Producing collotypes was a way of reproducing high-quality photographs in detail, with a continuous tone.
Collotypes were produced by coating a greyed glass plate with a photosensitive gelatin that would harden when exposed to light. The plate would be exposed to light through a reverse negative, the darker the area the less it would harden. The plate would then be washed to remove the areas which had not hardened. After this the process of reproducing the image is similar to that of a lithograph. Certain areas (the areas with hardened gelatine) would attract ink whilst the areas with less of the hardened gelatine would repel the ink.
Throughout its history different companies would experiment with different techniques, however this basic principle would be used up until the early 1990s when it would be replaced in favour of digital technology.