Dead Lustre Gilding, equal in appearance to the. best mercury gilding, is obtained - (a) With silver. An electro-silvered bath is prepared by dissolving in 2 1/5 gal. water, 5 1/4 oz. fused silver nitrate, and adding 9 oz. pure potassium cyanide; this at first produces an abundant precipitate, which soon dissolves. The filtered liquid is the silver bath, in which is steeped the zinc article previously coated with copper or . brass. Under the influence of a proper electric current, the silver deposited is of a handsome frosted dead lustre appearance, and perfectly white. The object is then rapidly and thoroughly riused, and dipped into an electrogilding bath. The dead gilding by this process is very fine and silky, but is soon darkened by the sulphuretted hydrogen of the atmosphere and of gaslight, which sulphurises the silver through the thin film of gold.
(6) The galvanoplastic process is both more durable and more economical than that with silver. Add to the necessary quantity of water, 1/10 its volume of sulphuric acid; in this dissolve as much copper sulphate as it will hold at the ordinary temperature. This solution will mark 20°-24° B., then add enough water to reduce its specific gravity to 16°-18° B. This galvanoplastic bath is generally held in large vessels of stoneware, slate, wood, or gutta-percha; and porous shells are immersed in it, filled with a weak solution of sulphuric acid and amalgamating salts. Plates or cylinders of zinc are put into these cells, and are connected by binding screws with one or more brass rods, which rest upon the sides of the trough, and support the articles which are to receive a dead lustre in this bath. The articles of zinc, previously coated with copper or brass, suspended from the rods, remain in the solution of copper sulphate until they have acquired a satisfactory dead lustre. A few seconds after the articles have been placed in the bath, withdraw and examine them carefully; should the previous coat of copper or brass be insufficient to resist the corroding action of the acid solution of copper sulphate, there is produced a muddy dark deposit, which is easily removed with the finger.
Should this occur, the object must be scratch-brushed and placed again in the former alkaline baths of copper or brass, in order to increase* the deposit which protects the zinc in the galvanoplastic bath. When the galvanoplastic dead lustre is successful, the deposit is perfectly regular, and of a pink shade which possesses great freshness. When it is irregular, marbled, crystalline, of a vinous or fire-red colour, and dull or earthy in appearance, these defects are due to the following causes: either the bath is in a bad state of con-ductability or of saturation; or the surface of the zincs is too large in proportion to that of the objects, and therefore too much electricity is given out; or the previous electro-deposits of copper or brass were insufficient or inferior in quality. The remedy for either of these inconveniences is easily found out, and only requires a little care and attention. The galvanoplastic dead lustre being satisfactory, two preliminary operations are needed to ensure the success of the gilding.
They consist in rapidly passing the object, after rinsing, through a solution made of water, 2 1/5 gal.; nitrate of binoxide of mercury, 1/3 oz.; sulphuric acid, 2/3 oz.; then, after rinsing, place it in another solution composed of water, 2 1/5 gal.; potassium cyanide, 14 oz.; silver nitrate, 3 1/2 oz. The object acquires a slightly white tinge in this liquor, and is again rinsed in fresh water, before being put into the following gilding bath: - Distilled water, 2 1/5 gal.; phosphate, 21 oz.; soda bisulphite, 3 1/2 oz.; potassium cyanide, pure, 1/3 oz.; gold chloride, neutral, 2/3 oz. The mode of preparation of this bath is given in the recipe for hot gilding bath. This bath should be nearly boiling, and worked with an intense galvanic current. The anode is a platinum wire, which, more or less immersed in the liquor,. allows of the regulation of the amount of electricity according to the volume, weight, and surface of the object to be gilt. This gilding requires an energetic electric action at the beginning; this is obtained by steeping the platinum wire deeply in the liquid to have the entire surface of the piece covered instantaneously; as the thickness of the deposit increases, the anode is gradually removed from the bath until it only dips in a little.
The gilding by this method has a remarkable freshness of tone. Before using the bath with battery, the zinc articles may be passed through a preparing bath; this is the same as a gold bath for dipping. Or the gilding may be done in two operations. After having deposited about half of the gold intended for the object, remove it from the bath, wash, pass again through the mercurial solution, and replace in the gold bath for finishing the gilding. After gilding, the articles are rinsed in clear boiling water for a few seconds to remove any saline matters; they are then dried in the stove, or in warm saw-dust of fir-wood. All friction should be carefully avoided, so as not to scratch the dead lustre. When parts of" this gilding are burnished, their colour is green if the frosted surface has been obtained in the silver bath, and red if the galvanoplastic bath has been employed. These inconveniences are remedied by dipping the burnished article, for a short time, into the gold bath; this last deposit of gold must be so thin as not sensibly to impair the brightness of the burnished parts. Dead lustre electrogilding upon zinc will only suit such objects as have no friction to bear, and which are not often handled; it is especially useful for clocks and similar articles, which remain under glass.
The dead lustre gilding here described can be applied to all metals and alloys, provided that those corroded by the solution of copper sulphate be previously coated with copper or brass; these previous coatings are always desirable, as they prevent the crystalline and irregular deposits often formed upon metals which are not corroded by the bath of copper sulphate. The galvanoplastic dead lustre upon copper is much finer when the pieces have been previously covered with copper or brass in the alkaline baths. Faded gildings may be renovated by dipping them into a weak tepid solution of potassium cyanide, and afterwards into very dilute nitric or sulphuric acid. Imperfect gildings may be removed by inverting the poles in a solution of cyanides,, connecting the gilt articles with the positive pole (carbon or copper), and the negative pole (zinc), with, the anode which becomes gilt. This process is employed for removing the gold from articles of iron, steel, and silver, which cannot be submitted-to the ungilding bath.
Silver, copper, and brass may also be removed by similar processes.
This operation consists in smearing, by means of a brush, the gilt and scratch-brushed, objects with a thin paste of potash nitrate, alum, and iron oxide, which have been well mixed and ground under the muller, and to which has been added a solution of saffron, annatto, or any colouring substance, according to the shade desired. If the gilding is strong and thick, the objects are heated until the previous coating curls over at the approach of a wet finger. If the gilding is a mere film, the mixture is simply allowed to stand upon the articles for a few minutes. In either case, the whole is rapidly washed in warm water holding in suspension a certain quantity of the materials for ormolu; they are then rapidly dried, when they appear of a darker shade; remove any portions too much coloured, by striking them vertically with a brush having long bristles. If the tint does not appear satisfactory, commence the operation afresh, after washing off the ormolu in dilute sulphuric acid.