On Fri, 19 Feb 1999, Gayle Callis wrote:

> 5gm gelatin in 1 liter distilled water, add 1 gm
> chromium potassium sulfate

   I'd like to add that it's best to dissolve the ingredients
   in separate lots of water and then combine the solutions.
   Otherwise you can end up with insoluble lumps of the
   Cr-gelatin complex, with most of the solution not
   "chromed." Once made, you cannot dilute it further,
   so make either a dilute version for the water bath
   (e.g. 0.1% gelatin, 0.05% chrome alum) or a stronger
   one for smearing onto slides with the finger-tip
   (1%:0.1%). The intermediate strength (0.5%:0.2-0.5%)
   is good for dipping slides then drying.

   These solutions do not keep well - they fill up with
   fungi after a few days. Once I aliquoted some 1%:0.1%
   chrome gelatin into injection ampoules, sealed them and
   autoclaved. These kept well, and I went on using them
   for over a year by the digital smearing method. No
   loss of adhesive power, but it was much more trouble than
   making a fresh solution every time!

   I use slides subbed with chrome gelatin for nearly
   everything. True, the gelatin layer does become
   coloured, but never strongly enough to interfere
   with examining the stained sections.

 John A. Kiernan,
 Department of Anatomy & Cell Biology,
 The University of Western Ontario,
 LONDON,  Canada  N6A 5C1