Collotype is a plangraphic, photo-mechanical, non-screen printing process that makes possible the faithful reproduction of half-tones and fine tonal gradations. Collotype was first used to produce printing plates by A. L. Poitevin in 1855. To make the plates for printing, a matt etched mirror-glass plate about 10 cm thick - similar to that used in screen printing - is coated with a light-sensitive emulsion (chromate gelatine), which makes it sensitive to light. The plate is exposed beneath a negative; the amount of light admitted in exposure determines the extent to which the emulsion is case-hardened (less light means less case-hardening). When the plate dries, a fine, puckered grain is left. The individual grains of this layer swell in water to varying degrees, matching the extent to which they repel the waxy collotype inks. Thus contrast can be increased in collotypes by moistening the emulsion (moistening to a greater extent lessens contrast whereas moistening to a lesser degree enhances it). Hence the necessity of uniform humidity for ensuring consistent results in collotype printing. Collotype is only suitable for small editions. It is the printing process that achieves the finest results in reproducing paintings.