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Full text of "An essay on the art of painting on glass. From the German"

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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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PAINTING ON GLASS. 19 

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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PAINTING ON GLASS. 21 

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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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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