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

Re: Gum Humidity Question

Actually on reflection I would be more inclined to blame some kind of thermal hardening process than a dark reaction, for both Kerik's and Chris's spontaneous hardening, although I'd emphasize again that neither works as a sufficient explanation in Kerik's case, because of the difference between sized and unsized paper. But perhaps "spontaneous hardening" never mind Mark, is the best generic label, since it's not clear whether it actually is a dark reaction or thermal hardening or something else entirely.

At any rate I've done some testing this morning. At the time I ran the tests, the humidity at the airport was 87% and even the junk hygrometer was reading 80% in my workroom. Since it's never read higher than what one would assume the actual humidity to be, always lower, and sometimes 20 percentage points lower, as when I checked it against the outside humidity last night, I think we can assume the humidity in the workroom to have been at least 80% at the time.

The test strip developed in 30 minutes and cleared easily, as always, and the exposure required was more than my usual exposure when the airport humidity is over 90%, which suggests that the normal humidity in my workroom is As I've been saying here for several years, it ain't the humidity, folks, it's the heat (to paraphrase an old expression). Or heat in combination with humidity. But not humidity by itself.

I also coated some strips and put them away unexposed, to see how long it would take to create a dark reaction. I intended to put one in water every 15 minutes, but then I went back to bed (got up too early the first time) and so the only two times I have are 15 minutes, and 4 hours. But neither of them produced spontaneous hardening, at all. So I don't think the dark reaction speeded up by humidity is probably the right explanation.
Katharine




On Sep 27, 2006, at 9:18 AM, Katharine Thayer wrote:

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.
Katharine