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496 J. I. CRABTREE AND H. D. RUSSELL [J. S. M. p. E.
quantity of developer added increased, while the hardening with alkaline film decreased. The hardening properties, after standing one day, were almost completely destroyed and were not revived by an adjustment of the acidity with acetic acid. In view of these facts, it was concluded that acetic acid was not satisfactory for use in chrome alum stop baths.
The effect of the addition of sulfuric acid on the hardening properties of a 2 per cent chrome alum stop bath is shown in Figs. 5 and 6. The hardening action produced with neutral film in a bath containing 0.1 per cent sulfuric acid (Fig. 5) increased and the acidity decreased on the addition of developer. A maximum degree of hardening, however, was produced with neutral film after the acidity of the solution was adjusted to a pH of 4.0. The age of the bath had no effect on the hardening properties. Alkaline film was hardened to a maximum degree, irrespective of the quantity of developer added up to 10 per cent. In the case of the 0.5 per cent sulfuric acid stop bath, the hardening action with alkaline film increased as the original acidity decreased. After the bath was three weeks old, the hardening behavior with alkaline film increased slightly. No appreciable hardening was obtained with neutral film until the acidity was adjusted to a pH of 4.0. A maximum degree of hardening was produced with both neutral and alkaline film after the acidity of the bath had been adjusted in this way.
From the results of these preliminary experiments recorded in Figs. 1 to 6, it was concluded that: (1) the hardening properties of a chrome alum stop bath are impaired by the presence of acetic acid, developer, and sodium sulfite; (2) the addition of developer and sodium sulfite decreases the acidity of the bath which usually decreases the hardening action; (3) the hardening properties of an acetic acid-chrome alum bath containing developer are almost completely destroyed after standing for one day, while baths containing an equivalent quantity of sulfuric acid produce satisfactory hardening on keeping; (4) the hardening properties of a plain 2 per cent chrome alum bath with the addition of developer over a period of several days are better than those of a 2 per cent chrome alum bath containing acetic acid; (5) sulfuric acid appears to be the most suitable acid for use in chrome alum stop baths; (6) the degree of hardening produced in a plain chrome alum bath or in a sulfuric acid-chrome alum bath depends upon the acidity of the bath in relation to that of the gelatin film, which is determined by the