THE BITUMEN METHOD.
This, the earliest photographic process, was discovered and described by Niepee even before the discovery of photography or silvered plates. It depends on the peculiar property asphalt possesses of losing its solubility in ether, benzine, or turpentine after prolonged exposure to light. If it be dissolved in turpentine in a photographically dark room and the solution poured upon a well-polished metal plate or lithographic stone, dried, and exposed under a reversed negative (otherwise the impression yielded in printing will be reversed) for two or three days[P] in sunlight, the picture may be developed with turpentine or benzine, either of which solvents will remove the layer from such portions as have been unexposed to the light and reveal a picture in asphalt that may be rolled or inked-in and capable of withstanding etching. This direct process, as applied to photo-lithography, is now of little practical value owing to the introduction and perfection of the transfer process. It is, however, most valuable in heliographic engraving or etching and for the production of zinc and copper printing plates.
[P] The bitumen most suitable is that variously known as Syrian asphalt, Jew’s pitch and bitumen of Judea. This is finely powdered and placed in a bottle with twice the bulk of ether and occasionally agitated for five hours, at the end of which time the ether may be poured off and fresh supplies added so long as any portion soluble in the ether remains. The purified residuum maybe dried and dissolved in benzole or chloroform; this will yield a solution which may be used as above, but poisoning, perhaps, twenty times its sensitiveness. The picture is developed with turpentine.