By coincidence, I was discussing this very topic a few months ago on
another list (or newsgroup - I forget) and I happened to save my answer
along with the original question. [Note, that when the magnificent
Weston exhibit "California and the West" was at the MFA in Boston a
couple of years ago, none of the some 200 vintage prints showed any
deterioration.]
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> The Westons were superb and they had two
> comparisions of vintage Westons with the newer Brett versions. There
> was a substantial difference in the moods of the prints (hard to describe).
> A few of the prints were beginning to show their age - no discoloration, but
> the high density areas were starting to get a silver patina that occurs with
> age. (Carson -- what is this effect caused by? Silver migration to the
> surface of the gelatin?)
It is an oxidation product, similar to the tarnish you see on silverware.
I looked through several sources trying to find a precise chemical
explanation and couldn't. (One nameless author said it was a process of
residual silver "exuding" from the gelatin... sigh) My sense is that
the silvering (which I also found described as "bronzing" and "plumming")
is caused by a variety of factors which may be present in sum or in part.
The most straightforward explanation is that it is incompletely fixed
(residual) silver reacting to environmental sulfur - either from fix
that hasn't been completely washed out of the print, or from high sulfur
storage materials (common in mount board up until about 20 years ago),
or from atmospheric pollution. The result is a silver sulfide compound
which is unstable and eventually fades. This oxidation can also happen
to image silver. If it is image silver, the discoloration will appear
primarily in the areas of highest density (shadows in a print, highlights
in a negative.)
I have sucessfully removed silvering from gelatin glass plate negatives
with a solution of thiourea and phosphoric acid (similar to the "magic"
silverware cleaning solution you see advertised on late night TV). I'll
append the formula at the end of this post for those who are interested.
I don't see why the process shouldn't also work on prints (provided they
aren't dry mounted :-) or negatives on acetate or mylar backing.
Glass Plate Cleaning Formula
The formula is from an article by James Enyeart which appeared in a 70's
edition of the SPE Journal. It was adapted from a similar formula published
by Eugene Ostroff and used for cleaning Daguerreotypes. I assume that it will
work on prints, but I have no experience with that application. Oh,
this formula will destroy glass colloidion (wet plate) negatives, so
be sure you know which is which before using it on a glass plate
negative (colloidion emulsions are brown in color compared to a more
neutral gray of gelatin emulsions).
Distilled Water 500 ml
Thiourea 50 grams
Phosphoric Acid (85%) 10 ml
(You can also add a small amount of wetting agent, such as photoflo,
if you are cleaning negatives)
1. If there is surface dirt, you can clean it first with tap water and a
dilute solution of Parsons Ammonia.
2. Place plate in a tray of the Thiourea solution and gently agitate
(if emulsion is cracked or peeling, better to swab the solution on
locally with cotton balls than immerse).
3. When silvering appears to have dissappeared, rinse in tap water for
one minute.
4. Fix for one minute (I don't recall having any problems with rapid
fix, but you might want to use a conventional, non-hardening fix to
be on the safe side.)
5. Rinse again for one minute.
6. Use washing aid for 30 seconds
7. Rinse with tap water for 2-3 minutes (This is for a film emulsion, don't
forget to wash longer if you are doing a print.)
8. Use photoflo and dry. If any silvering remains, you can repeat the process.
Hope this helps,
Carson Graves
carson@ileaf.com