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Fixing Baths— Grab tree and Hart 383
sodium sulfite, 1.0% acetic acid) required to produce a fixing bath having the above desired properties. This hardener may then be added to a given hypo solution and the fixing bath is then complete.
Fig. 3 was used for the above example merely for the purpose of simplicity. In order to compound fixing baths having properties other than the above it is necessary to refer to Figs. 4 and 5 or additional sets of curves for other concentrations of alum.
In choosing the alum concentration, the minimum quantity which will give the necessary hardening is desirable. The quantity of sulfite to insure the necessary sulfurization life (X days at 115° F.) is then determined as indicated above. The quantity of acid is decided upon by finding the set of curves (aluminium precipitation life and .sulfurization life) such that the space between them at a distance of two days* from the sulfite axis corresponds to one-sixth of the developer capacity. The acid concentration represented by this chosen set of curves gives the acid concentration to use in the formula. With this fixing bath it is therefore possible to add a quantity of sulfite X (as represented by a developer life of 6X) in addition to that already present in the formula before a precipitate of aluminium sulfite is formed.
The formula given above represents a suitable stable acid hardening fixing bath which might be used in cases where only a relatively small quantity of developer is carried into the fixing bath. Fig. 8 shows the effect of the addition of developer on the hardening properties, of this formula. It is seen that the hardening at first is not as high as desired, but with the addition of a small quantity of developer the hardening increases to the desired limits, and with the addition of further developer (about 5% MQ25) the hardening falls off and the bath is rendered useless by the formation of a precipitate.
6, The Effect of Excessive Hardening. Excessive hardening is apt to render the gelatin opalescent especially if the temperature of the fixing bath is slightly above normal. The opalescence is usually removed during subsequent washing and drying and is ordinarily unobjectionable. If, however, the opalescence persists after wash
* If the bath does not precipitate a sludge of aluminium sulfite in 2 days at 115 °F., it will not sludge at room temperature within a period of one to two weeks and is therefore satisfactory.