In the mid 1800's John Wesley Hyatt invented the first semi-synthetic polymer. Cellulose nitrate, better known as Celluloid, was created as an ivory substitute for billiard balls at Hyatt's Celluloid Manufacturing Company in Albany New York. It was also found that this material could be used as a substitute for animal bone and tortoise shell in combs. It was also used as fibers to produce the first rayon fabrics.
In the 1880's when George Eastman tried to find an alternative to cumbersome glass photographic plates, as well as for the delicate negative paper which had been the only plate substitutes so far, he found that the durable cellulose nitrate also worked well because it could be mixed with a solvent to form a liquid and then cast as a strong, extremely transparent film. So in 1885 Eastman introduced the first transparent photographic film. And in 1891 Edison used this film in the development of the motion picture camera, which would not have been possible if not for the invention of the film medium. (Imagine a movie shot on a series of thousands of heavy glass plates!) In 1896 the first film made with an emulsion coating especially for motion pictures was produced.
Until 1899 the film was manufactured by breaking down the cellulose nitrate in a solvent and spreading it very thinly on long glass tables where the solvents would evaporate leaving a transparent film layer. (Follow this link to see how you can cast your own film from polystyrene.) Then the film was coated with emulsion - a light sensitive material made with silver salts and color dyes - and removed from the glass to be cut to proper lengths and widths and then wound. Since then the process has been pretty much the same except the film is now rolled out on large uniform wheels where it is air dried as the wheel turns to add more film.
This new synthetic film material - made from short cotton fibers or wood pulp mixed with nitric and sulfuric acids - though versatile, was also extremely flammable. In the early Twentieth Century many lives were claimed in disastrous theater fires as well as in accidents in the factories where Celluloid film was produced. So in 1908 Eastman developed what is called "Safety Film," an alternative material with the same properties as cellulose nitrate, but made from the much less volatile cellulose acetate, which is made by the same basic method of mixing cotton or wood fiber with acetic acid, then mixing the precipitated plastic pellets with solvents to form a liquid "dope" and casting it as a thin layer of film. However it would not be until 1948 before acetate film would become the standard for motion picture camera film. In fact in 1950 the Eastman company won an Oscar for developing a safety film for the motion picture industry.
Incidentally, Eastman cellulose acetate has also been used for other things besides film: as a coating to smooth airplane wings in World War II, in unbreakable lenses, as well as to produce a synthetic yarn which was used in new non-flammable rayon fabric.
In 1960 a film made from the petroleum product polyester was created. At first it was to be used mainly in a sheet form for graphic art films and x-ray sheets, but later the assets of polyester would be put to use in the base for a durable alternative film for distribution prints of motion pictures. It is also used in amateur 8mm films which are not likely to be edited and need the added strength of polyester since they are often run through a cartridge mechanism. To day both cellulose acetate and polyester are used for film. So which material is better. Find out here.
Now we have the problem in film archiving and restoration of identifying the material from which that crusty old piece of film is made. Now that all nitrate films are over 50 years old telling the difference is not so hard as it once was. That's because nitrate film has a pretty high rate of decay. Although most nitrate films found today are in a pretty sad state, some has been well kept and will be hard to distinguish by sight alone. The film itself is usually extremely yellowed brittle and beginning to oxidize on the edges. It also lets off a lot of gases such as nitric oxide and nitrogen dioxide, and these make a pretty bad smell. They can also build up if the film is enclosed in a can. These gases and heat caused by the rapid decay when coupled with high surrounding temperatures can cause quite a fire hazard. Nitrate film has been known to spontaneously combust under extreme heat and pressure.
As for nitrate film which has been well cared for and is in pretty good condition, there are some ways to identify it for sure. A quick one is the size of the film. As most film stock from that era was made by Kodak it can be assured that any commercially produced 16mm films are not nitrate as it was never made in this size. There was also never any regular color film produced on nitrate base. The only "color" films ever made on nitrate base were Technicolor films printed on a special black and white nitrate stock.
So we're narrowed to black and white 35mm or 70mm films. If you're afraid it might be nitrate, check for extreme yellowing and see how it smells. Old film gives off acids and gases as it oxidizes and decays. In the case of Cellulose acetate the smell will be from the acetic acid, which is essentially vinegar, so that is how it will smell. Nitrate film, on the other hand gives off... you guessed it - nitric acid, which has a stronger "burning" smell.
If look and smell don't give you clear indication, the final test should. This would be to take a VERY SMALL sample of the film and burn it under isolated and controlled conditions. Nitrate film will burn quickly and with a bright flame, while acetate or polyester film will burn much more reluctantly and slowly. THIS TEST - IF PERFORMED - SHOULD BE DONE WITH VERY SMALL SAMPLES OF FILM IN A STRICTLY CONTROLLED LABORATORY ENVIRONMENT WITH APPROPRIATE SAFETY PRECAUTIONS TAKEN (PROPER VENTILATION AND EYE COVERING ARE ESSENTIAL).
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Major sources for this page include Kodak's The Book of Film Care, Eastman Kodak's website on their company history, and a bit from the National Plastics Museum website on the history of Celluloid.