Re: Gelatin-polymer blend

From: Katharine Thayer <kthayer@pacifier.com>
Subject: Re: Gelatin-polymer blend (was Re: Gesso sizing)
Date: Sat, 21 Jan 2006 13:23:23 -0800

> On Jan 21, 2006, at 12:57 PM, Judy Seigel wrote:
>
> >
> > However, I doubt gesso ever contained titanium -- too expensive for
> > the covering process, among other disabilities.
>
> I was surprised about that too, since titanium dioxide is so much
> more expensive than other white pigments; I would have expected them
> to use some kind of chalk for the whitener. But I went and looked at
> my can of acrylic gesso, and sure enough, PW6.

When gesso was strictly for old material like oil, the story was
probably different. Acrylic gesso products I tried are basically
polyacrylate binder containing a filler, a matting agent and a pigment
(besides biocides, preservatives, etc.) This composition may not look
too dissmilar but the proportion of each ingredient in terms of dry
volume is quite different from old material. Because polyacrylate
dries to a very thin film, most of the volume is taken up by the
filler and the matting agent. Titanium white pigment is a very small
fraction of all.

The reason why titanium white is used is very analogous to why RC
paper uses this pigment over baryta. Barium sulfate requires quite a
bit of thickness to ensure good white reflection and opacity to the
base support. Barium sulfate is a bulky part of
the subbing layer in fiber papers, but such a thickness is impossible
on RC paper. The resin layer has to be much thinner, yet it must
ensure good white reflection and opacity. The pigment must also be
compatible with hydrophobic resin rather than hydrophillic gelatin.

So for all these reasons, titanium white is a very natural choice to
incorporate in acrylic gesso.

When it comes to improving the property of the gelatin binder by
incorporating additives, the useful ingredient is a certain kind of
acrylic polymer, which is used in part in acrylic medium. Anything
else (filler, matting agent, pigment, etc.) is potentially harmful, or
questionably undesirable at best. This is why I use Rhoplex/Primal
AC-35 (cheap and easy to buy from Kremer Pigmente in NYC or Germany)
as a blend, and be done with this whole issue. I think discussing
ingredients of various gesso products is rather irrelevant to our
goal.

http://www.kremer-pigmente.de/bindem07.htm

I tried both AC-33 and AC-35 but for sizing layer AC-35 works best.
Received on Sat Jan 21 16:05:03 2006