Re: sizing with glyoxal

From: "Christina Z. Anderson" <zphoto@bellsouth.net>
Subject: Re: sizing with glyoxal
Date: Sun, 15 Feb 2004 19:11:09 -0500

> Using a 40% solution of glyoxal, 25 ml per 1000 ml sizing solution which
> contains 30g of gelatin, that would equate to 10g glyoxal per 1000ml. You
> are saying I only need 1-2% of the GELATIN amount, thus I only need .3 to .6
> g of glyoxal which equates to only about 1-2 ml of glyoxal 40%??

I have had a terrible cold since yesterday, and my brain cells are
barely intact with the help of ibuprofen and pseudoephedrine so I'm
not punching my HP calculator (or slide rule or abacus) but that
sounds about right, EXCEPT I meant glutaraldehyde in place of glyoxal.

I can't say how much glyoxal is needed or is enough because I never
seriously pursued it (I simply spent 30 minutes in library and found
the best aldehyde hardening agent, and searched for a bottle of
glutaraldehyde and stick with it). But the problem with glyoxal is
like this. Glyoxal is an inferior hardening agent to glutaraldehyde,
because individual crosslinks made by GO are weaker than those made by
GTA. So, if you want to achieve comparable overall hardness of
gelatin, you'd need more GO to bind to gelatin. But the link between
GO and gelatin is reversible and so there will be free GO if used in a
large quantity. The excess GO can cause yellowing problem.

> I wonder how I could test to see if this is sufficient hardening for
> gum printing, short of sizing a bunch of paper...

More GO is needed than GTA for comparable hardness at room
temperature. But adding more GO also increases free GO. This is the
problem. With GTA, there are fewer molecules needed, and these GTA
molecules are very tightly attached to gelatin, so there is not much
worry for GTA to come off gelatin and cause problems.

Another difference is that GTA is called "bifunctional" in the sense
that one GTA molecule can attach to two different gelatin molecules
and embrace the gelatin matrix, and this is a major advantage of GTA.

With my silver gelatin emulsions, I add a few per cents of
glutaraldehyde of gelatin. If I add too much more, the gelatin
becomes stiff solid even at a high temperature and soaked in hot tap
water. When I started making emulsion I did a quick casual search on
this list and a couple other places and found about the amount of
glyoxal people use, and tried to use it as a guide. I added a fraction
of what's suggested first, but in a moment, the gelatin became a stiff
solid, and I ruined the brush that was in the dish because there was
no way I could get the solid emulsion off of it. So I realized that
GTA was a lot more potent stuf, so I slowly started gathering more
information about hardeners.

If you want to stick with GO, you can empirically find the smallest
quantity that is useful for your goal. But gelatin is a molecule of
widely varying molecular size, and I personally don't prefer anything
that needs precision to function properly, whenever there's a
sure-fire alternative. Glutaraldehyde is great because even if I add
smaller quantity, I see some hardening effect, and all I need to worry
about is not to get greedy in hardening gelatin to the maximum
possible extent. I ruined a $0.42 foam brush instead of a $40 brush,
but it was enough of a lesson.

So you might want to try GO in smaller quantities, but again I can't
suggest any starting point as to the quantity of GO to use.

--
Ryuji Suzuki
"Reality has always had too many heads." (Bob Dylan, Cold Irons Bound, 1997)