Re: Kosar's Top 10 Gum Facts Redux-Dark Hardening

ericawd wrote:
>
> A while back Ms. Christina Z. Anderson wrote in Kosar's Top 10:
>
> "Completely dehydrated or fully swollen coatings do not show any light
> sensitivity at all, but in between the sensitivity is high when the humidity
> is high. Sensitivity doubles with increase of 30% humidity."
>
> I have found this to be true in practice. Dark hardening/humidity jumped up
> and bit me on the a... I am not sure about doubling with a 30% increase,
> however, since just this week I bought a hygrometer.
>
> Question 1: Are the terms dark hardening and dark reaction interchangeable?
> If I understand it correctly the term dark hardening is meant to describe
> when the exposed emulsion will not develop, rub or come off even with a
> brillo pad. And that this can be caused by among other things humidity.

I think yes, the terms are interchangeable. The dark reaction (or dark
hardening, if you will) is a reaction that is similar to the light
reaction of dichromated gum but begins as soon as the dichromate and
colloid are coated onto the paper and proceeds without exposure to
light. When it proceeds to completion, it results in all of the gum
being hardened. In other words, if you coated a piece of paper, dried
the coating, put it immediately into a dark place and waited long
enough, you would end up with a paper covered with hardened gum which
"will not develop, rub or come off even with a brillo pad."

This reaction is very slow under conditions of low humidity and much
faster under conditions of high humidity, but even in conditions of very
high humidity it's not so fast that it should affect your prints as long
as you print immediately after coating. The exception to this may be
when humidity is coupled with extreme heat; in that case it is my
understanding that the humidity/heat combination can result in "baked"
emulsions. I suspect that problem could be alleviated partly or entirely
by altering the exposure times, light intensity or dichromate
concentration to compensate for the extra sensitivity resulting from the
high humdity, but since I don't have those kinds of conditions I can't
experiment with it; someone who needs to know should do those
experiments.

I have a graph in front of me (not from Kosar) for the dark reaction in
gum arabic. The y axis shows the number of hours required to reach the
same level of completion of the reaction, and the x axis shows the
relative humidity. The line crosses the x axis (0) at 85%, but since
it's not clear what the level of completion is (25%, 50%, 100%?) I'm not
sure what this means. The line is almost vertical except for a shoulder
at the bottom; it goes from 0 at 85% to 10 hours at 70% to 140 hours at
45%, the lowest humidity recorded on the graph. It would certainly seem
to indicate that at 85% humidity you should see some effect of the dark
reaction immediately, even within the space of time it takes to expose a
print. But I can say from much experience printing in the 95% range, I
just haven't seen it. It would show in the highlights, certainly, and I
just don't get that kind of fogging that one might expect from this
graph at that humidity. Just another reminder to trust your experience
rather than someone else's test results.

I almost never get to print below the range of 60-99% relative humidity
and have no problem; in fact from my experience I'd venture that high
humidity coupled with cool temperatures may actually be the optimal
condition for gum printing.

>
> Question 2: Will dark hardening occur even in spite of the exposure
> corrections when the humidity reaches a certain point? Katherine Thayer
> wrote around the same time: "Well, the dark reaction is extremely variable.
> Referring back to the dark reaction test I posted the other day." Obviously
> that post is long gone. What was the conclusion?

This is the problem with posting things for a short time; I encountered
this problem myself when I was looking for a discussion in the archives
the other day, trying to remember the results of something I'd tested. I
found the post, but when I clicked on the link there was something else
there now. The problem is that my website and my incoming mail have to
fit in a 10MB space, so unfortunately I can't keep everything I ever
post on the site permanently. So from now on, I promise I will make sure
that all of the important information is on the post, so that even if
the picture is gone, you'll still be able to figure out what the results
were.

It would be helpful to have the context of my above remark, but if I
recall correctly, I was responding to someone who made some kind of
general statement about the dark reaction to the effect that you could
leave your coated papers overnight to slightly fog them using the dark
reaction. When I said that the dark reaction is variable, I meant
variable depending on the humidity especially, and the test I referred
to was a paper I had coated and put in the dark for a few hours, and
then developed. I think it was about four hours. Somewhere in that time,
the gum had totally hardened through the dark reaction alone, and the
whole paper was black or brown or whatever the pigment was. My point was
that you may be able to "flash" your paper by letting it sit overnight
only if your work area is fairly dry; it wouldn't work at all in mine.

 
>
> Question 3: Has anyone come up with an optimum humidity level? From reading
> past posts it can actually be too low.
>
> I had been humming along all winter having solved my sizing problems thanks
> to all the great input from "the group". Now that the humidity is reaching
> its spring/summer levels, I have run into a dark hardening wall. I am not
> really able effect the humidity level other than to measure it (on Friday RH
> ran from 65-70% while I was printing). The only alternative is to lower my
> exposure time and I really would rather not waste a lot of sized paper only
> to find that changing the exposure is not an option; that the darks will
> still harden. I understand that subject has been covered, but am totally
> addicted to gum printing, so stopping is not an option, any suggestions
> would be helpful.

 I can't say about the lower ranges of humidity, having no experience
with them, but I can say with confidence that there's no limit at the
upper end. For a couple of years I was printing in a dank cellar that
had a river running through it, with a little delta where mud was
deposited around the edges (I'm not kidding!) and I had to wear boots
and a heavy jacket to work in. The humidity must have been close to 100%
there all the time. But I made beautiful prints there.

The main thing about humidity is to understand the relationship between
humidity and sensitivity and adjust your exposures accordingly.
According to another graph I have here, gum requires 4x the exposure
units at 20% as it does at 80% to achieve the same degree of hardening,
which corresponds with to Kosar's statement about gelatin that
sensitivity doubles with a 30% increase in humidity. I daresay this is
why I can print in 1-5 minutes with a photoflood, whereas others,
probably in less humid climates, find the exposure with such a light
much longer. As I've said above, I think you'll find that adjusting the
exposure times to reflect the fact that gum is much more sensitive under
high humidity conditions will do you just fine, unless you are using an
especially intense or hot light, in which case you may also need to
adjust the dichromate concentration downward. Good luck,
Katharine
Received on Mon Apr 11 10:31:28 2005