That the good qualities of asphalt were even earlier recognized is proved by the fact that Negré, before the discovery of zinc etching, exposed a copper-plate coated with asphalt solution under a negative, washed out the places not affected by light, electroplated these blank places in a gold bath, then cleaned the plate and deep etched it with acid. He obtained in this way an intaglio printing plate similar to an etching.
By using a positive with this process a relief or typographic plate may be obtained, both of which processes are known as “gold etching,” and were practised in the beginning of the ’50’s in the K. K. Hof- und Staatsdruckerei.
The asphalt which is used for photo-lithographic purposes must possess certain qualities, of which the most important is as high a light sensitiveness as possible. With ordinary asphalt success will not be attained, since it is only slightly sensitive to light; the so-called Syrian asphalt is therefore generally used, which after having been prepared is dissolved in chloroform and then benzole and oil of lavender are added.
Professor Husnik prepares an asphalt from which all constituents not sensitive to light have been extracted. Herr Valenta produces his sulphurized asphalt. Both possess a far higher light sensitiveness than ordinary Syrian asphalt, and are specially suitable for photo-lithography.
The second, and considerably more extensively used method for photo-lithography—a transfer process—is based on the light sensitive property of the chromate salts in combination with gelatine, or albumen, or gum.
That the bichromate salts possessed light sensitive properties, and could be used for photo-lithography, Mungo Ponton discovered in 1839. He printed on an ordinary paper, which had been rendered sensitive to light in a solution of potassium bichromate, drawings and silhouettes, and obtained after fixing, which was effected by merely washing in pure water, brown images on a white ground.
Later Talbot found that the chromate salts in combination with organic substances under the action of light altered the property of the latter, and particularly that exposed chromated gelatine became insoluble in hot water. Poitevin used this discovery for carbon or pigment printing, and had produced in 1855 various pictures with this process.
Poitevin discovered, further, that exposed chromated gelatine {18} would not swell up in cold water and took up greasy ink, and therefore after exposure under a photographic negative a picture could be obtained exactly inversely corresponding to the tones of the negative; and that these places affected by light could be inked-up and printed from, or could be transferred to a zinc or stone plate, and from this pulls could be prepared. By this discovery photo-lithography and collotype were actually discovered.
Led on by his success, Poitevin used later, instead of gelatine, albumen and gum arabic, and prepared with these photo-lithographic prints, which were transferred to stone.
If a solution of gelatine is mixed with a solution of bichromate of potash, and a sheet of paper coated with this mixture and exposed, or if the paper is first coated with gelatine and then bathed in a solution of bichromate of potash, various chemical changes take place as the result of the action of light on the gelatine film, which we will now consider.