[pure-silver] Re: word from ilford
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Tue, 14 Aug 2007 20:38:41 -0700
----- Original Message -----
From: "Steve Nicholls" <gl1500@xxxxxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Tuesday, August 14, 2007 5:38 PM
Subject: [pure-silver] Re: word from ilford
Eric Nelson wrote:
I got a call from Mike Bain from Ilford/Harman, and
his opinion, as well as one other person he consulted,
is that these prints we've been discussing have
suffered oxidative bronzing.
Now to me bronzing has been a defect that presents as
a reflective mirror effect with little or no
discoloration such as an over exposed albumen print,
but Mike assured me this is different and is
attributed to pollutants of one kind or another.
I had three RC prints do virtually the same and it was the
glues used in the mounting. Not done by me. They were
mounted under glass.
[snip]
His only suggestion to prevent this from happening is
selenium toning for a few minutes in either 1:19 or
1:31 dilutions.
I had always thought that silver prints processed &
washed well would be fairly archival and that selenium
gives and added amount of stability for the truly
paranoid and also cools the image, but it seems from
this conversation that selenium is pretty much a
necessary part of the silver process when sending off
work to unknown environments and handling.
At the 1:31 dilution I doubt there would be any colour
change.
Afga Sistan is also recommend for similar reasons.
Apparently it has been re-released by AO ??
Steve
There seems to be no research data on Sistan. It may
very well work but I've never seen any proof. Fuji published
a research paper on Ag-Guard, another stabilizer. Ag-Guard
is different from Sistan so research on one does not
necessarily apply to the other. The Fuji research showed
that Ag-Guard was effective in protecting images against
action by oxidizers but less so than sulfiding toners.
Very dilute solutions of Kodak Rapid Selenium Toner do
not tone all densities uniformly. While early resarch showed
KRST to be an effective protective treatment it appears to
have changed in some way about 20 years ago. After that
change it was no longer effective in weak dilution. The
research showing this was carried out by the Image
Permenance Institute of the Rochester Institute of
Technology. At stronger dilutions KRST is effective but will
change the color or density or both of the image. The
recommended treatment is at a dilution of not more than 1:9
and for not shorter than 3 minutes at 68F.
A better treatment is a liver of sulfur type toner such
as Kodak Brown Toner. These toners convert silver uniformly
regardless of grain size so partial toning will have some
protective action on all densities. However, KBT does cause
a color change on many materials. It, and similar toners,
are now the recommended protective treatment for microfilm.
Gold toners are also quite effective but are expensive.
Gold toner produces a neutral to bluish image on neutral
tone paper and a blue color on warm tone paper.
Bronzing or silvering out is an effect of atmospheric
oxidizers on the image. The convert the silver to a very
finely devided Silver Oxide which then can migrate to the
surface of the emulsion where some of it is converted back
to metalllic silver by other polutants. The metallic silver
can be yellow to silver in color depending on how finely
devided it is. Silver oxide is black so there are also black
stains on the surface of the print. These stains can often
be removed but some silver will have been removed from the
image so it may be faded.
Metallic silver can also be sulfided from polutants in
the air or in the emulsion. Usually, this results in a brown
or yellow stain. Where there are sulfur containing compounds
in the emulsion, such as hypo or fixer reaction products,
the stain can be generalized, not just of the image. In some
cases the stains can be removed but some techniques
recommended in the past have been shown to accelerate future
aging of the image.
A very small residue of hypo remaining in the emulsion
can act as a stabilzer by partially sulfiding the silver.
This prevents further sulfiding or oxidation. However, other
than not using very extended washing there is no
recommendation and not much information is available about
how much hypo should be retained. This effect has been known
since about 1960. It may be one reason that some drugstore
processed snapshots have survived in relatively good
condition. It has been shown, however, that film and paper
processed to the old standard of having no residual hypo is
particularly vulnerable to oxidative attack, unless it has
been toned. Sources of peroxides are plentiful especially in
urban areas. For best life prints should be toned in a
Liver-of Sulfur toner, a fairly strong solution of KRST (as
indicated above), a Gold toner or fully toned in nearly any
sulfiding toner where the resulting image color is not
objectionable.
In general, prints should not be covered by tightly
fitting glass or plastic covers, especially RC prints.
Note that in the past RC prints were subject to
oxidation from a gas emitted by the Titanium Dioxide
reflective layer beneth the emulsion. This effect was
exagerated when the prints were mounted under glass or
plastic, which prevented escape of the gas. This gas
attacked both the image and the plastic layer causing it to
craze and even flake off. Modern RC papers contain
scavengers for these gasses and, in general, are about as
permanent as fiber base paper. They will still benefit from
proper protective toning or the use of stablizer.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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