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Encyclopedia Britannica - Main :: GOA-GRA |
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GOLDBEATING .The art of goldbeating is of great antiquity, being referred to by Homer; and Pliny (N.H. 33. 19) states that 1 oz. of gold was extended to 750 leaves, each leaf being four fingers (about 3 in.) square; such a leaf is three times as thick as the ordinary leaf gold of the present time. In all probability the art originated among the Eastern nations, where the working of gold and the use of gold ornaments have been distinguishing characteristics from the most remote periods. On Egyptian mummy cases specimens of original
The gold used by the goldbeater is variously alloyed, according to the colour required. Fine gold is commonly supposed to be incapable of being reduced to thin leaves. This, however, is not the case, although its use for ordinary purposes is undesirable on account of its greater cost. It also adheres on one part of a leaf touching another, thus causing a waste of labour by the leaves being spoiled; 'but for work exposed to the weather it is much preferable, as it is more durable, and does not tarnish or change colour. The external gilding on many public buildings, e.g. the Albert Memorial in Kensington Gardens, London, is done with pure gold. The following is a list
Name of Leaf. Proportion Proportion Proportion of Gold. of Silver. of Copper. Grains. Grains. Grains. Red . . . . 456-460 .. 20-24 Pale red . 464 .. 16 Extra deep 456. 12 12 Deep 444 24 12 Citron 440 30 10 Yellow . . 408 72 Pale yellow . 384 96 Lemon . . 36o 120 Green or pale . 312 168 . . White 240 240 The process of goldbeating is as follows: The gold, having been alloyed according to the colour desired, is melted in a crucible at a higher temperature than is simply necessary to fuse it, as its malleability is improved by exposure to a greater heat; sudden cooling does not interfere with its malleability, gold differing in this respect from some other metals. It is then cast into an ingot, and flattened, by rolling between a pair of powerful smooth steel rollers, into a ribbon of 11 in. wide and 10 ft. in length to the oz. After being flattened it is annealed and cut into pieces of about 62 grs. each, or about 75 per oz., and placed between the leaves of a " cutch," which is about a in. thick and 32 in. square, containing about 180 leaves of a tough paper . Formerly fine vellum was used for this purpose, and generally still it is interleaved in the proportion of about one of vellum to six of paper . The cutch is beaten on for about 20 minutes with a 17-lb hammer
in. thick, containing about 720 skins, which have been worn out in the finishing or " mould " process. The shoder requires about two hours' beating upon with a 9-lb hammer
instrument called a waggon, the leaf being laid upon a leathern cushion. The leaves thus prepared are placed into " books " capable of holding 25 leaves each, which have been rubbed over with red ochre to prevent the gold clinging to the paper. Dentist gold is gold leaf carried no farther than the cutch stage, and should be perfectly pure gold.By the above process also silver is beaten, but not so thin, the inferior value of the metal not rendering it commercially desirable to bestow so much labour upon it. Copper, tin, zinc, palladium , lead, cadmium, platinum and aluminium can be beaten into thin leaves, but not to the same extent as gold or silver.The fine membrane called goldbeater's skin, used for making up the shoder and mould, is the outer coat of the caecum or blind gut of the ox. It is stripped off in lengths about 25 or 30 in., and freed from fat by dipping in a solution of caustic alkali and scraping with a blunt knife. It is afterwards stretched on a frame
The dryness of the cutch, shoder and mould is a matter of extreme delicacy. They require to be hot-pressed every time they are used, although they may be used daily, to remove the moisture which they acquire from the atmosphere, except in extremely frosty weather, when they acquire so little moisture that a difficulty arises from their over-dryness, whereby the brilliancy of the gold is diminished, and it spreads very slowly under the hammer. On the contrary, if the cutch or shoder be damp
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