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THE EVOLUTION OF FILM 39
nitrate." This process, while not altering the physical appearance of the original cotton, does change it chemically so that it will be soluble in the various mixtures in which it will be deposited.
When nitration is complete, high speed rotation of centrifugals separates the excess acid from the cotton. Next the nitrated cotton is immersed in large tanks of water and drained and rinsed over a period of weeks. Other centrifugal wringers spun at high speed remove all the moisture before the cotton is ready for the solvent.
Huge drums or barrels are used to bring about a thorough mixing of the cotton and the wood alcohol which is the chief solvent. The drums are sealed and revolved for a period of several days, and the solution which results has the consistency of syrup or extracted honey. This is then pumped through mechanical filter-presses to render it absolutely free from any remaining dirt, dust, or foreign particles.
This "dope," as it is called at Kodak Park, is next piped to airtight tanks and is held ready to be converted into sheets. The solution, now glass clear, is poured on the surface of great polished wheels which run continuously night and day. One of these wheels now produces twenty-five times as much film base as the whole of the first Eastman factory.
As the film must be uniform in thickness, this operation calls for extreme care in handling, and the variation in thickness in a sheet 2,000 feet long and 3J/£ feet wide is imperceptible except with the most delicate instruments of measurement.
For easy handling the base is rolled on a core in large rolls similar to printing paper rolls, and in this form, after a period of aging, it is sent to the sensitizing rooms.
Silver is the active element in the sensitizing material, called the "emulsion," with which the film is coated. The pure silver bullion comes in bars, each weighing about 42 pounds. The bars are dissolved in nitric acid in porcelain dishes, and after crystallization pure crystals of silver nitrate are obtained. Other ingredients of the emulsion are potassium iodide, potassium bromide, and gelatine. If these bromide and iodide salts are dissolved in water, and if to the solution thus prepared silver nitrate solution is added, a soluble yellow salt is precipitated which is very sensitive to light and which turns black after a few minutes' exposure.
If this solution were coated on the base, the film would have very little sensitivity, and for all practical purposes it would be worthless. For this and other reasons the precipitation must be conducted in some material that will avoid these difficulties.
The material commonly employed is gelatin, a substance analogous to glue in composition and like glue in that it is extracted from the bones and hides of cattle. The gelatin is dissolved in water and the bromide and iodide solutions are carefully mixed with it. To this mixture, heated to the correct temperature, is added the silver nitrate solution. The precipitate of the sensitive silver salts is held in suspension by the gelatin.
All emulsion-making operations are conducted in rooms lighted with safelights. The actual operations of making the emulsion are