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CRABTREE, EATON, AND MUELHER [j. s. M. P. E.
Under average darkroom conditions the results were usually within 5 per cent when uniform agitation was employed and, although tungsten light provided the best lighting conditions, the light from a Series OA, 1, or 3 safelight, although less intense, did not alter the "apparent time to clear" when transmitted rather than reflected.
Since the removal of the last traces of residual silver is necessary for archival purposes, tests were made with the F-5 fixing bath and the negative film, Type 1232, to determine the best time of fixing
' (I) F-5 EXHAUSTED 375 FT. PER GAL pH MAINTAINED AT4
(2) F-5 EXHAUSTED 375 FT PER GAL pH AT 5.2S
(3) F-5 FRESH pH4.l
(4) F-5 FRESH pH 525
MULTIPLES OF "TIME TO .CLEAR11 IN F-5 FIXING BATH
FIG. 7. Effect of increasing time of fixation on the residual silver content of film with fixing bath pH maintained at 4.1; with fixing bath pH. at 5.25. Kodak Fixing Bath F-5. Eastman Super-JO" Panchromatic Negative Film, Type 1232.
to accomplish this. The fresh fixing bath at pH = 4.1 was compared with the fresh bath adjusted to pH — 5.25, with exhausted .F-5 at pH = 5.25, and with exhausted F-5 adjusted to pU = 4.1. The curves in Fig. 7 show that (a) either a fresh or exhausted bath at a pH above 4.9 permits the removal of the last traces of silver from the negative film after fixing for twice the apparent "time to clear" and (b) a fresh bath at its original pH (4.1) required four times the apparent time to clear in order to make complete removal of the last traces of silver possible, but an exhausted bath when adjusted to pH 4.1 or to any value below 4.9 caused the retention of a considerable quantity of silver. Similar results were obtained with the