High Quality Content by WIKIPEDIA articles! Collotype is a dichromate-based photographic process originated in Germany circa 1868 and was used for large volume mechanical printing before the existence of cheaper offset lithography. It can produce results difficult to distinguish from metal-based photographic prints because of its microscopically fine reticulations which comprise the image. Many old postcards are collotypes. While no longer a commercial process, its possibilities for fine art photography were first employed in the United States by Alfred Stieglitz. The collotype plate is made by coating a plate of glass or metal with a substrate composed of gelatin or other colloid and hardening it. Then it is coated with a thick coat of dichromated gelatine and dried carefully at a controlled temperature so it 'reticulates' or breaks up into a finely grained pattern when washed later in approximately 16 °C water. The plate is then exposed in contact with the negative using an ultraviolet light source which changes the ability of the exposed gelatine to absorb water later. The plate is developed by carefully washing out the dichromate salt and dried without heat.

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