Re: Old Postcard Silver Patina

----- Original Message -----
From: "BOB KISS" <bobkiss@caribsurf.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Saturday, January 07, 2006 3:36 AM
Subject: RE: Old Postcard Silver Patina

> DEAR LIST,
> Forgive my "Bobby come lately" entrance into this thread
> but I just caught
> up on reading my e-mails. Isn't the effect all of you
> have described
> referred to as "dichroic fog" and is the result of not
> using an acid stop
> bath (either going from dev straight into fix or using a
> water rinse w/o
> acetic acid to reduce the pH) and using an acid fixer? It
> appears that
> there is some reaction that results from putting a print
> which is still
> alkaline from the developer into an acid fixer. I have
> been told that this
> isn't a problem when using ammonium thiosulfate fixers
> which are of a higher
> pH. Ryuji, can you comment on this?
> I have been told by conservators that it isn't visible
> until after some
> time. I recall seeing some lovely
> 2 1/4 X 3 1/4 contact prints of NYC buildings by Bernice
> Abbot which had
> this "silvering" in the shadows.
> CHEERS!
> BOB
>
   The mirroring effect on old photos is a different
phenomonon than dichroic fog. Dichroic fog is produced
during processing. It is a uniform coating of very fine
silver on the surface of the emulsion and sometimes back
coating of film or paper. It appears as a stain which is
yellow by transmitted light and greenish by reflected light.
Very thin coates can have a rainbow effect like an oil film.
Dichroic fog can be caused by excessive silver in developers
or fixing baths.
   The mirroring effect is caused by degradation of the
silver image of properly processed material over a
relatively long period of time. The main cause is oxidation
of the silver by polutants in the air. The polutants cause
some silver oxide to be formed which can migrate through the
emulsion. Some reaches the surface and can be reduced back
to metallic silver by other polutants. This is a different
mechanism from the sulfiding of the image caused by lack of
proper fixing or washing. Well processed images are quite
suseptable to oxidation and it is probably the main threat
to unprotected silver images today because of the universal
presence of atmospheric polutants. Where silver halide or
reaction products are left in the emulsion from poor fixing
or washing they are also suseptable to oxidation, however,
the more usual effect is sulfiding by reaction to the sulfur
from retained hypo or its reaction products. This is
typically in the form of a yellow stain or browning and
fading of the image, or both. sulfiding can also be produced
by polutants.
   There is a very large literature on image permanence now
which will give those interested a much better idea of how
images become degraded and what can be done in both
processing and storage to avoid or mitigate the effects.
Toning in suitable toners, for instance, can significantly
improve the resistance of well fixed an washed imags to
oxidation or attack by sulfiding agents.
   My explanations above are necessarily very simplified, my
main purpose being to separate the processing effect of
dichroic fog from after effects and to point out the
multiplicity of after effects.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com