Film base

A film base is a transparent substrate which acts as a support medium for the photosensitive emulsion that lies atop it. Despite the numerous layers and coatings associated with the emulsion layer, the base generally accounts for the vast majority of the thickness of any given . Historically there have been three major types of film base in use: nitrocellulose (cellulose nitrate), cellulose acetate (cellulose triacetate, cellulose diacetate, cellulose acetate propionate, and cellulose acetate butyrate), and polyester (polyethylene terephthalate (PET) (Kodak trade-name: ESTAR)).

Nitrate film base was the first transparent flexible plasticized base commercially available, thanks to celluloid developments by John Carbutt, Hannibal Goodwin, and Eastman Kodak in the 1880s. Eastman was the first to manufacture this for public sale, in 1889. Unfortunately, nitrate also had the drawback that it was extremely flammable (being essentially the same chemically as guncotton) and decomposed after several decades into a no less flammable gas, leaving the film sticky and goo-like (and ultimately dust).

As this happened, the likelihood of auto-ignition increased even further. Projection booth fires were not uncommon in the early decades of cinema if a film managed to be exposed to too much heat while passing through the projector's film gate, and several incidents of this type resulted in audience deaths by flames, smoke, or the resulting stampede. An accident of this kind was recreated in Cinema Paradiso (1988).

The year 1978 was particularly devastating for film archives when both the United States National Archives and Records Administration and George Eastman House had their nitrate film vaults auto-ignite. Eastman House lost the original camera negatives for 329 films, while the National Archives lost 12.6 million feet of newsreel footage. Because cellulose nitrate contains oxygen, nitrate fires can be very difficult to extinguish. The US Navy has produced an instructional movie about the safe handling and usage of nitrate films which includes footage of a full reel of nitrate film burning underwater. The base is so flammable that intentionally igniting the film for test purposes is recommended in quantities no greater than one frame without extensive safety precautions.

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