812

PROPERTIES OF GOLD.

metallic lustre. It is not remarkable for its hardness, being, when in a pure state, nearly as soft as lead. Its ductility, however, is considerable, ranking next to silver, so that it may be drawn into very fine wire. As already mentioned, it is the most malleable of the metals, and so extreme is the thinness to which it may be reduced by hammering, that 280,000 leaves placed upon one another would be required to occupy the thickness of one inch. The thickness of the film may be still further reduced by floating it upon a dilute solution of cyanide of potassium. Faraday found that such a film when attached to a plate of glass still retained its power of reflecting yellow light and transmitting green: if, however, the temperature were maintained for a short time at a point not exceeding 600° F., the metallic lustre disappeared entirely, and the transmitted light became of a pure ruby red. The pressure of agate, or of any kind of hard body upon the film, changed the colour of the transmitted light at that spot again to green. (Phil. Trans. 1857.) Gold fuses at a temperature of 2016°. It cannot be advantageously employed for castings, as it shrinks greatly at the moment of solidifying. It is but very slightly volatile in the heat of the furnace, though by a powerful electric discharge, by the concentration of the sun's rays with a large convex lens, or by the intense heat of the oxyhydrogen jet, it may be dispersed in purple vapours. It is one of the most perfect conductors both of heat and of electricity. Gold suffers no change by exposure to air and moisture at any temperature. None of the simple acids, with the exception of the selenic, have any effect upon it, but it is dissolved by any mixture which liberates chlorine. Its usual solvent is aqua regia, which for this purpose is generally prepared by mixing 1 part of nitric acid and 4 parts of hydrochloric acid. The hydrated alkalies do not affect it; a crucible of gold is consequently a valuable instrument in the analysis of minerals which require fusion with the caustic alkalies. The metal combines directly with fluorine, chlorine, and bromine, without the aid of heat, and with phosphorus when heated.

(955) Preparation of Fine Gold.-Gold is best obtained in a state of purity by dissolving the metal in aqua regia, and evaporating the solution of chloride of gold thus obtained with an excess of hydrochloric acid, for the purpose of destroying the excess of nitric acid the solution is then largely diluted with water, to precipitate the chloride of silver, from which it is afterwards decanted. A solution of ferrous sulphate is next prepared and added to the solution of chloride of gold: I part of gold requires between 4 and 5 parts of the crystallized sulphate [6 FeSÐ ̧+2 AuCl ̧=

PREPARATION OF FINE GOLD-GILDING.

6

813

2 (Fe2 3 SO1) + FeCl + 2 Au]. Metallic gold is thus precipitated in the form of a finely divided powder, which, when suspended in water, is brown by reflected, but purple when viewed by transmitted light. For commercial purposes it would be sufficient now to collect the gold, dry it, and after fusing it with borax, to cast it into ingots; but when required to be perfectly free from silver, the gold is not melted at this stage, but the precipitated metal is boiled with hydrochloric acid of sp. gr. 1'1. The acid is decanted, and the residue is boiled twice with fresh acid without washing the gold between these successive additions of acid. The last traces of iron and nearly all the chloride of silver are thus removed. The gold is then washed, dried, and mixed with its own weight of acid sulphate of potassium, and fused in a Hessian crucible. The last portions of chloride of silver are thus removed, and the gold is perfectly pure. When thus prepared its surface often exhibits a crystalline appearance, being embossed with aggregations of tetrahedra if the metal be allowed to cool slowly.

Levol prefers to precipitate the gold from an acid solution of its chloride by means of an acid solution of terchloride of antimony: 3 SbCl ̧ +2 AuCl ̧=3 SbCl +2 Au. The hydrochloric solution retains any traces of chloride of silver which may be present.

Uses.-Gold is employed in its finely divided state for gilding porcelain, which is first painted with an adhesive varnish and allowed to become partially dry; the powdered metal is then dabbed on with a dry pencil (having been previously mixed with a fusible enamel), after which the article is fired; the gilt portions are subsequently burnished, and take a high polish. It communicates a fine ruby colour to glass, and is the colouring ingredient in the beautiful red glass manufactured in Bohemia. The uses of gold in the fabrication of ornamental articles and in coinage are well known; like silver, it is too soft to be employed in a pure

state.

(956) Gilding upon woodwork, papier-mâché, or plaster, is effected by means of gold-leaf, which is attached to the surface by an adhesive varnish, such as gold-size. Gilding upon metals is effected either through the medium of mercury, as in one of the processes for silvering (935), or by voltaic action, as in the process of electro-silvering already mentioned: for this purpose, a solution either of the cyanide of gold and potassium, or of sulphide of gold in sulphide of potassium, is used (296).

Some years ago, a means of gilding by immersion was introduced by Mr. Elkington, by which copper trinkets and stamped

814

GILDING UPON METALS BY IMMERSION.

articles can be coated with a thin film of gold; this method has been very largely practised. The process has been carefully investigated by Barral (Ann. de Chimie, III. xviii. 5). The gilding bath is prepared by dissolving I part of fine gold in aqua regia, and expelling the excess of acid by evaporation; the chloride is dissolved in a small quantity of water; to this solution 30 parts of the acid-carbonate of potassium (KHEO) are gradually added. This liquid is then mixed with a solution of 30 parts more of the acidcarbonate, dissolved in 200 parts of water, and the liquid is boiled for two hours: during this operation, the acid-carbonate of potassium is converted into the sesquicarbonate, and the yellow liquid passes into green; after this, the solution is ready for use. The trinkets having been annealed are cleansed from adhering oxide by a momentary immersion in a mixture of equal parts of sulphuric and nitric acids, to which, when the gold is intended to have a dead appearance, a little common salt is added. The articles are washed in water and then plunged into the hot gilding liquid, where they are left for about half a minute, after which they are washed in water and dried in hot sawdust. The layer of gold deposited in this operation is always excessively thin, and cannot be increased, because as soon as the alloy is once covered with a film of gold no further deposition occurs. This bath may also be employed for gilding on German silver, platinum, or silver, by immersing the objects composed of these metals in the liquid in contact with wires of copper or of zinc. During this process of gilding, a remarkable reaction occurs,-the gold imparts a portion of its chlorine to the excess of potash contained in the bath, forming chlorate of potassium; protochloride of gold is formed and is decomposed by the copper, cupric chloride being produced, whilst metallic gold is deposited upon the surface of the trinkets: 6 AuCl, +3 K‚¤Ð ̧+6 €u=6 €uCl2+5 KCl+ KCIO2+6 Au+3 C02. In the course of the operation a black powder is precipitated, which contains hydrated carbonate of copper, mixed with a small proportion of the purple of Cassius derived from the action of the gilding solution upon the tin contained in the solder of the trinkets.

3

With mercury, gold forms a semi-solid amalgam of a yellowish colour, which is soluble in an excess of mercury. This excess may be removed, as in the case of silver amalgam, by filtering, and squeezing it through chamois leather. It is this amalgam which is formed during the extraction of gold from its ores; it is also extensively prepared for the purposes of gilding. A combination of mercury with gold (Au.Hg) may be obtained crystal

ALLOYS OF GOLD-PARTING.

815

lized in brilliant four-sided prisms by acting with diluted nitric acid, aided by a gentle heat, upon an amalgam of gold containing about 1 part of gold to 1000 of mercury (T. H. Henry): these crystals are insoluble in nitric acid.

(957) Alloys of Gold.—The ductility of gold is much impaired by alloying it with other metals, though its hardness and sonorousness are increased: these alloys are generally formed without difficulty. If a proportion of tin, of cobalt, of nickel, or of zinc, greater than 2 per cent. of the mass be present in the alloy, it is unfit for coinage; and still smaller quantities of lead, of arsenic, of antimony, or of bismuth, render gold brittle. Palladium is a still more inconvenient impurity, since it not only renders the gold brittle, but it requires special treatment in order to extract it, as it is not removed by the ordinary operations of the refiner. A similar remark is applicable to the alloy of platinum with gold. Small quantities both of platinum and of palladium render the gold nearly white. The native alloy of osmium and iridium which frequently accompanies the Californian gold, does not combine with the metal, but remains disseminated through it in distinct grains after the gold has been melted. These grains occasion much inconvenience; they often escape notice until the metal passes through the coining press, where they make themselves apparent by their hardness, and by the injury which they consequently inflict upon the dies.

In

Silver and gold may be alloyed with each other in all proportions. The alloy which they form has a pale greenish-yellow colour, but it becomes nearly white when the quantity of silver exceeds 50 per cent. The malleability of gold is less diminished by the presence of silver than by that of any other metal. the arts it frequently becomes necessary to separate these two metals, and this is usually effected by the method termed quartation or parting. This operation depends on the solubility of silver in nitric acid, and the insolubility of gold in this liquid. It is necessary that the silver should amount to at least three times the weight of gold, otherwise portions of silver would be mechanically protected from the action of the acid, and the separation would be incomplete. If, therefore, the alloy be found to contain more than a fourth of its weight of gold, sufficient silver is added to reduce it to this proportion, and hence the origin of the term "quartation." The metals are fused together, granulated by being poured into water, and they are then digested in the acid. The gold is afterwards melted into ingots, the silver is precipitated as chloride, by common salt, and the chloride is reduced

816

ALLOYS OF GOLD-ASSAY OF GOLD.

either by zinc (940), or by fusion with an alkali (944). On the large scale sulphuric acid is usually substituted for nitric acid; it is much cheaper, and is quite as effectual in dissolving the silver if boiled upon it (947). Indeed, in refining upon the large scale, when sulphuric acid is used the gold may be obtained containing 998 or 999 thousandths of the pure metal; whereas when nitric acid is used it is seldom finer than from 993 to 995 thousandths.

The most useful alloy of gold is that which it forms with copper it is of a redder colour than pure gold, and considerably harder and more fusible, but it is less ductile and malleable. It is this alloy which is used for coinage. British standard gold contains 8:33 per cent. of copper, or 11 parts of gold to 1 part copper. The specific gravity of this mixture is 17.157, instead of 18.47, the two metals expanding a little when they unite. In France and in the United States the standard gold contains 10 per cent. of copper. Jewellers frequently alloy their gold with a mixture of copper and silver. The alloys of gold and copper, when once the materials have been well mixed, do not exhibit the tendency to liquation which occasions so much trouble in the case of silver (937). The solder used for uniting pieces of gold is an alloy of gold with copper, which melts at a lower temperature than pure gold.

(958) Assay of Gold.-In the assay of gold, a combination of the processes of cupellation and quartation is employed. In the cupellation of gold the quantity of lead which is needed is about double that employed for silver; I part of copper requiring about 32 parts of lead. The assay of gold furnishes results which are more accurate than those obtained in the cupellation of silver. The loss of gold by volatilization is very much smaller, and scarcely any of the metal is carried into the cupel by an excess of lead.

The following is an outline of the method adopted in the assay of gold :-The quantity of the alloy for assay having been accurately weighed, it is wrapped in a piece of paper, with a proportion of silver equal to about 3 times that of the gold which the alloy is supposed to contain, and this is submitted to cupellation

* An approximative estimate of the composition of the alloy is sometimes made by the use of the touchstone, though it is seldom employed by the practised assayer:-A number of pieces of alloy are formed containing known quantities of gold and copper, or of gold and silver: the first consisting of pure gold; the second of 23 parts of gold and 1 part of copper; the third of 22 of gold and 2 of copper, and so on: the assayer selects one of these alloys, or needles,' which from its colour he judges to approach nearest in composition to the alloy which he is about to assay; this he rubs upon a hard, black stone,