However, my experience with Vandyke brown was different. When I first
began VDB, I heat dried. The only way to avoid bronzing and get a decent
D-max on most papers was with a preliminary spray-starch size. (Attempts
to use a classic cornstarch size wrecked all prints, while subsequent
experience showed that spray starch was not archival, and not reliable.)
As soon as we stopped heat drying, the bronzing ceased. Perhaps this was
because the emulsion could sink more into the paper. But whatever the
explanation, my experience is that in these respects pl-pd and VDB work
differently.
I'll add that I have seen no evidence of VDB bronzing where the emulsion
is thin. If that were the case, the ends of brush strokes that trail off
would (presumably) be the most likely to bronze. I don't recall having
seen that. Where bronzing would typically appear was in the most heavily
exposed shadow areas. At times it actually followed the shadow outline,
which was often quite beautiful. Unfortunately, over time the metallic
effect would break down, to be replaced by blotches. That's in this
latitude of course.
As for humidifying a gum print, I've never tried it & would advise
proceeding with caution -- odds of gluing your negative to the paper would
be improved. In any event, high humidity is not good for gum printing, at
least in this latitude and near sea level.
Judy