3776. Chinese Bronze

3776.    Chinese Bronze. Take 2 ounces each verdigris and vermilion ; 5 ounces each alum and sal-ammoniac, all in fine powder, and sufficient vinegar to make a paste; then spread it over the surface of the copper, previously well cleaned and brightened; uniformly warm the article by the fire, and afterwards well wash and dry it, when, if the tint be not deep enough, the process may be repeated. The addition of a little sulphate of copper inclines the color to a chestnut brown; and a little borax to a yellowish brown. Much employed by the Chinese for copper tea-urns.

3777. Carbonate of Iron Bronze

3777.      Carbonate of Iron Bronze. Beautiful tints are produced by using plate-powder or rouge. After moistening with water, it is applied and treated in precisely the same manner as the plumbago. (See No. 3775 (Plumbago Bronze).)

3778. Black Bronzes

3778.    Black Bronzes. A very dark colored bronze may be obtained by using a little sulphuretted alkali (sulphuret of ammonia is best). The face of the medal is washed over with the solution, which should be dilute, and the medal dried at a gentle heat, and afterwards polished with a hard hair brush. Sulphuretted hydrogen gas is sometimes employed to give this black bronze, but the effect of it is not so good, and the gas is very deleterious when breathed. In these bronzes the surface of the copper is converted into a sulphuret.

3779. German Method of Bronzing Brass Black

3779.      German Method of Bronzing Brass Black. There are two methods of procuring a black lacquer upon the surface of brass. The one which is that usually employed for optical and scientific instruments, consists in first polishing the object with Tripoli, then washing it with a mixture composed of 1 part nitrate of tin and 2 parts chloride of gold, and, after allowing this wash to remain on for about 12 or 15 minutes, wiping it off with a linen cloth. An excess of acid increases the intensity of the tint. In the other method, copper turnings are dissolved in nitric acid until the acid is saturated ; the objects are immersed in the solution, cleaned, and subsequently Seated moderately over a charcoal fire. This process must bo repeated in order to produce a black color, as the first trial only gives a deep green; when the desired color is attained, the finishing touch is given by polishing with olive oil.

3780. Black Bronzes

3780.    Black Bronzes. Many metallic solutions, such as weak acid solutions of platinum, gold, palladium, antimony, etc., will impart a dark color to the surface of medals when they are dipped into them. The medal, after being dipped into the metallic solution, is to be well washed and brushed. In such bronzes the metals contained in the solution are precipitated upon the face of the copper medal, which effect is accompanied by a partial solution of the copper.