Re: Hardening gelatin

On Tue, 30 Mar 2004, Katharine Thayer wrote:
> ..... The
> open tooth is essential; if the tooth were clogged with gelatin, the
> paper won't be able to hold onto the hardened gum. For this reason one
> wouldn't want to use a very thick application of gelatin, and the recipe
> given by Ryuji a while back for his own work (10-15 grams of gelatin in
> 200 ml water) is more than twice as much gelatin as the 2-3% gelatin
> most often used to size paper for gum printing.

Extrapolating from one use of gelatin, or gum, or dichromate to a
different use is IME almost bound to be wrong -- as for instance
extrapolations made about gum bichromate from Kosar's citations about
carbon. Even if the chemistry is the same, or similar, the mechanics are
quite different.

> ... This, along with the
> cooler temperatures used in gum development (at least in my shop) may
> explain why the test he suggests for determining whether the hardening
> is successful did not successfully discriminate in my shop between the
> hardened and the unhardened gelatin-sized papers. It's possible that the
> lack of difference is an indication that hardening did not place, but
> it's also possible that it just can't be determined with this test, with
> this low percentage of gelatin, under these particular conditions.

The only test for adequate hardening that I find relevant would be if
there's pigment stain or not -- because that's the reason for hardening
our gelatin (whatever it may be in Ryuji's use) in the first place. I know
a printer who makes gorgeous gum prints without hardening her gelatin
size, and Katharine's point that the dichromate probably tans or hardens
the gelatin may apply -- although I'd guess that it only hardens where
there's been hardening by exposure, so the paper whites -- which is where
stain happens, or anyway where it's an issue -- wouldn't be hardened that
way.

> I've never hardened gelatin myself in practice; even back in the
> beginning when I routinely gel-sized my paper, I didn't harden the
> gelatin for two reasons:
> (1) I figured that if the paper manufacturers don't harden their gelatin
> sizing, why should I? That's a somewhat flippant comment, because the

I'm wondering if that's true -- don't most papers have alum?

> manufacturers do at least add a fungicide to slow the growth of mildew
> and mold in their unhardened size, which I didn't do. But more
> seriously and to the point: (2) while it's not at all clear to me that
> the literature explaining the mechanism of dichromated gelatin is
> applicable to dichromated gum arabic, one hopes that much of it is at
> least applicable to dichromated gelatin, in which case it seems
> reasonable to assume that the gelatin size on the paper should be
> hardened during gum printing, and an extra hardening step should not
> be necessary. This may be especially true, I should add, for a

Well again, I think the paper whites are the issue, and have the
impression (wrong perhaps) that the tanning action of dichromate depends
on the exposure to light, not just applying the solution. In the white,
or very light areas, exactly where it's needed, that hardening wouldn't
occur. Areas of even moderate image density have a layer of hardened gum
arabic on them, so hardening of gelatin for those areas wouldn't be at
issue.

Judy

> saturated dichromate solution, and may not be as true for a very weak
> dichromate solution where there may simply not be enough dichromate
> ions floating around to harden both image and underlying size. And of
> course it goes without saying that it would only work with images where
> there is some tone throughout the image.
>
> Katharine Thayer
>
Received on Tue Mar 30 16:38:19 2004