Oil pressure

A printing process that exploits the change in a colloidal layer when exposed to light. First, paper is covered with a layer of gelatin that has been made light-sensitive in a potassium bichromate solution. The gelatinized paper is now exposed under a negative. When exposed to light, the gelatine becomes water-insoluble in the exposed areas, i.e. it is tanned. By washing the paper in warm water, the gelatin layer creates a colorless relief made up of more or less water-containing areas. Finally, a greasy paint is dabbed onto the paper, which only sticks to the tanned areas of the relief. This creates a positive image that is not, strictly speaking, a print in the traditional sense.