up together.
It is clear from what we have just stated, that to succeed in
obtaining a beautiful plate depends very much upon the work-
man, for the overlaid glass always Remains thicker at the
mouth of the tube than at the opposite end of the bulb of
glass ; the plate is therefore always darker on one side than
on the other, and only the middle is uniform; indeed the
flashing is sometimes so thin at one end, that the colour
disappears and passes over into white. Dr. Engelhardt has
in his possession several ancient pieces of glass in which this
transition from dark to light has been made of the greatest
use in producing certain efiects ; among others, in the satin
drapery of a Judith. However, the workman may with some
practice acquire the knack of making tolerably uniform plates
of glass, and Dr. Engelhardt hopes soon to obtain this result
in a glass-house with which he has become connected for this
particular purpose.
A frit containing lead seems more likely to preserve the red
than any other frit ; Herr Engelhardt, however, says that his
experiments are not decisive upon this point.
Of the yeUow glass, — ^There are several compounds with
which glass may be coloured yellow, if they are mixed with it
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114 ESSAY ON THE ART OF
at the time of its manafacture. Among these are sulphuret
of antimony, antimonite of lead, and chloride of silver. The
method of coloaring with the latter has, in this case, nothing
in common with the production of the silver yellow. The
employment of chloride of silver, however, requires a very well-
refined glass which does not contain an excess of alkali.
Without this precaution the chloride would be decomposed,
the silver which is in the melted mass of glass, being now
reduced to the metallic state, would suffer a rapid a^l$>-
meration of its minute particles, and the colour would in
consequence disappear. Chloride of silver is not used on
account of its being very expensive. Sulphuret of antimony
and antimonite of lead are the only colouring materials in
general use« But as the yellow which is burnt in gives a
fresher and clearer colour, yellow pot-metal is not much
manufactured. We merely wish to mention one other yellow
here, which is obtained by an admixture of a carboniferous
substance with the pot-metal. This imperfect method, which
gives a glass of a disagreeable shade of colour and full of small
bubbles, has now fallen entirely into disuse.
Tn order to colour the glass blue, black oxide of cobalt is
employed, which causes the formation of the protosilicate.
. Fiolet glass is obtained by means of peroxide of manganese
with or without an admixture of oxide of cobalt.
A green is imparted to glass with oxide of copper, oxide of
chromium, or a mixture^ of antimonite of lead and oxide of
cobalt. Oxide of chromium gives a less transparent coloar
than oxide of copper.
Black glass is prepared from oxide of manganese, oxide of
iron, oxide of copper, and oxide of cobalt, in combination with
one another. This colour depends upon the mixture of the
three colours which are produced by these oxides ; that is to
say, the mixture of green, blue, and violet, which in proper
proportions will give a black.
Finally, glass is made white and opaque with stannic acid
or phosphate of lime, obtained by calcining bones.
Phosphate of lime, or calcined bones, is also used in the
manufacture of opalescent glass. The exact receipt for the
mixture will be given below. Opalescent glass is employed
with advantage in every case where the direct rays, or even
the reflected light of the sun, is prejudicial to workmen. This,
for instance, is the case in the workshops of goldsmiths,
jewellers, &c., in government offices, banking-houses, &c.,
which are situated on the ground floor, where it is often
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PAINTING ON GLASS.
115
necessary to use ground glass. So also this glass is of great
service for all kinds of lamp-glasses, since the grinding of con-
cave or convex surfaces is attended with difficulty, and makes
such glass expensive.
We will now give several receipts for the ingredients of
coloured glasses :
Mixtwrefwr RoBe^eoloured Okug,
Part*.
^^hitesand 100
Potash 48
Slaked lime 8
Purple of Gassius .... 6
Peroxide of manganese . . 4
Or.
"WWtesand 100
Minimn 78
Caustic potash 35
Nitrate of potash .... 7
Purple of Cassias .... 8
Peroxide of manganese . . 4
Sulphuret of antimony . . 4
Red OhM,
White sand . •
Minium . . .
Caustic potash .
Nitrate of potash
Purple of Cassias
Peroxide of manganese
Sulphuret of antimony
100
60
30
5
12
6
6
YeUow GUus.
White sand 100
Potash 50
Slaked lime* 8
Antimony - yellow, coloured
with oxide of lead ... 6
Or,
White sand 100
Potash 40
Lime 10
Antimony, yellow, coloured
with oxide of lead ... 10
Or,
White sand 100
Minium 80
Caustic potash 36
Crystallized nitrate of potash 12
Antimony -yellow, coloured
with oxide of lead ... 8
Blw Glass,
White sand .
Minium •. .
Caustic potash
Calcined borax
Oxide of cobalt
Or,
White sand •.
Potash . .
Slaked lime .
Oxide of cobalt
Or,
White sand .
Minium . .
Caustic potash
Nitrate of potash
Oxide of cobalt .
Parte.
100
150
35
10
4
100
50
6
1
100
80
40
8
1
Green Glass.
White sand 100
Refined pearlash .... 50
Slaked lime 8
Green oxide of chromium . 2
Or,
White sand 100
Refined pearlash .... 50
Slaked lime 9
Yellow oxide of antimony . 4
Oxide of cobalt, or zaffre . 2
Or,
White sand 100
Minium 75
Calcined potash .... 38
Nitrate of potash .... 4
Green oxide of chromium . 2
Or,
White sand 100
Minium 60
White pearlash 40
Oxide of arsenic .... 6
Glass of antimony .... 9
Oxide of cobalt .••••- 5
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THE ART OF PAINTING ON GLASS.
ParU.
Or,
White washed sand . . .100
Minium 85
Calcined potash .... 38
Nitrate of potash .... 8
Yellow oxide of antimony . 4
Oxide of cobalt 2
Violet Glass.
\niite sand 100
Pearlash ....... 48
Slaked time ...... 7i
Oxide of manganese . . .4-10
Or,
Whitewashed sand . . . .100
Minium 78
Calcined potash 35
Crystallized nitrate of potash 8
Peroxide of manganese . . 1-2
Black Glass.
White sand 100
White pearlash 66
Slaked lime 8
White glass, pulverized . . 70
Oxide of arsenic .... 6
Oxide of cobalt 10
Peroxide of manganese . . 10
Acetate of iron, or even iron
in the highest state of oxi-
dation ....... 5
Or,
White sand 100
Pearbsh ....... 48
Lime 6
Oxide of cobalt 4
Peroxide of manganese . . 3
Oxide of copper .... 3
Black oxide of iron ... 4
Or,
White sand 100
Minium 82
Calcined potash .... 38
Nitrate of potash . .
Oxide of cobalt . . .
Peroxide of manganese
Black oxide of iron .
Oxide of copper . .
Parti.
8
8
8
12
12
Opalescent Glass,
White sand 100
Refined pearlash .... 50
Slaked time 16
Oxide of silver 3-6
Phosphate of lime,or calcined
bones 6
Or,
White sand ...... 100
Purified soda 450
Slaked lime 160
Calcine,* or white broken
glass 500
Hydrochlorate of silver . . 10
Phosphate of lime from
mutton bones .... 60
Oxide of arsenic .... 30
White opaque Glass.
White sand 100
White pearlash 66
Slaked lime 8
White glass, pulverized . . 50
Oxide of lead 100
Oxide of arsenic .... 3
Or,
White sand 100
Calcined potash .... 50
Slaked lime 16
Oxide of tin 60
Or,
White sand 100
Minium 78
Calcined potash .... 30
Nitrate of potash, in crystals 8
White oxide of tin . . . . 62
* A calcined mixture of oxide of lead and oxide of tin in the proportion
of 4 of the former to 1 of the latter. — {Authot^s Note.) '
Hughes & Co., Printers, King's Head C(»urt, Oough Squaxe.
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