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668 J. I. CRABTREE AND H. D. RussEUv [J. S. M. P. E.
sulfuric acid, and sodium sulfite. The results at first obtained were very inconsistent and duplicable only within a small degree of accuracy so that further experiments were made to determine the reason for this inconsistency. It was at first observed that the time elapsing between the preparation of the bath and the time of testing apparently influenced the hardening produced by the bath. For this reason it was decided that the most satisfactory method of arriving at a suitable formula was to make a large number of tests with baths in which the quantities of sodium sulfite, chrome alum, and various acidifiers were altered independently and to determine the properties of the resulting baths. The following acidifiers were tested: acetic, citric, and sulfuric acids, sodium bisulfite and sodium bisulfate.
Method of Testing the Experimental Baths. — Since the most important factor under consideration was the retention of the hardening properties of the baths with keeping, only this property in addition to the sulfurization life of the experimental baths was determined in the preliminary tests. To facilitate storage of the large number of samples, most of the solutions were kept in stoppered bottles and then poured into trays for use.
The hardening properties of the various baths were determined as follows: Strips of motion picture positive film were exposed through a step tablet, developed, rinsed, and fixed in the various baths for five minutes at 65°F. to 70°F. After washing for twenty minutes the strips were pinned to a small wooden frame and placed in a beaker of water. A thermometer was suspended in the beaker and the water heated by means of an electric hot plate at the rate of about 5 degrees per minute. The frame was moved up and down about twice a minute in order to stir the water and remove air bubbles from the strips of film. The melting temperature of the gelatin on the various strips was taken as that temperature at which it flowed from the support.
Table I gives the composition (grams per liter), duration of hardening properties, and the sulfurization life of a number of experimental baths. The same quantity (240 grams per liter of sodium thiosulfate) was used in all the fixing baths. Several of the tests were duplicated and both series of results are given. In a few cases the solutions were stored in open trays instead of bottles and these are indicated by a "T" following the experiment number.
The following facts may be deduced from a study of Table I.