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One of the most versatile Alternative Processes, Gum Bichromate was invented in 1855, and became popular at the turn of the century. The process takes advantage of the fact that colloids, such as gum, gelatine and glue, when mixed with a bichromate, become hardened and insoluble when exposed to light. By mixing a coloured pigment in with the colloid, and then washing away the undeveloped areas with water, you are left with an image in the dye. Exposure times using daylight will be typically between 30 seconds and 2 minutes. Advanced Gum Bichromate workers build up a number of layers of varied pigment colours by re-sensitising, re-exposing and re-developing prints.