Silkscreen printing and photochemical machining originally were based on gum bichromate photosensitive materials, where exposure to intense ultraviolet light made previously-soluble gum or gelatin colloids insoluble; after exposure, the exposed surface was washed in water and the unexposed coating dissolved, leaving the hardened gum or gelatin to resist the passage of the silk-screen ink or the metal-etching solution. Offset printing can use either a negative plate, where the hardened, exposed photosensitive coating attracts ink and repels water, or a positive plate, where the exposed photosensitive coating decomposes or exposes the metal, water-attracting surface.
