What is a Collotype?  And how is it different from a Lithograph?

A collotype is surface printing technique similar to stone lithographs in many respects, but differing in its use of materials and in both the process of creating the image on the plate and printing.

For a stone lithograph, the stone’s ability to absorb water creates the area to repel the ink, while a lithograph crayon creates the area to repel water and accept ink. The ink is then transferred (printed) to paper, typically in a press.

For a collotype, a layer of gelatin or other colloid material is carefully cured on a glass or metal plate to create the area to absorb water, which repels the ink. A photographic negative is exposed onto to this surface which changes the image area so it repels water and therefore accepts ink. The ink is then transferred to paper. typically in a press.

Before the advent of offset lithography, collotypes were often used to reproduce photographs inexpensively. In the world of fine art, collotypes were, and are, often used to reproduce drawings.

See the Wikipedia article for historical information and a more detailed description of the collotype process: https://en.wikipedia.org/wiki/Collotype.