Re: An exchange with Mike Ware (on Argyrotype)

From: Loris Medici <mail@loris.medici.name>
Subject: RE: An exchange with Mike Ware (on Argyrotype)
Date: Fri, 24 Mar 2006 14:41:36 +0200

> Loris:
> Can you suggest a formula (most preferably using oxalic acid) that does
> this job?

I'm reluctant to suggest a formula without testing it in my hand,
especially I see MANY of those untested formulae nicely tabulated in
print and on web. But I can privide information, or collaborate with
others.

It's best to wash off most of the iron at low pH before they get to
make solid precipitate. Then if iron remains, you can try sodium
gluconate or galactonate solution at alkaline pH.

You might want to look into:

Mehltretter, C. L., Alexander, B. H., and Rist, C. E. 1953.
Sequestration by sugar acids, Industrial and Engineering Chemistry,
vol 45, 2782-4.

For the nature of gluconate, see:

Sawyer, D. T. 1964. Metal-gluconate complexes. Sorry, I can't find the
journal name right now, but it's somewhere in my database.

> Loris:
> Well, I seriously doubt I will manage to find the compounds you mention
> above.
>
> Ryuji:
> "One problem with residual iron is that it can generate hydroxyl radical
> in presence of air and moisture, and this WILL attack silver image. To
> solve this problem, it's most effective to incorporate iron ligand
> chelator that inactivates iron's redox catalysis, a radical scavenger in
> the paper base, or more preferrably both. Most effective ligands have
> some affinity to silver."
>
> Loris:
> Can you please decrypt the above statements for us mortal non-chemists?
> :) Which compound can do that job?

Well, this is no high school chemistry, and it's hard to know what
works and what doesn't, without doing a lot of experiment. This is the
kind of things that professional conservation scientists with money,
staff and other resources should be studying. (I only have ideas, and I
have limited money, no staff and no resource.)

In short, oxygen in air can make very potent and active oxidizing
agents, such as peroxide and hydroxyl radical. These are collectively
called "active oxygen species" and are the same kind of things that
they talk about one possible mechanism of aging. The thing is that,
the generation of radicals is much much faster when there is iron in
the environment, because iron is a very active catalyst for this type
of reactions.

There are several possible approaches to this. One is to keep your
work in vacuum, and probably in low or no light. But this is probably
not acceptable. Then, we should think about ways to inactivate iron's
catalysis. There are several agents that are known to do this, and you
want to find one that is compatible with cellulose and metallic
silver, and stable, practically colorless and nonvolatile in air.

Another is to sprinkle an agent that reacts with radicals faster than
cellulose or silver. Sort of sacrificial protector. It should be
compatible with the material and also meet the conditions above. Of
course, if the iron is not inactivated, or if air is not shut out, a
finite amount of this type of agent can do its job only for a while,
so it is best to combine two modes of protection. Agents like
thiosulfate, thiocyanate, sorbitol, mannitol, xylitol, salicylic acid,
citric acid, etc. can do this type of job reasonably, and they are
cheap. There are more efficient, fast-reacting agents, but they are
more expensive.

If your goal is to increase permanence of silver image, the simplest
method may be to use Fuji Ag Guard. It is a solution of an organic
thiol compound that acts similar to sepia toner but without converting
the silver to silver sulfide. Fuji people published test results with
silver gelatin prints, and they found degree of protection comparable
to gold toning. Fuji doesn't sell this product outside Japan, but
someone (conservation material supply houses??) should bulk purchase
this and split, or better make some sort of arrangement with Fuji.

Agfa Sistan is a solution of thiocyanate, but I don't know of good
test reports. Sistan is tricky because overdosing thiocyanate can harm
the image very badly, and the proper dose must be determined for each
material. Another problem with thiocyanate is that it is easily
soluble in water and humidity cycle of the print may weaken the
protecting effect. If it were used with silver-gelatin material, the
gelatin layer can hold some amount of thiocyanate close to the image
silver, but with non-gelatin processes, this story is very different
and must be tested. Ag Guard is not free from these issues, but the
window of effective dose is wider to tolerate some of these factors
and I think Ag Guard is a better approach.

> Can we use household 3% peroxide for this, or still need something
> stronger?

3% is plenty strong. The test is done at upward of 500ppm in air to
test archival property of toned silver prints. You might want to use
100ppm or lower for your goal. The RH is also buffered with saturated
solutions of salts, and it's usually 80 to 100%. The temp is regulated
in an oven, usually 30-50C range. But you can of course do the test at
room temp, which takes longer to conclude the test, but may work
better for your work.

Also, some tests use a small fan located in a sealed test chamber to
evenly agitate peroxide fume. The IPI method for microfilm uses fan
for first day and turn it off thereafter.

I'll dig papers that describe these protocols. I think IPI report also
provides details on this. Another place to look is ANSI standard.
Received on Fri Mar 24 15:46:12 2006