Collotype is a hybrid letterpress/lithography process. Lithography because it is planographic and the plates are printed damp. Letterpress because of the style of press. Books on thesubject are: Studio Collotype by Kent B. Kirby and The Practice of Collotype by Thomas A. Wilson.
The process in a nutshell: Dichromate activated gelatinis dried above its melting temperature in a lightproofoven. After being contact printed (UV), the plate is washed. The gelatin absorbs water and expands in opposite proportionto its exposure. This reticulation generates an inconsistent dotpattern which when printed can appear as continuous tone. Ink adheres to the hardened, exposed gelatin and is rejectedfrom the soft, saturated gelatin.
There are innumerable variables to this process which are outlined in the above books.Below are pictures of an oven, and how the press was adapted.