To precipitate the gold in a state of powder, a small quantity of the solution of sulphate of iron is poured into the glass containing the solution of gold, and a red powder immediately falls to the bottom from the nitromuriatic acid, leaving the gold to unite with the iron of the vitriol, for which it has a greater affinity. The fluid must be now poured off, and leaves the bright gold powder at the bot tom of the glass, which is then to be filled with clean wa ter, and the powder stirred up, to wash it from any remains of acid. It will soon settle again, and the water must be poured ar, to leave the powder as dry as possible. The re maining water is evaporated, by exposing the powder in the air upon a piece of glass.
Another method of producing gold powder, is to form an amalgam of gold with mercury, by the same process as described for wash-gilding, namely, heating the gold to red ness, and throwing it into mercury, heated nearly to its point of evaporation. The gold forms an amalgam with the mer cury, and after being separated from the superfluous mer cury, by passing through a piece of leather, the amalgam is put in a crucible or a glass, and exposed to a sufficient heat to evaporate the mercury, and this leaves the gold in a fine yellow powder. For this method, the mercury must be quite pure, and free from any admixture of lead, other wise the powder will be found in lumps, and discoloured.
Gold may be mechanically reduced to a powder by grind ing leaves of gold with a mullet' upon a marble stone, and working it up with fine clear honey. The fragments brush ed off by the leaf gilders will answer this purpose, and the grinding must be continued until the gold forms a perfect yellow paste with the honey. This paste being thrown into water, the honey is taken up by it, and the gold settles in a fine powder. It must be repeatedly washed till the honey is quite separated, and then the powder is dried. This does not produce so brilliant a powder as the former means, be cause the particles of gold are bruisod, and rendered dull by the action of the mullet. and stone.
The cement or size used for japanners' gilding is com posed of linseed oil and gum. The oil is boiled, and whilst upon the fire, piee.es of gum anime being put in, will be dissolved. The proportion of gum should he about one fourth (by weight) of the oil ; the mixture is boiled till it is reduced to the consistence of tar, and then strained through a cloth. When wanted for use, it must be ground with vermilion, adding as much oil of turpentine as will make it work with a brush. Some gliders add to the above composition asphaltum, red lead, and umber. These in
gredients are pounded and mixed with the gum, before it is put into the oil. Such additions give the size a good colour, which is more necessary in gilding with powder than for leaf gilding, because the size appears in innume rable small specks, which will diminish the lustre of the gold, unless the size is of a brilliant colour.
The operation of gilding with powder, is nothing more than to paint the work over with the size, mixed up with oil of turpentine; and when the size is so far dried that it feels adhesive without coming off when touched with the finger, the gold powder is applied, by dipping a piece of soft wash leather into it, and daubing it upon the work. When the work is small, it is best to strew the powder upon it, and shake off the superfluous gold.
Gilding upon Porcelain or Glass.—The gold is reduced to a fine powder by any of the means described under ja panners' gilding, and mixed with borax, adding as much gum water as will make it work with the pencil, with which it is to be laid upon the porcelain or glass; it must then be subjected to a sufficient heat to make the enamel soft, to which the borax contributes very much. This fixes the gold, and it can afterwards be burnished ; or leaf gold may be laid on with gum water, and fixed by burning. Borax is sometimes mixed with the gum.
To gild upon glass without burning, a cement or size may be made of amber, melted in drying oil, and boiled to a strong consistence. It must be tempered for working with the pencil by oil of turpentine, and laid on the glass or porcelain, and the gold leaf spread over it. When per fectly dry, it will bear a careful burnishing, and is durable.
False or Imitative Gilding—Wash gilding cannot be imi tated ; it must, as we have before mentioned, be of pure gold. Leaf gilding may be clone in Dutch leaf, which is made of copper covered with gold upon its surface, by the wash gilding process, and afterwards beat out in the same manner as the gold ; but the leaves are much thicker, be cause there is little inducement to reduce them so far, and they are more easily laid on. When firs: done, this work has all the appearance of true gold, bo: soon tarnishes by the air, and any dampness will produce spots in it ; if se i cured by a transparent varnish, it preserves its beauty as long as the varnish lasts, and in dims case becomes a very good substitute. The proper Tarnishes are made of mas tic sandal ac, white resin, or L opal, these alone being suffi ciently white and transparent.