Collotype

Collotype, a term including a number of photographic printing processes which, though differing in details, all depend on the action of light upon a gelatine film sensitised by means of potassium bichromate. Such a film loses, in those places where the light has acted, its power of absorbing water, but is able to receive greasy matter as printer's ink. A glass plate with the sensitised gelatine film upon it is therefore prepared and exposed to light under a photographic negative, which should be a "reversed" negative. It is then washed with cold water, and used as a "block" for printing from, only those parts which had been exposed to the light receiving the ink.