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Fixing Baths — Crahtree and Hart 397
air, otherwise in the absence of the neeessai'y sulfite the harcleuer will cause the bath to siilfurize.
1. Temperature of tlie Fixing Bath. Under normal conditions the bath should be maintained at a temperature between 65° to 70° F. for most efficient fixation. If the temperature is higher than this, the gelatin is liable to soften before the hardening materials have been able to harden the gelatin sufficiently to w^ithstand a higher temperature while if the bath is below 65° F., the rate of fixation is impaired and maximum efficiency is not obtained.
2. Time of Fixation. The times required to clear the emulsion of positive and negative motion picture film at 65° F, with and without agitation* during the active life of the bath (rack and tank method) are sho\^ai in Table lY. The bath w^as revived with acid at intervals but, of course, was continuously diluted by the w^ater carried over by the racks from the rinse bath.
The time of fixation wdth agitation may be taken as a rough measure of the times for fixing with machine development. Recent experiments have shown that the emulsion is fixed out when all signs of opalescence of the film have disappeared so that for all practical purposes the film is fixed as soon as it clears. However, wdth most hardening fixing baths a slight opalescence of the gelatin persists even after thorough fixation and washing but invariably disappears on drying. It is good practice, therefore, to fix the film for at least twice the time required to clear the emulsion appreciably.
3. The Degree of Hardening Produced. The melting point of positive motion picture film ranges from 120° F. to 160° F. throughout the active life of the bath when revived at intervals with acid as shown in Table IV. After each revival with acid, the degree of hardening rises from 20 to 30 degrees and gradually drops again as the bath is used and more developer is carried into the bath by the film. In this case, the bath was on the verge of sludging at the revival point so that the hardening increased on the addition of acid. In the case of the curves shown in Figs. 9 and 10 the hardening decreased on revival with acid because the bath was much more remote from the sludging point at each revival. The hardening curve for a fixing bath used in conjunction with the Eastman borax developer^^ (which contains a high concentration of sulfite) is somewhat different. Since the addition of an excess of sulfite to a fixing bath lowers the degree of hardening (Fig. 4) it is seen that if the