>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.
doesn't present any chemical problems: alum (unlike bisulphite) is *not* a
reducing agent, so the dichromate will not be changed to chromium(III) by
it;
> 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.
Certainly. The inorganic ions present in the alum solution must have
assisted the wash-out of the paper by opening up the pores in the cellulose
structure - performing the same function as 'washaids or hypoclear' (or
even sea-water) on silver gelatin papers. There is also a dependence on pH,
hence the use of acids to assist clearing, but it's as much a physical as a
chemical effect, and depends on the paper. Luis describes clearly in his
post how papers vary in 'washability'.
Mike