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TCHPAINTING ON GLASS AND PORCELAIN

AND

ENAMEL PAINTINGA COMPLETE INTRODUCTION TO THE PREPARATION OF ALLTHE COLOURS AND FLUXES USED FOR PAINTING ON GLASS,

PORCELAIN, ENAMEL, FAIENCE AND STONEWARE, THECOLOUR PASTES AND COLOURED GLASSES, TOGETHERWITH A MINUTE DESCRIPTION OF THE FIRING

OF COLOURS AND ENAMELS

OX THE BASIS OF PERSONAL PRACTICAL EXPERIENCEOF THE CONDITION OF THE ART UP TO DATE

BYFELIX HERMANNTECHNICAL CHEMIST

WITH I ILLUSTRATIONS

SECOND, GREATLY ENLARGED, EDITION

TRANSLATED BYCHARLES SALTER

LONDONSCOTT, GREENWOOD, & CO.PUBLISHERS OF THE(pofferg &aytte

10, -21, AND 23 LUDGATE HILL, CITY, E.C.1897


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ABERDEEN UNIVERSITY PRESS


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PREFACE.IN the preface to the first edition I remarked that,so far as my knowledge extended, there was no otherindependent work on the production of the coloursfor glass and porcelain painting, i.e., the chemicaland mechanical preparation of the pigments andmetals necessary for this class of painting, except,perhaps, Brongniart's Traite des Arts Ceramiques,which is now out of date. There are, it is true, afew works on the subject in the German language,but these are, for the most part, uncritical compila-tions of essays on glass and porcelain colours, andcontain recipes and instructions which I havepersonally found to be unreliable.My own practical experience in the subject, whichI was able to complete in the celebrated manufactoryat Sevres, has placed me in a position to produce acomplete work on the whole matter, and one that Ifeel will, in view of the facts referred to above,supply an existing deficiency and be welcomed bothby amateurs and by those engaged in the business.In preparing the first edition I naturally en-deavoured to include everything that was, in myopinion, of importance to our subject, whilst givingonly those instructions of whose practicability I hadbeen able to convince myself by personal investi-


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IV PREFACE.

gation. In the course of the years that have sinceelapsed, many novelties have been brought out inthis connection, few of which, however, possess anyactual practical value. This necessitated a carefulsilting of the materials at disposal in order to excludesuperfluities and useless matter, and maintain thevalue of the book for the practical man. The residueof actually valuable material was, however, so greatas to render the enlargement of the work necessary.

Since the manufacture of the coloured glass pastesand the production of coloured glasses stands_ in close ,'connection with that of the glass colours, exhaustivetreatises on these matters have been incorporated,and the technical side of glass painting thoroughlydepicted.The undersigned therefore hopes that his work, inthis new and greatly enlarged edition, will constitutea reliable and profitable guide to all who are engagedin the beautiful branches of industrial art treated ofin its pages.

THE AUTHOK.


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TABLE OF CONTENTS.PREFACE '. . . . . iiiINTRODUCTION

History of glass painting 1I. THE ARTICLES TO BE PAINTEDGlass 11Porcelain 14Enamel . . 17Stoneware . 19Faience 20

II. PIGMENTS1. METALLIC PIGMENTS 22Antimony oxide 22Naples yellow 23Barium chromate 25Lead chromate . . 26Silver chloride . .27Chromic oxide 29

1. Potassium bichromate and sulphur method . 292. Ammonium chromate method .... 313. Wet method 314. Mercurous nitrate and potassium bichromatemethod 32

Iron Oxide 341. Preparation of ferric oxide from ferrous

sulphate 362. Ferric oxide for fine (red) colours . . . 383. Vogel's iron red 40Ferrous chromate 40

Gold purple . 41Iridium oxide 46Copper oxide . . 48Copper protoxide 49Cobalt oxide 61Cobaltous silicate 55Cobaltous zinc phosphate 56Smalt 57Zaffre 59


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vi CONTENTS.

Manganese oxide ........ 60Uranium oxide ........ 62Zinc oxide . ........ 63Tin oxide.......... 662. EARTHY COLOURS ..... . .68Yellow ochre ......... 68Red ochre ......... 70Terra di Sienna ........ 70Umber .......... 71Mars yellow ......... 713. METALS ......... 73Gold........... 73Mussel gold ....... .74Mussel silver ......... 76Platinum........ . .77

III. FLUXESFluxes ........ . .79Felspar . ......... 80Quartz . . . . ...... 81Purifying quartz ........ 84Sedimentation......... 85Quenching ......... 86Borax .......... 87Boracic acid ......... 88Potassium and sodium carbonates .... 88Rocaille flux ......... 89

IV. PREPARATION OF THE COLOURS FOR GLASS PAINTING1. Black .......... 902. White.......... 933. Red .......... 954. Yellow.......... 1015. Green .......... 107


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CONTENTS. VilCHAPTER PAGEGold ruby glass (according to Fuss) .... 145

Copper ruby glass 14GLead-free (flash) glass 147Lead-ruby glass . 147Violet-coloured glass 148Black glass 148

VII. COMPOSITION OF THE PORCELAIN COLOURS. . . . 1491. Soft muffle colours 1522. Hard muffle colours . . . . . . . 1863. Colours for strong fire 189

VIII. THE ENAMEL COLOURS 216Enamels for artistic work 223IX. METALLIC ORNAMENTATION

Porcelain gilding 230Glass gilding 236X. FIRING THE COLOURS1. Eemarks on firing 244

Firing colours on glass 244Firing colours on porcelain 248

2. The Muffle 253XI. ACCIDENTS OCCASIONALLY SUPERVENING DURING THEPROCESS OF FIRING . . . . . . 275

XII. KEMARKS ON THE DIFFERENT METHODS OF PAINTING ONGLASS, PORCELAIN, ETC. . . . . . 282APPENDIX

Cleaning old glass paintings 295INDEX . 297


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PAINTING ON GLASS AND PORCELAIN.INTKODUCTION.

HISTORY OF GLASS PAINTING.ALTHOUGH in the subjoined historical particulars the datahanded down to us have been adhered to, we consider it,nevertheless, important to make thereon a few preliminaryobservations, which, indeed, involuntarily thrust themselvesupon one's mind when visiting large museums. In theEgyptian collections in these museums we find a largenumber of objects presenting an indubitable proof that theEgyptians who lived 4000 years ago were already masters ofthe art of producing white and coloured glass and of glasssmelting. On this account it is not improbable that theyalso understood how to apply certain permanent colours toindividual portions of glass surfaces, thereby laying thefoundation of the technical industry of glass painting.

However, the art (in the true sense of the word) of paint-ing on glass is very young, since it was only in recent timesthat it was brought to the state in which we now see it,although the commencement of the art decidedly dates backas far as the first centuries of our era.

The invention of true glass painting was made in theeleventh century. The ancients, however, were acquaintedwith the art of preparing their multi-coloured household anddecorative articles by fusing together portions of glasscoloured in the soft stage, as is shown clearly enough in the


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'2 PAINTING ON GLASS AND PORCELAIN.numerous specimens of Roman glass ware extant, such asbeads, vases, urns, tear bottles, etc. Still, at that time, theuse of glass for windows was as yet unknown, and, therefore,''glass painting in its true meaning cannot be consideredas being then existent.

The ordinary window glass, of which we first discoverundoubted traces in the fourth century of the Christian era,differed materially in its nature from that of the present day.It was only made in small irregular panes of considerable,though unequal, thickness, and yellowish (bottle) green incolour. Already, in the ninth century, a similar colouredglass was used in Rome for covering the windows ofchurches. This was, however, effected only by a purelychromatic grouping of dark coloured glasses in leaden frames.Subsequently the individual panes were arranged in sym-metrical order after the style of mosaics ; and finally, by thejuxtaposition of suitable coloured glasses cut to shape,pictures were produced, but it was only at a much laterperiod that these cut pieces were provided with outlines, andmore or less shading, by means of vitrified metallic coloursfired -into the surface of the glass.

Only a single pigment, the so-called black flux abrownish black, surface colour was at the disposal of theartist on glass in those days. Some highly interestinginformation on the technical processes of the oldest glasspainters is afforded by Theophilus Presbyter in his eleventhcentury treatise, Diversum Artium Schedule, lib. ii. Theglass painter of that day had not only to prepare his owncolours and designs, but also to make and cut the glass him-self. After the latter had been prepared he proceeded withthe preliminaries for the operation of painting. In the firstplace a wooden table of the size of the projected window wasmade, the entire surface of which was rubbed over withchalk, moistened with water, evenly distributed by the


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HISTORY OF GLASS PAINTING. 3aid of a cloth, and left to dry. On this table a sketch inoutline of the picture was drawn with red or black colour, orwith lead or tin, the different colours being indicated byletters, and in this way various divisions were mapped outand covered with sheets of glass of corresponding size. Inorder to make the glass sheets agree as closely as possible insize with the divisions, the outlines, visible through theglass, were traced thereon in white, and the glass cut alongthe tracing by means of a glowing iron, the edges beingthereafter smoothed with a large iron (grosarium ferrum] ;and then only were the various pieces placed together for thepurpose of being painted. The black flux a combina-tion of copper scale with green and blue lead glass servedas pigment, and the painter drew therewith the interiorcontour of his design. The shading was produced by carefulcross-hatching ; where light was desired the glass was lefttransparent. Damascened effects were produced, accordingto the artist's fancy, on garments and backgrounds, by lightlysizing the glass and removing as much of the coating, bymeans of an etching tool, as sufficed to produce various kindsof patterns.

For the process of firing the. colours (chapter xxii.)several rods were stuck into the ground in a corner of thehouse, and the equal sized ends of each pair were boundtogether into an arch about eighteen inches high and thesame breadth. This framework, some two feet or more inlength, was plastered over on the inside and outside, aboutfour inches thick, with a mixture of three parts of potters'clay and one part of horse dung well soaked with water andmixed with dry hay. A smoke-hole of about a handbreadthwas left open at the top, and a doorway in front, as well asthree corresponding apertures in each side, the latter largeenough to permit of the insertion of iron rods as thick as aman's thumb. A fire was kept burning in this furnace until


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4 PAINTING ON GLASS AND PORCELAIN.the walls were perfectly dry, and then an iron plate, twofingers' breadth shorter and narrower than the dimensions ofthe oven, was prepared and fitted with a handle. A thicklayer of dry or quick lime having been spread over this plateand levelled down by a flat piece of wood, the painted platesof glass were laid side by side thereon, so that the green andblue pieces lay on the outside, and the more refractory white,yellow and red pieces in the centre. The plate was thenplaced over the iron cross-pieces, and a moderate fire ofbeechwood lighted, which was gradually increased until theflames rose around the sides of the plate and met above theglass. As soon as the latter began to glow the fire wasquickly drawn, and, the openings being covered up, thefurnace left alone until cooled down. On taking the glassout it was tested with the nail to see if the colour wouldscratch off. If it resisted, the operation was finished ; other-wise the firing had to be repeated.

The next step was to lay the coloured pieces in properorder on the wooden cartoon, and join them together withstrips of lead, after which the whole was enclosed in awooden frame.

The oldest monumental buildings which have transmittedglass remains (veritable glass samples) to our own day, dateback to the twelfth and eleventh centuries. These remains,with surfaces deeply corroded into holes by the effects ofoxidation during the lapse of so many centuries, betray theirvenerable age by the texture of the glass and their warmgreen tinge. The fragments are unusually thick, five milli-metres (\ inch) being no unusual diameter. The size of theindividual panes of this antique window glass, with theiroriginal rough edges, is at the most barely twelve centimetres(4f inches) in superficial diameter.The artists on glass in those earlier ages at first had onlyfour kinds of window glass at their disposal, viz., red, blue,


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HISTOEY OF GLASS PAINTING. 5yellow and white (ordinary bottle green), to which wereadded in the twelfth century green and purple glass. Allthe glass paintings of this period are characterised by thevery sparing use of black flux, and the colours by a highdegree of transparency. However, as may be gathered fromthe description given, the pictures of that era would be farmore correctly denominated mosaics than true paintings onglass.

Finally, in the fifteenth century, glass painting graduallytook a fresh turn. The manufacture of glass had made con-siderable progress ; in addition to window glass colouredthroughout there were now produced flashed glass plates,consisting of plain glass for about three-fourths of theirthickness, the remaining fourth being coloured, i.e., blownin two lamellae direct from the crucible. This glass ex-hibited numerous advantages for the painter, being clearerand softer in tone ; and being shaded with darker tints inplaces owing to the running of the molten glass exhibiteda watered appearance, a circumstance utilised by the artistto represent the folding of draperies and garments.

By means of this practice of flashing numerous tints andgradations of all the mixed colours could be obtained, from apale flesh tint to deep blue violet. The glass painter had alsothereby a new means at hand for introducing ornamental de-tails previously unknown in this branch ; he could grind outthe most .delicate decorations in the coloured cover-glass andthereby leave beautiful lustrous patches in the clear glassunderneath.

These spaces he painted and shaded with a brownishblack palette pigment, and also understood how to applyyellow colour to the reverse of the glass, thus changing theclear silver white of the background to a deep, warm gold.

This second period of the glass-painting industry was alsoits golden age. Churches, palaces, town halls, guild halls


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6 PAINTING ON GLASS AND PORCELAIN.and public and private houses were embellished with histories,weapons, emblems and decorations, all in fine colours. 1The metallic oxides came into vogue as colouring matters ;the well-known copper scale was mixed with iron scale andused for making the black flux . Bed was compoundedfrom iron slag, gold-leaf, gum, ruddle and rocaille ; blue fromcobalt, or from smalt with an addition of minium, or fromzaffre a mixture of cobalt oxide and sand used for makingsmalt with clear glass, saltpetre and minium ; green, asformerly, from copper scale. A beautiful yellow the so-called art yellow was yielded by the oxides of silver.Respecting the substances capable of colouring glass therewere then at disposal a large number of those we nowemploy for the same purpose. The introduction of thesepigments necessitated, by reason of their varied degrees offusibility, a rearrangement of the qualitative and quantitativecomposition of the fluxes, as well as, occasionally, modifica-tions in the method of their employment ; and this was parti-cularly the case when the heat requisite to enable the metallicoxide to form with the flux, the chemical compound necessaryfor the production of a particular tint, was greater than couldbe employed for firing.

In consequence of these new relations between the pro-portion of pigment, flux and foundation, the theory of theglass pigments and their fixing media was developed, andhas remained almost unaltered down to the present time.The process and appliances for firing were also consider-ably improved; the crude and inconvenient furnace of thefirst period was now replaced by a more suitable arrange-ment, a regular furnace with a hearth and ash-pit ; notinfrequently we find the furnace already provided with aniron clay-covered cupola, the draught pipe of which con-centrated the fire and kept it steady. The iron plate, without

1 Thomas Garzoni, in bis Allgemeine Xchauplatz, p. 629. Frankfurt, 1641.


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HISTORY OF GLASS PAINTING. 7sides or cover, on which aforetime the goods were exposed tothe fire, was replaced by muffles of baked earthenware orstrong sheet iron, which afforded greater protection andensured the much greater success of the operation of firing.Instead of fixing a definite duration for the firing process,applicable to all classes of work, as formerly, certain indica-tions were recognised as denoting satisfactory progress andthe completion of vitrification, whereby the operation acquireda degree of security hitherto but too often lacking.

Finally, the employment of the diamond point, a dis-covery made in this period, whereby the safe and accuratecutting of glass could be effected, must be mentioned.The third period of glass painting commenced with theseventeenth century, and this period also witnessed its totaldecline and extinction. The wars and religious convulsions ofthat century eradicated the art, the practice of which felleverywhere into desuetude owing to the absence of anydemand ; and even the art itself, with its empiric technicalsecrets, was almost totally lost. The last blow was deliveredby the extraordinary advance made in the production of pro-gressively clearer and finer plain glass, and of larger panes,especially as a result of the labours of J. Kiinkel. 1 A similarstate of desolation existed throughout the eighteenth century,the arrangement of colour in the glass paintings of that agebeing as crude and destitute of taste as the colours of thecostumes of the period. The chromatic work undertaken inglass painting was mostly confined to three primary colours :a glaring brownish yellow, an extra deep red and a violet.The art of encaustic painting on glass (actual painting)was really discovered anew in the nineteenth century. Butwhereas as we have seen the old art of glass paintinggradually developed into perfection from its own resources,the new industry exhibited a reversal of this mode by basing

J Ars Vitraria, 1689.


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8 PAINTING ON GLASS AND PORCELAIN.itself on the developed arts of porcelain and enamel painting,and transferring the technique of the former to a transparentground the translucent surface of window glass. In otherwords the glass painting of the nineteenth century is nodaughter of the older art, but an encaustic, petrified offshootfrom the art of painting in oils and water-colours, occupyingan intermediate position between them in regard to thetreatment of colours and mode of application. This closesthe history of glass painting.

Painting on earthenware was known to both the ancientEgyptians and Phoenicians, and was subsequently developedby the Arabs and Persians. Independently of these nationsthe Chinese have produced painted porcelain from time im-memorial.

The existing art of decorating earthenware with per-manent colours is chiefly due to the French chemistsCalvetat and Brongniart, who, by their incessant researchesand labours, brought the industry up to a high state ofperfection. After the renowned porcelain of Sevres, that ofDresden, Berlin and Meissen is most prized in Germany, onaccount of its beautiful lustrous and durable colours. Theporcelain ware produced in the now defunct porcelain worksof Vienna is already of priceless value to collectors by reasonof the beauty of the colours employed.

Further particulars of porcelain and enamel painting willbe given in the proper chapter, and it only now remains tosay a few words about painting on glass and porcelain.

The method of decorating the surface of certain objectsmade of glass, porcelain, enamel, etc., with such colours asonly attain their lustre and adherence by the influence ofhigh temperatures, is called encaustic painting . In thecase of glass the pigment may be applied partly to colour-less, partly to coloured transparent glass (already colouredin the frit stage).


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HISTOEY OF GLASS PAINTING. 9The whole of the pigments employed are metallic oxides

or their compounds, a few other substances capable of re-sisting fire being also used. The rule is that only suchcolours should be employed as are able to withstand theinfluence of a high temperature.

Porcelain painting differs considerably from painting onglass, in that, whereas in the latter case verifiable metalliccolours are employed, which by the action of fire are fusedonly into the surface on which they are laid and leave thesame more or less transparent in porcelain painting thecolours and the porcelain glaze, when fired, fuse and runtogether, thus permeating the whole of the glaze withcolour.

Two chief classes of pigments are differentiated in bothglass and porcelain painting, viz., the colouring matter(colouring foundation or the oxide, applied in its originalcondition by means of an earthy vehicle), and the flux.The latter serves to render the former vitreous, and (beingitself of a glassy nature) when fused gives the colours theirbrilliance and the power of adhering to the surface of theobject they decorate.

It is important to remember this difference betweencolouring matters and fluxes and to keep them separate.According to Gessert the colours requiring fluxes are pro-perly further divided into two classes, viz., glass-painters'fluxes and glass-painters' colours. The latter is the namegiven to pigments wherein the oxide, in an unaltered con-dition and merely mixed with the flux, is applied to the glass.By glass-painters' fluxes are meant those compositions inwhich the oxide and flux have been previously vitrified byfusion before application to the glass. For the sake of clearunderstanding we will adhere to this classification in thepresent treatise.

It should also be remarked that chemical combination


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10 PAINTING ON GLASS AND POBCELAIN.does not always result from the fusion of the colouringmatter with the flux. For example, in the case of the oxidesof iron and chromium the flux is probably merely a vehiclesurrounding the colouring matter and attaching it to theobject but in no wise contributing to the development of thecolour. It is somewhat different in the case of the oxides ofcopper and cobalt, which produce the colour only throughchemical combination with the flux. In this instance it isthe silicates and borates of the respective oxides that appearas colouring matters and are employed as pigments.More extended details will follow in the respective sectionsof the book : we come now, in the first place, to the objects,to be decorated.


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CHAPTEK I.THE ARTICLES TO BE PAINTED.

GLASS.RESPECTING the invention of glass by the Phoenicians littleneeds to be said, most readers being already acquainted withPliny's account of the Phoenician mariners who, on thesandy coast near the mouth of the Belus, employed (in theabsence of stones to support their cooking pots) some lumpsof natural soda they had on board, and found after the firehad been put out that the soda had become fused with sandto form glass. Moreover, Pliny himself ascribed but littleprobability to this fable, which, for chemical and technicalreasons, appears to us altogether incredible. On the otherhand the fact remains that the oldest specimens of glass, ofwhich we have any account, originated in Phoenicia andEgypt, and glass blowers are depicted in full work in thereliefs on the royal tombs at Beni Hassan, dating from about1800 B.C. So much for the history of glass. 1 We will nowproceed to make a few remarks on the manufacture of thearticle, and briefly notice its properties, so far as they relateto our subject.Glass is divided commercially into four groups accordingto its chemical composition, viz. : 1. Potash or Bohemiancrystal glass, perfectly colourless, extremely refractory andhard ; 2. Soda-lime glass, also called French glass,

1 This subject is treated of fully by Deville, Histoire de I'art


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1'2 PAINTING ON GLASS AND PORCELAIN.window glass, bluish green in colour, and somewhat harderthan the previous class ; 3. Potash-lead glass, crystal glass,soft, fusible, colourless, brilliant, and rings well ; 4. Aluminaglass, bottle glass, poor in alkali ; often contains considerableamounts of manganese and iron, and frequently magnesia inplace of potash. It is reddish yellow or dark green incolour.

In preparing glass which, for certain purposes, is moreperfect in proportion as it can resist the action of water andchemicals, only just as much potash or soda should be takenas suffices to completely fuse the silica used. On an averagethree parts of purified potash are reckoned to four parts ofquartz sand. Should the glass contain too much potash itis more readily fusible, but has so little power of resistingacid that it is often corroded and eaten right through by con-centrated sulphuric acid. Glass made from pure silica andpurified carbonate of potash is perfectly colourless andtransparent.

The mixture itself, from which the glass is made byfusion, is technically known as the charge or frit . Inthe present high state of development, of the glass-makingindustry, the greatly improved furnaces and the purer con-dition of the materials now employed, the fritting is abolished ;the glass, with few exceptions, being fused at one operation;Instead of perfectly pure silica (rock-crystal), which cannotbe used on a large scale, sand is used, quartz sand being thebest, but this must be, as far as is possible, freed fromextraneous admixtures, and especially from oxide of iron.Well-known deposits of glass sand occur at Nivelstein(near Aachen 99'97 per cent, silica), Lemgo, Namur,\\ ight, 1 and in many other places in America and Australia.

Various decolorants ( glass-makers' soaps ) are used inmaking colourless glass, and act in different ways. Thus, for

1 Isle of Wight.


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THE ARTICLES TO BE PAINTED. 13example, manganese peroxide produces an amethyst tinge bythe formation of manganese silicate, but this coloration canbe neutralised by the pale green imparted by ferrous silicate,so that the glass appears colourless. Protoxide of nickel isstill more serviceable, gradually supplanting the manganeseperoxide. Other decolorants are : zinc oxide, antimony oxide,and cobalt oxide. Those mentioned so far act physically bycomplementing the colour to form white, whereas minium,sodium nitrate, barium nitrate, and arsenious acid decoloriseby oxidation.

Respecting the colouring of glass it may be stated thatblues are produced by means of cobalt compounds (smalt andcobalt oxide) ; a clear, perfectly transparent yellow is obtainedby uranium, whilst the same metal gives a somewhat turbidyellow coloration in potash glass, with a green shimmer dueto fluorescence Copper oxide colours blue-green, but ismostly used along with chrome oxide, the yellow-green ofwhich it renders bluer. In presence of reducing agentscopper oxide is reduced to protoxide which gives a shining,blood-red colour. Silver, although producing very fine paleyellow tints, is but seldom employed for mass colours. Forthe preparation of white (so-called alabaster ) glass tinoxide is used, and the beautiful ruby glass is obtained bymeans of gold compounds. The oxides of iron, mixed invarious proportions, are capable of producing all the glasscolours.

Glass must not be regarded as a simple chemical com-pound, but rather as a variable mixture of certain silicateswhich remain homogeneous after being fused together andrapidly cooled. When heated, glass gradually passes fromthe solid to the liquid condition ; at about glowing heat itmay be bent and drawn out, and at the commencement ofred heat may be blown out by a current of air and spun outinto the finest fibre. At bright red heat it is inclined to flow


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14 PAINTING ON GLASS AND PORCELAIN.and becomes thenceforward more and more fluid, but evenat white heat retains the consistency of a thin syrup. Itsbehaviour in this respect differs considerably with alterationsof composition ; whereas silica makes it more refractory, leadoxide produces the opposite effect and boracic acid and fluor-ine also render it more fusible. When glass is kept for sometime at the temperature at which it softens, devitrificationsets in, the result being the production of an opaque, crystal-line, stony, very hard and but slightly brittle mass. If theinterior of a piece of glass be exposed by grinding, etc., it isless able to resist the action of chemical reagents than thenatural surface of the glass as produced by heat. Boilingwater, when the exposure is prolonged, gradually attacksglass to a greater or less extent.

Glass plates that have been stored for a long time indamp places mostly become coated over with an alkalineliquid, which constantly renews when wiped away ; the glassin time loses its lustre and becomes blind (opaque); anumber of small scales separate from the surface, or thelatter becomes covered with a tender, highly iridescent skin.

Blind glass can be made clear again by washing withpotash lye or hydrofluoric acid.

This subject is treated more exhaustively in K. Gerner'swork on Glass Making (Die Glas-fabrikation), and W. Merten'sGlass Making and Refining (Die Fabrikation und Raffinirung desGlases).

PORCELAIN.As far back as the year 442 A.D. porcelain was generallyknown and used throughout China ; but it was only in the

fourteenth century that the Portuguese introduced it intoEurope. It is supposed to derive its name from them, butso many versions are current of the origin of the word thatwe prefer not to pin our faith to this one. Many were the


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THE AETICLES TO BE PAINTED. 15attempts to imitate porcelain, all of which failed untilJohann Friedrich Bottger, 1 originally an apothecary's assis-tant and afterwards a so-called gold-maker, succeededby accident. The first successful experiments were madein 1703 with a red clay mass, called red porcelain ; but whenin 1709 Schnorr, a smith, discovered, also by accident, a de-posit of kaolin near Aue (Saxony), Bottger succeeded in pro-ducing white porcelain as well. The Government of Saxonydid everything possible to keep the process of making porce-lain secret, but unsuccessfully, although the factory at Al-brechtsburg was treated like a veritable fortress and thedrawbridge let down only by night. The secret of themanufacture spread with a rapidity surprising for that age.For instance, a porcelain factory 'was set up in Vienna in1710, and in the following year white porcelain was offeredfor sale at the Leipzig fair ; works were started in 1756 atBerlin ; in 1757 at Drankenthal ; in 1758 in Thuringia, andshortly afterwards in Bohemia.

Porcelain is extremely hard, with a fine, dull fracture ; istranslucent in thin Jayers, and withstands rapid changes oftemperature, especially when unglazed. The glaze is sointimately incorporated with the mass that no border linebetween them can be detected. The characteristic of porce-lain is that it consists of a substance which, at the highestattainable furnace temperatures, fuses and envelops theextremely fine particles of infusible porcelain earth. Thechief constituent of porcelain is the so-called porcelain earth(kaolin), a pure clay (composed solely of alumina and silica,mostly resulting from the weathering of felspar) which con-tains no or but very little lime, and is infusible at thefiercest furnace heat. Kaolin or porcelain earth differs inphysical properties from ordinary clay mainly in its inability

1 Born in Schleiz (Voigtland) in 1682 and died at the age of thirty-fiveyears.


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16 PAINTING ON GLASS AND PORCELAIN.to retain water and form a plastic, tough paste. In itsnatural, unwashed condition, kaolin falls apart in waterto a fine powder like fuller's earth.

After the kaolin has been broken down by passing itbetween rollers, and well washed, mixed with silica, quartzor sand, and gypsum or felspar, and also roasted, calcined,ground and sifted, the materials are made into a thin gruelwith water and mixed with the powder obtained by grindingfragments of white porcelain, the whole being then inti-mately mixed and ground, kneaded, cut up and kneaded overagain. The manufacture of the ware on the wheel or inmoulds, by pressing or beating, is then proceeded with, theoutline being made sharper, after drying, by tooling in thelathe and polishing by means of small plates of horn andivory. Hollow, human or animal figures are pressed intoseparate moulds for the two halves (lengthwise) of the figure,the halves being joined after removal from the moulds, andthe traces of the junction obliterated and smoothed down.

The air-dried ware is enclosed, for protection from smokeand soot, in refractory capsules, in which it is supported atthe bottom and sides by colombines and pernettes to preventit from setting crooked or sticking in the baking, and is firedin a kiln made of fire-brick, frequently cylindrical in shape,and divided either longitudinally or transversely into threesections. When the capsules with the contained ware areproperly inserted, and the fire gradually raised to a maximum,the progress of the firing is observed by taking out test-potswhich are examined in the light outside the kiln. Onlywhen the kiln has been suitably cooled down, a processrequiring several days, are the capsules containing the waretaken out. In this condition the porcelain is termed biscuit . Now comes the glazing. The glaze is preparedfrom the same materials as the porcelain itself, only that itis rendered more fusible by the employment of a larger


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THE ARTICLES TO BE PAINTED. 17proportion of gypsum, felspar or porcelain sherds. Againenclosed in capsules and placed in the kiln, the glaze isfused on to the ware, test-pots being also used in this case toenable the completion of the operation to be determined.The ware on removal from the cooled kiln is classifiedinto fine, medium, outshots (defective but still usable) and wasters (unusable and destined to be broken up again).A good deal of ware comes out of the kiln possessed ofvarious defects which may be remedied by placing thethicker articles in a hotter situation and the thinnerarticles in less highly heated positions, or by employingvarious fusible or refractory mixtures, the former for thickand the latter for thin goods.

In firing the ware decreases in size by about one-seventhin circumference, so that this shrinkage must be taken intoconsideration in making articles intended to have a definitesize when finished.

ENAMEL.It is difficult to define strictlythe difference between enamel

and glass. Under the former term is, however, understooda fusible opaque or transparent glass flux (often coloured bymeans of metallic oxides), which is mostly employed forcoating metallic objects. In the case of transparent enamelsall the constituents are completely fused, whereas opaqueenamel is rendered so by the milky turbidity resulting fromthe admixture of bodies that are not readily fusible (chieflyoxide of tin).Enamel is employed for two separate purposes, one beingthe decoration of articles de luxe and the other the pro-duction of a protective covering to metallic appliances fordomestic or technical use. We are only concerned with theformer.

B. Bucher of Vienna describes the enamelling of metallic2


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18 PAINTING ON GLASS AND PORCELAIN.objects as follows : The chief component of the majority ofenamels is a glass rendered fusible by a large admixture oflead oxide and occasionally by borax or some other flux, sothat, without becoming too thinly fluid, it forms a smooth sur-face at a moderate red heat. In enamelling the mass shouldnot actually become perfectly fluid but should assume a pastyconsistency, so that the enamel powder on the surface of themetal unites to form a cohesive covering which, on cooling,will look as though it had really been liquefied. When ametallic surface is to be only partly coated with enamel aboundary is formed by soldering metal wire or by depressingthe surface in any convenient manner for the reception ofthe enamel (cloisonne enamelling). In order to facilitate theadherence of the enamel to the metal the surface of the latteris often covered by a network of cross-hatchings or workedup as rough as possible. The metal is then boiled in potashlye, rinsed with weak acid, washed with water and coveredwith a thick layer of moist enamel previously ground to agranular powder. It is then dried in the air, heated overglowing coal until the evolution of fumes ceases, and then,without being cooled, transferred to the strongly heatedmuffle of the enamelling furnace. As soon as the enamel isevenly fused the article is carefully (so that it can only coolslowly) removed from the muffle and washed with very dilutenitric acid and cold water, after which it is covered with afresh layer of enamel powder and again fused. When thethird coating has been affixed in the same way, the surfaceparticularly when large and flat is rubbed over with wetsandstone and the article returned to the furnace in order toproduce the requisite smoothness. The surface may then bepainted, and when this is dry the colour is fixed by a finalfiring in the muffle.

Now-a-days almost any colour can be imparted to theenamel mass. White and thereby always opaque enamel


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THE AETICLES TO BE PAINTED. 19is obtained by using oxide of tin, and if other colours aremixed with white enamel the opacity remains. Blue is pro-duced by cobalt oxide ; white enamel gives with this oxideforget-me-not blue. The various yellow colours are producedby silver chloride, lead antimoniate and zinc oxide, or byzincite, antimony oxide and specular iron ore, or by ferricoxide or uranium oxide. Green is obtained by chromic oxideor cupric oxide, nickel oxide, copper antimoniate, or cobaltantimoniate.The finest red colours are obtained by means of goldpurple, whereas cuprous oxide, ferric oxide or silver sulphidewill produce a cheaper but less handsome red. Manganeseoxide gives violet ; the same oxide, with chromic or ferricoxide, brown ; and a mixture of violet, dark blue and greenproduces black. The intensity of each colour naturallydepends on the amount of colouring matter employed, andthe most diversified tints can be prepared by altering theproportions of the mixtures. Fuller information on thepreparation and use of enamel for artistic and technicalpurposes will be found in P. Eandau's work on the subject(Die Fabrikation der Emaille und, das Emailliren, 2nd edition).

STONEWARE.Stoneware is not very often painted, and in the descrip-

tion of this material we will therefore confine ourselves tothe most important particulars.

This ware was invented by Josiah Wedgwood about themiddle of the eighteenth century. It is. a hard, densematerial (so much so that fire may be struck from it), and canendure remarkable alterations of temperature without crack-ing. It is prepared from white clay (or rather from clay thatburns white) and ground flint-stones intimately mixed to-gether and fired not merely burned hard, but caused to


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20 PAINTING ON GLASS AND PORCELAIN.winter (commencing to fuse) by intense heat on whichaccount it exhibits a clean glassy fracture. Stoneware ismade of various colours, chiefly white, yellow and black ;and blue or green ware is not infrequently met with. Theunglazed stoneware, with a dull alabaster-like surface, istermed biscuit.

Moulding and firing are performed in the same manneras for porcelain, and the method of painting and firing thecolours is also the same.

FAIENCE.This class of ware is named after Faenza in Italy, whence

it was for a long time obtained, and is made of a finerwashed clay than that used for ordinary earthenware. Claythat will burn white or yellowish in colour is selected, andfaience pottery is prepared with great care and accuracy,with a very lustrous glaze, and is generally decorated withfine paintings, or with pictures applied by the transferprocess.The washed clay is mixed with sand or powdered ala-baster in the proportions ascertained as most suitable byexperimental firing, and prepared in the best possible mannerby treading, kneading, scraping, etc. ; and is then made upinto ware on the wheel or by moulding. Goods made on thewheel are, however, when air-dried, tooled again in a latheto ensure greater accuracy, and then fired in a faience kiln,which is generally divided into three superimposed sections,communicating with each other by perforated partitions.In the two upper divisions the pottery is fired, the lowerserving as the furnace. The ware is enclosed for firing inrefractory fire-clay cassettes or capsules, and is first half-finished without glaze. A coating of the latter is thenapplied by dipping the ware into the finely ground glazestirred up in water, and fired until complete, the progress of


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THE AETICLES TO BE PAINTED. 21the operation being watched by taking sample pots throughsuitable apertures in the kiln, in order to see whether thegoods are properly fired and the glaze well fused. Aftercooling down, follows painting (see porcelain painting), orprinting with pictures.

Those interested in the decoration of porcelain or otherearthenware with pigments, require to study the specialproperties of the various classes of ceramic ware, and areadvised to refer to Ludwig Wipplinger's work on ceramics(Die Kcramik).


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CHAPTEK II.PIGMENTS.

THE pigments employed for painting on glass and earthen-ware may be grouped in three classes :

1. Metallic oxides and salts.2. Earths or earthy bodies.3. Metals.The metallic oxides and salts are the actual verifiable

pigments, whereas the earths (ochre Terra di Sienna) areeither coloured by metallic oxides or are white ; they aregenerally termed coaters, since they are opaque and do notassume a glassy appearance until covered by a coating ofglaze. The metals are only used in the metallic conditionand in an extremely fine state of division for metallicdecorations or lustre.

I. METALLIC PIGMENTS.The coloration of glass masses and earthenware is mostly

effected by means of metallic oxides which are either pre-viously fused, or simply mixed, with soft fusible glass theflux or binding material. The fluxes are soft fusible glasseswhich become fluid at moderate temperatures and cause thecolouring matter, pigment, or oxide to adhere to and combinewith the glass.

ANTIMONY OXIDE.The pigment employed under this name in glass painting

is altogether wrongly designated, since it is really the bi-


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PIGMENTS. 23(acid) antimoniate of potash. Metallic antimony is thefundamental substance employed in the production of thispreparation. It is tin-white in colour, very lustrous, andexhibits a radially laminated structure ; is hard, but verybrittle, and easily reduced to fine powder. The specificgravity is 6'702-6'860 ; it glows and fuses at 700 C., andsublimes when raised to white heat in closed vessels.

If metallic antimony be calcined along with six parts ofsaltpetre, and the mass maintained at strong red heat forsome time, the acid antimoniate of potash our wronglynamed antimony oxide is produced. The product ispowdered and extracted several times with cold water (inorder to remove the wholly or partly decomposed saltpetre) ;the resulting white powder is then boiled with water for anhour and the solution filtered. It does not become turbidwhen cooled.

The insoluble residue is impure biantimoniate of potash,and must be re-washed with boiling water. On the otherhand the solution contains only potassium antimoniate,which, however, can be converted into the pure acid salt bypassing a current of carbonic acid gas through the liquid.The resulting salt forms a white powder, insoluble in water.

This substance cannot by itself be used for the productionof a yellow pigment, but when mixed with zinc oxide andferric oxide in suitable proportions it gives very fine yellows,from the palest sulphur to reddish orange tints. The lattershade is, however, obtained more effectively with lead chro-mate or uranium oxide.

NAPLES YELLOW.Many glass painters replace antimony oxide, for cer-

tain purposes, by commercial Naples yellow (lead antimoni-ate), which can be ground along with the flux and used


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24 PAINTING ON GLASS AND POBCELAIN.direct. The commercial substance is, however, not alwaysof constant composition, the quantitative ratio of its compo-nents varying to such an extent that its employment shouldbe decidedly discountenanced. Some Naples yellow will fuseat red heat, whilst others require three times the quantity oflead flux before being usable. Since certain definite shadesof colour are required, it is evident that the necessary testingof the fusibility of this substance causes considerable loss oftime.

One part of antimony oxide and three parts of purifiedlead oxide or minium will give a very fine pale yellow ; thefirst named used alone produces a yellowish white. InBrunner's method one part of perfectly pure tartar emetic ismixed very intimately (by grinding together) with two partsof lead nitrate and four parts of common salt, the mixturefused in a Hessian crucible at a moderate red heat, and thefluid mass poured on to a cold iron plate. When cold themass is extracted with boiling water, which leaves the leadantimoniate behind as a more or less dark yellow powder.

It is, however, not at all an easy matter to attain thisfavourable result with certainty in all cases. If a certain de-gree of heat be exceeded, though merely by a little, a hard,solid mass results, which cannot be brought to fine powderby boiling ever so long, but remains as a granular mass ofinferior brightness of colour.

Even when the operation succeeds, the shade of the colouroften varies, so that at one time a sulphur yellow and atanother an orange yellow product is obtained. As a rule, itmay be assumed that with lower temperatures the productswill be lighter in colour, and darker (with a red tinge, so thatthey may be classified as orange) when greater heat has beenused.

According to another recipe, the mixture consists of twoparts of tartar emetic, four parts of lead nitrate and eight


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PIGMENTS. 25parts of common salt. If the fused mass be treated for a longwhile with very dilute hydrochloric acid, a certain amount oflead oxide is removed, whereby a product of greater brilliancyis obtained. Great care is, however, necessary in carryingout the hydrochloric acid treatment, since, by the use of acidthat is too strong, the whole product can easily be renderedvalueless for the purpose in view.

Naples yellow may be prepared, by the so-called Parisianmethod, as follows : Metallic antimony is oxidised by fusingin air, and to each twelve parts of antimony are taken eightparts of minium and four parts of tin oxide, and the wholefused together at a moderate red heat.

BARIUM CHROMATE.Barium chrornate is not often employed. Its chief uses

are for porcelain and faience painting, and it gives very fineyellow colours. The preparation is not now met with incommerce, but is not difficult to make. To this endbarium chloride is prepared by dissolving natural barium car-bonate (Witherite) in hydrochloric acid to a saturated solu-tion, which is filtered and slowly evaporated at a moderateheat, whereupon octahedral tabular crystals separate out andare from time to time freed from the mother liquor by pres-sure.

An aqueous solution is made of this salt, and, if acid, isneutralised by the addition of sodium carbonate until a pre-cipitate commences to form. A solution of neutral (yellow)potassium chromate is added so long as any precipitate comesdown, and the latter is then washed and dried.

The resulting preparation is a canary yellow salt insolublein water, but completely soluble in nitric acid. It contains59'88 per cent, of barium, and 40'12 per cent, of chromicacid.


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26 PAINTING ON GLASS AND PORCELAIN.LEAD CHEOMATE.

Lead chromate, commercially known as chrome yel-low, does not generally consist of pure chromium oxide orlead chromate, but mostly contains admixtures of leadsulphate, lead chloride, or gypsum (sulphate of lime), addedto tone down the colour, although the fusion of the com-pound is thereby rendered more difficult.

Even if the preparation is obtained in a pure state thevariable composition of the different commercial grades is.too great to allow the article to be used with any degree ofsecurity.

The renowned former director of the Sevres' porcelainworks, A. Brongniart, gives the subjoined analyses of severalcommercial chrome yellows :

ANALYSES.Chrome Yellow.


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PIGMENTS. 27The lead chromate comes down as a beautiful dark yel-

low powder ; neutral (yellow) potassium chromate gives anorange yellow, whilst acid (red) potassium chromate gives ayellowish red or dark red precipitate.Some people assert that chrome yellow requires to befused before being employed as a pigment, but this is alto-gether superfluous.

In order to produce the finest chrome yellow, which shallleave nothing to be desired in point of beauty of tint, thefollowing method should be adopted : Lead acetate is dis-solved in water, and the solution diluted with an equalvolume of water ; then mixed with vigorous stirring with asimilarly diluted solution of neutral or acid potassium chro-mate. The precipitate immediately produced rapidly subsideson account of its great weight, and must be washed withpure water as long as any soluble matter is taken up, and isthen spread on cloths and dried in the air.The finest product is obtained by working with thefollowing proportions :

Lead acetate solution ... 100Potassium chromate solution - - 50 (red bichromate), or

- - 40 (yellow ).

SILVEE CHLORIDE.Although silver chloride is frequently used in glass paint-

ing for the production of fine, transparent yellows, it is lessextensively employed in painting on porcelain, and that onlyfor purple and carmine not for any other colour.

Silver chloride is prepared as follows : Pure metallicsilver is dissolved in nitric acid by the aid of warmth. Inorder to avoid having an excess of acid, rolled silver is addedto the acid until no more is dissolved, i.e., the solution issaturated. The succeeding operations are best carried on in


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1>S PAINTING ON GLASS AND PORCELAIN.a dark room to prevent the silver turning black. The silvernitrate solution, after being suitably diluted with water, iswell shaken up and placed in a large glass flask, and to it isadded hydrochloric acid until precipitation ceases. After afew hours' rest the water is decanted from the precipitateinto another flask in order to give the silver salt, still insuspension, time to subside. The curdy precipitate in thefirst flask is washed five or six times with fresh, clean water ;the washing to be continued so long as a white precipitate(indicative of hydrochloric acid) is produced on the additionof silver nitrate to the washings, or a red brown precipitate(copper ferricyanide) with potassium ferricyanide. 1When the water ceases to become turbid the silverchloride is dried by being enclosed in doubled dark blottingpaper and dried on a flat plate in a warm oven ; when dry itis stored in black glass bottles. If this precaution be neg-lected, the silver chloride will turn black under the influenceof light, and it will be impossible to obtain perfectly pureyellow or fine carmine and purple therefrom.

Silver nitrate (so-called Lapis infernalis) being now obtain-able in a very pure state commercially, it is better to employthis preparation when making silver chloride.The nitrate is dissolved in distilled water and dilutehydrochloric acid is added (by artificial light) so long as a pre-cipitate forms ; the latter is then repeatedly washed withwater and finally dried in the dark.

Silver chloride is an insoluble white powder, at firstcaseous and balky, but on settling collects to a heavy snow-white mass ; it is soluble in hydrochloric acid, particularly inconcentrated acid, so that an excess of hydrochloric acid

1 Copper, even though in small quantities, in the silver chloride exerts ahighly prejudicial influence on the colours, and also contaminates them. Itis also strongly advisable to employ distilled water in order to avoid theintroduction of lime into the preparation.


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PIGMENTS. 29must be avoided in precipitating. When heated, silverchloride first turns rose red, then fuses to yellow, but turnsto white on cooling, and when cold may be cut like horn,from which circumstance it is known as horn silver. Thepercentage composition is : Chlorine 24*67, silver 75'33.

CHEOMIC OXIDE.Chromium (from xpw/jLa, colour), so called from the pro-

perty inherent in its oxides of forming pigmentary compounds,was discovered, almost simultaneously, by Vauquelin andKlaproth in the year 1793, and was first prepared from redSiberian cerussite. The metal is whitish grey, fairly lustrousand of medium granular structure, very brittle and breakingunder the lightest blow.

Chromic oxide forms an important constituent of chromeiron ore. The pure oxide is of a fine dark grey colour, theshade being, however, dependent on the method of prepara-tion. The oxide, heated to bright redness, is crystalline andvery dark coloured, and in this condition is impervious tothe action of acids, but alkali in a state of fusion converts itinto alkali chromate, especially in presence of saltpetre.The various methods of preparing this are subjoined :

1. Potassium Bichromate and Sulphur Method.The cheapest way to produce chromic oxide is by heating

potassium bichromate and sulphur to redness, extracting themass with very dilute sulphuric acid and washing theresidue. The sulphur reduces the chromic acid ; sulphurdioxide is evolved on the application of sulphuric acid to thehot mass, and the sulphide and sulphate of potassium pass intosolution, leaving pure chromic oxide behind. The larger theamount of sulphur employed the paler will be the colour ofthe resulting product. The purity of the potassium bi-


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30 PAINTING ON GLASS AND PORCELAIN.ehromate used exerts particular influence on the fineness ofcolour of the chromic oxide produced by this method. Shouldthe former contain iron in quantity the product will alwaysbe off-coloured and never of fine quality.

It is advisable to take nineteen parts of potassium bi-chromate and four parts of sulphur ; these will produce 9'33parts of chromic oxide. If no bichromate free from iron canbe had, the colour of the product can be somewhat improvedby treatment with dilute hydrochloric acid, wherein ferricoxide is more readily soluble than chromic oxide, the latterbeing especially when heated to redness soluble only withdifficulty.

There are many prescriptions given for the preparation ofchromic oxide by the aid of sulphur, but in all of them theabove-mentioned rule applies : the more sulphur used thepaler the product obtained.

According to A. Casali, a chrome green, fulfilling allrequirements, may be prepared by heating to bright rednessa mixture of one part of potassium bichromate and threeparts of calcined gypsum, the mass being subsequentlyextracted by boiling with very dilute hydrochloric acid. Thereaction is expressed by the following equation :

7 + 2CaS04 = 2Cr2 3 + 21^804 + 2CaO + 302The lime is dissolved by boiling with hydrochloric acid,

and when, after prolonged boiling, the liquid exhibits adecided acid reaction, it is poured off and the residual chro-mic oxide washed with hot water and dried.

The potassium bichromate should be finely powdered andmixed with one half its weight of powdered sulphur. Thismixture is placed in crucibles or capsules, and heated to palered heat in a blast furnace. The spongy green mass remain-ing in the crucible when cold consists of chromic oxidemixed with potassium sulphate, which latter must be re-


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PIGMENTS. 31moved a tedious operation owing to the sparing solubilityof this salt in water. It may be effected much quicker bythe addition of a little sulphuric or hydrochloric acid. Whencompletely purified the pigment is filtered on linen filtersand dried on boards. One hundred parts of potassium bi-chromate and fifty parts of sulphur will yield about sixty-eight parts of chromic oxide, provided the pure salt has beenused ; should it, however, as occasionally happens, contain adeal of potassium sulphate the result will not be so good.

Brongniart recommends a proportion of one part ofpotassium bichromate and two of sulphur, but this al-ways results in the production of a larger amount of sul-phate, and concurrently sulphide, of potassium. Theresulting chromic oxide must, for the purposes of theglass and porcelain painting industry, be re-heated toredness, whereby the oxide on attaining a certain tem-perature parts with its water and immediately in a fewseconds takes fire, whereupon the temperature againfalls, without any loss being occasioned.

2. Ammonium Chromate Method.The chromate is heated very gradually, and when a

certain temperature is attained the salt glows and im-mediately changes into a dark-green, almost black, mass,very similar in appearance to rolled-up tea leaves. This,when extracted and ground, yields a fine green, the qualitybeing in inverse ratio to the temperature employed fordecomposition.

3. Wet Method.Chromic oxide may be prepared in the wet way, but the

product cannot be compared in point of fineness of colourwith that from dry methods. When a solution of chromealum is mixed with a solution of soda a grey-green pre-


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3'2 PAINTING ON GLASS AND PORCELAIN.cipitate of hydrated oxide is formed, which, when washedand heated to redness, leaves pure chromic oxide behind.

In a similar fashion chromic oxide may be obtained bymixing potassium bichromate solution with hydrochloricacid and continuing to add small quantities of alcohol solong as any reaction results and the green colour of theliquid progressively deepens. A solution of chromiumchloride is produced from which the hydrated oxide maybe thrown down by soda as before.

According to Brongniart this gelatinous precipitate canbe advantageously employed in the unroasted condition forthe production of the bluish-green pigments obtained bymixing chromic oxide and cobalt oxide. The moist hy-drated oxide is mixed on a glass plate with moist cobaltoxide and then dried and strongly heated. Very fine, purebluish-green colours are thus obtained, the shade varyingwith the proportion of cobalt oxide employed.

4. Mercurous Nitrate and Potassium Chromatc Method.A very fine chromic oxide can be prepared by this method,

but it should be remarked that the process is rather expen-sive and not unattended with danger.

In the first place mercurous nitrate is prepared by dilutingpure nitric acid with three to four times its weight of waterand immersing mercury in the mixture. Solution goes ongradually and is attended with effervescence, which sub-sequently decreases as the greater part of the free acid be-comes saturated with mercurous oxide. Pointed needlesthen begin to form in the liquid and may be separated bypouring off from the undissolved mercury and dried by verymoderate warming. The crystals consist of

Mercurous oxide - - - - - 74-29Nitric acid 19-29Water - , . . . 6'42

100-00


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PIGMENTS. 33The mercurous nitrate is now dissolved in water to which

has been added a little nitric acid in order to prevent thedecomposition of the oxide to a basic or acid salt, and anaddition of successive portions of a dilute solution of potas-sium chromate is then made. The resulting red, flocculentprecipitate is purified by a large quantity of water, thendried and finally calcined in a crucible, whereby the mer-cury is volatilised and the chromic acid reduced to chromicoxide which possesses a delicate green shade of colour.

The chief condition necessary for the success of the pre-paration is the careful washing of the precipitate with boilingwater until the washings run off quite pure.

Excess of mercurous nitrate should always be avoided,since it greatly increases the cost without improving thecolour. The best effect is obtained when the potassium bi-chromate is in excess. Delong demonstrates that the bi-chromate in excess exerts a favourable influence on thecolour by increasing the fineness of division of the chromicoxide. 1

Another method of preparation is by heating a mixtureof potassium bichromate and ammonium chloride (sal am-moniac) and extracting the mass with water.

According to Jean 2 the waste chrome alum from the ani-line green and aniline violet works is now used for preparingchromic oxide. To this end one part of chrome alum isstrongly calcined with three parts of carbon ; sulphurousacid is evolved and a mixture of potassium sulphate andchromic oxide remains behind, and can be separated bymeans of water.

1 It is worthy of note that silica freshly precipitated from water-glasssolution forms, when mixed with chromic acid, a rose-red compound, in-soluble in water and undergoing no change at the temperature of theporcelain kiln.

2 Comptes rendus, vol. Ixviii., p. 198.3


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34 PAINTING ON GLASS AND PORCELAIN.Chromic oxide has an extraordinary power of withstand-

ing heat, and can therefore be employed both in glass andporcelain painting for entire groundworks.A fine, brilliant chrome green cannot be prepared withcertainty unless the potassium bichromate be quite freefrom iron. Even a very small proportion of this body inthe salt exerts an unfavourable action on the brilliancy ofthe colour. Direct experiments have shown that the bi-chromate may be freed from iron by recrystallisation,without any particular difficulty. The best way to effectthis is by making a saturated solution of bichromate inboiling water, and, after filtering boiling hot, cooling downthe solution as quickly as possible, with continual stirring.The fine crystalline meal thus obtained is placed in a funneland left until all the liquid has drained off, and is thenrinsed with a little cold water to drive out the motherliquor. By this simple operation an extremely pure saltis obtained and will yield a fine shade of chrome green.

IRON OXIDE (FERRIC OXIDE).Ferric oxide occurs in nature as red ironstone (haematite),

specular iron ore, brown haematite, martite and glim-mer, as well as in small quantities in many other mineralswhich frequently owe their colour to this oxide. It occursin many varieties of haematite forming a valuable iron ore ;the specific gravity of the pure natural ferric oxide varies be-tween 519 and 5'23. The natural oxides are not, however,suitable for direct employment for painting ; their composi-tion is, for the most part, very variable, and, as their fusi-bility corresponds, they are unsuitable for use.

Pure ferric oxide may be prepared artificially as anamorphous red powder, by calcining the hydrated oxide or


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PIGMENTS. 35the oxalate of iron ; by heating anhydrous ferrous sulphatewith common salt, or the amorphous oxide with ammoniumchloride and extracting the residue with water, it remainsbehind as black (frequently magnetic) scales which, when re-duced in a current of hydrogen, yield the metal in the sameform. When ferrous or ferric sulphate is strongly calcined aresidue is left consisting chiefly of ferric oxide, but alsocontaining appreciable traces of sulphuric acid. This isthe chief constituent of the well-known capvt mortuum .No other preparation yields so many shades of colour asferric oxide, since it produces orange red, blood red, flesh red,carmine red, lake red, violet red, brown red, red brown andblack, and that, too, from the pure oxide alone, the modifica-tions being entirely dependent on the stronger or fainter,longer or shorter action of fire. It is manifest that noordinary care and skill are requisite to hit upon the correctmethod of producing each shade exact to standard. More-over, the shade may be affected in the highest degree by thequality and suitability of the preparation itself, as also by theuse of a warm or cold concentrated or dilute solution and bythe means employed for solution and precipitation.

Although ferric oxide may be used in all instances forglass painting, and is frequently employed for yellow, red,brown and black colours in equal proportion, this is not thecase in porcelain, faience and stoneware painting. The pre-paration is also used here for the same colours but can onlyendure the same heat as the muffle colours, whereas in thehigh temperature of the porcelain kiln the colour would beentirely or partly destroyed, since ferric oxide combines withthe silica of the felspar to form the almost colourless com-pound ferrous silicate.

There are two methods of preparing ferric oxide for glassand earthenware painting, differing according to the objectin view ; for all colours except red the ferric oxide from com-


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36 PAINTING ON GLASS AND PORCELAIN.inercial ferrous sulphate is used, but for the latter colour itis better to prepare the iron salt oneself.

1. Preparation of Ferric Oxide from Ferrous Sulphate.The oxide for yellow-brown colours for glass painting is

prepared as follows : Pure ferrous sulphate (green vitriol) isdissolved in hot water and poured into a porcelain evaporat-ing basin, which is then covered with a thin cloth to keep outdust, dirt, etc. After fifteen to twenty days, during whichtime the liquid has been stirred twice a day with a glass rod,the water is poured off and the residue used. This latteractually consists of basic ferric sulphate, the name ferricoxide being therefore only a technical term.

Another way of preparing ferric oxide is by dissolving twoparts of pure ferrous sulphate in five parts of distilled waterin a porcelain basin, and, when solution is complete, addingthereto three parts of iron filings previously cleansed by hotwater. The whole is left to stand for four hours, with inter-vals of stirring ; then filtered through paper and transferredto a perfectly clean iron pan, where it is evaporated over aslow fire down to about one quarter its original volume.The whole is then placed in a crystallising basin and storedin a cool place. The liquid, originally very turbid, becomespaler in colour after a few days, and finally crystals areformed which are removed and dried on blotting-paper.They are then carefully calcined and finely ground, andlastly heated to redness in a wide pan which is set in thefire and kept there until the powder has been exposed to adark, brown-red heat for a quarter of an hour, the powderhaving been kept stirred with a spatula. (The operatormust take care to protect himself from inhaling the acidvapours.) To ascertain if the operation has succeeded, alittle of the powder is taken out of the crucible and thrown


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PIGMENTS. 37into a porcelain basin, the bottom of which is moistenedwith cold water ; should the powder show a yellow colora-tion the preparation is only half converted into oxide, but. ifit produces a fine red, without any tinge of yellow, then theprocess is complete. This heating is the most difficult partof the operation, for whereas, on the one hand, an in-sufficiently-roasted preparation will never produce a finecolour and quite one-half will be wasted in lixiviation, onthe other hand over-roasting results in very dark and dullcolours.

Ferric oxide may also be prepared by using ferric sul-phate and potassium sulphate, the procedure being asabove, except that the precipitate must be heated to rednessimmediately ; the method is, however, not recommended.

Ferric oxide for black can be simply prepared as follows :Pure ferrous sulphate is finely crushed and exposed for be-tween fourteen days and three weeks to the heat of the sunin summer or stove in winter. The water of crystallisationthus evaporates spontaneously and the residual powder canbe at once placed in a crucible and exposed to a red heat.After cooling, the product is ground to a pasty mass with oldolive oil and again heated to redness, whereupon the mixturequickly takes fire, the oil burns away and a deep black pow-der is left, which is ground down fine in water and stored foruse. Should the powder not attain perfect blackness, theabove operation with olive oil and burning is repeated.The occasion is suitable for a few remarks on ferrous sul-phate itself. This substance is rarely met with in a purestate in commerce, the ordinary impurities present beingmagnesium sulphate, manganous oxide and zinc oxide. Asthese impurities cannot be detected by external appearances,it becomes essential to determine at any rate the amount offerrous sulphate in the substance, the impurities havingmostly a secondary influence only. To determine the con-


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38 PAINTING ON GLASS AND PORCELAIN.tent of iron fairly accurately, without much trouble, Genteleadvises weighing twenty-five grams of the sulphate into atared porcelain basin, which is then warmed slowly over agas or spirit flame to drive off the water and afterwards keptat a good red heat for about half an hoar. The ferrous sul-phate is thereby decomposed into sulphuric and sulphurousacids, which are evolved, and ferric oxide, which remains be-hind in the crucible. Should the residue weigh more thanseven and a half grams, it may confidently be assumed thatit is impure, and that in addition to ferric oxide there are pre-sent sulphates (such as those of the metals referred to above)undecomposable at the temperature employed. The redmass is then moistened, filtered through a tared filter,washed, dried and weighed as red ferric oxide. Thirty-nineparts are the exact equivalent of 138 parts of ferrous sulphate,and the percentage of pure ferrous sulphate in the originalsubstance can easily be calculated. For instance, if theresidue weighs 25 the amount was 39 : 138 = 25 : x, x = 88'5per cent.

Good ferrous sulphate is pale green, bluish green notyellow green in appearance, and is readily soluble both incold and hot water.

2. Feme Oxide for Fine (Red] Colours.It is advisable to prepare the ferrous sulphate for colour-

making oneself, and have it perfectly pure. To this end afew kilograms of ordinary shoe nails, which are made of verypure iron, are placed in a large bottle and suffused with dilutesulphuric acid (one part acid to ten parts water), small addi-tions of acid being made from time to time until merely asmall residue of iron remains undissolved. The solution isfiltered and evaporated down at moderate heat, to about one-quarter the original volume, in a porcelain basin containing a


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PIGMENTS. 39few more nails. On cooling, crystals of pure ferrous sulphatewill separate out, and, the liquid (which on further concentra-tion will yield another crop of crystals) being poured off,may be dried on bricks and stored in properly closed glassreceptacles.

The preparation so obtained is employed for the produc-tion of ferric oxide by being crushed and slowly calcined byexposure to moderate warmth for a long time, so that thecrystals do not melt. When all the water of crystallisationhas passed off, the residual white mass is ground to very finepowder ; the finer the powder the better the result. Thispowder is now spread out regularly, and not too thickly, overthe bottom of a wide, flat porcelain capsule, and then heatedto redness in a muffle as slowly as possible. The colour as-sumed by the oxide must be scanned very closely, and whenthe desired shade is attained the fire is drawn and the muffleallowed to cool down gradually. The testing of the colour iseffected by taking small samples from the muffle at shortintervals by means of an iron spatula.When the ferric oxide is thoroughly cooled down it iswashed several times with boiling water and dried. Duringcalcination the oxide becomes first reddish yellow, then redderand redder, the yellow tinge disappearing progressively, untilat last the colour is violet red.

A very useful ferric oxide pigment may be produced inthe following manner : A solution of seventeen parts ofsodium carbonate in sixty-eight parts of water is preparedand raised to boiling in an iron pan, and stirred whilst tenparts of crystallised ferrous sulphate are added, a little at atime. The boiling and stirring are continued until the fer-rous sulphate is completely dissolved, and the resultinggreenish-white precipitate is then left to subside. This pre-cipitate, consisting of ferrous carbonate, is washed severaltimes with water and then spread out in thin layers exposed


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40 PAINTING ON GLASS AND PORCELAIN.to the air. Already, during the washing process, the pre-cipitate will have begun to turn yellow, and after a very shorttime in the air will become converted into ochre (ferrichydrate).

The product, when dried and heated to redness, yields afine red powder of pure ferric oxide, the shade of which is,however, dependent on the roasting temperature employed.The higher this is carried and the more the operation is pro-longed, the darker, as a rule, will the resulting product be.

3. Voyel's Iron Red.This preparation, which is distinguished by its peculiar

brilliancy, and is, therefore, highly suitable for a painter'scolour, is produced by heating a solution of ferrous sulphateto boiling and then adding a saturated solution of oxalic acid.A greenish yellow precipitate of ferrous oxalate is formed,which, after being collected on a filter, is well washed withwater. After drying, the precipitate is placed in a flat ironbasin and heated to a temperate of about 200 C., whereuponit decomposes and is converted into a particularly soft pow-der of

bright red colour, consisting of pure ferric oxide. Fromthis powder the various shades of colour can be prepared byexposure to red heat in covered crucibles.

Many esteemed glass painters and technical men haveupheld the opinion that the substitution of organic acids inplace of inorganic acids for dissolving the iron in the pre-paration of ferric oxide would facilitate the production of themost extensive variety of shades of red oxide, but this view iserroneous.

FERROUS CHROMATE.The substance often employed under this name, in the

glass and porcelain painting industry, for the preparation offine brown colours, owes its designation to its discoverer,


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PIGMENTS. 41Brongniart ; the production of actual ferrous chromate is im-possible, owing to the immediate reduction of the acid tochromium oxide, and our preparation is nothing but a com-pound of chromic oxide and iron (Cr2 3 , Fe O), whichoccurs in nature in dense, black, heavy masses and occa-sionally in the form of octahedral crystals as chrome iron-stone.

The method of preparation is exceedingly simple. Potas-sium chromate is dissolved in distilled water, diluted withthrice the volume of similar water and then left to stand,care being taken to prevent access of dust. Meanwhile asimilar solution of pure ferrous sulphate is prepared, and onthe two solutions being united a precipitate is produced,which, after five or six careful washings, is dried, heated toredness and finally ground on removal from the cold crucible.

GOLD PURPLE.Among the metals precipitating gold from solution, 1 tin

exerts the most notable action, in that the precipitate is of apurple colour, and is therefore called gold purple. If a rodor leaf of tin be inserted in a solution of gold a sedimentalpurple cloud immediately forms around it. When metallictin is used the precipitate, is more inclined to brown, thecolour being finer if a solution of tin in hydrochloric or nitroushydrochloric acid is employed. This purple precipitate is acompound of tin oxide with the protoxide of gold in the pro-portion (according to Berzelius) of 28'2 per cent, of gold to64 of tin oxide. The tin, or protoxide of tin in the solution,continues to deprive the gold of oxygen and itself becomesperoxide of tin, whilst the gold is reduced to a lower stage of

1 Gold, as is well known, is only soluble in aqua regia a mixture of nitricand hydrochloric acids. Further particulars follow.


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42 PAINTING ON GLASS AND PORCELAIN.oxidation, and both come down in a condition in which theyare insoluble in the acids present.

Several conditions are essential to the production of afine gold purple. The gold solution must be free fromnitric acid, since this acid favours higher oxidation of thetin. As the latter metal must exist in the tin solution asprotoxide the tendency towards higher oxidation must beguarded against by dissolving the perfectly pure tin inconcentrated hydrochloric acid, with exclusion of air andany higher temperature than a moderate degree of warmth.The simplest way to prepare the solution of protochloride oftin is by pouring concentrated hydrochloric acid over thetin, taking care that there is always a certain amount of tinin an undissolved state in the liquid. The clear solution ispoured into a bottle containing a stick of metallic tin andtightly stoppered in order to preserve the contents from theoxidising influence of the air.

Another circumstance whereon the beauty of the colourin great measure depends is the degree of dilution of thegold and tin solutions ; the more dilute the solutions thegreater the beauty and purity of the resulting gold purple.The gold solution may be diluted until of a faint yellowtinge, but in the case of the tin solution care is necessarythat the dilution be not carried to excess. The safest planis to dilute the tin solution with eighty volumes of water,then set apart three or four portions in as many test glassesand attenuate these still further. Then by dipping a glassrod moistened with the gold solution into these test portionsit can readily be seen which of the precipitates has the finestpurple shade and the bulk can then be diluted accordingly.The flocculent precipitate of gold purple forms and graduallysubsides ; when all has settled down the supernatant liquidis poured off and the precipitate dried after several carefulwashings.


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PIGMENTS. 43We now come to the precise description of the methods

generally adopted for the preparation of gold purple, or, as itis also called, purple of Cassius . 1 Most of the modificationsof the process are concerned with the production of the aquaregia ; each glass painter employs a particular ratio whichhe treats as a great secret and regards as producing the bestsolvent. The most renowned combinations are given below.

P. Eobert employs the following proportion : Four parts36 nitric acid and one part hydrochloric acid.The purple is obtained by dissolving 0'63 gram of finegold bullion in 3O59 grams of the above aqua regia. Onthe other hand 3'19 grams of chemically pure tin are dis-solved in 22'94 grams of the same solvent diluted with anequal volume of water in order that the attack may begradual.

When solution is complete a further dilutionwith an equal bulk of water is performed, and the liquid,after filtration, added to the gold solution, which has alsobeen greatly diluted.

Bunel prepares aqua regia from four parts nitric acid, onepart hydrochloric acid, and ten parts distilled water.The chloride of gold solution he makes by dissolving fivegrams of gold in the aqua regia, which must not be inexcess ; the protochloride of tin solution is prepared bydissolving fifteen grams of tin in aqua regia and immediatedilution with five grams of water.

Buisson uses an aqua regia composed of three parts 36nitric acid and one part ordinary hydrochloric acid, in whichhe dissolves two parts of tin. The aqua regia for dissolvingthe gold is different, viz. : one part nitric acid and six partshydrochloric acid, this being employed for the solution ofseven grams of gold. No more solvent should be used thanis absolutely necessary. He also prepares another solution

1 From Cassius of Leyden, who first prepared it, in 1683.


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44 PAINTING ON GLASS AND PORCELAIN.of tin by one part rasped tin and three parts hydrochloricacid.

The gold solution is further diluted with three and a halflitres of distilled water, the nitrohydrochloric solution of tinadded and the purple precipitated by adding the tin chloridesolution drop by drop until the precipitate has become of thecolour of old red wine.

In order to more easily obtain the proper mixture of thetwo chloride compounds of the tin, P. Bolley makes use of pink salt (consisting of 70*80 per cent, of tin protochlorideand 29'20 of ammonium chloride), which must be free fromwater and of constant composition. Ten grams of pink.salt are mixed with 1'07 grams of metallic tin and thewhole warmed, with an addition of forty grams of distilledwater, until the tin is all dissolved, whereupon a further140 grams of water are added. This liquid is employedto precipitate a gold chloride solution made by dissolvingT34 grams of gold in aqua regia (without excess of acid)and diluting with 480 grams of pure water.

At the Sevres Porcelain Works the following method ofpreparing gold purple .is adopted. The aqua regia consistsof 10'200 grams 36 nitric acid and 16'800 hydrochloric acid.One gram of fine gold is dissolved in eighteen grams of thisacid, and when solution is effected the liquid is filtered anddiluted by twenty-eight litres of water, which gives it thewell-known straw-yellow colour. Thirty-six grams of theaqua regia are now taken and shaken up with ten grams ofdistilled water (in warm weather, Or six grams if it is cold),and the vessel kept as cool as possible by immersion in abeaker of water. Six grams of tine Malacca tin filings arethrown into the vessel and are slowly and completely dis-solved. The solution is passed through filter-paper andstirred into the gold solution ; the precipitate subsides inthe course of an hour or so and is carefully washed with


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PIGMENTS. 45boiling water after the supernatant liquid has been decantedoff.

Brongniart recommends this preparation with good reason,since, as he remarks, by means of this method, wherein every-thing is accurately weighed, an equally good result can alwaysbe obtained, provided the directions be complied with. Thisis confirmed by the author's own experience.

Several analyses of gold purple, showing that preparationsmade according to different prescriptions differ materially incomposition, are appended. At any rate it is proved thatthe preparation may yield a fine red colour of variousshades, such as scarlet, carmine,1 rose, flesh colour, etc.,or a violet or brown, according to the larger or smallerproportion of tin present and the lower or higher stage ofoxidation existing in the solutions.

Gold Purple.


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4(5 PAINTING ON GLASS AND PORCELAIN.acquires a greenish tinge. It is then diluted with waterand added gradually to the somewhat dilute solution ofgold. The flocculent, pale-red precipitate is carefullywashed and dried.

C. F. Capaun describes another mode of preparation byadding to a solution of ferric chloride three parts of wateruntil the mixture assumes a greenish tinge. Six parts ofwater are then added and the flask containing the liquid setin a cool place until the gold solution is ready. The gold tobe dissolved has the pure hydrochloric acid poured over it,and when this has been heated to boiling the nitric acid isgradually added until solution is complete. In this case alsono excess of acid is permissible. The solution is now suit-ably diluted with distilled water and the iron-tin solutionslowly added until no further precipitate forms, the additionbeing accompanied by continued stirring. The precipitate isbrown and produces a beautiful purple colour.

IRIDIUM OXIDE.The employment of this oxide in our branch of painting

is limited to black, in which colour it yields finer and morestable pigments than can be obtained by the usual mixturesof ferric and cobalt oxide, or ferric and manganese oxide.

Iridium occurs in platinum sand, partly in the actualplatinum granules, partly in separate granules associatedwith osmium ; the black residue left after dissolving platinumsand in aqua regia, consists of iridium and osmium togetherwith mechanical admixtures of chrome iron and titanic acid.

Iridium is readily obtained from the black residue fromplatinum ore, by fusing the latter with potassium nitrate ina porcelain retort and washing out the mass with water. Itis then distilled with nitric acid on the water bath, a gooddeal of osmium passing over. (The latter forms very noxious


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PIGMENTS. 47vapours, so precautions should be taken against them.)Hydrochloric acid is added slowly and distillation proceedsuntil the odour of osmium ceases to appear in the test samples.Then follows the gradual evaporation of most of the acids,and extracting with water, the residue being treated anew withpota

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