3716. Taking Silver from Copper, Etc

3716.     Taking Silver from Copper, Etc. First by stripping or dissolving it off; this is done by putting into a stoneware or copper pan some strong sulphuric acid (vitriol), to which a little nitrate of potassa is added; the article is laid into this solution, which will dissolve the silver without materially affecting the copper; nitrate of potassa is added by degrees, as occasion requires; and if the action is slow a little heat is applied to the vessel. The silver being removed, the article is well washed and then passed through the potash solution, and finished for plating. When the sulphuric acid becomes saturated with silver it is diluted, and the silver is precipitated by a solution of common salt; the chloride of silver formed is collected and fused in a crucible with carbonate of potash, when the silver is. obtained in a metallic state, as a knob or button. The crucible should not bo over two-thirds full, and should be kept in fusion till effervescence ceases. The crucible is then removed from the fire, and, when cool, it is broken. (See No. 3702 (To Recover Silver from Solution).) The article thus stripped by acids often shows a little roughness, not from the effects of the acid, but because the copper under the silver has not been polished; it is therefore a necessary practice in the electroplating factories to polish the articles before plating. This is done by means of a circular brush, more or less hard as required, fixed upon a lathe, and a thin paste made of oil and pumice-stone ground as fine as flour. By this process the surface of any article can be smoothed and polished; but a little experience is required to ensure success, and enable the operator to polish the surface equally without leaving brush marks. After this the article must be cleaned in potash before it is plated.

3717. To Recover Silver from Copper

3717.   To Recover Silver from Copper. Instead of stripping off the silver by means of acid, as in No. 3716, it is a more common and preferable mode to brush off the silver by the operation just described. In this case the brushings must be collected, dried,.and burned; this may be done in an iron pan. keeping it at a red heat until all carbonaceous matters are consumed ; the remainder is fused with carbonate of soda or potash, when the silver is obtained, in combination with a little copper.

3718. Cyanide of Silver and Potassium, its Decomposition During the Plating Process

3718.    Cyanide of Silver and Potassium, its Decomposition During the Plating Process. The silver salt in the plating solution is a true double salt, being, as already described, a compound of 1 equivalent of cyanide of silver, and 1 of cyanide of potassium - two distinct salts. In the decomposition of the silver solution by the electric current, the former, cyanide of silver, is alone affected; the silver is deposited, and the cyanogen passes to the positive plate or electrode. The cyanide of potassium is therefore set at liberty upon the surface of the article receiving the silver deposit, and its solution, being specifically lighter than the general mass of the plating solution, rises to the top; this causes a current to take place along the face of the article being plated. If the article has a flat surface, suppose that of a waiter or tray, upon which a prominence exists, as a mounting round the edge, it will cause lines and ridges from the bottom to the top. Newly formed solutions are most subject to produce this annoyance.