At temperatures above 68° F., alum fixers tend to decompose spontaneously with deposi tion of sulphur ; such mixtures are therefore only made up in such quantities as are needed for immediate use. 2 In trade practice it is advantageous during the repeated use of a hardening fixer, to com pensate for the additions of alkali (developer impregnating the emulsion) by carrying out periodically re-acidification by adding diluted acids (sulphuric acid for chrome alum ; acetic acid for ordinary alum). The technique has been described by J. I. Crabtree and his col laborators (1029-193o).
410. Fixation in Practice. Inasmuch as the ill effects of faulty working are not immediately evident, fixation and the washing processes which precede and follow it are not always carried out so carefully as is the case with development.
It is essential to realize that no visible sign shows that fixation is perfect ; a negative may be quite clear after fixing and drying and yet may not be properly fixed ; it may not have been allowed to remain long enough in the fixer, or the fixing bath itself may have been exhausted. Images (negatives or positive prints) which are required to be kept for a long time must be fixed in a manner which conforms minutely to the following directions ; these directions are much less important in the case of images which have merely passing interest.
411. The negatives, after being rinsed free of developer (§ 375), are placed in one of the fixing baths already prescribed, generally an acid fixing bath, or, in warm weather, in a fixing-hardening bath compounded with alum.
Non-actinic light should be used, at any rate when putting plates in the fixer ; in the case of neutral fixing solutions, negatives should be allowed to remain in the bath for at least three minutes before being exposed to white light ; this period may, if necessary, be reduced to one minute in the case of acid fixing baths.
On account of the injurious action of the least trace of hyposulphite on certain developers, the various dishes employed must be so arranged that no splashes or drops of hyposulphite can fall into the developer. 1 The fingers should be rinsed in water after every time they have come into contact with hypo.
Fixation being generally slower than develop ment, it is advisable, in the case of continuous working, to provide larger dishes or tanks for this purpose than for development. For exam
ple, when working with dishes, one would use for fixation those of a size capable of taking two or four of the negatives under treatment.
During fixation, negatives must be well covered by the solution.
After some minutes in the fixing bath, an examination of the back of the negative shows that the milky coating of the silver halides under the developed image is beginning to dissolve, the disappearance of the milkiness occurring as a rule more quickly under the denser parts of the image where least silver bromide remains.
From the time when the last traces of milki ness have disappeared the negatives are kept in the bath for a time equal to that already taken, or, preferably, when two-bath fixation (§ 4o6) is employed, they are transferred to the fresh bath and allowed to remain there for an equal time. When fixation is complete, negatives are washed.
If fixation is effected by the single-bath method, the solution must be frequently renewed so as not to pass the safety limit. In case of doubt, any of the tests described earlier (§ 408) may be used.
In large commercial establishments periodical re-acidification of the fixing baths is necessary.' The control of the exhaustion can sometimes be effected automatically by measuring the amount of sensitive material treated (especially with cinematograph films) or by the application of empirical rules, which can be worked out on the basis of experience. When these methods cannot be applied, re-acidifying with acetic acid requires the previous determination of the residual acidity by the works chemist or by a foreman who has been trained to carry out this simple test. On the other hand, re-acidification with bisulphite may be done without special precautions.
A thin, metallic-looking film of silver sulphide sometimes appears on the surface of partly-used fixing solutions which are kept undisturbed for some time in open tanks. This is due to the reaction of suiphuretted hydrogen on the silver hyposulphite dissolved in the bath. There is the liability of particles of this film adhering to the gelatine of negatives it may be removed by skimming the surface with a piece of filter paper or a cloth, according to the size of the tank.