Gelatin-polymer blend (was Re: Gesso sizing)

From: Katharine Thayer <kthayer@pacifier.com>
Subject: Re: Gesso sizing (Katharine)
Date: Thu, 19 Jan 2006 11:53:56 -0800

> I use about 30-40 ml gesso in 150-200 ml 3.5% gelatin.

What's the polymer content of your acrylic gesso?

> > Does this mixture need hardening?

Whether it needs hardening is a different question, but blending this
type of polymer is not a substitution for hardening. In slightly
acidic cold water condition, the swelling of gelatin-acrylate blend is
lower than with plain gelatin, and this itself may help the sizing
layer to hold up, but I wouldn't rely on it at all, at least for
silver gelatin process (emulsion is coated at 35-40C).

I've tried gesso mix but one objection I had was that the acrylic
gesso is gritty because of matting agents in the mix. Another
objection I found is that the gesso contained titanium white and other
things I would not want to have in my prints unless archivality is
proven in the particular combination. Titanium white is very
photoactive and generates a lot of radical chain reaction in the
material when exposed to UV, and this was the main cause of problems
in early days of RC papers. Photographic industry learned enough and
today's products have good amount of means to prevent the problem, but
I am not sure of gesso in sizing layer.

Modern photographic films blend gelatin with hydrophobic polymer
containing carboxyl groups, like poly(methyl
methacrylate-co-methacrylate) in poly(acrylic acid). According to Naoi
of Fuji Photo Film, this helps in both rapid diffusion of chemicals in
development process, and fast drying of the material after washing. In
terms of chemistry, apparent molecular size of the polymer blend
changes depending on the pH of the environment (the apparent molecular
size observed by GPC is much larger than the starting molecules). Naoi
explains this in terms of polyion complex formation between primary
amino group of gelatin and carboxyl group of polymer. (That is,
isoelectric point and swelling properties of binder blend found in
today's film and paper are not like what you find in old literature.)

The blender I use is Roam and Haas Rhoplex (US) or Primal (elsewhere)
AC-35, in the quantity of 0.5 ml per each gram of gelatin. You can buy
this product in small quantity (cheap) from Kramer Pigmente. It has no
questionable pigment or gritty matting agent, and blends very well
with gelatin. Among polymer blends I tried, this was one of the bests
and is readily available in all sorts of quantity. Then I harden the
mixture with the bisepoxide hardener at pH 6.
Received on Thu Jan 19 16:26:27 2006