of bone- ashes, and called a cupel, is used, into which is put the alloyed gold with about twice its weight of silver and some lead. Heat is now applied, which oxydizes the lead, and then vitrifies it, at the same time it forms a similar vitreous oxyd with the other metals in the alloy. The cupel then absorbs this glassy fluid, while the gold and silver, not undergoing the process of oxydizement, remain untouched. The silver is afterwards separated by another process, which will be presently noticed. We have heretofore mentioned, that gold is distinguished, with respect to its purity, by the carat standard. This is the method. employed by goldsmiths and coiners. Pure gold they suppose divided into 24 parts, called carats.*

Gold of 24 carats means pure gold; gold of 23 carats means an alloy of 23 parts gold, and one of some other metal; gold of 22 carats means an alloy of 22 parts of gold, and two of another metal. The number of carats mentioned, specifies the pure gold; and what that number wants of 24, indicates the quantity. of alloy. Thus gold of 12 carats would be an alloy containing 12 parts gold, and 12 of some other metal. In Great Britain the carat is divided into four grains; among the German's into 12; and by the French it was formerly divided into 32.

Parting or quartation. Gold is purified, if it should exist with silver, by the process called parting or quartation. The latter term is applied when three parts of silver are added to the alloy, and making of course a fourth part of the mixture. When they are melted, sulphur is added, which combines with the silver, and the gold falls to the bottom. Gold is purified from silver in the humid way by the nitric acid, which dissolves the latter, but has no action on the former. From the nitric acid, the silver is separated by precipitation. See Silver.

The methods of purifying gold, at large, may be seen in the 44th chapter of La Grange's Manuel, in Nicholson's Principles of Chemistry, and in the metallurgical work of Monsieur Sage.

• Lewis's Philosoph. Commerce, p. 116.

Gold unites with most of the metals. With arsenic, bismuth, nickel, and antimony, it is rendered brittle. It unites with lead and tin, and loses in a great measure its ductility. A small portion of tin to gold, in the opinion of Mr. Alchorne, is not so injurious as workmen have imagined. Mr. Tillet, however, is of the contrary opinion. With iron, it forms a very useful alloy; and by copper, it is more fusible, and rendered of a redder colour. This alloy is employed for coin, toys, gold-plate, &c. It is rendered very pale by silver. This alloy forms the green-gold of goldsmiths. With platina, gold forms a malleable and ductile mixture.

Gilding. This is an art of spreading or covering certain bodies with gold, either in leaf or powder, or in amalgam with quicksilver. This art was known at an early period. The mode of gilding at first was very expensive. We read in Homer's Odyssey, 1. iii. v. 492, that the horns of a ram bought by Nestor, as an offering to Minerva, were covered with gold. Pliny speaks of gilding; and Seneca observes (Epist. 115.) that "when we cover our houses with gold, what else do we than rejoice in deception? for we know that coarse wood is concealed under that gold." Gilding is mentioned in Exodus. There are several methods of gilding in use, viz. gilding on oily size; gilding on a water size; gilding by the fire, which is peculiar to metals; gilding of books, &c. For a particular account of the different kinds of gilding, for different purposes, I would refer the reader to Reese's Cyclopedia, article Gold and Gilding, Observ. sur les Coulumes de l'Asie (on the art of gilding on porcelain.) Handmaid to the Arts, Smith's School of Arts, Commercium Philosophico-Technicum, Beckman's History of Inventions, Philosophical Transactions of the Royal Society, Philosophical Magazine, ix. 20. Macquer's Dictionary, and Aikin's Dictionary of Chemistry, art. Gilding.

The works already mentioned contain a full description of the processes of gilding. But as the following general remarks of Mr. Accum* may prove interesting, we shall insert them.

* Vol. ii. p. 21.

Shell gold, or gold powder,

For painting, may be obtained by uniting one part of gold with eight of mercury, and afterwards evaporating the latter by heat, which leaves the gold in the form of powder; or otherwise the metal may be reduced to powder by mechanical trituration.

Cold gilding by friction.

Steep a fine linen rag in a saturated solution of muriate of gold, till it has entirely imbibed the fluid; this rag must then be dried over a fire, and afterwards burnt to tinder. When any thing is to be gilt, it must be previously well burnished; a piece of cork is then to be dipped, first into a solution of salt in water, and afterwards into the black powder; and the piece, after being rubbed with it, must be burnished. This pow der is frequently used for gilding delicate articles of

silver.

Gilding of brass or copper.

Fine instruments of brass, in order that their surface may be kept longer clean, may be gilt by immersing them several times in a solution of muriate of gold, free from excess of acid, and afterwards burnishing them.

Water gilding.

This term was probably at first confined to such processes as demand the use of a solution of gold in nitromuriatic acid, and means a chemical application of gold to the surfaces of metals. If a solution of gold be copiously diluted with ardent spirit, a piece of polished steel will be gilt by being repeatedly steeped

therein.

An improved process for gilding steel.

This method, which is less known among artists than it deserves to be, may prove useful to those who have occasion to gild steel.

The first part of the process consists in pouring into a solution of gold in nitro-muriatic acid, about twice as much sulphuric ether. In order to gild iron or steel, the metal must be highly polished. The ether

which has taken up the gold is then to be applied with a small brush; it evaporates, and the gold remains on the surface of the metal. In this manner all kind of figures may be delineated on steel, by employing a pen or fine brush. Lancets, razors, &c. may be gilt in this manner. See Experiment 24.

Gilding of iron, by means of heat,

Is performed by cleaning and polishing its surface, and then heating it till it has acquired a blue colour. When this has been done, the first layer of gold-leaf is put on, slightly burnished down, and exposed to a gentle fire. It is usual to give three such layers, or four at the most, each consisting of a single leaf, for common works; or two for extraordinary ones. The heating is repeated at each layer, and last of all the

work is burnished.

Grecian gilding.

Is performed in the following manner: equal parts. of muriate of ammonia, and muriate of mercury, are dissolved in nitric acid, and a solution of gold is made in this fluid: upon this the solution is concentrated, and applied to the surface of silver, which becomes quite black; but on being exposed to a red heat, it assumes the appearance of gilding.

D 2

Method of gilding silver, brass, or copper, by means of an alloy of gold and mercury.

Eight parts of mercury and one of gold are alloyed together by heating them in a crucible. As soon as the gold is perfectly alloyed, the mixture is poured into cold water, and is then ready for use.

Before the alloy can be laid upon the surface of the metal, this last is brushed over with diluted nitric acid, in which it is of advantage that some mercury may have been dissolved. Some artists then wash the metal in simple water, and scour it a little with the finest sand, previous to the application of the gold; but others apply it to the metal while still wet with the nitric acid. But in either case, the alloy must be laid on as uniformly as possible, and spread very even with a brass wire-brush, wetted from time to time with water. The piece is then laid upon a grate, over a charcoal fire, or in a small muffle or furnace, adapted to this purpose; the heat drives off the mercury, and leaves the gold behind. Its defects are then seen, and may be remedied by successive applications of more alloy, and additional application of heat. Expert artists however make these additional applications while the piece remains in the furnace, though the practice is said to be highly noxious, on account of the mercurial fumes. After this it is rubbed with gilders' wax, which may consist of four ounces of bees' wax, one ounce of acetite of copper, and one ounce of sulphate of copper; they then expose it to a red heat, which burns off the wax'; and lastly, the work is cleansed with the scratch brush, and burnished, if necessary, with a steel tool. The use of the wax seems to consist merely in covering defects, by the diffusion of a quantity of red oxyd of copper, which is left behind after the burning.