This continuous tone printing process invented in 1855 began to be used commercially in 1868. It starts with a glass plate coated with a photosensitive gelatin that puckers and cracks as it dries. When exposed to light through a reverse negative, the lit areas harden into an insoluble non-absorbent finish. It is the areas within the reticulated cracks that harden the most because they are the thinnest part of the emulsion. They in turn will print the darkest in proportion to their exposure to the tones of the original image. The emulsion in areas with little or no exposure to light is washed out from the gelatin. Since there is some relief it is not a true planographic printing method but the plate is printed in a similar manner to that of a lithograph. A solution of glycerin and water is spread over the plate’s surface, which is absorbed by the remaining gelatin. Areas that are to carry the dark tones absorb little or no moisture while areas for the lighter tones and non-image areas absorb the most. When greasy ink is rolled over the gelatin on the plate, the non-image areas holding the most moisture repel the ink, and the dry hardened image areas attract the ink. Once printed the reticulated pattern creates a somewhat light continuous toned image of incredible detail. Many variations on this process were developed in the 19th century. In 1896 a way to mount a collotype emulsion onto an aluminum plate was discovered and this medium was finally adapted to the rotary press. Most black & white postcards, especially those from Europe were made this way. Even though collotypes can reproduce sharper details than any other technique, the process was abandoned by the 1980’s.
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