The Cine Technician (1939)

L'l'.l INK < I \ I E0HN101AN Vfarch-April, L938 EMULSIONS and GRAINS By ALBERT DYAS IN ilu manufacture "I cinematograph film, the mixture of silver halide gelatine and water, constitutes what is known as emulsion. When fully prepared and finished, it is coated over a flexible transparent base of celluloid and is used either .is a negative or a positive, as the ease may be. Both negative and positive emulsions are somewhat similar in their chemical construction, but aetualh there is a vast amount of difference in the structure of the silver lialide. Negative emulsion is made up of larger, irregular grains, taster in speed than the positive, In the slower positive, the grains are smaller and more uniform in size, this having a larger effect on the contrast. To record the correct density of coloured objects, the negative emulsion requires correct colour sensitivity. This necessitates the processing oi the exposed negative in complete darkness, whereas the positive can be processed in a fair amount of green or red safety light. Naturalh the methods of preparation of various film stocks are very diverse. Each manufacturer has his own particular and complicated formulas, hence the description here is in a simplified form and only the basic operations are described. The silver halide of a negative emulsion, explained simply, is formed by a mixture of gelatine and potassium bromide in a warm solution, with the addition ot sil\ r nitrate. The bromide and gelatine are held together in a solution at an even temperature in a container, of which the inside is coated with silver. The silver nitrate is dissolved in water and heated. When this solution is added to the bromide and gelatine, double decomposition occurs and throws out a milk-like fluid containing the insoluble light sensitive silver bromide, the reaction being: Silver Nitrate + Potassium Bromide= Silver Bromide + Potassium Nitrate. The extraction ot the soluble salts, namely potassium nitrate, is made after cooling down to a jelly. The emulsion is either cut up into shreds or compressed through holes and thoroughly washed. After washing, it is reheated, ripened and coated on the celluloid base. An ordinary silver bromide emulsion similar to this wovdd be of little use as a negative, as the sensitiveness of pure silver bromide to light is very small. For colour correctness it is only sensitive to the blue rays of the spectrum and is almost insensitive to yellow, green and red. The preparation of high-speed panchromatic negative emulsions demands a large amount of accuracy in quality and manipulation of its components. The purity and quality of the gelatine is important, as sensitivity depends upon the particular kind used. Gelatine, which is made from the skins of calves, has natural properties of its own. namely mustard oil, which contains sulphur. With the addition of other organic compounds, the gelatine possesses t li ioearl iati I ides which are active sensitizers to the silver bromide grains. A tine grain emulsion, with an absence of granularity, is essential. Owing to the enormous enlargement of the positive print to the screen, any clumping of grains would obviously show to a certain extent when the negative is printed, and the absence of detail would be noticeable. The smaller the grains, the lesser the graininess. the more rendering of detail will be possible. The alleviation, as tar as possible, ot clumping is partly due to the concentration ot the solutions and the precise way in which the silver halide is formed. This has a large influence on the grain size characteristics of the emulsion. By treating the emulsion with s< nsitizing dyes known as " isocv aniues" it is rendered panchromatic. The absorption of these dves by the silver halide gn ns has the attributing factor ot making the emulsion sensitive as n< ir as possible to all colours ot the visible spectrum. What happens when tin emulsion is exposed to light is still a controversial point. It is not known with certainty whether the action of light is of a chemical, electrical or physical nature. On the surface of the silver halide grains are tiny areas which are the sensitive centres or nuclei which somehow or other are affected by light, and produces invisible specks ot metallic silver from which development starts. The other parts which are unaffected merely supply the silver to the nuclei to make possible the building up of the visible image. Alter the light action, there is not any apparent v -ible effect on the emulsion, the light having produced an invisible or latent image. The latent image becomes visible with the aid of a developing solution, which reduces the silver halide compounds to metallic silver. The structure of the developed image depends very much on the developer. Negative developers, being fine grain. break up the grains so much that it bears practically no relation to the original emulsion grains: therefore our resulting negative should be free from granularity. When the exposed negative is immersed in the developer and the latter has penetrated into the gelatine around the grains, the reduction begins first at the sensitive centres and spreads slowly over the light affected areas. Where the greater intensity ot light has registered. there are more deposits of black metallic silver grains. the unexposed parts of the silver halide remaining practically unaffected by the developing action. When the development is completed, the negative is fixed in a hypo, solution. The silver grains which were not reduced by the developer are dissolved away, leaving only the metallic silver grains, which is our negative image. When the negative has been washed and dried, and is finally ready, it remains to be printed. If we examine the negative, wherever light has fallen there remains a black layer of silver grains ; the lesser the intensity of light, the smaller will be the deposits. Theretore everv conceivable shade ot light that has affected the exposed emulsion will be reproduced on the finished negative in the form of small and large deposits of silver grains. When the negative is finished, the emulsion ot the negative is placed into contact with the emulsion side of the positive stock. The amount ot light from the printing lamp penetrating into the bromide emulsion of the positive is determined by the deposits of silver grains in the negative, the greatest amount ot light getting through where the deposits are least, the least where the negative is darkest. Thus the light and shade of the scene that was photographed will be the same on the positive aboriginally registered by the negative emulsion