Re: Gelatin hardening question

From: Keith Gerling <Keith@gumphoto.com>
Subject: Gelatin hardening question
Date: Thu, 15 Jul 2004 03:52:45 -0500

> I've been having some sizing problems lately. Problems I never have
> in months that are cold and dry. I harden gelatin with chrome alum
> and give it a day to harden. Searching through the archives, I
> found a post where Philippe Monnoyer mentions that chrome alum takes
> up to a month to completely harden. Can someone elaborate on this?

It's well known that gelatin hardening can take some time with agents
like chrome alum and formaldehyde. I posted references to gelatin
literature here a while back, and many of those discuss this
issue.

> The degree of hardening per unit of time is obviouslynot linear.
> What degree of hardening takes place in 24 hours? How about in a
> week? Would glyoxal harden faster? Lastly, is higher heat and
> humidity known to have a factor in the hardening process?

If you use more of a (slow working) hardening agent, you get more
hardening in the short time, but that approach is likely to lead to
different kinds of trouble some time later.

Glutaraldehyde hardens gelatin very fast. By the time the coated layer
dries halfway, most of the hardening has taken place. So just enough
amount of glut can be added to the gelatin solution to obtain desired
hardening. This way, you don't need to worry about further hardening
to occur while the paper is sitting around. There is no need to rinse
out unreacted hardeners either.

In order to get good hardening effect, it is best to use high gelatin
concentration, to use gelatin of high bloom strength, and chill the
coated material immediately after coating to gel the gelatin before
drying. These factors contribute to increase the dry strength
(abrasion resistance, etc.) of the coated gelatin as well as more
efficient hardening reaction. Drying at high temperature is not a
good idea though some humidity is ok.

However, for sizing of large papers it's hard to use gelatin that is
too thick, or to chill the paper in refrigerator after coating. So
I've been using 3% of 225 bloom gelatin with glut without chill
setting but I don't have any problem. The gelatin doesn't melt with
subsequent coating or in processing solution. The material does not
develop stain like others reported for glyoxal. My darkroom is hot
and humid these days (26-30C).

(As long as the material is small, I routinely refrigerate freshly
coated material to chill set the gelatin (gelate) before taking it out
to room temperature for drying. I do this to all dry plates.)

Christina posted a comparison of glyoxal and glut hardened sizing for
gum dichromate process some time ago. She uses 2.5% glut solution that
came from the same source as mine. So you might be interested in
digging the archive for that post. Both she and I found that much less
glut is necessary than the amount of glyoxal that is commonly used to
harden gelatin for sizing.

--
Ryuji Suzuki
"You have to realize that junk is not the problem in and of itself.
Junk is the symptom, not the problem."
(Bob Dylan 1971; source: No Direction Home by Robert Shelton)