Early Photographic and Printing Processes

Collotype

1870s

History

Collotype was a printing process "half way between lithography and photography".  It was used in the 1870s

Process

I believe the process was approximately as follows:

-  A gelatin coating is added to a plate, then dried in the dark.

-  Place a negative over the plate and expose to light

-  The gelatin hardens in proportion to the light that hits it.

-  Wet the plate, and the soft areas will absorb more water.

-  Make prints from the plate.  The ink will not mix with the water, so more ink will adhere to the harder part of the plate. 
i.e. more ink will adhere to those parts that received the most light through the negative, and hence a positive image is produced.

Result

In Edinburgh 

Some details of the process above are based on a presentation by Harry Magee, September 2004