Wet-collodion process, also called collodion process or tintype, is an early photographic technique invented by Englishman Frederick Scott Archer in 1851. The process involves adding a soluble iodide to a solution of collodion and coating a glass or metal plate with the mixture. In the darkroom the plate is immersed in a solution of silver nitrate to form silver iodide. The plate, still wet, is then exposed in the camera. After, it is developed by pouring developer over it and fixed with a strong solution of sodium thiosulfate, for which potassium cyanide is sometimes substituted. Immediate developing and fixing are necessary because, after the collodion film had dried, it became waterproof and the reagent solutions cannot penetrate it. The process is valued for the level of detail and clarity it allowed.