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THE art of enamelling consists in the application of a smooth coating of vitrified matter to a bright polished metallic surface. It is, therefore, a kind of varnish made of glass, and melted upon the substance to which it is applied; affording a fine uniform ground for an infinite variety of ornaments, which are also fixed by heat.
The only metals that are enamelled are gold and copper; and with the latter the opaque enamels only are used. Where the enamel is transparent and colored, the metal chosen should not only have its surface unalterable when fully red hot, but also be in no degree chemically altered by the close contact of melted glass, containing an abundance of some kind of metallic oxide. This is the chief reason why colored enamelling on silver is impracticable, though the brilliancy of its surface is not impaired by mere heat; for if an enamel, made yellow by oxide of lead or antimony, be laid on the surface of bright silver, and be kept melted on it for a certain time, the silver and the enamel act on each other so powerfully that the color soon changes from a yellow to an orange, and lastly to a dirty olive. Copper is equally altered by the colored enamels; so that gold is the only metal that can bear the long contact of the colored glass at a full red heat, without being altered by them.
A piece of thin sheet-copper, hammered to the requisite convexity, is first accurately cut out, a hole drilled in the middle for the axis of the hands, and both the surfaces made perfectly bright with a brush. A small rim is then made round the circumference with a thin brass band rising a little above the level, and a similar rim round the margin of the central hole. The use of these is to confine the enamel when in fusion, and to keep the edges of the plate quite neat and even. The substance of the enamel is a fine white opaque glass; this is bought in lump by the enamellers, and is first broken down with a hammer, then ground to a powder sufficiently fine with some water, in an agate mortar; the superfluous water being then poured off, the pulverized enamel remains of about the consistence of wetted sand, and is spread very evenly over the surface of the copper plate. In most enamellings, and especially on this, it is necessary also to counter-enamel the under concave surface of the copper plate, to prevent its being drawn out of its true shape by the unequal shrinking of the metal and the enamel on cooling. For this kind of work, the counter-enamel is only about half the thickness on the concave, as on the convex side. For flat plates the thickness is the same on both sides.
The plate, covered with the moist enamel powder, is warmed and thoroughly dried, then gently set upon a thin earthen ring that supports it only by touching the outer rim, and put gradually into the red-hot muffle of the enameller's furnace. This furnace is constructed somewhat like the assayfurnace; but the upper part alone of the muffle is much heated, and some peculiarities are observed in the construction, to enable the artist to govern the fire more accurately.
The precise degree of heat to be given here, as in all enamelling, is that at which the particles of the enamel run together into a uniform pasty consistence and extend themselves evenly, showing a fine polished face, carefully avoiding on the other hand so great a heat as would endanger the melting of the thin metallic plate. When the enamel is thus seen to sweat down, as it were, to a uniform glossy glazing, the piece is gradually withdrawn and cooled; otherwise it would fly by the action of cold air.
A second coating of enamel is then laid on and fired as before, but this time the finest powder of enamel is taken, or that which remains suspended in the washings. It is then ready to receive the figures and division marks, which are made of a black enamel ground in an agate mortar to a most impalpable powder, worked up on a pallet with oil of lavender, and laid on with an extremely fine hair brush. The plate is then stoved to evaporate the essential oil, and the figure is burnt in as before. Polishing with tripoli, and minuter parts of the process, need not be here described.
Take of sulphur, saltpetre, vitriol, antimony and oxide of tin each, 1 lb.; minium, or oxide of lead, 60 lbs. Mix all together in a crucible and melt in a furnace; next take it out and wash it, to carry off the salts; after melting in the crucible, add 19 oz. of rose copper, 1/2 oz. of prepared zaffre, 1 1/2 oz. of crocus martis, made with sulphur, 3 oz. of refined borax, and 1 lb. of a composition of gold, silver and mercury.
When all are well combined, the mass is to be stirred with a copper rod, and the fire gradually diminished, to prevent the metals from burning. The composition thus prepared is finally to be put into crucibles and placed in a reverberatory furnace, where they are to remain 24 hours. The same composition will answer for other colors, by merely changing the coloring matter. This composition has almost all the characters of real stone, and when broken exhibits a vitreous fracture.
Mix 100 parts of pure lead with from 20 to 25 of the best tin, and bring them to a low red heat in an open vessel. The mixture then burns nearly as rapidly as charcoal, and oxidizes very fast. Skim off the crusts of oxide successively formed, till the whole is thoroughly calcined. Then mix all the skimmings and again heat as before, till no flame arises from them, and the whole is of a uniform gray color. Take 100 parts of this oxide, 100 of white sand, and 25 or 30 of common salt, and melt the whole by a moderate heat. This gives a grayish mass, often porous and apparently imperfect; but which, however, runs to a good enamel when afterwards heated.
The sand is previously calcined in a very strong heat with a fourth of its weight; or if a more fusible compound is wanted, as much of the oxides of tin and lead as of salt are taken, and the whole is melted into a white porous mass. This is then employed instead of the rough sand, as in the preceding process.
The above proportions, however, are not invariable, for if more fusibility is wanted, the dose of oxide is increased, and that of the sand diminished; the quantity of common salt remaining the same. The sand employed in this process is not the common sort, however fine, but a micaceous sand, in which the mica forms about one-fourth of the mixture.
Melt together pulverized feldspar, 27 parts; borax, 18 parts; sand, 4 parts; potash, nitre, and potter's earth, 3 parts each. Then add 3 parts of borax reduced to fine powder.
From the trial which the Society of Arts in London ordered to be made of this enamel, it has been found superior to any hitherto known. It is easily and uniformly applied, and spreads without producing bubbles or spoutings out; it neither covers nor impairs even the most delicate colors, It incorporates perfectly with them, and the porcelain which is covered over with it may pass a second time through the fire without this enamel cracking or breaking out.
Neri, in his valuable treatise on glass making, has long ago given the following proportions for the common material of all the opaque enamels, which Kunckel and other practical chemists have confirmed: Calcine 30 parts of lead with 33 of tin, with the usual precautions. Then take of this calcined mixed oxide 50 lbs., and as much of powdered flints (prepared by being thrown into water when red-hot, and ground to powder), and 8 oz. of salt of tartar; melt the mixture in a strong fire kept up for ten hours, after which reduce the mass to powder.
Mix 6 lbs. of the compound with 48 grs. of the best black oxide of manganese, and melt in a clear fire. When fully fused throw it into cold water; then re-melt and cool as before 2 or 3 times, till the enamel is quite white and fine.
The most beautiful and costly color known in enamelling is an exquisitely fine, rich red, with a purplish tinge given by the salts and oxides of gold; especially by the purple precipitate formed by tin in one form or other and by nitromuriate of gold, and also by the fulminating gold. This beautiful color requires much skill in the artist, to be fully brought out. When most perfect it should come from the fire quite colorless, and afterwards receive its color by the flame of a candle.
Other and common reds are given by the oxide of iron, but this requires the mixture of alumina, or some other substance refractory in the fire, otherwise at a full red heat the color will degenerate into black.
Take of saturnus glorificatus, 1 lb.; natural crystal, calcined to whiteness, 1/2 lb.; salt of pulverine, 1 lb. Mix them together, and bake in a slow heat for about 12 hours; then melt the mass, and pulverize the same in an agate mortar, or any other proper vessel, which is not capable of communicating any metallic or other impurity.
Take litharge of white lead, put it in a pan, pour on distilled vinegar, stirring it well over a gentle fire till the vinegar becomes impregnated with the salt of the lead; evaporate half the vinegar, put it in a cool place to crystallize, and keep the crystals dry for use.
Take of copper-dust, 1 oz.; sand, 2 oz.; litharge, 1 oz.; nitre, 1/2 oz. Or, copper, 2 oz.; sand, 1 oz.; litharge, 2 oz.; nitre, 1 1/2 oz.
Mix them with equal parts of flux, or vary the proportions of them as may be found necessary, according to the tint of color required.
Another. - Take of opaque or transparent enamel, 10 parts; oxide of chromium, 1 to 2 parts.
Take of calcined iron, cobalt, crude or prepared, each 1 oz. Or, zaffre, 2 oz.; manganese, 1 oz.
Mix them with equal parts of flux, by melting or grinding together.
Take of lead and tin ashes, litharge, antimony, and sand, each 1 oz.; nitre, 4 oz.
Calcine, or melt them together; pulverize, and mix them with a due proportion of flux, as the nature of the glass may require; or take more or less of any or all of the above, according to the depth of color desired. Or, opaque or transparent enamel, 6 parts; chloride of silver, 1 to 2 parts.
Take of prepared cobalt, sand, red-lead, and nitre, each 1 oz.; flint-glass, 2 oz.
Melt them together by fire, pulverized and fluxed according to the degree of softness or strength of color required.
Take of the blue, as prepared above, 1 oz.; black, 1/2 oz.; yellow, 1/2 oz. Grind them for use. If necessary add flux to make it softer.
Take of tin, prepared by aqua-fortis, and red-lead, each 1 oz.; white pebble-stone, or natural crystal, 2 oz.; nitre, 1 oz.; arsenic, 1 dr.; with equal parts of flux, or more or less, as the softness or opacity may require. Melt together, calcine, or use raw.
Take of opaque or transparent enamel, 12 parts; purple of Cassius, 1 to 2 parts, regulated with sal ammoniac. Put it in a sand-heat for about 48 hours, to digest the gold. Collect the powder, grind it with 6 times its weight of sulphur, put it into a crucible on the fire till the sulphur is evaporated, then amalgamate the powder with twice its weight of mercury; put it into a mortar or other vessel, and rub it together for about 6 hours with a small quantity of water in the mortar, which change frequently; evaporate the remaining mercury in a crucible, and add to the powder 10 times its weight of flux, or more or less, as the hardness or softness of the color may require.
Take purple as prepared above, mix it with 30 times its weight of flux, and 1-100th part of its weight of silverleaf, or any preparation of silver, or vary the proportion of the flux and silver as the quality of the color may require; or any of the other preparations for purple will do, varying the proportions of the flux and silver as above; or any materials, from which purple can be produced, will, with the addition of silver and flux, answer.
Take of red-lead, 1 oz.; calcined iron, 1 oz.; antimony, 2 oz.; litharge, 2 oz.; zaffre, 1 oz.; sand, 2 oz.
Calcine, or melt together, or use raw, as may be most expedient; or vary the proportions of any or all the above, as tint or quality may require.
The preceding colors may be applied to vessels of glass in the following-manner, viz., by painting, printing, or transferring, dipping, floating, and grounding.
By Painting. - Mix the colors (when reduced by grinding to a fine powder) with spirits of turpentine, temper them with thick oil of turpentine, and apply them with camelhair pencils, or any other proper instrument, or mix them with nut or spike oil, or any other essential or volatile oil, or with water in which case use gum Arabic, or any other gum that will dissolve in water, or with spirits, varnishes, gums of every kind, waxes, or resins; but the first is conceived to be the best.
By Printing. - Take a glue-bat, full size for the subject, charge the copper plate with the oil or color, and take the impression with the bat from the plate, which impression transfer on the glass. If the impression is not strong enough, shake some dry color on it which will adhere to the moist color; or take any engraving, or etching, or stamp, or cast, and, having charged it with the oil or color, transfer it on the glass by means of prepared paper, vellum, leather, or any other substance that will answer; but the first is the best. Any engravings, etchings, stamps, casts, or devices may be charged with waters, oils, varnishes, or glutinous matters of any kind, reduced to a proper state, as is necessary in printing in general. Any or all of these may be used alone, or mixed with the colors. When used alone the color is to be applied in powder.
By Dipping. - Mix the color to about the consistency of a cream, with any of the ingredients used for printing, in which dip the glass vessel, and keep it in motion till smooth.
By Floating. - Mix the color with any of the ingredients used for printing, to a consistency according to the strength of the ground required, float it through a tube, or any other vessel, moving or shaking the piece of glass till the color is spread over the part required.
By Grounding. - First charge the glass vessel with oil of turpentine, with a camel-hair pencil and while moist apply the color in a dry powder, which will adhere to the oil; or, instead of oil of turpentine, use any of the materials used for printing; but the first is the best.
In making these enamels, the following general cautions are necessary to be observed.
1st. That the pots be glazed with white glass, and be such as will bear the fire.
2d. That the matter of enamels be very nicely mixed with the colors.
3d. When the enamel is good, and the color well incorporated, it must be taken from the fire with a pair of tongs.
Powder, sift, and grind all the colors very nicely, and first mix them with one another, and then with the common matter of enamels; then set them in pots in a furnace, and when they are well mixed and incorporated, cast them into water and when dry set them in a furnace again to melt, and when melted take a proof of it. If too deep colored, add more of the common matter of enamels; and if too pale add more of the colors.
Mix some chloride of platina, dissolved in water with neutral nitrate of mercury, and expose the precipitate, which will be formed, to a heat simply sufficient to volatilize the proto-chloride of mercury; there will be obtained a black powder which, applied with a flux, gives a beautiful black enamel.
Take 1 part of pure silver, 2 of copper, and 3 of pure lead; fuse them together, and pour the amalgam into a longnecked earthenware matrass, half filled with levigated sulphur; let the mouth of the vessel be immediately closed, and the contents left to cool. The mass which results, when levigated and washed, is ready for the purposes of the artist. The cavities left by the fusion having been filled with it, the plate is to be held over a small furnace, fed with a mixture of charcoal and wood, taking care to distribute the enamel with the proper instrument. As soon as fusion has taken place, the plate is to be removed, and when sufficiently cooled, is to be cleared by the file, and polished by fine pumice and tripoli.
The enamel painter has to work, not with actual colors, but with mixtures, which he only knows from experience will produce certain colors after the delicate operation of the fire; and to the common skill of the painter, in the arrangement of his palette and choice of his colors, the enameller has to add much practical knowledge of the chemical operation of one metallic oxide on another; the fusibility of his materials; and the utmost degree of heat at which they will retain, not only the accuracy of the figures which he has given, but the precise shade of color which he intends to lay on.
Painting in enamel requires a succession of firings, first of the ground which is to receive the design, and which itself requires two firings, and then of the different parts of the design itself. The ground is laid on in the same general way as the common watch-face enamelling. The colors are the different metallic oxides, melted with some vitrescent mixture, and ground to extreme fineness. These are worked up with an essential oil (that of spikenard is preferred, and next to it oil of lavender) to the proper consistence of oil colors and are laid on with a very fine hair brush. The essential oil should be very pure, and the use of this rather than of any fixed oil, is that the whole may evaporate completely in a moderate heat, and leave no carbonaceous matter in contact with the color when red hot, which might affect its degree of oxidation, and thence the shade of color which it is intended to produce. As the color of some vitrified metallic oxides (such as that of gold) will stand at a very moderate heat, whilst others will bear, and even require a higher temperature to be properly fixed, it forms a great part of the technical skill of the artist to supply the different colors in proper order; fixing first those shades which are produced by the colors that will endure the highest, and finishing with those that demand the least, heat. The outline of the design is first traced on the enamel, ground and burnt in; after which the parts are filled up gradually by repeated burnings, to the last and finest touches of the tenderest enamel.
Transparent enamels are scarcely ever laid upon any other metal than gold, on account of the discoloration produced by other metals. If, however, copper is the metal used, it is first covered with a thin enamel coating, over which gold-leaf is laid and burnt in, so that, in fact, it is still this metal that is the basis of the ornamental enamel.
Mosaic work consists of variously shaped pieces of colored glass enamel; and when these pieces are cemented together, they form those regular and other beautiful figures which constitute tessellated pavements.
The enamel, consisting of glass mixed with metallic coloring matter, is heated for 8 days in a glass-house, each color in a separate pot. The melted enamel is taken out with an iron spoon and poured on polished marble placed horizontally, and another flat marble slab is laid upon the surface, so that the enamel cools into the form of a round cake, of the thickness of three-tenths of an inch.
In order to divide the cake into smaller pieces, it is placed on a sharp steel anvil, called tagliulo, which has the edge uppermost; and a stroke of an edged hammer is given on the upper surface of the cake, which is thus divided into long parallelopipeds, or prisms, whose bases are three tenths of an inch square. These parallelopipeds are again divided across their length by the tagliulo and hammer into pieces of the length of eight-tenths of an inch, to be used in the Mosaic pictures. Sometimes the cakes are made thicker and the pieces larger.
For smaller pictures, the enamel, whilst fused, is drawn into long parallelopipeds, or quadrangular sticks, and these are divided across by the tagliulo and hammer, or by a file; sometimes, also, these pieces are divided by a saw without teeth, consisting of a copper blade and emery, and the pieces are sometimes polished on a horizontal wheel of lead with emery.
Gilded Mosaic is formed by applying the gold-leaf on the hot surface of a brown enamel, immediately after the enamel is taken from the furnace; the whole is put into the furnace again for a short time, and when it is taken out the gold is firmly fixed on the surface. In the gilded enamel used in Mosaic at Rome, there is a thin coat of transparent glass over the gold.
The wrought and cast-iron vessels which are to be placed on the fire are often covered with enamel, which protects the liquid from metallic contact with the sides.
Two compositions are generally employed for this purpose, one having for its base silicate of lead, and the other boro-silicate of soda. These enamels are applied to the scoured surface of the metal in the form of a powder, which is fixed by heating it to a sufficiently high temperature to fuse it; it then spreads over and covers the metal with a vitreous varnish.
The boro-silicate of soda enamel possesses great superiority over that of silicate of lead, for it is unattacked by vinegar, marine salt, the greater number of acid or saline solutions, even when concentrated, and resists the action of the agents employed in cooking or chemical operations.
The silicate of lead enamel is whiter and more homogeneous, which explains the preference given to it by the public, but it gives up oxide of lead to vinegar or to common salt; it acts upon a great number of coloring matters, and it is attacked by nitric acid, which immediately communicates a dull appearance to it. On evaporation the liquid leaves a white crystalline residue of nitrate of lead. This enamel is instantly darkened by dissolved sulphides, and also by cooking food containing sulphur, such as cabbage, fish, and stale eggs. It is very easy to distinguish these two enamels by means of a solution of sulphide of potassium, sodium, or ammonium. On allowing of one of these reagents to fall on the vessel to be tested, the lead enamel darkens in a few moments, whilst the boro-silicate of soda enamel retains its white color.
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