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PHOTOGRAPHIC IMPORTANCE OF CHROMIUM COMPOUNDS 101
We have to mention another process of photo-lithography, based on the use of asphalt. We have already described this in our first chapter as a sensitive substance, and have also described a process called heliography, which produces, by means of photography, copper plates and steel plates for printing. Asphalt serves also for photo-lithography. A lithographic stone is sprinkled with a solution of asphalt in ether, allowed to dry in the dark, and exposed under a negative. The asphalt becomes insoluble on the exposed places, and is retained upon treating the stone Avith ether or benzine. If the stone is then damped, the moisture only penetrates where no asphalt covers the stone. On rolling it after this with an inked roller, the fatty ink is not retained by the damp places, and only adheres to the asphalt — that is, to the picture ; thus a stone giving impressions is obtained. This method gave good results in the hands of several practitioners, and was preferred by many to the gelatine process, though asphalt is much less sensitive than the prepared gelatine.
Porcelain Decorations. — Photography has become allied to almost all the multiplying and descriptive arts, though it was at first looked upon as their rival. It is not surprising, therefore, that it has become a help in porcelain painting and decoration. The original method was invented by Poitevin, and subsequently was materially improved by Joubert in London and Obernetter at Munich. It consists in this : a mixture of gum, honey, and bichromate of potash is poured on glass ; the film is carefully dried in the dark, and then exposed under a positive. This film of gum, when freshly prepared, is sticky, and retains any colouring matter strewn over it in powder, but after exposure to light the film loses its stickiness. If this exposure takes place under a drawing with black strokes, the film under them will retain its stickiness, but will lose it beneath the white, transparent parts of the paper.