The hot electrogilding baths for small quantities of liquor are kept in porcelain dishes, but for large baths use enamelled cast-iron kettles. The temperature may vary from 120°-175° F. Small articles, such as jewellery, are kept in the right hand with the conducting wire, and plunged and agitated in the bath. The left hand holds the anode of platinum wire, which is steeped more or less in the -liquor, according to the surface of the articles to be gilt. Large pieces are suspended from one or more brass rods, and are not moved about. The gilding is very rapid, and a sufficient thickness is obtained after a few minutes. The shade of the gold deposit is modified by the amount of the platinum anode dipping into the liquor. If it dips but a little, relatively to the surface of the articles, the gilding is pale; by immersing it more, the shade will become deeper and deeper until it is red. The platinum anode is connected by a conducting wire to the positive pole of the battery, and the conducting wire starting from the negative pole, touches or supports the articles to be gilt. As a rule, it is preferable to replace the impoverished baths by fresh ones, instead of keeping up their strength by additions of metal,-especially for small articles.
When gilding large pieces, maintain the strength of the baths by successive additions of gold chloride, or, what is better, of equal parts of gold ammonium and pure potassium cyanide. In this manner baths may be made to last a long time, but they are open to the inconvenience of furnishing a red or green gilding, if many articles of copper or of silver have been gilt in them. Articles of copper, or its alloys, should be perfectly cleansed, and may be passed through a very diluted solution of nitrate of binoxide of mercury.
(i) Silver requires to be heated, dipped, and perfectly scratch-brushed. For this metal the gilding 'should be strong, in order to prevent the corners and raised parts from becoming white and bare; and it is a good precaution to give it a coat of copper or brass, or a first gilding in an old bath. Soda phosphate, 14 oz.; soda bisulphite,, 3 1/2 oz.; potash bicarbonate and caustic potash, 1 3/4 oz. each; potassium cyanide and pure gold for neutral chloride, 1/2 oz. each; distilled water, 2 1/5 gal. All the substances except the gold chloride may be dissolved together, and filtered if necessary; then the solution of gold chloride is added. This bath is heated at 120°-140° F. and produces a very fine gilding, but it requires an intense electric current. It does not suit for the direct gilding of iron or steel.
(k) Yellow prussinte of potash, 5 1/4 oz.; potash carbonate, pure, 1 3/4 oz; ammoiria hydrochlorate, 2/3 oz.; pure gold for neutral chloride, 1/3 oz.; water, 1 gal. Dissolve the first three salts in hot water, and filter the solution; after cooling, add the gold solution, boil for 1/2 hour, taking care to replace the evaporated water.
(l) Pure potassium cyanide, 1 3/4 oz.; pure gold, for neutral chloride, 1/3 oz; water, 5 pints. Dissolve the gold chloride in the whole of the water, and add the cyanide, which dissolves and makes the liquor colourless. This bath may be employed with little re* gard to temperature, and is simple in its ingredients. Unfortunately it is not uniform in its working, as it will ungild one face of the object while the other face becomes gilt, or may produce a red gilding at the bottom and a yellow one at the top. These inconveniences will partly disappear by a long ebullition.
(m) Distilled water,17 pints; potash or soda pyrophosphate, 28 oz..; hydro-cyaujo (prussjc) apjd, 1/3 oz.; crystallised gold perchloride, 2/3 oz. The soda pyrophosphate is most generally employed, and is obtained by melting, at a white heat, the ordinary crystallised soda phosphate. The soda pyrophosphate maybe obtained in the form of crystals,| which is a proof of definite composition. The quantity of chloride represents a little more than 1/3 oz. pure gold treated by aqua regia. Put 16 pints distilled water in a porcelain vessel, or an enamelled cast-iron kettle, and add, by small portions at a time, and stirring with a glass rod, the pyrophosphate; heat, filter, and let it cool down. The gold chloride is prepared by introducing into a small glass flask pure gold finely laminated, 1/3 oz.; hydrochloric acid, pure, nearly 1 oz.; nitric acid, pure, 1/2 oz. The flask is slightly heated, effervescence and abundant nitrous vapours result, and in a few minutes the gold has entirely disappeared, leaving a reddish-yellow liquor. The flask is then put upon a sheet of iron, with a hole in its centre, and supported by a tripod.
The whole is heated by a gas or spirit lamp to evaporate excess of the acids; too much acidity may cause great irregularities in the working of the bath, and even prevent its action altogether. An excess of nitric acid causes a jumping of the heated liquors, and may overthrow the whole; it is preferable to have the hydrochloric acid predominating. The evaporation is finished when vapours escape slowly from the flask, and when the liquid has become of an oily consistency and of a deep red colour. The flask is then removed from the fire by wooden pincers, and set to cool upon a ring of plaited straw. If a more rapid evaporation is desired, heat the flask over ignited charcoal, or the spirit lamp; agitate the liquid to prevent any of the gold from returning to the metallic state. Well-prepared gold chloride, when cold, forms a saffron-yellow crystalline mass. If the colour is red, it has been too much evaporated, and will do very well for electro-baths, but for dipping baths it must be heated again after a small addition of the two acids. If the gold perchloride, by too protracted a heat, hat passed to the state of insoluble protocbloride, or even of metallic gold, the treatment must be begun again with the indicated mixture of pure nitric and hydrochloric acids.
The perforated sheet of iron, upon which the flask rests, is intended to prevent the action of heat upon the sides of the vessel, which will decompose the films of gold chloride wetting the flask at these places. When the gold chloride is cold and crystallised, dissolve it in the flask with a little distilled water, and pour the solution through a paper filter held in a glass funnel into a clean bottle; this is to separate a small quantity of silver always found in the gold of the trade. Rinse the flask and filter with the unemployed water, so as to get all the gold into the bath. Pour the filtered solution of gold chloride into the cooled one of pyrophosphate, and stir with a glass rod. Lastly, add the hydrocyanic acid, and the bath is heated nearly to the boiling point for use. If the solution of pyrophosphate is still tepid, add the hydrocyanic acid before the gold chloride. Hydrocyanic (prussic) acid is not absolutely necessary; but, without it, the bath is too easily decomposed, and the gold is too,rapidly precipitated upon the objects placed in it. When the solutions are mixed in the cold, the liquor is yellow or greenish-yellow; but becomes colourless by the increase of temperature.