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400 Transactions of S.M.P.E., Yol. XIII, No. 38, 1929
mined by the limits of the permissible times of fixation, and the actual quantity of hypo to add can only be ascertained by trial. Under no circumstances should solid hypo be added to the bath because this may tend to cause sulfurization.
With such revival, the point at which the bath should be discarded is determined mainly by the propensity of the bath to stain the film.
6. When to Discard the Bath. In practice, a fixing bath is discarded for one or more of the following reasons: (a) the bath sulfurizes; (b) a sludge of aluminium sulfite forms in the bath; (c) the bath becomes dark in color, stains the film or froths excessively ; and (d) the time of fixation is excessive.
(a) The sulfurization life of the bath F-2 with use is shown in Fig. 10. When freshly mixed, a deposit of sulfur will commence to form after storing for one and one-half days at 115° F. but with use the acid content decreases and more sulfite accumulates so that the sulfurization life rapidly increases to three days at the above temperature. On revival with acid, the sulfurization life drops again to one day so that when reviving with acid in warm weather when the temperature of the bath may be higher than normal, the acid should be added very slowly and the bath should be put to use within one or two days after revival. Once the bath has deposited sulfur, it is impossible to redissolve the precipitate and the bath should be thrown away.
(b) If the bath is not revived with acid, it will precipitate a sludge of aluminium sulfite after processing about two hundred feet of positive film per gallon although this precipitation point depends upon the alkali and sulfite content of the developer and the degree of rinsing between developing and fixing. If the bath is revived with acid at intervals as recommended, no sludging will occur.
(c) After the bath becomes alkaline the developer carried over by the film oxidizes and stains the bath, but if revived with acid at intervals, the bath does not stain up to the point when the time of fixation becomes excessive.
(d) The fixing bath is usually discarded at the point when the time of fixation (twice the clearing time) is excessive. The limiting time of fixation which can be tolerated depends upon circumstances, but average maximum fixing times (twice the time to clear) for negative and positive motion picture film are 20 and 6