The bichromate of potassium hydroxide produces with the gelatin an insoluble combination in the presence of the light (KO 2crO3)
Principe :
Potassium dichromate (and ammonium dichromate) Possess the property to deoxidize under the effect of the light, as soon as they is in the presence of an organic matter susceptible to oxidize at his expense, such as the gelatin. There is then formation of chromate and chromic acid : Cr2O7K = CrO4K2 + CrO3
The chromic acid gives up to the organic matter a part of its oxygen to be transformed into chromate of oxide of chromium : 3CrO3 = 3O + Cr2O3, CrO3
The brown of the image just after the exposure in the light is due to the brown color which possesses the chromate of oxide of chromium.