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382 Transactions of S.M.P.E.; Vol. XIII, No. 38, 1929
4. The Application of Hardening Curves to the Preparation of Fixing Baths Having Required Properties. From the curves (Figs. 3, 4, and 5) it is possible to compound a fixing bath having any given properties. The following three factors must be known' before a suitable fixing bath may be compounded: (a) the hardening desired; (b) the developer capacity, and (c) the sulfurization life. With this information available, the series of curves (Figs. 3, 4, and 5) which gives the maximum hardening desired is selected and from these curves the necessary quantity of alum is determined. Then, by drawing a line parallel with the sodium sulfite axis at a distance from it representing the desired sulfurization life as indicated on the axis ''life in days," and determining the limits of the sulfite content between the sulfurization and aluminium precipitation curves, the quantity of acid necessary is indicated by the curves whose sulfite limits correspond to one-sixth of the developer life desired. Then, by projecting the point of intersection of this parallel line and the sulfurization life curve to the sodium sulfite axis, the quantity of sulfite sufficient to give the necessary sulfurization life is indicated. This gives the complete hardening formula which may be added to a quantity of hypo as determined by the amount of film to be fixed and the rate of fixation desired.
5. Example of Use of Curves in Compounding a Fixing Bath Formula. If it is desired to compound a fixing bath having the following properties; relative hardening 160° F., sulfurization life 2 days (110° F.), developer capacity 6% MQ25, proceed as follows:
It is seen from Fig. 3 that the maximum hardening produced by an 0.5% potassium alum solution with varying concentrations of sodium sulfite is approximately 160° F. Draw a line parallel with the sodium sulfite axis at a distance from it represented by two days on the sulfurization life curve at B and the aluminium precipitation curve at C. Project the points B and C to the sodium sulfite axis to the points E and F, respectively. The difference in the sodium sulfite concentration represented by these two points multiplied by 6 gives the relative developer capacity for the acidity (1.0%) which is represented by these curves; that is, 1.5% — 0.6% == 0.9% ; 0.9% x 6 = 5.4% equivalent developer capacity. The quantity of sulfite represented by point E (0.6%) is the quantity necessary to give the desired (2 days) sulfurization life. This information gives the proportion of the various ingredients (0.5% potassium alum, 0.6%