Re: chrome alum

[Richard Knoppow <dickburk@ix.netcom.com>]

At 09:15 PM 04/30/2002 -0600, you wrote:
>      Sandy King (or anyone else in the know), a while back you said you
>usually use chrome alum for hardener, sometimes potassium alum, too.  You
>mentioned mixing 2 g of chrome alum (the purple stuff)  per 1000 ml of
>gelatin solution.  This is about 1/2 tsp.  How much *gelatin* do you use in
>your 1000 ml--my formula calls for 2 tsp Knox gelatin, does yours, too?
>     I'm starting to work with tiles and glass to get liquid emulsion to
>stick, and a student wants to try to sub glass with the chrome alum solution
>for gum; she tried it already with varnish and a mild sanding and the gum
>totally came off.  She was inspired by my telling her a while back about
>Sarah Van Keuren's students doing gum on glass. Short of a sandblaster,
>we're going to keep trying.
>Chris
>

  FWIW, there are two forms of "alum" used in potography. 
"Alum" is Potassium Aluminum Sulfate
"Chrome Alum" is Potassium Chromium Sulfate
 The aluminum hardener is the one generally used in fixing baths for silver
photography.
  Chrome alum is more effective in hardening gelatin but has a short life
in fixing baths and can leave a green stain, so is used mainly for high
temperature processing and in the manufacture of emulsions. Both need to be
in a specific range of pH, on the acid side, to work. I don't know the
appropriate amounts to add to alt processes gelatin or other sensitive
coatings. 
----
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com