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

Re: Gum Humidity Question

Yes, sorry I didn't make that clear in the general observation I made below. I did go into more detail in a subsequent post, but I know that I'm not always getting all the posts, or sometimes I get them days after the fact, so I wouldn't be surprised if you didn't get all of mine over the last couple of days.

I got out the 15 or 20 test strips I made in the beta portion of this ill-starred humidity project and divided them into humidity ranges. I found that at each humidity level, there was an optimal exposure that gave the best test strip, and those exposures varied linearly from 2 minutes at over 90% to 3.5 minutes at the 60-70% range. For each of the humidity ranges, the optimal exposures produced identical- looking test strips, with 7 steps (for the particular coating mix I've used for all these tests) and the DMax and DMin apparently visually identical, in other words both the number of steps and the apparent contrast range is the same across all the humidity levels, as long as the exposure has been adjusted for humidity.

Exposures that were more than the optimal for that particular humidity level (3 minutes vs 2, or even 3.5 vs 3) produced test strips that didn't open up even after 2 hours development, and therefore exhibited little apparent contrast. The unexposed steps did clear, though. Since a lesser exposure in each case did give the optimal 7 steps, I assumed that the low contrast strips were not a function of humidity but of overexposure.

I just want to add that I've never had to deal with humidity in combination with heat, and I certainly haven't meant to leave the impression that I think I would find it a piece of cake. I've said many times that I think I'm very lucky to be a gum printer in a cool humid climate, which I personally think is ideal for gum printing, and I don't envy those living in less conducive environments. I'm working at high RH but cool temperatures,where the water content of the air is less than for people working at high RH and high temperatures, as Francesco pointed out, and so I can't speak from my own experience about your situation; all I'm doing here is posing the question. As I said, someone else will have to do the experiment.

Good luck,

On Sep 29, 2006, at 11:11 AM, ericawd wrote:

Did you change the exposure to compensate for the change in humidity?

Candace Spearman

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@usask.ca>
Sent: Tuesday, September 26, 2006 5:24 PM
Subject: Re: Gum Humidity Question

P.S. I will say, however, just as a general anecdotal observation,
that I haven't seen noticeable differences in contrast between prints
I've made at 100% humidity (in the air, not in the gum coating) and
those I've made at 60%.

On Sep 26, 2006, at 2:31 PM, ericawd wrote:

According to Christina Z. Anderson and Kosar:

7. Humidity:  The presence of a certain amount of moisture in a
coated and
dried layer is necessary for the hardening reaction.  When dry, the
remaining varies with relative humidity.    Completely dehydrated
or fully
swollen coatings do not show any light sensitivity at all, but in
the sensitivity is high when the humidity is high.  Sensitivity
doubles with
increase of 30% humidity.

In my experience, I have found the above to be true.  My question
is, in the experience of the gum printers, what effect might
humidity have on contrast?  Lower humidity equals higher contrast
or vice versa?  All other things being as equal as they can be with

Candace Spearman