Re: About softening gelatin.

From: Sandy King <sanking@clemson.edu>
Subject: About softening gelatin.
Date: Sat, 15 Nov 2003 16:20:20 -0500

> Does anyone have a suggestion as to how I can soften to the maximum
> the hardened gelatin of photographic papers? If possible I would
> like to have it almost slimy to the touch.

Modern films and papers are designed to minimize excessive swelling
even with rapid processing solutions and at a higher temperature, so
what you are trying is right against the manufacturer's design
goal. Gelatin is often made hardened very much by all kinds of clever
application methods of hardening agents (involving mixing of hardening
agent in the coating machine, or coating hardening material as an
additional step) and optimal blending of binder (some non-gelatin
binder is blended to enhance binder's response to hardener). Plus,
modern coating machines are capable of coating emulsions of very high
viscosity, which enhances gelatin's response to hardening agent, as
well as improve dry strength of the material. In addition, materials
are chill set and ofted dried at a low temperature to enhance all
these hardening properties as well as dry strengths. So all these give
you some idea of what are effective control variables if you want to
coat paper with gelatin for your purpose.

If you want to reverse hardening of commercial material, it would
require drastic procedure which is damaging to the material, yet you
might not attain enough effect. Paper products from different
manufacturers may respond differently -- try some of small
manufacturers. Gelatin is a complex material and once hardened
effectively in the factory, it is very difficult to undo the hardening
effect. (This is why hardening agent is unnecessary with regular tray
processing of prints these days.)

You have already posted sodium hydroxide solution worked to increase
gelatin swelling. If you desire to amplify the effect further, I
suggest to try:

1. weak sodium hydroxide bath
2. brief rinse
3. weak hydrochloric acid bath
4. brief rinse
5. repeat at step 1 as necessary

The idea is to expose the gelatin to very high and very low pH regions
(both sides of isoelectric point, the farther the better) without
appreciable salt content in the solution. Both sodium hydroxide and
hydrochloric acid are strong irritants and need care while handling,
but they are effective (or harsh) for the purpose.

If hydrochloric acid is not in your darkroom stock, you could use some
(not all) household rust remover. CLR is a blend of mineral acids and
its pH is below 1 even when diluted 1:10 or so. (It's not a very good
rust remover though.)

Triethanolamine is ineffective for this purpose.

Long soaking in plain water may work but the disadvantage is obvious...
it takes long time. Warm water should help though.

Either way, if you judge the degree of swelling by the feeling to the
fingers, be aware that alkaline solution will give slimy touch to the
skin, and there may be other factors like mineral deposition or
bacterial growth if you choose to soak the print in plain water for a
long time. In industry, the swelling factor (see below) or wet
abrasion strength is used to measure hardness of coated gelatin
materials. Perhaps abrasion is easiest to adapt for your testing if
necessary. The standard is ANSI PH4.35-1972 "Method for determining
the resistance of photographic films to abrasion during processing."
The abrader is a sapphire of radious 0.38mm, and force is varied until
damage to the specimen is seen in water or processing solution at a
specified temperature. About 0.5-2N of force is usually necessary.

The term "hardening" in the context of gelatin is a process to reduce
swelling of the material in aquaous solution. It has nothing to do
with mechanical rigidity in dry state of the material. The degree of
hardness is the reverse of degree of swelling tendency, and swelling
factor is measured by the ratio of the thickness of coated gelatin
after long soaking to that in dry state (or some other reference
state). So hardening of gelatin means reduced swelling. Dry mechanical
strengths are different issues. The word "hard" may sound to people as
if it were linked to the dry strength, but this is not the case in
photographic gelatin literature.

--
Ryuji Suzuki
"Reality has always had too many heads." (Bob Dylan, Cold Irons Bound, 1997)