SILVERING, as a handicraft, is not always an example of eilver working, as the name would seem to imply ; for in silvering looking. glass, no silver whatever is used, in the ordinary methods.
One mode of silvering, first adopted at Sheffield, is that of rubbing an amalgam of silver and mercury on the surface of any article of copper ; by the application of heat the mercury is driven off; the sil ver, remaining behind, adheres firmly to the copper, and is susceptible of receiving a high polish. This is, chemically, analogous to the process of metal-gilding, or so-called " water-gilding,' described under GILDING. The details, in the same article, concerning the application of leaf-gold, will nearly apply also to that of leaf-silver in the silvering of articles made of wood.
In an ordinary looking•glass, the reflection is derived from a film of mercury or quicksilver, in contact with the hinder surface of the glass, and fixed by amalgamating with a sheet of tiu-foil. Silvering is here not a correct term, for there is no silver employed. The process is nevertheless a highly curious one. In the first place, a large sheet of tin-foil is unrolled, and laid down on a perfectly flat and very smooth stone table. Liquid mercury is poured on the foil, from the iron bottles in which it is imported, and made to float over the entire sm• face. The glass, made perfectly clean, is laid upon the mercury with a peculiar sliding movement, which suffices to remove the slight film of oxide which soon forms upon the surface, and also any air bubbles. The glass is then entirely covered with heavy leaden weights, which could not be done with safety unless the glass and the stone were perfectly flat. After remaining a day or two in this state, with the stone slightly inclined, it is found that all the superfluous mercury has been pressed out from between the glass and the foil ; and, more. over, that the mercury has chemically combined with the foil, in such a way that both adhere firmly to the back of the glass. .
In Mr. Drayton's process, nitrate of silver is combined with spirit and certain liquids, and is poured on the clean surface of the glass : a border of putty or some other substance being laid round the glass to retain the liquid. After remaining thus a few hours, the liquid is
poured off, and a sediment of silver is found to be left adhering to the glass. This sediment or film is secured in its place by a varnish of bees' wax and tallow.
In Kidd's embroidered glass, the peculiarity of the process is that the patterns have the appearance of being in relief, or embossed on the exterior surface, and illuminated in frosted and burnished silver, whereas the whole of the processes are effected on the under surface.
Dr. Thomson's silvered vessels display great brilliancy and beauty. Glass vessels of any shape, and made of glass of any colour, are silvered within in such a way as to yield a reflection of great lustre. Some of the specimens, in which green and ruby glass are thus silvered, produce an effect which can hardly be paralleled in any othpr manufacture. The surface is often richly cut and diversified, and the silvering may be made to appear at any spots selected by the workman. The silvering agent is one of the salts or compounds of silver, as in Drayton's pro cess; but arrangements of an intricate kind arc requisite to the due production of the required effects.
Dr. Faraday bas described at the Royal Institution a mode of silvering introduced by M. Petitjean. A solution of oxide of silver, ammonia, nitric acid, and muriatic acid is employed. A cast-iron table-box is provided, with water within and gas underneath. The sheet of glass to be silvered is laid upon the iron plate and heated to 140° Fehr. by gas flames acting through or on the water. The solution is poured on the glass, together with a little polishingposvder. NVIien well floated, the solution is rolled with an India-rubber roller to expel all air-bubbles. Then more solution Ls poured on, and more heat applied, until a precipitate of pure silver is deposited on the glees, which takes place in about a quarter of an hour. The surplus liquid being poured off, and the silver washed, it is found to present a beauti ful reflecting surface. No mercury being hero employed, the process is less harmful than that usually adopted.