Re: APIS, hydroquinone hardening

From: Kate M <kateb@paradise.net.nz>
Subject: RE: APIS, hydroquinone hardening
Date: Fri, 15 Jul 2005 09:21:43 +1200

> Chris (and others), since we are already coating with a mix of
> gum/dichromate, don't you think it would be sufficient to just coat
> with unhardened gelatine + hydroquinone, then let the dichromate in
> the emulsion do the hardening? Just a thought...in theory I don't
> see why it wouldn't work.

This is one thing discussed between Chris's and my posts. If you let
hydroquinone in contact with dichromate in the gum-dichromate mix, I
strongly suspect that you will get raised fog density. So you want
excess dichromate in the size before coating gum-dichromate
mix. However, mixing chrome alum (a trivalent chromium) is an easier
and cleaner way to get the same net effect.

The reason is as follows.

1. Dichromate (or hexavalent chromium) does not have hardening
   property.

2. Dichromate can be reduced to trivalent chromium, which has strong
   hardening property. Hydroquinone is a relatively strong reducing
   agent; when hydroquinone and dichromate are brought into a contact,
   trivalent chromium is produced, and hydroquinone is oxidized.

   (Oxidation products of hydroquinone probably include a dozen
   compounds of varying lifetimes and reactivities. We'll set these
   guys aside because they are very complicated and not very necessary
   for this explanation.)

If you size paper with gelatin and hydroquinone, and coat dichromated
gum over it, the dichromate can diffuse into the sizing layer and
cause the above reaction to harden the gelatin. At the same time, the
moisture in the gum will dissolve hydroquinone and the hydroquinone
can diffuse into the gum layer to cause the above reaction in the gum
layer as well.

Now whether trivalent chromium itself can harden gum molecules at a
reasonable reaction rate is a question. However, it's very reasonable
to expect it can potentiate the effect of very weak exposure.

Exactly how high will the fog level be? I don't know. It will depend
on how much hydroquinone is present in the sizing layer, the
dichromate concentration in the coat, the temperature of the coating
solution, method of gum application, etc. If I were a gum printer I
wouldn't waste my time trying to figure out how to get this hardening
technique to work. Arent there enough things to worry about before
such a complicated technique?

If anyone is so strongly attached to this cumbersome technique, I
suggest to use more mild reducing agent rather than hydroquinone. Find
one that reacts with dichromate only very slowly, so that you can
dissolve gelatin, dichromate and the reducing agent in one solution
(dichromate should be present in excess). The actuall reaction
shouldn't happen until the stuff is coated and half dried. At this
point the reactions listed above should occur. This way you can get
easy coating and well hardened size. This method allows you to use a
lot more chromium hardener than you can with straight addition of
chrome alum.

But even then, I would say what's the point. Glutaraldehyde does the
job very well, very cleanly, and in one step.

> ......unfoirtunately I'm in a writint phase
> right now and have scads of hardened paper sitting by, so don't have
> time to test this out myself.

Photography is already complicated. Subject matters, composition,
lighting, etc. etc. etc. Chemistry is only a tool. To me it's a tool
to find the simplest and most effective solution to the problem. I
wouldn't waste time for hardener when we have lots of agents that work
well by simply mixing them with gelatin before coating. I use
glutaraldehyde because it's one of the most effective and most readily
available and least expensive. But I've tried several more agents and
glut is my #1 pick.

By the way, some time ago I was mounting a 20x20 print in a shadow box
frame with two S-hinges on top. What is the ideal adhesive to glue the
heavy print to the hinge? I tried my starch glue on scrap print to
test but it didn't seem to be strong enough. So I blended in some PVA
(like bookbinders customize glue-PVA mix depending on the paper), but
if there are conservation experts here, I would like to know the safe
and strong glue for this. (This print is hung in a gallery in Harvard
Square, Cambridge, MA for a few more weeks, if anyone is in the area.)
Received on Thu Jul 14 21:28:56 2005