Cellulose nitrate film base

Cellulose nitrate, produced from cotton waste or wood pulp treated with nitric acid, was initially used as gun cotton as an explosive, and first marketed in a solid form as Xylonite. It can be dissolved in organic solvents and laid down in
a transparent thin sheet.

The very first flexible film base, invented in England as Parkesine, and first used for photographic base as Celluloid about 1888.

Celluloid was invented in USA in 1870 as a colloidal mixture of cellulose nitrate and camphor. In many ways cellulose nitrate base provided all the requirements needed for photography, except for the hire fire risk, its tendency to shrink by loss of plasticiser, and the decay that occurs. It was the first synthetic plastic, is thermoplastic, very tough, with a high tensile strength, and could be handled mechanically somewhat like a metal. It could be fabricated, and polished, and was used for a wide range of plastic products from toys to telephones, as safety glass as well as film base.

It was used for film until the beginning of the 1950's, when it was discontinued due to the high flammability. However almost all the film produced from 1896 until 1950 was cellulose nitrate and in some respects is more stable with time than some of its successors.

Nitrate base decay is a process that is dependant on temperature and humidity. The initial signs are severe shrinking, but up to 5% in the length, but chemical decay can also occur well before serious shrinkage is obvious. The first visual sign is a cloudiness of the base, thickening of the film, and brown powdery deposits, especially from the film edges. The film convolutions start to stick together and the aromatic smell characteristic of nitrate increases. Eventually the film may set in a block, which may crack across the radius, and finally the stickiness becomes almost liquid. It does not seem that the material becomes more inflammable during decay and may even lose its flammability to some extent.

Cellulose nitrates apparently spontaneously combusts, although this has not been observed. Several of the major film base fires appear to have started spontaneously often in unusually hot weather. Nitrate has been observed to burst into flames due to excessive friction, but most nitrate fires were in cinemas and laboratories where the ignition occurred in projectors. If the film stopped in the projector gate the heat from the projector lamp ignited the film, unless a dowser, an asbestos [usually] shutter, dropped between the lamp and the gate. Occasionally, though rarely, nitrate film base is found in narrow gauges.