The silver articles, previously cleansed and scratch-brushed, are boiled for about 1/2 hour in the gold bath of pyrophosphate, to which add a few drops of sulphurous acid, or, preferably, hydrocyanic acid, in excess of the quantity needed by the primitive bath. This gilding is very fine, but without firmness. The deposit is rendered more rapid and thicker when the silver articles are stirred with a rod of copper, zinc, or brass.
Preparing Silver Parts, - Marks of the file are obliterated by a rubbing upon a wet stone, and lastly upon an oilstone. Any oil or grease is removed by boiling the parts for a few minutes in a solution made of 100 parts water and 10 of caustic soda or potash; rinse in clean water, which should wet them entirely if all the oil has been removed. The articles are threaded upon a brass wire; cleanse them rapidly in the compound acids for a bright lustre, and dry them carefully in white wood saw-dust. The pieces are fastened upon the even side of a block of cork by brass pins with fiat heads. The parts are then thoroughly rubbed over with a brush, entirely free from grease, and charged with a paste of water and very fine pumice powder.
Move the brash in circles, in order not to rub one side more than the other; thoroughly rinse in clean water, and no particle of pumice should remain upon the pieces, or the cork. Next place the cork and the pieces in a weak mercurial solution, which very slightly whitens the copper, composed of - water, 2 1/5 gal., nitrate of binoxide of mercury, 1/14 oz., sulphuric acid, 1/7 oz. The pieces are passed quickly through the solution, and then rinsed. This operation gives strength to the graining which, without it, possesses no adherence. Silver Powder, - Silver powder is obtained by immersing cleansed copper plates in a very dilute solution of silver nitrate made with distilled water. The more dilute the solution is, the finer is the precipitate of silver upon the copper, and the more easily it is removed. In a glass or porcelain vessel, 2/3 oz. crystallised silver nitrate are dissolved in 2 1/5 gal. distilled water, and 5 or 6 bands of cleansed copper 3/4 in. wide are placed in it. These bands should be long enough to allow of a portion being above the liquid. The whole is kept in a dark place for 24 hours, and now and then stirred with the copper bands. This motion is sufficient to loosen the deposited silver, and present fresh copper surfaces to the action of the liquor.
When no more silver deposits on the copper, the operation is completed, and there remains a blue solution of copper nitrate. The silver powder is washed by decantation, or upon a filter, until there remains nothing of the copper solution. It is then carefully dried, avoiding contact with hard bodies. Nuremberg powder is produced by grinding a mixture of honey and silver foil upon a ground-glass plate with a muller until the proper fineness is obtained. The silver is separated by dissolving the honey in boiling water, and washing the deposited metal in a filter, until there is no remaining trace of honey. The silver is then carefully dried at a gentle heat.