The grocery store "alum" is Ammonium Aluminum Sulfate, whereas photographic
alum is Potassium Aluminum Sulfate. The ammonium salt has two problems: the
ammonium radical (NHsub4) is not stable like the potassium, leading to (2)
in an acid (low pH) the "alum" breaks down. This does not happen with the
potassium compound. There are other problems with it discussed in the
literature. So for photographic purposes, use photographic alum (Potassium
Aluminum Sulfate) or chrome alum (Chromium Aluminum Sulfate) both of which
should be available from professional dealers, and the mailorder outfits
that sell chemicals in small quantities for photographic purposes.
Incidently, in the small quantities we photographers use, formalin
(formaldehyde gas - it is a gas - dissolved in water) is not all that toxic.
The main reason restrictions have been put on its sale is its use in making
drugs, as I understand it.
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net