Hardening gelatin

Christina Z. Anderson wrote:
>
  So far, therefore, glut looks
> equally as good as glyoxal, even one layer instead of two, but as Katharine
> said a while back, how would I know, really? It *seems* to behave similarly
> to glyoxal, but that is as scientific as I can get.
> However, it may be that 6ml of 2.5% glut is a bit overkill for 1000ml
> gelatin, or maybe not.

Ryuji Suzuki wrote:
>
>
> I would soak a small strip of sized but uncoated paper in water of
> highest temperature you ever use, and try to scratch the paper surface
> with fingernail. If gel's still firmly attached and not swollen too
> much, it's well hardened!
> (You can't tell hardening of gelatin when it's dry.)

Hello all,

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.

When paper is properly sized for gum printing, IMO, the
gelatin size does not sit on top of the paper but soaks in, leaving
the tooth of the paper open and no visible gelatin on the surface. 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. 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.

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
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
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 15:54:03 2004