Husnik (1837-1916) improved the collotype proces, a flatbed photomechanical printing proces. This is the only commercial proces which gives a true and continuous tone. It is very expensive and demands great skill from the printer and so is rarely used. The image is transferred onto the printing plate which has been coated with a light-sensitive emulsion of bichromated gelatin. Collodion, a gummy substance was used originally, hence the name collotype.
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