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4 FUNDAMENTALS nitrate. There are other bases used, but they are in such small sup- ply and usage that they will be ignored since it is extremely unlikely that television personnel will encounter them. Most 35 mm film is made of the nitrate base, which is highly inflammable. In fact, it might almost be called explosive since it requires very little air to burn fiercely with intense heat. On the other hand, the acetate base used for 16 mm film is slow-burning and is considered non-inflam- mable. It can be caused to catch fire but requires the application of heat for a comparatively long time. Safety film has the words "safety film" printed along its edges so there is no mistaking it. If these words do not appear, it must be treated as nitrate stock. Wherever 35 mm film is used the local fire ordinances are very strict in their application to the premises in which it is used. Steel, fireproof projection booths are required, and a number of other restrictions also apply to complicate matters. As a result of this, and cost factors which are discussed in the next chapter, 16 mm film is used almost exclusively for television in all but the largest stations. The question will at once be asked: Why not use acetate base for 35 mm film? There are three answers to this. One is that the acetate base is not as clear as the nitrate. Instead of being completely trans- parent and with no identity of its own, it tends to produce pictures that are slightly opaque where the only coloring, or shading, should be from the emulsion. Then, also, the acetate base is slightly more brittle than nitrate. This is a big factor in the theatre world. A film print for the movie theatre has to be projected a certain number of times before it is scrapped, and the acetate film wears out very rapidly by comparison. Acetate dries out in the air very much more quickly than nitrate and during the run through the projector is exposed to considerable heat. This hastens the drying (the heat in a 35 mm projector is very much greater than in a 16 mm, because lower-powered lamps are used in the smaller, semi-professional projectors), and the film is more likely to break in the machine. As far as cost is concerned, there is very little difference; if there is any, it is in favor of nitrate. Finally, the acetate film must be kept in cans with humidifiers to keep the film moist. The 35 mm film is perforated on both edges and has four