3618. To Silver Specula and Other Glass Surfaces

3618.     To Silver Specula and Other Glass Surfaces. Make a solution of am-monio-nitrate of silver, of the strength of three grains to the ounce. Render it very slightly turbid by excess of nitrate of silver, and then filter it. Just before using, add to each ounce of the foregoing solution 21/2 grains of Rochelle salts. Having scrupulously cleaned the glass intended to be silvered (see No. 3621 (To Clean the Surface of Glass for Silvering)), place it in a convenient vessel about one inch from the bottom, supported on three little cones of white wax. The glass plate may be suspended; but in that case there is more difficulty in avoiding vibration, the absence of which is essential to success. Expose to a northern light, or any other subdued light, and in about two hours the deposit of silver will be sufficiently thick. It must now be carefully removed, washed, and dried. When the surface next the glass is to be used as the reflector, the glass side should bo cleaned by nitric acid if the state of its surface, after the silvering, so require; and the silvered side should receive a protecting coating of a good tough black varnish.

3619. Liebig's Process for Silvering Glass Mirrors

3619.    Liebig's Process for Silvering Glass Mirrors. The process of silvering glass generally rests on the reduction of metallic silver from a solution by means of glucose or some other organic substance. By Liebig's method the deposit of silver is produced by the action of a mixture consisting of 50 parts by measure of a silver solution, and 10 parts of a reducing solution, this latter previously diluted with 250 to 300 parts water. The components of the silver solution are: 140 parts of a solution containing 10 per cent, of nitrate of silver ; 100 parts of a solution of nitrate of ammonia (free from chlorine) of 1.115 specific gravity (or a solution of sulphate of ammonia of specific gravity 1.105-1.106 ;) lastly, 750 parts of caustic soda lye of specific gravity 1.050. In case sulphate of ammonia is used, its solution must be added to the silver solution, not as in the case of nitrate. The reducing solution consists of 1 part by measure of sugar, liquor and 1 part of copper liquor. '

The sugar liquor is prepared by dissolving 50 grammes (7711/2 grains) white sugar in water to a thin syrup, kept for 1 hour at a boiling heat with 31/10 grammes (48 grains) tartaric acid ; the solution is then diluted to measure 500 cubic centimeters (l1/10 pints).

The copper liquor consists of a solution of 2857/1000 grammes (44 grains) dry tartrate of copper in water, by the aid of a caustic soda solution added by drops until the blue salt is dissolved; the whole is then diluted with water to measure 500 cubic centimeters (l1/10 pints).

The glasses to bo silvered, if for mirrors, are placed upright on their edge in the silvering tank and held together in pairs by clamps; when for optical purposes, they are held in a horizontal position, just touching the surface of the fluid. In cold seasons the temperature must be kept at 68° to 84° Fahr. The quantity of silver necessary for a square yard of surface is from 46 to 54 grains.