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376
Transactions of S.M.P.E., Vol. XIII, No. 38, 1929
4. The cost of the fixing hath. Although with negative motion picture film a 40% solution of hypo gives the most rapid fixation as compared with a 30% solution for positive motion picture film, the question arises as to whether the extra cost of the more concentrated solution is justifiable by the slightly shorter time of fixation. For average use, a 25% solution of hypo is satisfactory for fixing both positive and negative motion picture film though if time is the most valuable consideration a 40% solution for negative film is desirable.
VI. The Hardening Action of Fixing Baths Potassium and chromium alum are the most desirable hardening agents for use in an acid fixing bath but the present discussion will be confined exclusively to the hardening properties of fixing baths compounded with potassium alum.
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EFFECT or SODIUM SULPHITE
ON THE
PROPERTIES OP A FIXING BATH
HARDENING
POTASSIUM AL.UM-0.5Vo
SODIUM sulphite: xvo
ACETIC ACID [GL.) -l.OV. HYPO -30%
\ ALUMINUM PPT. LIFE
O.S 1.2 1.& 2.0 2,4
PERCENT SODIUM SULPHITE
Figure 3. Effect of tlie addition of sodium sulfite to a fixing bath on (a) the hardening properties, (b) the sulfurization life, and (c) the aluminium precipitation life.
At the outset, it was considered that a fixing bath formula might be compounded by merely determining (a) the quantity of alum necessary to produce satisfactory hardening (b), the quantity of acid necessary to prevent the precipitation of aluminium sulfite on the addition of a given quantity of developer to the alum solution,