Re: Saving old negatives

From: savitrika@sify.com
Subject: Re: Saving old negatives
Date: Thu, 06 Apr 2006 12:09:19 +0600 (IST)

> The Kodak publication 'Conservation of Photographs' conveys the
> possibilities of using thiourea treatment and/or ammonium hypo
> reducer treatment for silver tarnish and stains.It also lists the
> formula for the cleaning solution comprising of ammonia, thiourea
> and phosphoric acid.

I would be extremely cautious about those recommendations. In
particular, thiourea was used as the cleaning agent in daguerreotype
cleaning solution in 1950s and those images were badly damaged in
10-20 years. It's often referred to as "Daguerreian measles."

This is said to be because thiourea decomposes in presence of light
and oxygen to form dicyandiamidine sulfate. (Kurth, D. G., 1999. The
reaction of thiourea to dicyandiamidine sulfate on silver surfaces
investigated by reflection-absorption infrared spectroscopy, Fresenius
J Anal Chem, 365, 465-6.)

The problem with this type of treatment is that, the metallic silver
surface is strongly adsorbed by thiourea and thiourea cannot be washed
out by water wash.

Ammonium thiosulfate with metabisulfite and citric acid is a nice
trick to remove fine silver stain from developed out silver material,
but it does remove some of the image silver, especially if it is very
fine grained.

At any rate, aged gelatin is quite fragile so wet processing would be
the very last thing to consider.
Received on Thu Apr 6 00:44:37 2006