A collotype is a type of print in which a glass or metal plate is covered with a light-sensitive substance.  The substance is exposed to an image on a negative, much like a photograph, which modifies the substance on the plate and, in turn, creates the image.  The image would vary in hardness according to the amount of light that reached it through the negative.  The softer areas accepted more ink.  Collotypes were first employed for fine art photographs in the United States by Alfred Stieglitz.