Briefly: heavily pigmented gum arabic coated onto a well gelatin sized
paper. Dried and sensitised with 5-10 % solution ammonium bichromate.
Exposed under a negative in the usual way. Developed in a soup of
sawdust and water.
I also have a description by LeClerc from a slightly later book but
which mentions fresson.
If anybody wants fuller information I'm sure I can oblidge.
Vincent