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378 Transactions of S.M.P.E., Vol XIII, No. 38, 1929
Preliminary experiments indicated, however, that the hardening produced by a fixing bath was not merely a function of the alum or acid content but w^as determined hj the relation between the concentration of acid, sulfite, and alum and w^as independent of the quantity of hj^po. Therefore, in order to determine the correct proportion of the various constituents to produce an acid hardening fixing bath having any desired properties, it was necessary to study the effect of each constituent in the presence of varying proportions of the other two constituents.
In Fig, 3, the full line represents the progress of hardening on the addition of sodium sulfite to a bath containing 0.5% alum, 1.0% acetic acid, and 30% hypo. The ''dash" line represents the tendency of such a bath to precipitate sulfur with various concentrations of sulfite w^hile the "dash and dot" line indicates the tendency of the bath to precipitate, aluminium sulfite with increasing additions of sulfite. The last two curves are plotted in terms of days of storage at 115° F. indicated on the right side of the chart.
Figs. 4 and 5 represent a multiple set of curves corresponding to those shown in Fig. 3 for fixing baths containing 0.5% and 1.0% of potassium alum, varjdng concentrations of acetic acid (0.5 to 3.0%), and increasing concentrations of sulfite.
1. Factors Influencing ilie Hardening Properties of Potassium Alum Fixing Baths. From a study of Figs. 3, 4, and 5, it is seen that (1) the hardening produced is a function of the relative proportion of the acetic acid, sodium sulfite, and potassium alum; (2) for given quantities of sulfite and alum the hardening decreases as the concentration of acid increases; (3) for given quantities of acid and alum the hardening increases to a maximum with increasing concentration of sulfite after which the hardening decreases, that is, there is a critical quantity of sulfite for each concentration of acid which gives maximum hardening; (4) the maximum hardening produced in any case increases when the concentration of alum is increased; (5) in any particular case, the maximum hardening obtainable with a given concentration of alum decreases as the concentration of acid increases. The decrease is not appreciable with acid concentrations below 2.0% but it is more marked as the acid concentration is increased above this quantity.
The effect of temperature on the hardening produced by a fixing bath is shown in Fig. 6 which indicates that the hardening increases by the following table.