Re: Hardening gelatin

From: Katharine Thayer <kthayer@pacifier.com>
Subject: Hardening gelatin
Date: Tue, 30 Mar 2004 13:57:03 +0000

> I tried Ryuji's test yesterday with unhardened gelatin vs
> glutaraldehyde-hardened gelatin, and I honestly couldn't tell any
> difference. On neither paper strip did the gelatin loosen or swell
> noticeably in the cold water I use for gum development; in neither case
> did a fingernail leave a scratch on the paper surface.

If your unhardened gelatin size is undamaged by a treatment that is
rougher than anything you could possibly (and maybe accidentally) do
during processing, why do you bother to harden gelatin then? If you
never use water above 20C, and your gelatin is high bloom and also you
dry the coated material at a low (near refrigerator) temperature, this
is a good possibility. But unless you are absolutely sure that you
adhere to such a practice, I'd harden gelatin.

> 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.

Depending on support material and coating technique, the gelatin
content can vary widely, but in my silver gelatin emulsions gelatin is
(almost always) above 3% when coating. When the emulsion is coated on
hot pressed watercolor or printmaking paper, and when the emulsion
dries up, I can feel the paper surface. But during wet processing, I
feel the texture of the coating, not paper. This way, finished images
look like the image is in the paper not on the paper. (and the hue can
be like Loris' toned cyanotype, and could be black, purplish brown or
orange brown, depending on chemistry and toning.)

> 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.

One major difference between gelatin and gum is that unhardened gum is
very soluble even in cold water, whereas unhardened but gelled gelatin
is insoluble in cold water. In dichromated gelatin process, the
exposed material must be washed by water that is hot enough to
dissolve away the unexposed gelatin while retaining exposed gelatin
and sizing material.

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