Quote Originally Posted by Jorge
They most likely are nitrocellulose based film. Acetate films did not come until much later. If they are not brittle dont worry about it, they should be fine.
Of course a way to test is to snip a small part of the negative and IN A SEPARATE room light it up, if it burns fast, you know for sure.
True ... the "burn" test is an fairly effective way to identify plastics.

Nitrocellulose is (probably) the *first* of what we call "plastics". It was developed by an entrant into a contest seeking an alternative to ivory, used in the manufacture of billiard balls.

Other names for nitrocellulose were "cellophane" and "celluloid".
The stuff is definitely flammable. When ignited with an ordinary cigarette lighter or match, it will burn with a clean, white flame, with very little ash residue.

It is also the principle ingredient in "smokeless" (gun) powder, which is not all that dangerous to have around. It will not explode (read: burn *very* fast so as to create a LOT of pressure), unless tightly contained, as in a rifle or pistol chamber.

I checked: the roll of Ansco Plenachrome (although it says "chrome" it is, in fact, a black and white negative film) DId have a nitrocellulose base.

Many of the old-time movie theaters burned down; having a roll of flammable nitrocellulose in close proximity to a carbon arc lamp, or *very* hot tungsten lamps was an "iffy" proposition, to say the least.