They are particularly well known for their use of pigment processes, especially gum bichromate. The main product was potassium dichromate, known then as bichromate of potash. The bichromate is decomposed using ammonia. Addition of potassium dichromate to this solution precipitates acridine bichromate. Printing-out paper' or other daylight-printing material such as gum bichromate may also enable outdoor exposure. Gum bichromate is a 19th-century photographic printing process based on the light sensitivity of dichromates.