Submitted by Al …
(Adapted from The Encyclopedia of Photography ©1963)
Bitumen, also known as asphalt, is a semisolid product formed by the partial evaporation of certain petroleum. It is used for photographic purposes because it is sensitive to light.
Syrian asphalt is found to be most useful because it has a higher sensitivity to light. The sensitivity may be increased by the addition of sulphur, which is dissolved in bisulfide of carbon added to powdered asphalt.
It was over a hundred years ago that Joseph Niepce first discovered that Bitumen of Judea, or asphalt, becomes insoluble when exposed to light. Niepce used his discovery to produce the first fixed photograph, in the ordinary sense of the term. In 1827, he made a photograph of Kew Church on an asphalt coated plate in a camera. The process did not prove satisfactory because of the enormously long exposure that was necessary.
Niepce also discovered the forerunner of these photomechanical processes when he dissolved asphalt in oil of lavender and spread it on metal plates which he exposed behind a drawing. He next dissolved the asphalt from the parts which had been protected from the light by the lines of the drawing, then the metal beneath was etched in an acid bath. This forerunner of the modern process he called heliography.
The steps of the process are outlined below:
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