The emulsions used for printing papers are of three general types : bromide papers, which are made with a washed emulsion essentially identical with that used for coating upon glass and film for use in making lantern slides and positive motion pic tures; unwashed chloride emulsions in which the emulsion is coated on the paper in the form in which it is mixed without any removal of the soluble salts by washing; and emulsions also made without washing but in which the silver salts are a mixture of chloride and citrate, and in which there is free silver nitrate present. These papers darken visibly when exposed to light and thus give a printed-out image which is then toned and fixed.
The most usual size of plates are 8"xio", I I"X 14", and 14"X I7", in the United States; 44"X 31" ("quarter plate"), 61"X 4i" ("half plate"), 8" X 64" ("whole plate"), 12"X o", and is"X i2", in Great Britain (where the larger dimension is given first for inch sizes, the contrary practice being used in the United States) ; and 6 X 9 cm., 9X 12 Cm., 10X 15 cm., 13)518 cm., 18X 24 cm., 24 X30 cm., and 30X40 cm., in countries using the metric system.
For coating both film and paper the same type of machine may be used. (See Plate II., fig. I.) The stock roll of the base is car ried on a mandrel, freely revolving, and is led over and under rollers that keep it taut, to the coating trough into which the fluid emulsion is fed continuously and automatically at the rate at which the base takes it up. The base passes under a roller, dipping into the emulsion, and it hugs this so tightly that no emul sion finds its way on to the back. The temperature and viscosity of the emulsion are kept constant, and the thickness of the coat is determined by the rate of travel of the base through the emul sion—the faster the travel, the thicker the coat, and vice versa. The coated base then passes up a vertical run-up, which allows the film of emulsion to even itself out, or it passes over a chill roll and then through a refrigerating chamber, where cold air is blown on to the emulsion surface and thoroughly sets it. Leaving this chamber, the film is automatically looped into festoons varying from five to twenty feet in length and travels slowly through the drying tunnels. To these is supplied heated air which has been thoroughly washed with water sprays to remove all dust and dele terious gases. The humidity and temperature of the air used for drying is adjusted very carefully and it is usually recirculated through the system, being driven in at one end of the drying room and exhausted at the other, and then reconditioned before entering the drying chamber again. Frequently the festooning
apparatus makes a right-angled turn so that the prepared material is reeled up side by side with the coating machine ; the operations of coating, drying, and reeling are continuous and automatic.
Motion picture film is slit from the original roll to the exact width required, 35 mm. (Li in.) in the case of standard film, and before packing is perforated on the edges in accordance with the standard gauge used for the cameras and projection machines. The maker's name and private mark are light impressed on the margins, becoming visible only on subsequent development, while on negative film every foot is numbered in order to aid in the identification of the scenes. Negative film is supplied not only coated with ordinary emulsion, but also panchromatized, which gives far better rendering of all colours and permits considerable shortening of exposures, especially under the artificial illuminants, such as the mazda and other lighting systems now so much in vogue in the studios. The speed of the emulsion is very high and a special hypersensitised film is obtainable, which places in the hands of an operator the possibility of photographing poorly lighted subjects. Positive film is supplied either on nitrate base, that used most generally, or on acetate safety base, which is slow burning. For amateur motion picture photography film is sup plied 16 mm. in width on slow-burning acetate base only and is supplied in lengths of ioo ft. packed with paper leaders at the ends and on a special protective spool so that the cameras can be loaded in daylight. This film is made to be reversed directly to a positive so that the same film that is used in the camera is em ployed for projection.
The camera or cartridge film is slit to the required widths, cut to the desired lengths, and spooled on wooden cores with light excluding metal flanges. A light-protecting apron of black paper is wound with the film, so that the cartridge may be inserted in the camera in daylight ; or red paper with an inset of carbon paper is used instead of the black, so that records may be made on the film after exposure. Film is also cut into sheets, to each sheet being attached a numbered tab, and the sheets are assembled into a film-pack, which is usable in any ordinary plate camera. Af ter each exposure, the tab is pulled, which draws the exposed film to the back of the pack, and the tab is torn off and thrown away.
For X-ray work, the film is coated on both sides with emulsion; this gives greater contrast and better rendering of shadow detail, for the rays can penetrate both emulsion layers with but little loss of action. The film is used in holders like those for plates, except for dental work. Films for this purpose are packed in special flexible, light-tight holders ready for use.
Paper is cut from the roll into sheets, and the sheets stacked on the top of each other are cut by means of guillotines to the size required for the market, each sheet being inspected for defects before packing. (G. EA.)