Re: Old Postcard Silver Patina

----- Original Message -----
From: "Michael Koch-Schulte" <mkochsch@shaw.ca>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Wednesday, January 04, 2006 7:53 PM
Subject: Old Postcard Silver Patina

> I'm trying to identify the patina that is on the surface
> of this old
> photo-postcard. It looks to me like (and probably is)
> silver when held to
> the light at an angle. What would cause this? Bad fixing?
> Could it have been
> deliberate? The card dates between 1910 up to the first
> world war. Purchased
> recently in a market in Zagreb for $1. There is a mark on
> the centre right
> which shows the original black tones. Possibly a scuff. Is
> it
> possible/advisable to clean the silver off this card? Is
> it possible to
> stabilize the image using modern stabilizers? Thx. Any
> history on
> photo-postcards would also be appreciated.
>
> ~m
>
> http://www.mondotrasho.ca/pages/alt-photo/cards/Postcard.jpg
>

   It looks to me like typical "mirroring" from oxidation of
the image silver. The silver is converted to silver oxide by
polutants in the air. Some of this oxide migrates to the
surface of the emulsion and some is converted back to silver
by other polutants. Oxidation is not a sign of poor
processing, in fact a very small residue of hypo has been
shown to prevent oxidation to some extent. It is the result
of oxidizing gasses, often peroxides, present in the air.
   There are ways of removing the silver and silver oxide
from the surface but they carry a risk of destroying the
original image and there will be some fading of the original
image beause some of the silver is gone.

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