When reading references about autochrome plates, I saw "pine pitch" mentioned - Damar Gum could well be the material that was used. You should be able to purchase small quantities from a traditional artist's supplier such as Cornelissen. Damar will dissolve in pure turpentine (not a substitute).
would be cellulose varnish - Also available from an artist supplier or anyone selling automotive paints. Very flamable stuff, but not explosive"nitrate de cellulose"