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May, 1930] STOP BATHS AND FlXING BATHS 507
alum baths but the use of chrome alum as a hardening agent is desirable when processing at high temperatures or when rapid drying at high temperatures is necessary.
As it is somewhat difficult to maintain a uniform degree of hardening with most chrome alum fixing baths and since their hardening properties frequently fall off before the fixing powers of the hypo are exhausted even with suitable revival, it is more economical to use a chrome alum stop bath because this is cheaper and can be renewed more frequently. Also, a stop bath hardens the gelatin more rapidly than a fixing bath so that at high temperatures the gelatin is hardened before it has a chance to swell excessively.
The purpose of this investigation has been to determine the more important factors which influence the hardening properties of chrome alum solutions and to recommend suitable stop bath formulas.
It has been found that: (1) plain chrome alum solutions are more suitable for hardening motion picture film than those containing acid; (2) the addition of developer decreases the acidity and the hardening properties; (3) the acidity or pH value of a chrome alum solution must be maintained at a value of 4.0 for neutral film and between 3.8 and 3.0 for alkaline film if the maximum degree of hardening is to be obtained; the hardening properties are constant, irrespective of the concentration of chrome alum above 0.5 per cent when the pH is adjusted to these values ; (4) sulfuric acid is the most suitable acid for maintaining this acidity; and (5) the hardening properties of a bath containing an excess of developer decrease with age.
Practical exhaustion tests with and without revival with acid made with 2 per cent and 3 per cent chrome alum baths have shown that: (1) as the degree of exhaustion increases, the acidity and hardening properties decrease; (2) the addition of sulfuric acid at regular intervals increases the hardening properties and prolongs the life of the bath; (3) with a 3 per cent chrome alum bath more uniform hardening properties are obtained with use than with a 2 per cent bath; (4) more uniform hardening properties are also obtained at 85°F. than at 70°F.; and (5) the D-76 developer has a greater effect on decreasing the hardening properties than D-16.
PRACTICAL RECOMMENDATIONS
The following formula is satisfactory for use as a stop bath during the processing of motion picture film.