PLASTER OF PARIS. (See GxPsuie.) PLATED MANUFACTURE. The sil ver in this case is not applied to ingots of pure copper, but to an alloy of copper and brass, which possesses the requisite stiffness for the various articles. The furnace used for melting that al loy, in black-led crucibles, is a common air-furnace, like that for making brass. The ingot-moulds are made of cast iron, in two pieces, fastened together ; the cavity being of a rectangular shape, 8 inches broad, 11 thick, and 18 or 20 long. There is an elevated mouth-piece or gate, to give pressure to the liquid metal, and secure solidity to the ingot. The mould is heated, till the grease with which its cavity is besmeared merely begins to smoke, but does not burn. The proper heat of the melted metal for casting, is when it assumes a bluish color, and is quite liquid. Whenever the metal has solidified in the mould, the wedges that tighten its rings are driven out, lest the shrinkage of the ingot should cause the mould to crack. (See BRAss.) The ingot is now dressed carefully with the file on one or two faces, according as it is to be single or double plated. The thickness of the silver plate is such as to constitute one-fortieth of the thickness of the ingot ; or when this is an inch and a quarter thick, the silver plate applied is one thirty-second of an inch ; being by weight a pound troy of the former, to from 8 to 10 pennyweights of the latter. The silver, which is slightly less in size than the copper, is tied to it truly with iron wire, and a little of a saturated solu tion of borax is then insinuated at the edges. This salt melts at a low heat, and excludes the atmosphere, which might oxydize the copper, and obstruct the union of the metals. The ingot thus prepared is brought to the plating fur nace.
The furnace has an iron door with a small hole to look through ; it is fed with coke, laid upon a grate at a level with the bottom of the door. The ingot is placed immediately upon the coke, the door is shut, and the plater watches at the peep hole the instant when theproper solder ing temperature is attained. During the union of the silver and copper, the sur face of the former is seen to be drawn in to immediate contact with the latter, and this species of riveting is the signal for re moving the compound bar instantly from the furnace. Were it to remain a very lit tle longer, the silver would become alloy ed with the copper, and the plating be thus completely spoiled. The adhesion is, in fact accomplished here by the for mation of a film of true silver-solder at the surfaces of contact.
The ingot is next cleaned, and rolled to the proper thinness between cylinders ; being in its progress of lamination fre quently annealed on a small reverberatory hearth. After the last annealing, the sheets are immersed is hot diluted sul phuric acid, and scoured with fine Calais sand ; they are then ready to be fashioned into various articles.
In plating copper wire, the silver is first formed into a tubular shape, with one edge projecting slightly over the oth er; through which a red-hot copper cyl inder being somewhat loosely run, the silver edges are closely pressed together with a steel burnisher, whereby they get firmly united. The tube, thus completed,
is cleaned inside, and put on the proper copper rod, which it exactly fits. The copper is left a little longer than its coating tube, and is grooved at the ex tremities of the latter, so that the silver edges, being worked into the copper groove, may exclude the air from the sur face of the rod. The compound cylinder is now heated red-hot, and rubbed brisk ly over with the steel burnisher in a longitudinal direction, whereby the two metals get firmly united, and form a solid rod, ready to be drawn into wire of any requisite fineness and form ; as flat, half round, fluted, or with mouldings, accord ing to the figure of the hole in the draw Such wire is much used for mak • ing bread-baskets, toast-racks, snuffers, and articles combining elegance with lightness and economy. The wire must be annealed from time to time during the drawing, and finally cleaned, like the plates, with diluted acid.
The greatest improvement made in this branch of manufacture, is the introduc tion of silver edges, beads, and mould ings, instead of the plated ones, which from their prominence had their silver surface speedily worn off, and thus assumed a brassy look. The silver destined to form the ornamental edgings is laminated ex ceedingly thin ; a square inch sometimes weighing no more than 10 or 12 grains. This is too fragile to bear the action of the opposite steel dies of the swage above described. It is necessary, therefore, that the sunk part of the die should be steel, and the opposite side lead, as was observed in the stamping ; and this is the method now generally employed to form these silver ornaments. The inside shell of this silver moulding is filled with soft solder, and then bent into the requi site form: The base of candlesticks is generally made in a die by the stamp, as well 85 the neck, the dish part of the nozzle or socket, and the tubular stem or pillar. The different parts are united, some with soft and others with hard solder. The branches of candlesticks are formed in two semi-cylindrical halves, like the feet of tea-urns. When an article is to be en graved on, an extra plate of silver is ap plied at the proper part, while the plate is still flat, and fixed by burnishing with great pressure over a hot anvil. This is a species of welding.
The last finish of plated goods is given by burnishing-tools of bloodstone, fixed in sheet-iron cases, or hardened steel, finely polished.
The ingots for lamination might proba bly be plated with advantage by the deli cate pressure process employed for silver laid on by electric deposition; and old articles are re-silvered by this process which can not occur in the old way. For a description of the process, see the article ELECTRO-METALLtRGY.