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A complex mixture of hydrogen carbide which is brown or black in colour and can be had in solid or liquid form. Closely related to it is the substance called Syrian asphalt which has played an important role both in etching and in photography. When bitumen is mixed with clay it is usually called asphaltum. Once purified, bitumen becomes the base for etcher's ground and is mixed with turpentine and various gums*. It is also used in aquatint* in the form of a very fine powder which is used for graining* aquatint plates.(Fine particles composed of heavy mineral or coal-tar residues. Asphaltum has a melting point of approximately 200°F. Syrian asphalt was also (and still is) used to make photographic plates light sensitive as this substance becomes white and insoluble when it is exposed to light. After a great many tests on other substances, Niepce took to using it. In 1827, at the time of his very first "helioqraphies" he used Syrian asphalt and lavender oil (as a solvent) to make his first pictures. Bitume is also used to make lithographic crayons*.