There has been found no other way of cleaning bronze statues, when blackened by smoke and soot, than that of washing with plenty of clean water, accompanied with mechanical friction; but even this simple treatment is very undesirable; because the friction, however slight, accompanying the washing, destroys, or tends to destroy, the sharpness of the outlines; and the sulphurous and sulphuric acids of the prevailing smoke would be certain to rapidly corrode the surface of any bronze statue which is constantly being washed. It has been a matter of much debate whether the soot-blackened surface of a bronze statue is not more pleasing to the eye than the metallic lustre of a new, or newly-cleaned statue.
Weber finds that a dilute solution of caustic alkalies removes overlying dirt, and allows the green patina to become visible. Where the metal was not originally oxidised, the alkali simply cleanses it, and does not promote any formation of green rust.
By dipping fustian in soluble glass, and washing it with soap directly afterwards, we get a fabric largely impregnated with silica, which will be found very well adapted for cleaning bronzes, etc.
The method of restoring a bronze tea-urn turned black in parts will depend, to a great extent, on the metal and the colour. Clean the surface, first of all, with whiting and water, or crocus powder, until it is polished; then cover with a paste of graphite and crocus, mixed in the proportions that will produce the desired colour. Heat the paste over a small charcoal fire. If the bronzing has been produced by a corrosive process, try painting a solution of sulphuret of potassium over the cleaned metal. The bronzed surface may be polished; but it cannot be bright unless the surface of the metal itself is polished, and then covered with transparent lacquer to preserve the brightness.
Boil the articles in ordinary soap-boilers' lye, rinse in water, and roll in bran or sawdust. If the bronze is pressed, the lye must be mixed with common salt and the article thoroughly brushed, but no water must touch the back. (6) Bronze which has become dirty by oil, fat, tallow, or other greasy body, is boiled in an infusion of ashes, and cleansed with a soft brush dipped in a fluid of equal parts of water, nitric acid, and alum. Each piece is then dried with a rag, and slightly heated. To cleanse clock pendulums, and free them from the substance called by the gilders "mercury-dust," heat them moderately, touch the stain with a brush dipped in nitric acid, rub with a linen rag, and again heat moderately.