The Reducing Action of Fixing Baths on the Silver Image

The extent of the reducing effect of fixing baths on the silver image during the progress of fixation is greater than has been generally supposed. For example, in sensitometric work it is dangerous to prolong the fixation of motion picture positive film in the average fresh potassium alum fixing bath beyond S minutes at 65° F. and with certain highly acid chrome alum baths a measurable degree of reduction occurs even in this short space of time. — Since little or no reduction of the image occurs in an alkaline hypo solution, sensitometric tests should be checked against images fixed in a 25 per cent solution of hypo containing 1 per cent of sodium carbonate (anhydrous). — In regular laboratory work the degree of reduction which takes place in the normal time for fixation is usually of no practical importance with the baths in common use. In any given bath the rate of reduction increases with the acidity, the temperature of the bath, and degree of agitation of the film. — During use, the reducing action of a fixing bath falls off because it becomes more alkaline and accumulates silver thiosulfate which tends to retard the reduction. — In order to insure the minimum degree of reduction, baths having a minimum degree of acidity should be used although such baths have a short life and often do not harden satisfactorily. It is therefore necessary to revive such baths either by adding further quantities of acid or hardening solution at intervals during use, otherwise if the film is not rinsed in water before fixing an objectionable sludge will form in the fixing bath. — The nature of the reduction with the negative emulsions tested was found to be almost strictly proportional, and some of the more active baths enumerated could therefore be used advantageously for reducing the contrast of photographic images.

Print ISSN

0097-5834

Published

1932-03

Content type

Original Research

DOI

10.5594/J05549