Nitrate

Cellulose nitrate was the first man-made polymer and was originally developed in 1850s as a substitute for natural horn for brush handles and other small products. The product was then known as parkesine, named after the inventor. Later cellulose nitrate plastics became known simply as celluloid, a name which became synonymous with transparent plastic and later associated with motion picture film.

Kodak first marketed celluloid as a photographic film base in 1889. Celluloid was the original motion picture film base material and was used for professional motion picture film base from 1895 to approximately 1950 when high acetyl cellulose film base, commonly referred to as cellulose triacetate, was widely introduced.

Cellulose nitrate was manufactured using cotton linters 1, a by-product of the cotton industry, reacted with a mixture of nitric and sulphuric acids to form a polymerising ester. The polymer was dissolved in solvents with additives to improve the polymers properties and the resulting mass was “cast” on a highly polished flat bed. This produced a thin layer of polymer in solution. The residual solvents evaporated leaving a thin flexible plastic film.

Fully nitrated cellulose nitrate is an explosive and although the cellulose nitrate used for film has a lower degree of nitration, the polymer is still highly flammable. A common additive used in nitrate film, camphor, is also highly flammable further compounding the problem. Flammability was one of the main drawbacks with nitrate motion picture film.

The first clue to identifying cellulose nitrate based films in a collection is gauge or width of the film. While some scant examples of small formats such as 8, 16 mm film may exist, largely cellulose nitrate was only used for 35mm film (all emulsion types) and as the base for some 28mm negative materials.

Differentiating between cellulose ester bases can be very difficult and often it is an assumption made on edge markings such as date codes or the style of the font used that may match a particular base material to an era when it was in use. For example, it is unlikely to find a cellulose triacetate base on a 1920s film.

Nitrate glossary image 1

Approximate dates for film base polymers


Nitrate film usually has NITRATE printed on the edge. This may be a good indicator, however this can be a print through from a nitrate film copied onto a safety base duplicate and other evidence may be required for a more definitive identification.

Nitrate glossary image 2

Typical nitrate edge marking


Storage and transportation

Nitrate film is classed as a 'Dangerous Goods’ using criteria described in the UN Manual of Tests and Criteria . As such, some countries may require a license to store nitrate film and there are restrictions on projection, packaging and transportation. Check national and local regulations regularly for obligations regarding the storage and transport of cellulose nitrate based film.

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References

1 E. K. Carver, The Manufacture of Motion Picture Film, SMPE Journal, Vol XXVIII No 6, June 1937