POWDER PROCESS This is better known as the pro cess (which see).
The following, however, was introduced as a powder process, and is based on the reduction of the persalts of iron into proto-salts by the action of light. It was invented in 1858 by H. Gamier and A. Salmon, of Paris, in competi tion for the award offered by the Duc de Luynes for the purpose of solving the problem of pro ducing absolutely permanent photographs. Well sized paper is coated with a strong solution of ferric ammonio-citrate in water ; having been dried in the dark, it is exposed under a positive transparency to daylight until the image is faintly visible. The paper is pinned to a flat board, and, by diffused daylight, very dry and fine lampblack or other pigment is brushed over the paper with a pad of cotton-wool or a camel hair brush. By breathing upon the paper the parts not acted upon by light become somewhat sticky and retain the black pigment ; thus, details of the image appear and in time the pic ture is complete. The print is fixed by immers
ing in water, which washes out the sensitive iron salts. The print is then dried and varnished.
In process work, there are several methods of powdering to form acid resists. For instance, the so-called " dry enamel " method is a powder process, development being performed by dust ing a hygroscopic film of gum and bichromate so that the parts which have not been acted upon by light absorb the powder. These parts are readily cleared away after the plate has been burnt in. There is also the dragon's-blood powdering process, which is very largely used for zinc etching. The Austrian process of zinc etching depends on a method of building up the image with asphaltum powder.