Re: alum as paper size
Luis Nadeau (nadeaul@nbnet.nb.ca)
Tue, 1 Aug 1995 17:05:48 -0300
>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
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
When acidified (remember it does not keep well) it is almost as effective
as gelatine hardener as alkaline formaldehyde. Its full effect is not
immediate however.
Luis Nadeau
NADEAUL@NBNET.NB.CA
Fredericton, New Brunswick, Canada
>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