Prior to the mid 1940s 2 thin films of celluloid were use. The first, opaque film, had a printing or litho on it. The next film to complete the sandwich was a clear film to act as a protection for the litho.

After the mid 1940s Burt used plastic, although I believe that Burt used celluloid for a couple of years before switching over.

Celluloid was quite flamable and toxic.