Although this process is called “silvering,” yet no silver is used; the substance at the back of the glass is a mixture of tin and mercury, called an “amalgam,”—indeed, the term “amalgam” applies to all mixtures of mercury with other metals. The process is as follows:—A sheet of “tin-foil” (tin rolled out to about the thickness of paper) the size of the glass to be silvered is placed on a perfectly level table, covered with cloth; upon the tin-foil some mercury is poured, and spread evenly and quickly over the surface with a hare’s foot. The plate of glass is in the meanwhile to be made perfectly clean and dry—not the slightest speck or smear must remain. A sheet of tissue paper, also clean and dry, is laid over the surface of the mercury, the plate of glass is placed on the paper and made to correspond with the mercurialised tin-foil beneath. Weights are now placed on the plate of glass to keep it firmly down, and the sheet of paper is drawn out steadily and slowly; as it passes over the surface of the mercury it brings away all film or dust, and the surface being left perfectly bright, adheres so firmly to the dry glass that it is not easily removed. A great difficulty is to prevent air-bubbles from finding their way between the glass and mercury. The table is now raised slightly at one end, and the superfluous mercury allowed to drain off. After a few hours the tin-foil will be found to be completely united all through with the mercury, and will be so brittle that it can be scraped off in powder from the glass. Great care and practice are required to silver large plates, but any one by a few trials may succeed perfectly with a piece of glass a few inches square. Glass globes are silvered inside by shaking in them a mixture of mercury and tin filings until it adheres to the surface of the glass, which must first be made perfectly dry and warm.
SILVER PLATING.
Plated goods consist of metallic articles coated with a thin plate of silver; the metal is made of a mixture of brass and copper, which is cast into flat slabs or ingots about an inch-and-a-half thick, the surface on one or both sides is filed flat and smooth, and a plate of silver of about the thirtieth part of an inch thick, but a little smaller than the metal, is applied smoothly to it, the edges are covered all round with borax ground fine with water and the plates tied tightly together with wire. The whole is then put into a furnace and closely watched till the silver begins to melt, when it is at once taken out and allowed to cool; by this mode of treatment the silver adheres so firmly to the metal that they become as one piece. It is then passed between steel rollers and rolled out to the required substance, the silver and metal both becoming thinner in about the same proportion, so that on a plate of metal, of whatever thickness, the silver is somewhere about a fortieth or forty-fifth part of its thickness; these plates of metal coated with silver are worked by stamping, punching, or passing between rollers the edges of which have mouldings, curves, &c., cut on them, and the parts of each article when moulded are afterwards soldered together so as to form what is intended. Wires of various forms are plated in the same way and afterwards drawn out by means of draw-plates (see “[Wire-drawing]”). Electro-plating has to a great extent superseded this process (see “[Electro-plating]”).
ELECTRO-PLATING AND DEPOSITION OF METALS.
This art has for its objects the coating of metallic articles with other metals of more value, beauty, or durability, such as gold, silver, or copper, by means of electricity, and the formation (by the same means) of other articles by the deposition of metals, from liquids containing them, upon moulds or engraved surfaces capable of modelling them. When the deposit forms a coating intended to be permanent, and which adheres to the article so as to be incapable of removal, it is called “electro-plating,” but when a fac-simile of any surface is required, or a cast of a mould which may be removed, forms the object to be produced, it is called “electrotyping” or “electro-depositing.”
FIG. 1.