If the gilding is dull and irregular in colour, melt together in their water of crystallisation, at about 212° F., equal parts - of iron, zinc, alumina, and potash sulphates, and saltpetre. Cover the articles with the mixture, and put them into a cylindrical and vertical grate. This is placed in the centre of a furnace, where the charcoal burns between the sides and the grate which holds the articles. When the moistened finger is presented to one piece, and a slight hissing sound is heard, the heat has been sufficiently raised; put all the articles rapidly into very dilute sulphuric acid, where the coating of salts is quickly dissolved; the articles present a warm, uniform shade of colour. If the copper articles are not entirely gilt by the first operation, the ungilt portions will show themselves by a red colouration, and the articles must then be deprived of gold, cleansed, and gilt anew. Sometimes, when the first gilding is imperfect, instead of colouring by the process just described, the articles are placed for a few moments into the electro-bath. For articles which require a good plating there is an easy method by this process of obtaining as good results as by the battery; it consists in gilding several times, by dipping; before each dipping, the article is passed through solution of nitrate of binoxide of mercury.
(a) These shades may be graduated at will, and are obtained by adding, drop by drop, until the desired shade is arrived at, to the bath of double pyrophosphate of soda and gold, a solution of silver nitrate. For the solution of silver nitrate, dis-ap'.ve in 5 oz. distilled water, 1/2 oz. silver nitrate, crystallised!, or lunar caustic. Before gilding green or white, yellow gild the objects in the ordinary bath, then pass them rapidly through, the mercurial solution, and, lastly, dip them into the gold bath holding the. silver nitrate, which parts rapidly with its silver upon the first articles steeped in it. It is necessary to maintain the. constancy of the shade by the addition of a few drops of the silver solution when required.
(b) Add to one of the above baths a solution of the double cyanide of silver and potassium, until the desired shade is obtained. The tints will vary from a leek-green to a very pale whitish-yellow, This kind of gilding mixed upon the same articles with red, yellow, or pink gold, will produce splendid effects of contrast, especially upon chased parts, where the green gold has a velvety lustre.
(c) 1 oz. 10 dwt. saltpetre; 1 oz, 4 dwt. sal ammoniac; 1 oz. 4 dwt. Roman vitriol; and 18 dwt, verdigris. Mix them well together, and dissolve a portion in water as occasion requires. The work roust be dipped in these compositions, applied to a proper heat t<a burn them off, and then quenched in water or vinegar.
(a) Mix in suitable proportions the electro-copper bath already described with one of the baths for electrogilding; or use an old bath in which a great many copper articles have been gilt, with an intense current of electricity. Yellow gilding may be made to pass to red, by heating it after it has been covered with a paste of copper acetate, cream of tartar, and common salt. Plunge the heated piece into weak sulphuric acid, and carefully scratch-brus't afterwards.
(6) To 4 oz. melted yellow wax, add 1 1/2 oz. red ochre in fine powder; 1 1/2 oz. verdigris, calcined till it yields no fumes; and J oz. calcined borax. It is necessary to calcine the verdigris, or else, by the heat applied in burning the wax, the vinegar becomes so concentrated as to corrode the surface, and make it appear speckled. Pink - Ting kind of gilding is the most difficult to obtain on account of the different tendency of the various metals to galvanic decomposition. Pink gilding, to be perfect, should present at the same time the red, yellow, and white shades, in such a manner that a practised eye will distinguish them. The articles are first gilt yellow by the pyrophosphate bath for dipping, or by the hot electro-bath. Then, without drying, but keeping them in fresh water, small packages are made weighing 1-2 oz. each; pass lightly through the mercurial solution, and then red gilt in an old and hot bath, where a great deal of copper has already been gilt, or in a new bath composed of 10 parts hot electro-gilding bath, and 3 to 4 parts of the first coppering solution, with battery.
For imparting the whitish tint of articles gilt by stirring and of the gold alloy for jewellery, the red gilding is passed through a boiling and nearly exhausted bath of pyrophosphate, to which add 1/10 - 1/30 of its volume of a silver bath, or simply a few drops of a concentrated solution of silver nitrate. In either case a blush of silver is deposited upon the red gilding. This gilding should be scratch-brushed or burnished, and may be chased, but the lustre soon disappears on account of the proportion of copper. To obtain the proper pink gilding, if the first deposit is unsatisfactory, plunge the articles for a few seconds into a mixture of 5 parts sulphuric to 1 of nitric acid. The copper and silver are dissolved, and the yellow gilding reappears, upon which the operation may be begun anew. Besides the variations of colour in gilding due to the dipping of the anodes more or less into the bath, and to the strength of the electric current, moving the articles about in the bath will at all times enable the operator to vary the colour of the deposit from pale straw yellow to a very dark red.
The temperature of the solution likewise influences the colour of the deposit, the colour being lightest when the solution is cold, and gradually becoming darker as the temperature increases.
6 oz. saltpetre, 2 oz. copperas, 1 oz. white vitriol, and 1 oz. alum. If it be wanted redder, a small portion of blue vitriol must be added. These are to be well mixed, and dissolved in water as the colour is wanted.