U of S | Mailing List Archive | alt-photo-process-l | Re: Gum Humidity Question

Re: Gum Humidity Question

On Sep 27, 2006, at 4:01 AM, Katharine Thayer wrote:

On Sep 26, 2006, at 8:48 PM, Christina Z. Anderson wrote

Down in SC I would notice spontaneous hardening of emulsion from the first
to the last gum print when I was doing 9 at a time; here I can go a very
long time (did 13 at once this weekend) and no noticeable effect on prints
at the 33% humidity level.

Chris, I'm curious why you refer to this as "spontaneous hardening" rather than "dark reaction."
Dark reaction has an even steeper relationship to humidity than speed of the light reaction does; for gum the relationship is close to vertical, for this reason I never let coated paper sit around in my humid environment.
Sitting here staring vacantly at this graph (of the effect of humidity on the dark reaction in gum) in front of me, I notice that the line crosses 10 hours at 70% and 0 hours at around 82-83%, (in other words there's a toe in the graph) but at 60% it takes 50 hours for the dark reaction to reach the same degree of completion. Unfortunately we're not told what that degree of completion is, so it's impossible to translate the graph into terms that relate to gum practice. But if your inside humidity was never more than 60%, I'm surprised you'd get this much dark reaction.

Theoretically, given that the line does cross 0 at 82% or so, one could posit that part of Kerik's problem could be due to dark reaction, but if that were a sufficient explanation, it should be the same on unsized paper as sized.

Back to the graph: the line goes off the top of the scale (140 hours) at 55-56%, so it's impossible to guess how long it would take the dark reaction to proceed to the same degree of completion at 30% , but it's certainly consistent with the graph that you see no dark reaction at 30% within the time it takes to print a number of prints.