"Jim Stewart" <jstewart@[EMAIL PROTECTED]
> wrote in message
news:ktadnZwKuup4J8PanZ2dnUVZ_oKhnZ2d@[EMAIL PROTECTED]
> I'm in trouble.
>
> I sent my mother-in-law a really nice B/W
> print that I made and after about a year,
> the black areas turned an ugly mottled gold
> color.
>
> She wants a new print and I need to know
> what to do so that the problem doesn't happen
> again.
>
> I used Ilford chemistry and Mitsubi**** Gekko
> paper.
This is called silvering out and is common on very old
photographs. It comes from oxidation of the silver image.
The silver oxide is very finely devided and can migrate to
the surface, where it can be again reduced to metallic
silver by other polutants. Very finely devided silver
(colloidal silver) is bright yellow, hense the gold color.
The problem comes from oxidants in the atmosphere or
from mounting materials. Sources of peroxides are plentiful,
automobile exhaust, vapours from fresh paint, many other
sources.
Well fixed and washed prints are vulnerable. Actually,
a very small amount of hypo left in the emulsion acts to
protect the image by creating a layer of silver sulfide on
the individual silver grains. However, a much better way of
protecting the image is toning. The best toners for image
protection are sulfiding toners, selenium toners, and gold
toner. While it was found that Kodak Rapid Selenium Toner,
in very high dilutions, was not effective for microfilm a
stronger dilution _is_ effective for pictorial films and
prints where some change in image color or density is
acceptable. According to Dr. Douglas Ni****mura, of the Image
Permanence Institute of the Rochester Institute of
Technology, a minimum of three minutes in a solution of 1:9
KRST will protect the image. Another very good toner is
Kodak Brown Toner, a polysulfide toner. Polysulfide tones
all densities evenly so any amount of toning which results
in a visible effect on the image will protect it. Gold
toning is similar but is expensive so its not much used
although its still the standard for microfilm.
While inadequate fixing and wa****ng can cause image
degradation the effect is most often a brown or yellow
staining. Lack of complete fixing causes overall staining
because the remaining insoluble reaction products are
distributed throughout the emulsion. Inadequate wa****ng can
result in an overall stain but often shows up more as an
imagewise stain. While both stains can be removed the
process can further damage the image and the resusults are
not always permanent. Once an incompletely fixed image is
allowed to age for a few weeks it can no longer be
completely fixed by re-fixing because the reaction products
continue to change in the emulsion, eventually becoming
impossible to remove.
Despite the problems with inadequate fixing and wa****ng
oxidation of the image is caused by external polution and
well fixed and washed images are particularly vulnerable.
About 15 years ago some RC papers had problems with
image oxidation caused by peroxides being emmitted by the
titanium dioxide used in the reflective layer. This problem
was solved by adding peroxide scavengers to the reflective
coating and emulsion. The scavengers are self-regenerating
so they remain effective essentially for the life of the
print. These same peroxides also attacked the plastic layer
in which they are suspended causing it to crack and flake.
This has not been a problem for RC for many years now.
Silvering out can sometimes be removed by a mild silver
bleach, a treatment similar to removing dichroic fog. The
safest method is to treat the print is a bath of fresh rapid
fixer to which is added citric acid in the amount of about
15grams per liter. The print must be watched carefully
because this treatment will also bleach out the image.
However, the best solution for your problem is to make
new prints and tone them as suggested above. The toning will
protect the prints against even pretty severe oxidative
attack.
--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@[EMAIL PROTECTED]
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