Movie Makers (Jan-Dec 1930)

IM«»'%'IE IVaymiCERS that it actually regenerates the reducing agent. At any rate, we find that the addition of sulphite does prevent the rapid oxidation of the developing solution, so it is termed the preservative. We find that we now have a developing bath which works quite satisfactorily, although we will find the film has a very definite inclination to fog. This trouble is aggravated if the developer is used at a higher temperature or in more concentrated form. The addition of a minute quantity of potassium bromide prevents the tendency to fog and, for this reason, it is known as a restrainer. Thus we find our developing bath is composed of a reducer, to develop; an accelerator, to speed up the action; a preservative, to keep the solution clear; a restrainer, to check the tendency to fog. When an exposed film is placed in such a solution the microscopic grains of silver bromide which have been exposed to light are changed to metallic silver. This process is not instantaneous but extends over an appreciable period of time and is cumulative. When every exposed grain has been reduced to silver, the number of the grains and the density of their deposit is proportionate rinse in this and then placed in a solution of sodium thiosulphate which is commonly but incorrectly referred to as sodium hypo-sulphite or "hypo."' This salt has a property of dissolving the silver bromide rapidly while it has no appreciable action upon the silver of the image, unless it is allowed to continue for an extended period of time. As in the case of the developing solution, the fixing agent or hypo is not used alone. When used alone it will perform its proper function in an entirely satisfactory manner but has a very short life. Other chemicals are added for quite different purposes than to aid in the elimination of the excessive silver bromide. One of the most common alterations of the bath is the use of some kind of acid salt which prolongs its life. The acid also prevents yellow oxidation stains which not infrequently follow development. One of the acid salts commonly used for this purpose is potassium metabisulphite. A still more common fixing mixture consists of hypo, acetic acid, sodium sulphite and common alum. In this mixture the hypo does the actual fixing. The acid reduces stain, clears the film and increases the rate of fixation. The sulphite prevents stains and oxidation of the solution, at the same time protecting the hypo from decompos'ition by the acid, while the alum hardens the is allowed to remain in contact with the silver of the photographic image, a chemical change takes place in the combining of the sulphur of the hypo with the silver. This compound of silver and sulphur has a very unpleasant yellow color and, when the change from' the black of silver to this yellow of the silver sulphide has taken place, we say that the picture has faded. To prevent this, it is essential that all the hypo be removed from the emulsion before the film is allowed to dry. In perfectly circulated water the hypo will be entirely removed from photographic emulsion in from three to twelve minutes, depending upon the type of emulsion. The emulsion of positive films can be washed much more quickly than the heavy orthochromatic and panchromatic negative emulsions. One danger in washing is that of wrinkling or reticulation which is commonly attributed to high temperature. The fact is that reticulation occurs very often at temperatures which would not seriously affect the film were washing to be carried out in water of the same temperature as that of the hypo. Reticulation is due to the fact that the gelatin of the emulsion will absorb water more rapidly than the solution of hypo is diffused. This actually crowds the capacity of the gelatin until it is forced to swell against itself, giving rise to the characteristic "orange peel" texture of reticulation. Naturally, as is the case with other chemical reactions, hypo elimination is effected to the amount of light which falls upon the film. Naturally, we have to start with an emulsion in which the silver bromide is uniformly distributed. When we secure a non-uniform deposit of reduced silver which forms the image, it is evident that we shall have a certain amount of silver bromide which has not been reduced to silver and which must necessarily still be sensitive to the action of light. Obviously, this light sensitive material nmst be removed from the film before it may be used for printing or projection. For this reason, the film is submitted to the action of a fixing bath. Fixing. For purposes which we shall learn later, the fixing bath has an acid reaction while the developer is strongly alkaline. In order to prevent undue weakening of the fixing bath and to stop the action of the developer instantly, the film is removed from the developer and placed in a weak solution of acid. It is given only a brief gelatin, preventing frilling and excessive softening. The hardening bath is quite essential as it not only prevents the actual deterioration of the gelatin while in the solutions but, if hardening is properly carried out, the emulsion, when the film is removed from the fixing bath, will be firm enough that the film may be stripped through the fingers without injury. As motion picture film cannot be successfully spotted or retouched, it is essential that every precaution be taken to prevent scratches and breaks in the emulsion. Washing. There is an unfortunately common impression that as soon as a film is fixed it is beyond danger. For this reason, washing, drying and polishing are likely to be neglected. When the film is completely fixed it is charged with hypo. If hypo more rapidly at higher that at lower temperatures. When the film has been thoroughly washed the fact may be determined by a very simple test. A very weak solution of potassium permanganate has a distinct rose color. If a few drops of this solution are poured into water containing even a trace of hypo, the rose color is rapidly changed to a straw or amber yellow. A little water taken from the washing vessel and tested in this manner will instantly indicate the extent of the hypo elimination. f Continued on page 45) 25 X