Re: Hardening gelatin

From: Katharine Thayer <kthayer@pacifier.com>
Subject: Re: Hardening gelatin
Date: Thu, 18 Mar 2004 10:16:16 +0000

> While thinking about my experiment in hardening gum, I got to wondering
> how people who harden gelatin for sizing actually know that hardening
> has taken place.

I can usually tell by the viscosity of the mixture.

But the real difference is when I do hand test of emulsion. I coat a
piece of small paper, and put it in freezer at various points in
emulsion making. After cooling it down, I take out the piece and air
dry at room temperature, expose, and process. The gelatin is very
fragile. If I leave the processed test strip on my sink, the surface
gets washed out.

After the emulsion making is completed, I take a portion for a few
sheets on a dish and mix in hardener, wetting agent, antifoam, and
plasticizer. Then I coat a few small strips for test exposures and a
few large sheets for printing. The small pieces gets geled and dried
in the same way as above. But these strips are much more durable than
the ones without hardener added. I can touch the surface of wet prints
and "feel" the texture without damaging it, though I would not like to
do this on real prints just in case.

> But when you're hardening gelatin, how do you know? Since, like Judy,
> I've long questioned the wisdom of assuming that the chemistry of gum
> and the chemistry of gelatin are interchangeable,

For chrome alum hardener, chemical reaction underlying hardening is
analogous. But I don't know for aldehyde hardeners. Aldehyde hardeners
in gelatin act on free amino groups. But gums shouldn't contain any
appreciable amino groups. I didn't investigate how aldehyde would
harden gum, and frankly I was a bit surprised that you said glyoxal
worked for gum. Maybe there are other positively charged groups that
can strongly bind with aldehyde.

> it occurred to me that
> this might be one place where I could compare the two in some kind of
> quasi-systematic way, since I have these hardening agents that are
> usually used for gelatin, that I'm using to harden gum with. If gelatin
> and gum behave similarly in this case, then I would have to consider
> softening my resistance to the idea that gum chemistry = gelatin
> chemistry.

Well, I won't be surprised if results vary across different colloids.

--
Ryuji Suzuki
"All the truth in the world adds up to one big lie." (Bob Dylan 2000)