[alt-photo] Re: HYDROSCOPIC!

[Ryuji Suzuki]

From: Marek Matusz <marekmatusz at hotmail.com>
Subject: [alt-photo] Re: HYDROSCOPIC!
Date: Tue, 11 May 2010 01:19:47 +0000

> Interestingly enough just before final collapse of B&W
> chemistry Agfa introduced a silver image stabilizer that was
> thiocyanate based, but I did not really follow that and I
> think Agfa dropped their silver B&W chemistry. So perhaps
> both components can act to stabilize silver image, but gold
> will also change the colour

I'm sure AGFA Sistan existed from at least 1990s. I used to
use Sistan for work prints but stopped using it as soon as I
understood the models of silver image degradation proposed
by various scientists.

Thiocyanate, in very low concentration, makes very sparingly
soluble silver salt, which is stable and inert. So, by
infusing the gelatin layer with a low concentration of
thiocyanate, you have a reserve of something that mops up the
free silver ion generated by oxidative attacks. The
disadvantage is that, if the concentration is too high, it can
accelerate image deterioration!

Also, the effect of Sistan (thiocyanate) is lost once you
rinse the treated print.

Another example. What else makes very sparingly soluble
compound with silver ion? Potassium iodide. If you treat
silver gelatin prints in a weak solution of iodide as a final
rinse solution, it presents very similar protective effects to
Sistan... not that I recommend it. Iodide can be hygroscopic
and there are other considerations as well.

In my view, a much better approach was taken for AG
Guard. It uses a compound that adsorbs on the surface of
silver. It covers the silver with invisible molecules so that
oxidative attachs won't easily reach silver. Then even if free
silver ion gets generated, the same compound can mop it up and
stabilize it.

I think AG Guard was not very widely used in North America
because Fujifilm did not export it. It was also sold in small
bottles of relatively low concentration (used with low
dilution ratio) and it was somewhat expensive.

The approach I took, described in my paper, is to use
2-mercaptobenzimidazole. This compound is much like the one
used in AG Guard, but the virtue of it is that it has been
used in emulsion making and also in processing chemicals to
some extent, so it's not a new compound. It's also cheap and
traded in large quantities because it's one of intermediate
ingredients in industrial chemistry. Once treated in a
solution of this compound, the silver image is very much
slower to bleach out in ferricyanide bleach, and almost
unaffected by strong peroxide fuming tests. It also slows down
the speed of brown toning.

The image protecting effects of AG Guard and my solution are
not completely lost by rinsing the treated prints, suggesting
that active ingredients are stuck on silver.

Also, both AG Guard and my solution can be harmlessly used at
higher concentrations as recommended.

--
Ryuji Suzuki
"Don't play what's there, play what's not there." (Miles Davis)