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1949 COLOR PROCESSING CONTROL 11
adopted to machines used for 16-mm film processing as described by Forrest,3 for 35-mm film processing as described by Harsh and Schadlich,4 and for rack-type sheet-film processing machines varying from a large Pako machine to small vane-agitated 3V2-gallon tanks. The value of replenishment is questionable for hand-agitation systems. The greatest control normally is obtained with the larger size machines that are in constant rather than intermittent use. It is desirable to maintain continuous filtration systems in both color and first-developer tanks as the build-up of gelatin particles, specks of oxidized developer, and other foreign material hasten the chemical breakdown of solutions. Proper filtration will keep both first developer and color developer clear and light in color after months of operation.
A. Modification of Processing Solutions to Change Color Balance
The widely varying agitation conditions existing in the different types of processing equipment introduce a complicating factor because variations in agitation can produce different color balances. Partial compensation for these balance differences can be obtained by increasing or decreasing developing tunes. However, in order to achieve the closest possible matches in speed, gradation, and color balance, it is sometimes necessary to make slight chemical changes in the processing solutions themselves.
The most convenient tools for modifying color-balance differences resulting from different agitation conditions are variations of the thiocyanate and iodide concentrations in the first developer solution. Chemical analysis of No. 502 first developer has shown that iodide accumulates during film development, and, depending somewhat on the type of film processed, exposure level and volume of replenisher added, normally reaches an equilibrium of from 3 to 6 milligrams per liter of developer. Iodide-analysis methods and a discussion of iodide equilibrium for black-and-white film developers were given by Evans, Hanson, and Glasoe.5» 6
Practical tests with color film show that even a small concentration of iodide exerts an appreciable restraining effect on the yellow and magenta layers giving an effective speed loss in these layers and a shift in the over-all color balance toward the brown. It can be shown that accumulation of iodide is responsible for a large part of the colorbalance shifts that occur when a first developer is used. If small