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Fiximj BatJis — Crahtrec <nid Hitrl 401
minutes, respectively, when it is usually more economical to discard the bath.
In the case of the F-2 bath containing 25% of hypo and without agitation of the film, the clearing time exceeds 10 minutes after fixing 200 feet of negative motion picture film per gallon. With positive film the clearing time exceeds 3 minutes after fixing 600 feet of film per gallon. With a degree of agitation corresponding to that existing with machine development at a moderate speed, with negative film the time to clear exceeds 10 minutes after fixing 300 feet per gallon while with positive film the time to clear is less than 2 minutes after fixing 700 feet per gallon. If the bath is revived by the addition of hypo at intervals the above footage figures will be increased accordingly.
7. Importance of Rinse of Stop Bath. Rinsing in water previous to fixing prevents the carrying over of developer into the fixing bath and this retards the formation of a sludge of aluminium sulfite in the absence of revival with acid. An excess of developer also lowers the hardening properties and renders the bath alkaline and liable to stain. Rinsing, therefore, prolongs the life of the fixing bath and insures that its various properties remain more nearly uniform throughout its life.
In the case of some processing machines when the film would be otherwise exposed to the air for several seconds after leaving the developer tanks and before entering the fixing bath, it is desirable to arrest development by the use of an acid stop bath consisting of a 2.5% solution of sodium bisulfite or a 0.75% solution of acetic acid. For hot weather processing, a 2.0% solution of chrome alum is desirable.^^ Such stop baths must be kept acid at all times. Fixing baths to be used in conjunction with such acid stop baths should obviously contain a minimum of acid in the first place.
8. Use of Two Fixing Baths. The practice of using two fixing baths in succession is to be recommended. When the first bath is exhausted it is replaced by a comparatively fresh second bath which in turn is replaced by a new one. This scheme insures more rapid fixation than the use of only a single bath. For example, if the film just clears in X minutes in the first bath when practically exhausted and is allowed to remain for X minutes in the second bath, it will
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be likely to be fixed more thoroughly than if it remained for 2X