If the liquor becomes violet, it is an indication that there is too little hydrocyanic acid; add it, drop by drop, until the liquor becomes colourless. An excess of this acid is objectionable, but there is a very simple method of keeping the baths in good working order, by adding prussic acid gradually to those too rich in gold; or correcting any excess of prussic acid with a small proportion of gold chloride, until the gilding is produced without difficulty and of the proper shade. Thus prepared, the bath will produce very fine gilding upon well-cleansed articles, which most also have passed through a very diluted solution nitrate of binoxide of mercury, without which the deposit of gold is red and irregular, and will not cover the soldered portions. The articles, are supported by a hook or in a stoneware ladle perforated with holes, or in brass gauze baskets; they must be constantly agitated whilst in the bath. Gilders usually employ three baths, placed in close proximity to each other, and heated upon the same furnace; the first bath is one deprived of gold by a previous operation, and is used for removing all excess of acid which may remain upon the articles: the second bath still retains some gold, but not enough to give a sufficiently rich gilding.

The pieces passed through it begin to receive the deposit, which will be finished in thickness and shade in the third bath. A gas furnace, easy to manage, and clean in its working, may be arranged by having a properly supported sheet-iron plate, with holes cut out where the kettles are to stand. Under each kettle place suitable gas burners; when the baths have been heated nearly up to boiling point, lower the gas, so as not to increase the temperature. This method produces much more gilding with a given quantity of gold, than one bath alone. The gilding is done in a few seconds; the finishing operations consist in rinsing in fresh Water, drying in dry and warm saw-dust, and burnishing, if desired. , .

(n) The following solution, to be used at a temperature of 120°-180° F., is recommended by Rod: - crystallised soda phosphate, 60; soda bisulphate, 10; potassium cyanide, 1; gold chloride, 2 1/2; distilled or rain water, 1000 parts by weight. To prepare this bath properly, the water should be divided into three portions, viz., one of 700 parts and two of 150 parts. The sodic phosphate is dissolved in the first portion, the gold chloride in the second, and the soda bisulphate and potassium cyanide in the third. The two first portions are gradually mixed together, and the third is afterwards added. With this solution Rod uses a platinum anode (a wire or strip), adding fresh portions of the gold salt as the solution becomes exhausted. (o) Dr. Ebermayer gives a formula for gilding metallic articles so as to look like polished gold, by simply dipping them into a Warm solution. Dissolve 10 grm. gold in 40 grm. hydrochloric acid and 15 grm. nitric acid; stew down, letting as much of the acid 'escape as possible; then throw down the gold as fulminating gold by means of spirit of ammonia; filter, and wash. In the meantime dissolve 100 grm. potassium cyanide in as little water as possible,and then dissolve the gold upon the filter with the cyanide solution.

Pour this solution again and again over the filter until all the brown particles are dissolved, when the gilding solution is prepared by the addition of 1 litre distilled water. Into this solution, while warm, dip the metallic object to be gilded, and when drawn out it will have all the appearance of polished gold. (p) The bath is prepared in a cast-iron kettle, turned clean and smooth inside on the lathe, and gilt by the protracted ebullition of nearly spent gold baths. Water, 3 1/2 gal.; potash or soda bicarbonate, 1/3 oz.; pure metallic gold, transformed into chloride, 4 1/4 oz. The whole is boiled for at least 2 hours, and fresh water is added to replace that evaporated. A part of the gold, in the form of a violet-black powder, precipitates, and requires the cooling and decanting of the liquor. This is boiled again, and the gilding is proceeded with, in the same manner as before described, except that the mercurial solution should be more diluted than for baths of pyrophosphates. The operation is finished when about half of the gold in the liquor is deposited. The remainder goes to the saved waste.

The bicarbonate process is inferior in most respects to the pyrophosphate, and is now rarely used.

(g) This bath should be employed only as a complement to the cleansing process, before a more resisting gilding, as its results have little durability. Water, 2 gal.; potash bicarbonate, 7 oz.; caustic potash, 63 oz.; potassium cyanide, 3 oz.; metallic gold to be transformed into chloride, 1/3 oz. The whole is brought' up to the boiling point, and a pale gilding is obtained even upon articles imperfectly cleansed, and without using nitrate of binoxide of mercury. It is possible to add 1/6 oz. gold chloride several times to this bath without any other substance. Afterwards maintain it at the proper strength by additions of gold and salts in the above proportions, and it will last for an indefinite period. This bath will gild about 140 oz. of small jewellery with 1/30 oz.gold, whereas a pyrophosphate bath gilds only about 35 oz. of small articles with the 1/30 oz. gold extracted from the liquor.

Dissolving Gold In Aqua-Regia

Take aqua-regia, composed of 2 parts of nitric acid, and 1 of muriatic acid; or of 1 part sal ammoniac, and 4 of aquafortis; let the gold be granulated, put into a sufficient quantity of this menstruum, and exposed to a moderate degree of heat. During the solution, an effervescence takes place, and it acquires a beautiful yellow colour which becomes more and more intense, till it has a dark golden or orange colour. When the menstruum is saturated, it is very clear and transparent.

Management Of Hot Baths

The oaths may be more concentrated, the quantity of water may be diminished, without changing the proportions of the salts and of the gold. But it is preferable to use dilute solutions, which deliver the metal in smaller quantity in a given time, but more homogeneous in substance. The articles should be kept in constant agitation; there is then no difference of specific gravity among the layers of the liquor, and the gilding possesses a uniform colour. A foil or a wire of platinum is preferred to a soluble anode of gold when electro-gilding by the aid of heat, as it is not dissolved, and is more handy for regulating the intensity of the current, by immersing it more or less in the liquid. Thus with the same bath and battery three different shades can be obtained ■; a pale colour, with the anode dipping but slightly; a yellow colour, when the immersion is greater, and a red gold, if the whole anode is in the liquor. In a bath of pink gold, composed of gold, copper, and silver, by increasing or diminishing the length of the platinum anode in the liquor, the deposit will have a white, yellow, or red shade, as the various metals require different degrees of intensity for their reduction in the galvanic current.