 The collotype plate is made by coating a plate of glass or metal with a substrate composed of gelatin or other colloid and hardening it.Then it is coated with a thick coat of dichromated gelatin and dried carefully at a controlled temperature (a little over 50° Celsius) so it "reticulates" or breaks up into a finely grained pattern when washed later in approximately 16 °C water.The plate is then exposed in contact with the negative using an ultraviolet (UV) source which changes the ability of the exposed gelatin to absorb water later.