I hesitate to jump in here, since my last (sucessful) chemistry class
was in secondary school, but years ago when I was gum printing, I
followed the "holy writ" (a dilute alum solution) for a final clearing
bath and got a pale, but definitely yellow solution left after
soaking in it.
Even after reading all the discussion, I can't help but believe that
there was dichromate coming out of the print, dichromate that wasn't
being removed after prolonged "development" in plain water. I can't
deny that it might have been the effect of another (mostly) water bath
that I was observing, and not the action of the alum, but then why
wasn't the final development bath as yellow as the clearning bath?
Also, all of the chemicals so far discussed: alum, sodium bisulfite,
and sodium metabisulfite are hardeners. Would this treatment also be
beneficial to the gum emulsion (I have always assumed so) or do they
just harden gelatin?
A bit puzzled -
Carson