3705. To Cleanse Articles for Electroplating

3705. To Cleanse Articles for Electroplating. Articles that are to be plated are first boiled in an alkaline lye, to free them from grease, then washed from the ]ye, and dipped into dilute nitric acid, which removes any oxide that may be formed upon the surface; they are afterwards brushed over with a hard brush and fine sand. (See No. 3381 (To Frost Watch Movements).) The alkaline lye should bo in a caustic state, which is easily effected by boiling the carbonated alkali with slacked lime, until, on the addition of a little acid to a small drop of the solution, no effervescence occurs. (See No. 101 (To make Caustic Potash).) The lime is then allowed to settle, and the clear liquor is fit for use. The lye should have about 1/2 pound soda-ash, or pearl-ash, to the gallon of water. The nitric acid, into which the article is dipped, may be diluted to such an extent that it will merely act upon the metal. Any old acid will do for this purpose. In large factories the acid used for dipping before plating is generally afterwards employed for the above purpose of cleaning.

3706. To Prepare Articles for the Decomposing Cell

3706.    To Prepare Articles for the Decomposing Cell. The article being thoroughly cleaned and dried, has a copper wire attached to it, either by twisting it round the article or putting it through any open part of it, to maintain it in suspension. It is then dipped into nitric acid as quickly as possible, and washed through water, and then immersed in the decomposing cell containing silver solution, suspending it by the wire which connects with the zinc pole of the battery. The nitric acid generally used and found best for dipping has a specific gravity 1.518, and contains 10 per cent, sulphuric acid. The article is instantaneously coated with silver, and ought to be taken out after a few seconds and well brushed. On a largo scale, brushes of brass wire attached to a lathe are used for this purpose; but a hard hair brush with a little fine sand will do for small work. This brushing is used in case any particle of foreign matter may bo still on the surface. It is then replaced in the solution, and in the course of a few hours a coating of the thickness of tissue paper is deposited on it, having the beautiful matted appearance of dead silver. Any thickness of silver may be given to a plate by continuing the operation a proper length of time. 11/4 to l1/2 ounces of silver to the square foot of surface will give an excellent plate about the thickness of ordinary writing paper. "We may remark that, in depositing silver from the solution, a weak battery may be used; though when the battery is weak the silver deposited is soft, but if used as strong as the solution will allow, the silver will be equal in hardness to rolled or hammered silver. If the battery is stronger than the solution will stand, or the article very small compared to the size of the plate of silver forming the positive electrode, the silver will be deposited as a powder. Gas should never be seen escaping from either pole; and the surface of the article should always correspond as nearly as possible with that of the positive electrode, otherwise the deposit runs the risk of not being good; it requires more care, and the solution is apt to be altered in strength, because if the positive electrode be large compared with the negative, the solution will become stronger in silver, while if smaller in proportion the solution will become exhausted of silver.