There are three alums that are in common use. The first, available from
pharmacies (UK - chemists), and commonly called "alum" is Ammonium aluminum
(UK - aluminium) sulfate (old spelling, "sulphate" - I want to be precise
here for the worldwide audience). This is an inferior compound for
photographic purposes, even though it does have some hardening properties.
In the presence of alkalies, the ammonium becomes ammonia, which befouls the
atmosphere in addition to destroying much of the effectiveness of the
chemical. It is very rarely found in photo formulas for this reason, among
others.
On the other hand, photographic "alum" is Potassium aluminum (aluminium)
sulfate, which, though it reacts with alkalies, does not pollute the
atmosphere when it does. This has hardening properties similar to
formaldehyde, but does so by creating a cross-linked aluminum/protein chain,
which may not be desireable in many of our alternative processes. Remember,
particularly, that photo alum has a low pH, i.e. it is acidic, and should
not be used in either an alkaline environment or with materials not happy
with an acidic environment.
The least used alum is Chrome alum, which has the greatest hardening
properties of all. This is Chromium aluminum sulfate, easily identified as
the crystals are green. It is rarely used today, not only because of cost,
but because it is very easy to over-harden gelatin with this stuff,
resulting in crazing. It is also very sensitive to alkaline environments,
and the breakdown results in a gorgeous mess of mostly brown precipitate.
(There are lots of stories about this stuff, but let them rest.)
"Alum" is also an abbreviation for "alumnus," but I think that's the wrong
topic. The trouble with popular names, like abbreviations, is they are not
always precise. For photography, I always use the term "potassium alum" so I
don't get the wrong item.
Frankly, except for some off-color jokes, I don't know of any use for
ammonium alum.
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net