With all the vintage photography being sold these days, I've come across some negatives that are on nitrate stock including a few George Burke negatives from the early 1930's. Cellulose nitrate was introduced as a flexible transparent film base back in the 1890's. The problem with nitrate is that it's unstable and breaks down, becoming sticky, gooey, then ultimately turns into dust all the while releasing nitric acid. Worst still it is extremely flammable and if deteriorated can spontaneously combust. There were a number of incidents and deaths in movie theatres in the 1920's when projected films burst into flames, and in the 1970's both the National Archives and the George Eastman House lost films they were storing when they self-ignited. Because of the hazards of nitrate, "Safety" acetate film was introduced it the 1930's but did not replace nitrate entirely. I'm not sure when nitrate was phased out as a still film but nitrate movie stock continued to be made until the early 1950's.
I'm not an expert on this but it seems to me if you collect vintage negatives, you have to be aware that nitrate film can be hazardous and if you decide to keep them, you have to be able to store them as safely as possible. There's quite a bit of information about nitrate film on the web. Any negative marked "Eastman-Nitrate-Kodak" is nitrate. Unfortunately some marked with the words "Safety" can apparently also be nitrate. Here's a webpage that can help identify the film stock:
http://crossover.ellison.net/NegsID.htm Wouldn't want anyone's collection (or house) to go up in flames.