PHOTOGRAPHIC MANUFACTURE The manufacture of photographic sensitive materials is a very specialised industry. Approximately 40,000 people are employed in the whole photographic industry throughout the world, about 20,000 being engaged in the manufacture of materials sensitive to light, with which this section deals, and the remainder in the manufacture of cameras and in the wholesale distribution of the products to the retailer.
With the introduction of the gelatine dry plate, the preparation of materials by photographers for themselves diminished very rapidly, and the manufacture of ready-prepared material took its place. The first dry plates appear to have been placed upon the English market in 1877. In 188o the manufacture of gelatine dry plates was general in England and was commencing in Belgium, Germany, France, and the United States.
Until 19oo the photographic industry was usually carried on in small factories under the direct personal control of the founder. At the present time, however, photographic manufacture is or ganised chiefly in modern factories manufacturing on a large scale, and using specially designed machinery at every step of the pro cess. In the largest of these factories, the output of motion picture film alone exceeds i so,000 miles yearly. Over 5,000,000 lb. of cotton are used each year for the manufacture of film, and over three tons of pure silver bullion are used each week. The total power required exceeds 20,000 h.p., and the consumption of coal is over 500 tons a day.
The process of manufacture may be divided into three distinct sections: (I) The preparation of the base ; (2) the preparation of the emulsion ; (3) the coating of the emulsion on the base and the cutting and packing of the material so prepared.
Photographic glass is made very largely in Belgium in the fac tories near Charleroi, and in England at St. Helens. A con siderable amount is also made in the United States. The glass is supplied cut to sizes which are multiples of those used by photographers. The glass arrives packed in straw in boxes holding usually about two gross sheets, and after unpacking and pre liminary inspection, is cleaned by an automatic machine into which it is fed. The glass plate is received between rollers and passes through a strong solution of soda between revolving brushes which remove the dirt from the surface, and at the end of the machine is coated with a substratum, usually a weak solution of gelatine containing chrome alum, by means of which the emul sion is made to adhere to the glass. The glass is then dried in a heated oven, examined for defects, and if passed is packed in a tray and transferred to the coating room.
Film Base.—In the manufacture of film base the first step is the preparation of cotton linters for nitrating, the treatment con sisting of cleansing the linters very thoroughly and drying them.
They are then nitrated in centrifugal machines, and when they have absorbed the necessary amount of nitrogen, 12 to 13%, the resulting nitro-cotton is soused in water and the washing com menced. This washing is a long process, and is continued until the material is entirely free from absorbed acid, after which it is dehydrated, the water being then displaced by alcohol. The dehydrated nitro-cotton is then dissolved in tumbling barrels or mixers in suitable solvents, those commonly employed being ace tone and methyl alcohol, and at the same time the so-called softeners, such as camphor, are added, these resulting in a flexible film. The viscous nitro-cotton solution, which appears rather like honey, is known in the United States as dope. This is now spread out into a thin film by coating it on a travelling surface.