ELECTROTYPE

Syn. Electro-met′allurgy , Galvan′o-plas′tic . The art of working in metals by the aid of electricity. Strictly speaking, the term electrotype is only applicable to one branch of ‘electro-metallurgy’—that which relates to the production of copies of engraved plates, medals, coins, and other works—but it is now commonly employed in the sense indicated by our definition. According to this extended signification of the term, the art of electrotype includes ELECTRO-PLATING , and ELECTRO-GILDING . General Principles. —If a current from a voltaic battery be pa ed, by means of platinum electrodes, through water to which some sulphuric acid has been added, electrolysis, takes place, hydrogen appearing at the cathode, and oxygen at the anode. If into the acid liquid some crystals of sulphate of copper be now thrown, electrolysis will still go on, but only one of the elements of the water, namely oxygen, will be evolved; for the hydrogen, on being released, will take the place of the copper in the solution, and the copper thus liberated will be deposited on the platinum plate or wire which constitutes the negative electrode. This experiment may be continued until all the copper is extracted from the solution. Let this experiment be repeated with a copper plate for the positive electrode, and it will be found that neither of the gases will be evolved. The hydrogen, as before, will take the place of the copper in the solution; the oxygen, instead of escaping at the anode, will combine with the copper of the electrode and the sulphuric acid to form sulphate of copper. The chemical forces called into action by the current are so beautifully balanced, that in the last experiment the quantity of copper supplied by the positive electrode exactly equals the quantity withdrawn from the solution and deposited upon the negative electrode. The whole art of electrotype consists in applying the metals thus released from their solutions to artistic or useful purposes. To obtain compact and brilliant deposits, many precautions have to be observed. The solutions must be kept saturated, or nearly so; the mould to be copied, or object to be coated, must not be too small, or out of proportion to the size of the zinc plate of the battery; in fine, the power employed must be carefully regulated according to the work to be done. In all arrangements the moulds or objects which receive the deposits act as negative electrodes, and are consequently in connection with the zinc of the battery or generating cell. Electrotype Proce es. Although reguline deposits of many metals can be obtained through the agency of voltaic electricity, we shall only treat of those of copper, silver, gold, and platinum. When copper is deposited, the object is generally to produce a substantial copy of a medal, an engraved plate, or other work of art; but when solutions containing the precious metals are electrolysed, the deposits are nearly always used for covering the surface of inferior metals. We shall notice the operations connected with the deposition of copper, and those relating to electro-plating under separate heads. 1. Deposition of copper : The moulds or models intended to receive the deposited metals may be formed of various materials. For medals and similar small works, moulds of fusible metal, white wax, stearine, stearic acid, and gutta perch a, are commonly used. The first are formed by dropping or pre ing the medals to be copied upon the melted metal, taking care that the former are quite cold, and that the surface of the metal is bright or free from oxide. To make a mould in gutta perch a, the material must be softened in warm water, and then pre ed upon the medal by means of a strong screw pre . With the other materials the manipulation is very easy. A ribbon of cardboard or thick paper is placed round the medal, so as to form a rim; the material, which has been melted in an earthen ve el, is then poured on, and allowed to remain until quite cold and hard, when it is cautiously removed. For large works, moulds of plaster of Paris are usually employed; these require to be saturated with wax or tallow, by standing them in a shallow ve el containing these substances in a melted state. For copying seals and small coins, impre ions in ordinary sealing-wax may be used as electrotype moulds. Non-metallic moulds must be coated with some substance which has the property of conducting electricity before they can be used as negative electrodes. The substance commonly employed is plumbago or black-lead. It must be in the condition of an impalpable powder. It is rubbed briskly over the surface of the mould (wax, stearine, plaster, .) by means of a strong fine camel-hair brush, till the whole presents the well-known black-lead polish. The adhesion of the plumbago may be often promoted by breathing slightly on the mould. To cause it to adhere to sealing wax impre ions, the wax may be slightly moistened with spirits of wine, or exposed to the vapour of ether. Delicate moulds and objects, which cannot well be black-leaded, may be covered with a conducting film of silver, by first dipping them in bisulphuret of carbon holding about 1 ⁄ 20 th part of phosphorus in solution, and then, after a few seconds, immersing them in a weak solution of nitrate of silver, and allowing them to dry in the light. Metallic moulds require no preparation. The voltaic apparatus used may now be described. The single-cell arrangement, used for small works, is formed on the principle of Daniell’s Constant Battery. It consists of a ve el of gla , earthenware, or wood, containing a smaller cell of thin biscuit ware, or other porous material; a rod or plate of amalgamated zinc, placed within the porous cell, and a wire connecting the zinc with the mould to be copied; the latter being placed in the outer ve el. The annexed figure represents a convenient form of the single-cell:— The battery arrangement has many advantages over that described above, and should always be employed when large objects are to be electrotyped, or when a number of small moulds are to be operated upon. In this arrangement the copper solution is electrolysed in a separate ve el, termed the decomposition cell, and the current generated by one or more cells of a Daniell’s or Smee’s battery. This arrangement is shown in the following engraving:— IMG:596405606043057898_i628-1.png: a. An oval ve el of salt-glazed earthenware or wood nearly filled with a saturated solution of sulphate of copper. b. A porous diaphragm, containing the cylinder or plate of zinc ( c ), and filled with dilute sulphuric acid. d. A small bar of bra or copper fastened to the ve el by the binding screws ( e, e ), and supporting the plate of zinc ( c ), by the hook of copper wire ( f ), and the mould ( g ), by the hook ( h ). i. A small shelf or partition to support crystals of sulphate of copper, to keep up the strength of the solution. IMG:596405606043057898_i628-2.png: a. A constant battery cell. b. Decomposition cell, a cubical ve el made of wood or earthenware, and filled with a mixture of 1 part of dilute sulphuric acid (1 acid + 9 water), and 2 parts of saturated solution of sulphate of copper by measure. c, c, c. Moulds suspended to the bra rod ( f ), and connected with the zinc or positive element of the battery ( a ), by means of the screw ( g .) d, d. Pieces of sheet copper suspended on the bra rod ( h ), and connected with the zinc end of the battery, by means of the screw ( i ), employed to keep up the strength of the cupreous solution in the decomposition cell. To connect the moulds with the zinc or positive element, stout copper wires or strips of thin sheet copper are employed. In the case of a non-metallic mould, the wire must lead directly to the plumbagoed surface, or, what amounts to the same thing, the plumbago must be extended to the point of attachment. The connecting wires, and the backs and edges of metallic moulds, must be covered with sealing-wax varnish, or other non-conducting substance, to prevent them receiving the deposit. Before a mould is placed in the copper solution it is advisable that everything should be arranged, so that the immersion may occasion immediate voltaic action. If the connection between the zinc and the mould is not effected until after the immersion, the solution may act chemically on the surface of the mould, and cause the deposit to appear dark and dirty. When a mould has remained in the solution long enough to receive a complete coating of copper, it may be lifted out with impunity for examination. If everything is going on well, the deposited metal will present a brilliant, light, copper-coloured surface. When sufficiently thick, the deposit is removed with care, washed and placed to dry. Electrotype medals may be polished with wash-leather and the plate brush, or bronzed. Various natural objects such as insects, fruits, .; small works of art, such as busts and statuettes; chemical ve els, particularly gla flasks and retorts; and numerous cla es of articles, may be rendered le fragile by coating them with copper by the electrotype proce . II. Deposition of the precious metals — The solutions generally employed as electrolytes from which silver and gold are respectively separated, are those of the argento-cyanide and the auro-cyanide of pota ium. These compounds are what chemists call double salts; for instance, cyanide of pota ium is simply a compound of pota ium and cyanogen; but argento-cyanide of pota ium is cyanide of silver united with cyanide of pota ium. When a solution of this double salt is electrolysed silver appears at one electrode and cyanogen at the other, while a proportionate amount of the simple cyanide of pota ium is formed in the solution. But if the positive electrode is of silver, the cyanogen combines with it, and forms cyanide of silver, which unites with the liberated cyanide of pota ium, and so keeps up the strength of the solution. As in the deposition of copper, the apparatus used for plating or gilding may be the single cell or the decomposition cell and battery. The nece ity of economising solutions of silver and gold has, however, led to certain modifications in the apparatus. The single-cell arrangement consists, as before (see above ), of an outer ve el of gla or earthenware, containing a cell of porous biscuit ware; but the object to be silvered or gilded is placed, with the cyanide solution in the latter, while the zinc is placed in the outer ve el, with the dilute sulphuric acid. [273] The zinc is usually employed in the form of a cylinder, completely surrounding the porous cell. In the battery arrangement the decomposition cell may be of porcelain or gla ; the silver or gold employed to keep up the strength of the solution may be in plates, wires, or ingots. For plating small objects, a single cell of a Daniell’s battery will afford ample decomposing power; gilding may be better accomplished by using three such cells. The battery arrangement is much more convenient, effective, and economical than the single-cell arrangement. [273] The strength of the acid water acting upon the zinc must be regulated according to the work to be done. If the action between the acid and the zinc be too energetic, the electricity developed will be more than sufficient to release pure metal, and hydrogen will be evolved, which will interfere with the deposition. On a large scale, electro-plating is carried out in oblong vats, occasionally holding from 200 to 250 gallons of solution. Silver plates connected with a powerful voltaic or magneto-electric battery, are placed at intervals in the vats; they form the positive electrodes, and correspond in extent of surface with the articles to be coated, and face them on both sides. The articles (tea-pots, cruet-frames, forks, spoons, .) act as the negative electrodes, and are suspended by copper wires from bra rods laid lengthways over the vats, and connected with the battery. The articles plated are usually formed of nickel silver or German silver, which is chosen on account of its silvery whitene , a quality of great importance when portions of the coating of noble metal have been worn away by use. To prepare the articles for plating, they are first boiled in a solution of pota a, to free them from grease; they are then quickly dipped in red nitrous acid, to remove any oxide that may have formed on the surface, and after this well washed in water, to remove every trace of acid. They are then suspended from copper wires, and dipped into a solution of mercury in cyanide of pota ium, or some other mercurial solution, and afterwards washed in water, as before. The amalgamation of the surface effected by the last operation promotes the adhesion of the film of silver. The articles having been weighed, are now immersed in the silvering solution, and left until a sufficient amount of silver has been deposited on them. Their condition at any time may be ascertained by weighing a test-object removed from the solution. In some electro-plating establishments the silvering solution is kept constantly stirred by simple mechanical arrangements; in others, continual motion is given to the suspended articles. On being removed from the vats the plated articles are well brushed with brushes of fine bra wire attached to a lathe, and cleaned with fine Calais sand; they are afterwards polished on revolving brushes with rottenstone, then by hand with soft leather and rouge, and, lastly, with the naked female hand. A lasting polish is given to some articles by burnishing with a burnisher formed of highly polished hardened steel, bloodstone, agate, or flint. The proce of electro-gilding on the large scale is nearly the same as that of electro-plating or silvering, but, of course, plates of gold are suspended in the solution instead of silver plates. Various solutions for silvering, plating, and platinising, have been recommended. We give below those generally employed. 1. Solvent solution. Cyanide of pota ium, 2 oz; distilled water or rain water, 1 pint; di olve. Other proportions may be employed. Used as a general solvent for salts of silver, gold, and platinum. 2. Silver solution. Oxide of silver? [274] (not dried), 1 oz.; the solvent solution (No. 1), 1 pint. Used for the single-cell apparatus, its strength being maintained as the deposition proceeds by a fresh supply of oxide from time to time. [274] Precipitated from pure solution of nitrate of silver by exce of lime water. It should be well washed, and preserved in bottles with distilled water. Cyanide of silver di olved in solvent solution (No. 1). This is the solution generally employed for plating with a separate decomposition cell. 3. Gold solution. Add to a pint of No. 1 oxide of gold, 1 ⁄ 4 oz. Used in the same manner as the second silver solution. Cyanide of gold di olved in solution of cyanide of pota ium (No. 1). Used as last. 4. Platinum solution. The double chloride of platinum and pota ium, di olved in solution of caustic pota a. Other solutions have been proposed, but this appears to be decomposed with the greatest ease. The above sketch of the electrotype art is nece arily very imperfect. For minute details respecting manipulation, the reader is referred to the excellent treatises on the subject that have been written; more particularly to Ernest Spon’s valuable work, entitled ‘Workshop Receipts.’