Mechanical agents will not, like acids, act simultaneously on every part of the object, and it is impossible to entirely prevent the action of air, steam, gases, and acid fumes. Heat the object to a dull red heat upon a slow fire. If the silver is pure, it becomes covered with a thin bluish film; but if, as is nearly always the case, the silver is alloyed with a variable proportion of copper, the latter becomes oxidised, and covers the piece withagreyish-black coating. While the piece is still hot, plunge it into a boiling pickle of water and sulphuric acid, which dissolves the oxide. If the heat has been sufficiently protracted for oxidising all the copper on the surface, the object, when removed from the pickle, is of a perfectly dead white. It is greyish if the heating has been too short, and the operation must be repeated as many times as are needed for a perfect lustre. Or the silver may be placed in sheet-iron boxes filled with a mixture of powdered borax, lime, and charcoal dust. The borax dissolves the copper oxide as soon as formed. If the objects to be cleansed are hollow, it is necessary, before heating, to make a small hole which will allow of the escape of the air expanded by the fire. Without this precaution, the piece will burst open.

When the piece is put into the pickle, the acid liquor enters through the hole, and takes the place of the air between the shells, and is difficult to remove. In order to prevent the spotting of the piece by this liquor, it is dipped for a few minutes into a very dilute solution of ammonia or of soda crystals, which prevents the action of the acid upon the silver. Then place the article between layers of dry and warm fir saw-dust, which will absorb the saline solution. Nitric, instead of sulphuric, acid, may be used for the pickle bath. In this case, the water must be distilled, and the acid free from chlorine or hydrochloric acid, otherwise the silverware will be covered with a bluish-white film of silver chloride. This method will not suit articles in which iron or zinc may be present. In such cases, employ alkalies, and polish afterwards with very fine sand or pumice, with the aid of a stiff and short brush, or with a scratch-brush alone. Perfectly cleansed silver may directly receive a metallic deposit which will have the same dead lustre as the object itself, but it is customary, before introducing the articles into the plating bath, to scratch-brush them.

Bright Lustre For Small Articles

Very small articles, which cannot be scratch-brushed, receive a bright lustre by mutual friction. The operation is generally performed with the hands. The articles to be brightened are introduced, together with boxwood saw-dust, bran, or sand, into a bag; the ends of the bag being gathered into the hands with the thumbs inwards, the bag is shaken to and fro. As this operation becomes very fatiguing, mechanical means may be employed to effect the shaking.