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 film stock. Since the late 19th century, there have been three major types of film base in use: nitrate (until about 1951), acetate, and polyester. Nitrate In the literature of photography "nitrate" is used as a synonym for the chemical nitrocellulose. It is also referred to as "cellulose nitrate". Nitrocellulose is guncotton, the first replacement propellant for gun powder in firearms. Film stock with a nitrate 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 the film stock for public sale, in 1889. Unfortunately, nitrate also had the serious drawback of being extremely flammable, the characteristic that made it a suitable substitute for gunpowder. 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. As this happened, the possibility of auto-ignition increased 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 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). 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 are impossible 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 under water. 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