ELECTROTYPE

It has been stated under the article on Voltaic Electricity, that when metallic solutions are subjected to the transmi ion of the electric current, the metallic oxides which they hold are in certain cases decomposed; so that the metal in its pure state is deposited upon the cathode, or upon what is usually termed the negative electrode. In these cases the metallic deposition is usually a secondary effect, arising out of the action of ihe hydrogen evolved at the cathode upon the oxide of the metal. A good illustrative experiment of this metallic precipitation consists in plunging two pieces of clean platinum into a solution of sulphate of copper: nothing happens. But if the electric current be transmitted through the solution, so that these platinum plates may form the electrodes, copper is immediately precipitated upon the cat electrode or negative surface, the positive surface or an electrode remaining clean. 11 the current be now reversed, the cop1309 VOLTA TYPE. per will be removed from the platinum plate upon which it had just been deposited, and will be transmitted to the clean plate; and thus by reversing the direction of the electric current, the copper may be sent backwards and forwards, it always being deposited upon the negative pole, or, in other words, upon that surface by which the electric current leaves the electrolyte. A solution of sulphate of copper is particularly well adapted to this experiment, especially if it be slightly acidulated by sulphuric acid, inasmuch as it deposits the copper in a clean metallic state; and by a continuance of the electric current, and keeping up the strength of the solution by the occasional addition of fresh portions of the salt of copper, the metallic film upon the cathode may be obtained of any required tlilckne , and may afterwards be peeled off the platinum surface. The texture of the deposited copper varies with the power employed and with the temperature and strength of the solution; so that it may be obtained hard, brittle, and crystalline, or malleable and tough, or even pulverulent, according to the management of the operator. A low electric power, or rather a current of low intensity, a moderately strong acidulated solution of sulphate of copper, and a temperature not below 60° or 70°, are the circumstances under which the best deposit of copper is usually obtained for the purposes to which this proce is commonly applied. When the negative pole or cathode, instead of being, as above represented, a plane surface of platinum, is irregular, or worked into a pattern, or a medal or coin for instance, an exact impre ion of the irregularities of surface, or of the device upon the medal or coin, maybe taken off on the precipitated copper; and to copies or plates of this kind the term volta type, or electrotype, has been more particularly applied. Gold, silver, and other metals may, by proper management, be substituted for copper, and thus a variety of volta types may be obtained; or, if the precipitated metal be left upon the surface on which it is thrown down, gilding, silvering, and coppering may be extensively and beautifully effected. Thus it is that anew art of plating has arisen also out of these electro-chemical actions. We shall now proceed briefly to point out the manipulation in a few of the principal applications of this new art, and to describe the apparatus commonly employed; illustrating the subject, in the first instance, by one of the simplest cases, in which we will suppose that an impre ion in copper is to be obtained from a copper medal, which, being in relief, will of course yield an impre ion in indent: the original being a cameo, the copy will be an intaglio. The medal is first slung in a loop of annealed copper wire, so that it maybe conveniently suspended and made electro-negative in the metallic solution; the following simple voltaic arrangements may then be used. A is a gla cylinder (a common lamp-gla answers the purpose), closed at bottom with a pieceuof bladder, or with a plug of plaster of Paris, and filled with dilute sulphuric acid (1 of acid to 8 of water by measure); Z is a piece of zinc immersed in the acid, and connected by a copper wire with the medal M; C is a jar nearly filled with a strong solution of sulphate of copper, acidulated by the addition of about a tenth part of sulphuric acid. In this arrangement, the acid in the generating cell is prevented mixing with the solution in the jar by the diaphragm of bladder or plaster; which, however, when thoroughly wetted, suffers the electric current to pa from the zinc to the medal (or negative pole), and so through the wire, in the direction of the darts, back to the zinc. In this way a film of metallic copper is gradually thrown down upon M, which goes on increasing, and which, when it is suflBciently thick (perhaps in 24 or 30 hours), the wire with Z and M at its ends is lifted out; and by the point of a knife carefully applied, and aided, if nece ary, by a file, the deposited copper is pulled off of M, of which it presents a faithful and minute copy or impre ion. To prevent the deposition of copper upon the edges, and upon the reverse of the medal, those parts should be covered with a varnish not acted upon by the solution; and the connecting wire should also be similarly varnished, to prevent its receiving an unnece ary incrustation of copper. In this way, by careful management, any number of impre ions may be taken from either side of a medal without materially injuring the original; and for the medal any other metallic plate may be substituted, provided its dimensions and weight be not such as to render it inadmi ible or inconvenient in this form of apparatus. But it often happn;ns that the article to be voltafi/pcd. ^-C^ [s. 1322]