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508 J. I. CRABTREE AND H. D. RUSSELL [J. S. M. p. E.
Chrome Alum Hardening Bath (Formula SB-3) Metric Avoirdupois
Chrome alum 30 grams 13 pounds
Water to make 1 liter 50 gallons
The chrome alum should be dissolved in water at a temperature lower than 150°F., otherwise the solution will tend to become more acid due to hydrolysis and its hardening properties will be affected.
After development, immerse for three minutes in the above hardening solution, taking care to agitate the film for thirty to forty-five seconds. This will tend to prevent blisters, streaks, and chrome alum stains.
If the film is not agitated as above, the alkali in the developer is apt to precipitate a sludge of chromium hydroxide on the film although with developers which do not contain more than 2 per cent of sodium carbonate no trouble is usually experienced.
A less expensive bath may be compounded using a 2 per cent solution of chrome alum in place of the 3 per cent solution (Formula SB -3) above. This has less tendency to give blisters or green stains at high temperatures, but its life is much shorter than that of the 3 per cent bath.
The baths will keep indefinitely without use but with use the hardening properties fall off as a result of the addition of developer carried over by the films and in the case of any bath containing a given quantity of developer the hardening properties continue to decrease as the bath ages. In most cases, however, the hardening properties of fresh or old baths which have been impaired by the addition of developer can be restored by the addition of a quantity of sulfuric acid necessary to bring the pH value (degree of acidity) to 4.0 for neutral film and between 3.0 and 3.8 for alkaline film.
Blisters may tend to form if the film is swollen on immersing in the bath, as a result of decomposition of the carbonate in the developer by the chrome alum which is normally acid, but agitation will tend to prevent their formation.
The hardening properties of the bath depend upon (a) the acidity of the bath and alkalinity of the film, (b) the age of the bath and quantity of developer carried over to it by the film, and (c) the time of immersion. The time of immersion in the hardening bath should never be less than three minutes which is usually sufficient to give maximum hardening. Films which have been treated with a chrome alum hardening bath should always be wiped carefully or squeegeed after washing and previous to drying, otherwise any chromium scum