Postby dognose » Thu Oct 29, 2020 11:58 am
PRACTICE OF ELECTRO-METALLURGY
Mr. Napier, superintendent of the scientific department of Messrs. Elkington's Works, has explained at the Royal Institution, some curious points in the practice of electro-plating,—a new application of electricity in reducing metals from ores which, like that of copper, can be fused by a flux.
1. As to Electro-plating: —Mr. Napier commenced by noticing the known difference between solid and liquid conductors of electricity — i. e., that the latter are decomposed by the current which they convey ; on this property the principle of all the electro processes rests. The mode in which the copper is deposited from the liquid sulphate on a surface connected with the zinc terminal of the battery was then illustrated theoretically by a diagram, and practically by a large sheet of cloth covered by these means. The object of this fabric is to furnish a roofing for houses, lining and ornamenting rooms, and covering railroad carriages ; —not only waterproof, but also not liable to be set on fire by sparks falling on it. Mr. Napier here noticed the difficulty of maintaining that equable diffusion of the copper-salt through the solution which should insure the uniform deposit of the metal. This can only be effected by keeping either the liquid or the article in constant motion, or else by placing the latter horizontally at the bottom of the former—care being always taken to insure the solution being constantly saturated by suspending crystals of the salt in it. In plating goods with gold or silver, recourse is had to the cyanides of those metals. The preparation of the cyanide of potassium from the common yellow ferro-cyanide was described. This salt separates silver from the nitrate, and gold from the chloride, forming the required cyanide. An instantaneous gilding of several articles was effected before the audience. The subsequent process of brushing and burnishing, by which the soundness of the work is tested, were then exhibited.
2. As to the fabrication of solid silver articles: —On a model of metal, or plaster of Paris, or any other suitable material, is poured a compound of 12 parts of glue and 3 of treacle, melted together. This, when cooled, forms a perfectly flexible mould, from which any sculptured surface, even if there be much under-cutting on it, can be easily detached. Into a mould so prepared is poured a mixture of 3 parts tallow, 1 wax, and ½ resin. This dissolves at a low temperature ; and when liquid, and previously to being poured into the mould, it receives half an ounce of phosphorus, dissolved in sulphuret of carbon. This, diffused through the melted mass thus described, gives it the property of reducing silver from its nitrate. The new model, then, taken from the mould, is moistened with nitrate of silver, and becomes covered with a thin film, of that metal, on which copper is deposited by the battery-current. When this second mould is considered sufficiently thick, the fusible compound is melted away, the copper mould is protected at the back by non-conducting surface, generally a mixture of pitch and tar. Silver is then deposited within this mould, of any required solidity, from the solution of cyanide of silver and the battery-current ; and, finally, the copper mould is dissolved by the perchloride of iron, leaving the silver pure. By the same process, delicate organic textures are gilded. In all cases where it is desired to insure perfect metallic coating, the article, (after having been washed with the sulphuret of carbon and phosphorus,) is immersed first in a solution of nitrate of silver, and then of chloride of gold, both very dilute.
3. As to the reduction of copper, etc. from their ores by electricity : —Mr. Napier has proposed the following process for applying this discovery to practical purposes. He mixes the roasted ore with soda and lime, and places the whole on a bed of blacklead tiles in a reverberatory furnace ; these are connected with the zinc terminal of the battery, and the surface of the mass, when fused, is covered with an iron plate, which is put into connexion with the copper terminal. At the expiration of a period depending on the power of the battery, the complete extraction of the copper takes place, which is found collected at the bottom, amounting to, according to present experience, from 12 to 16 times the weight of the zinc dissolved in the battery-cell. Whether these results are dependent throughout on the direct electrolyzation of the fused mass, or if electrolyzation be the primary effect, deranging the constitution of the compounds, and which, in connexion with the intense heat, produces the results referred to, Mr. Napier cannot as yet say.
Source: The Year-Book of Facts in Science and Art - 1846
Trev.