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Fixing Baths — Crahtree and Hart 395
if both rapid fixation and excessive hardening are desired the fixing bath must harden more rapidly than if a longer time of fixation were permissible.
It is not absolutely necessary to harden the gelatin coating of motion picture film, but a hardened negative film has a less propensity to become finger marked during handling, while with positive film hardening during fixation is desirable because (a) this permits more rapid drying of the film; (b) if the film is developed by the rack system, the hardened gelatin coating is less liable to be abraded when wiping the film during transference to the drying reel;* and (c) if the film is relatively moist during projection, there is less tendencj^ for the film, if hardened, to stick in the projector gate.
Non-hardening Fixing Batlis
If the temperature of the various developing solutions can be maintained at a temperature not higher than 65°F. and if ample drying time is allowed so that relatively cool drying air can be used, the following non-hardening fixing bath is satisfactory:
Avoirdupois 100 lbs. 4 lbs. 10 lbs 50 gallons
The bisulfite serves to keep the bath acid and prevents stains while the sulfite prevents precipitation of sulfur if the bath is stored previous to use at too high a temperature. The bath should be maintained acid by reviving with a 25% solution of sodium bisulfite at intervals or in the case of machine processing, by allowing this reviving solution to flow into the tank continuously during use.
An alternative method of revival consists in passing sulfur dioxide gas into the bath. Although this method is efficient in practice, there is danger of accidental escape of the suffocating gas into the. atmosphere of the laboratory.
Acid Hardening Fixing Baths
In most cases it is desirable to harden motion picture film to a moderate degree during processing and a satisfactory acid hardening fixing bath for both negative and positive motion picture film should possess the following properties:
Metric
Hypo
240 grams
Sodium sulfite (desiccated)
10 grams
Sodium bisulfite
25 grams
Water to make
1 liter