Film base

A film base is a transparent substrate which acts as a support medium for the photosensitive emulsion that lies atop it.

Since the late 19th century, there have been three major types of film base in use: nitrate (until about 1951), acetate, and polyester.

It gradually decomposes producing a flammable gas, becomes sticky and, at a late stage of decomposition, the film oozes a gooey fluid, ultimately leading the substance to become dust.

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 flame, smoke, or the resulting stampede.

An accidental fire caused by the film jamming in the gate formed a significant plot point in the movie Cinema Paradiso (1988).

Eastman House lost the original camera negatives for 329 films, while the National Archives lost 12.6 million feet (3.84 million meters, or very roughly the distance between the two coasts of the continental United States) of newsreel footage.

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.

The smoke produced by burning nitrate film is highly toxic, containing several poisonous gases and can become lethal if inhaled enough.

Despite the dangers of the nitrate film base being known practically since its development, it was used in virtually all major motion pictures prior to 1952, when Kodak completed a four-year conversion program to the sole manufacturing of acetate base film stocks.

With exposure to heat, moisture, or acids, the acetyl groups are broken from their molecular bonds to the cellulose.