In the United States the coating is generally done on large wheels about four feet across and about fifteen feet in diameter which rotate slowly about their axes so that by the time one rotation is completed the solvents have been evaporated suffi ciently to set the film, which can be stripped off the wheel, and dried further by passing over drums. The operation is thus con tinuous, the film base, as it is called, being prepared in rolls of about 2,000 feet. In Europe it is customary to coat a film about 24 inches wide upon a metallic belt from which after one rotation the film can be stripped in the same way as from a drum. The surfaces of these belts and drums must necessarily be prepared very carefully, the wheels being usually made with a highly pol ished nickel surface, while the makers of the belt machines recommend that they be coated with gelatine. The film has a high polish on both surfaces and may be used at once or stored or seasoned until required. Before coating with emulsion, one side is treated with a substratum, which is really an etching solu tion that slightly roughens it, and gives a tooth that holds the emulsion firmly. For camera film, the back is coated with a non curling coat of hardened gelatine, which, imposing an expansion strain on this side, equalizes that of the emulsion on the other. Camera or roll film base is about 34 thousandths of an inch thick; motion picture film, about thousandths; and cut film, such as portrait and X-ray, which are used flat, about 71- thousandths. Still thicker cut films are sometimes used, about i o thousandths, but the celluloid for these is cast into blocks and then sliced, the knife marks being removed by heat polishing.
Since cellulose nitrate is highly inflammable, itself supplying the greater part of the oxygen which it requires for combustion, much work has been done oil the preparation of film which is not inflammable. The most promising substance for this purpose is cellulose acetate, prepared by treating hydrated cellulose with ace tic acid and acetic anhydride in the presence of a suitable cata lyst. Acetate film burns with about as much difficulty as thick paper, and should it catch fire it is very easy to extinguish. This acetate base is used under all conditions where proper precautions against fire cannot be taken, as where films are used in homes or schools. Sixteen millimetre film for amateur cinematography is made only on acetate base.
Photographic paper base is generally made by mills which spe cialise in its manufacture, but in the United States the bulk of the photographic paper base is made in the factory where the rest of the production of photographic paper is carried on. The paper is prepared very largely from rag stock, although refined cellu lose from wood is sometimes added. After inspection the raw rag stock is washed and digested and is then treated in the beater until it will make a homogeneous, strong sheet. Great care is neces
sary to exclude traces of impurities and especially of metals such as copper and iron from the paper stock. The composition differs with the purpose for which the paper is used. The finest kinds are prepared from linen rags, which are bleached with gaseous chlorine or hypochlorites, any excess being removed by treatment with sulphites or thiosulphates which are then washed out. Any residual trace of colour is neutralized by the careful addition of a complementary colour, such as indanthrene blue, or if a chamois or toned base is desired the bleaching may be omitted or indan threne yellow added. A resinous soap is usually incorporated with the fibres and precipitated with alum, and the paper may be later subjected to a sizing with gelatine. Throughout all its treatment great care must be exercised to avoid the occurrence of metallic particles, which would act prejudicially on the sensitive emulsion. The paper is generally cast about 41 in. in width and from 500 to i,000 yd. in length. It is calendered either to give it a closer texture or higher surface or to impart some special linen or fabric-simulating finish.
The paper is usually given a coating of baryta before it re ceives the photographic emulsion. In this process a suspension of baryta (barium sulphate) in gelatine is coated over the surface of the paper, several coatings being sometimes applied, and not infrequently the paper is calendered after baryta coating in order to get a perfectly smooth glossy surface. Baryta-coated paper is transferred to the emulsion coating-room in rolls of about 2,000 feet.