Collotype

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Collotype is a dichromate-based photographic process developed for large volume mechanical printing before the widespread use of cheaper offset lithography. It can produce results difficult to distinguish from metal-based photographic prints. Many old postcards are collotypes. No longer a commercial process, its possibilities for fine art photography were first exploited by Todd Walker in the 1970s.

The collotype plate is made by coating a plate of glass which has already been prepared with a thick coat of dichromated gelatine which is then dryed carefully at a controlled temperature (a little over 50º Celsius) so it 'reticulates' or breaks up into a finely grained pattern. The plate is then exposed in contact with the negative using an ultra-violet UV light source, hardening the exposed gelatine, developed by careful washing and dried without heat.

To manufacture prints, the plate is dampened with a glycerine/water mixture which is slightly acidic , then wiped and blotted before inking with lithographic ink using a roller. Paper is then put on top of the plate and covered before being printed using relatively light pressure, either in a lithographic press or by hand using a firm roller.