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Fixing Baths — Crahtrei and ILu-f 375
120 seconds witli no agitation in a 35% hypo solution. Conditions of agitation analogous to those produced by brushing rarely exist in practice.
With positive motion picture film agitation has little effect under the same conditions although with a 10% hypo solution the time of fixation is cut in half by brush treatment. The rate of agitation produced in the average processing machine corresponds approximately to that produced by hand agitation.
V. The Efficiency of Acid Hardening Baths
The practical efficiency of a fixing bath, is governed by the following factors :
1. The ahsoUUe times of fixation tvhen the hath is fresh and ivhen exhausted. In practice with, motion picture negative emulsions a fixing bath is considered exhausted when the time required to clear the film is excessive, the limit being usually from 10 to 15 minutes ; that is, it is considered better economy to prepare a fresh bath, than to waste tim.e waiting longer than 20 to 30 minutes for the film to fix, although at this point the bath is not actually exhausted because an emulsion could be satisfactorily fixed by prolonged treatment. (In practice the rule ''time of fixing = twice the time to clear" is usually observed.)
3. The quantity of emulsion fixed during the active life of the hath. The active capacity may be defined as the quantity of film fixed up to the point when the time required to clear the emulsion is greater than a definite limit. This varies according to the particular emulsion to be fixed, the active capacity being much less in the case of slow fixing emulsions than in the case of rapid fixing emulsions which are usually more fine grained.
3. The staining limit of the hath, that is, the quantity of emulsion fixed before the bath stains the film with average manipulation. Staining in a fixing bath may be a result of (a) insufficient fixation or (b) lack of agitation especially in the presence of free developer. If the film is insufficiently fixed any residual silver thiosulfate ultimately decomposes producing a bro\\T:i stain. Also, if the bath becomes alkaline because of developer carried into it by films, the free developer reduces the silver thiosulfate in the film to finely divided metallic silver in situ causing dichroic fog.^