Page:The American Cyclopædia (1879) Volume VII.djvu/612

 600 GALVANISM ting and gilding, and electrotyping. In electro- plating and gilding the deposited metal is usually retained upon the surface it is deposited upon, while in electrotyping it is subsequently re- moved from such surface, which is used as a mould of which the deposit forms a reverse copy. Electroplating and Gilding. Gilding was formerly done by covering the metal to be gilt with an amalgam of gold and mercury and volatilizing the latter metal, and the same process was employed in silvering. Brugna- telli, a pupil of Volta, is said to have been the first, in 1803, to gild the baser metals by means of the galvanic current ; but De la Kive was the first to make the process successful. Its present state of perfection, however, is due to Elkington, Ruolz, and others. The pro- cesses of electroplating and electrogilding are almost identical, and depend upon the power of inducing an electro-negative condition upon a surfaqe, usually metallic, which causes it to attract the electro-positive or metallic con- stituent of a salt; and also of inducing an electro-positive condition in a neighboring sur- face, causing it to attract the electro-nega- tive or non-metallic constituent. The positive plate in a battery, or the one upon which the chemical action takes place, and which is con- nected with the negative electrode, must there- fore be joined by the latter to the plate upon which it is desired to deposit the metal, the other or negative battery plate being connected with the plate in the bath upon which an elec- tro-positive condition is to be induced. The tendency in the bath, as the liquid is called which contains the metallic salt from which by electrolysis the metal is deposited, is usually to create an electromotive force acting in a di- rection contrary to the battery current ; there- fore the electromotive force of the latter must be sufficient to overcome the former, and also to effect decomposition. In choosing a bath solution, therefore, it is desirable to take one the tendency of whose action is to create as little opposing electromotive force as possible, and this is usually done by choosing an alka- line instead of an acid solution. It requires a powerful battery to cause a deposit of metallic gold to be formed from a solution of the chlo- ride upon the perfect metallic surface of an iron plate ; but by employing a cyanide of the metal dissolved in cyanide of potassium, the deposition is practicable with the use of a small battery or a single couple. If a clean piece of iron is dipped in a solution of sulphate of copper, it becomes coated with a film of the latter metal; but this is not an example of true electropla- ting, the copper not being deposited upon the clean metallic surface of the iron, but upon a film of the oxide or of the sulphate of that metal which is formed by the action of the sulphuric acid contained in the sulphate of copper. No chemical action must be suffered to take place upon the iron plate ; but while in a clean con- dition it must have negative electricity induced upon it sufficient to cause it, instead of attract- ing the acid, to attract the metallic constituent of the solution. It is usual in practice, in electro-silvering or gilding iron, to first deposit a thin coating of copper, which is more easily attached to the iron than the more precious metals, and forms a better surface for their deposition. An article of copper, which may be a cast of some object, or a piece of iron or one of the baser metals, after having been first electroplated with copper from a solution of the sulphate or of the cyanide, is plated with silver in the following manner: Cyanide of potassium being carefully added to a solution of nitrate of silver, a precipitate of cyanide of silver, AgOy, is formed, which, being washed in distilled water, is dissolved in a solution of cyanide of potassium, by which there is formed a double salt of cyanide of silver and potas- sium, AgKCya ; or lime water may be added to the solution of nitrate to precipitate oxide of silver, which may then be dissolved in cya- nide of potassium. Another mode of forming the solution is to add a solution of common salt to that of nitrate of silver, and dissolve the chloride in cyanide of potassium. In either case AgKCy 2 is formed. The object to be FIG. 30. Simple Bath. plated is connected with the negative elec- trode of a battery or a magneto-electric ma- chine and suspended in a hot solution of the cyanide, and a plate of silver connected with the positive electrode is suspended near it, as represented in fig. 30. The object to be plated, now forming the negative electrode of the electrolyte cell, attracts the metallic silver of the cyanide; the potassium, the most electro- positive of the three bodies, remaining com- bined with the cyanogen, the two forming the electro-negative constituent of the compound AgKCy 2. The reactions which take place may be represented in the following diagram : Copper or I Ag | KCy 2 Ag I KCy 2 Ag IKCy, I Silver iron plate | + | + | + | | plate. The body KCy 2, which is liberated upon the surface of the silver plate, immediately com- bines with that metal, and thus the solution is continually replenished with the double cya- nide. For gilding, a solution of auro-cyanide of gold, AuKOy 2, corresponding to the silver salt, may be formed in a similar manner. A plate of gold forms the positive, and the object which is to be coated the negative electrode.