Plating or Plated Manufacture. The art of covering other metals with silver. The method known by the name of French plating was usually applied to articles made of brass, after they were, in other respects, finished. After the goods were polished, and perfectly free from grease, etc, the part to be plated was heated to a temperature somewhat short of changing the colour of the metal. Leaf silver was now laid upon the part, and, while hot, was rubbed on with a hardened steel burnisher, perfectly dry and clean. By this means the silver adhered firmly to the brass, which, from the action of the burnisher, assumed a fine polish: these had much the appearance, in colour and lustre, of those of the present day; but they possessed but little permanence this art is, therefore, scarcely now practised, from the introduction of the superior plan of plating upon ingots of copper, and forming the utensils out of the sheets and the wire made from the ingots.
The plated manufacture is divided into three departments, to each of which there is a distinct set of workmen. Those employed in making vessels such as are required to be raised by the hammer, are called braziers. The second sort are called candlestick-makers, being exclusively employed in making all the varieties of these articles. The third are called pierce workers; these were originally employed in making articles with ornamental open work, such as bread-baskets and trays of different kinds; but this species of work has now become obsolete, since the invention of plated wire. The articles in which pierce work had been made, are now formed by the varied intersections of wires, which give great lightness and elegance, with less waste, and more expedition.
Previously to describing the different branches of this art, we shall give the method of preparing the plated sheets and wire, of which all the different articles are made. The ingots on which the silver is laid are not pure copper, but an alloy, consisting of copper and brass; this gives it a degree of stiffness greater than that of copper, which renders it less liable to be deformed when in use. The metals are melted to a proper heat, in a peculiar furnace appropriated to that purpose. The heat of the metals, and the temperature of the mould when the metal is poured, are of great importance as far as regards the soundness of the ingot. When the metal is too cold, and its liquidity of course imperfect, the impurities cannot freely ascend, which causes imperfection in its substance. The same effect may take place from the moulds being cold; this, with the great conducting power of the metal mould, rapidly robs the metal of its caloric, and lessens its liquidity. The proper heat for the moulds is somewhat short of burning the fat with which they are greased on the interior surface. For the ordinary kind of work these ingots are generally cut in two in the middle, being more convenient for plating than longer pieces.
The next process is to dress the face of the ingot for the purpose of receiving the silver on one or both sides, as it may be intended to be single or double plated. This is effected by filing, which is continued till the surface becomes entirely free from the least blemish: this is so important that the naked eye should not be depended upon; the surface of the copper should, therefore, be minutely examined by a magnifier before the silver is laid on. The thickness of the silver to be laid on the copper will be best known when it is understood that the silver, in single plated metal, or that plated on one side only, is from eight to ten pennyweights to the pound troy of copper; and, of course, double quantity when plated on both sides. When the plate of silver is cut to a little less than the size of the copper surface, made flat and scraped perfectly clean, the copper surface being equally clean, they are laid together, and the silver plate is tied down with wire. A little of a saturated solution of borax is now insinuated under the edge of the silver plate on each side; this fuses at a low red heat, and prevents the oxygen of the atmosphere from affecting the surface of the copper, which would prevent the adherence of the silver.
In this state the ingot is brought to the plating furnace; this furnace has a grate on a level with the bottom of the door. The fuel consists of cokes. The ingot is laid upon the bare cokes, and the door shut. When it has acquired nearly a proper degree of heat, the plater applies to the hole in the door to observe the proper point when the process is finished. When the silver and copper are uniting, the surface of the former begins to be rivetted, and this is the sign to remove the ingot from the fire as quick as possible. The ingot being now plated, is made perfectly clean, and is ready to be rolled. The first rollers employed for plated metal are of cast iron, similar in size and construction to those employed for sheet iron and sheet copper. The metal is rolled cold, and annealed from time to time. When it has gone through the rollers a certain number of times, it acquires a certain degree of hardness, so that the rollers have not much effect upon it; and if the rolling were continued, the metal would crack. To remedy this evil the metal is taken to a reverberatory furnace: it is laid upon a hearth of brick or fire-stone, and the flame of coal is made to pass over it; the heat, however, is not intense, since the metal is required to be slowly heated to a dull red.
It may now be cooled in the quickest way possible, to save time, as quenching in water does not harden it, as is the case with steel. It now passes through the rollers as before, till it becomes hard, and then annealed and rolled again, till it is reduced something short of the size required. This being done, it is again annealed and passed through a pair of rollers faced with cast steel, and finely polished: this gives the surface great smoothness and truth. It is now annealed for the last time: after this the sheets are immersed in hot dilute sulphuric acid, then scoured with fine Calais sand, which fits them for the workmen to shape into different articles.