post development hardening
If after a plate is fixed and washed, can it be hardend in a plain Alum solution? If so how much, how long, and wash for how many minutes after?
I like the idea of washing off a plate with warm water if it is not acceptable.
Some old manuals say to use citric acid in the solution.
Alum hardeners should be acidic at about pH 4.5 to work effectively.
Alum hardening can be reversed (See Mees, Mees and James and Haist for information). It can also cause a milky appearance to prints treated in alkaline solutions after hardening. (this does not include wash waters that are just slighly alkaline)
Citric acid is not a hardener per-se, but more a method of reducing swell in bone gelatins. Bone gelatins have their minimum swell at acidic pH values and are therefore most hard in that region.
Modern Kodak, Ilford and Fuji films do not need additional hardening. Hand coated glass and film can be hardened and often need it, but I find that the biggest problem is not hardening but rather adhesion to the substrate. A good hardening with chrome alum or glyoxal in the emulsion will do a good job on hand coated emulsions on plates and films but they must also attain good adhesion.
A good process however, would include an acidic fixer with alum hardener such as KRLF.