(long) Re: Permanence of Bleached Silver Prints

Barbara N. Brown (bnbrown@mail.utexas.edu)
Wed, 18 Feb 1998 09:27:54 -0600

Christopher -
Your inquiry is an interesting one, and I passed it along to photographic
chemist I know. He is not on this list, so I am forwarding his reply to you
and the alt-photo-process list.
Hope the following helps.
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I'm not on the alt-photo list, but your question was forwarded to me by
someone who is. (Although I'm sure that if Mike Ware is still on there,
that he would be a good one to answer your questions.) As a research
scientist at the Image Permanence Institute at RIT, this person thought
that I might be able to help.

> The bleach I use is out of Keepers of Light
>(Copper Sulfate, Acetic Acid, Potassium Bromide, Potassium dichromate and
>water.) Of course, when these prints are then fixed, the effect which I like
>disappears as the paper turns completely white.

What you have here are two bleaches, and bleaching aid, and a
"stabilizer". The copper sulfate reduces to form a Cu+ salt (which, of
course causes the metallic silver to oxidize into Ag+). The potassium
dichromate also reduces (forming a Cr+3 salt) and also oxidizing the
silver metal into Ag+. As a general rule, the electrochemistry is such
that we tend to promote oxidation in acidic solutions (which is why most
bleach baths are acidic.) (Similarly we tend to reduce in the chemical
sense things in alkaline baths which is why developers tend to be
alkaline. In case you don't have the basic chemistry, oxidation is a
process in which electrons are removed from something while reduction is
the process of adding electrons. Metals in their native state have a
formal charge of 0 so if we oxidize them we form positive ions and if we
have a silver salt (formal charge on silver +1) we reduce it to silver
metal (formal charge 0.)

The purpose of the halide salt (potassium bromide) is to anchor the
silver salts that are formed. Consider what would happen if you formed a
very soluble silver salt (like table salt) in the print. While the image
sat in the bleach bath, the soluble silver salt would be a) washing out
into the solution and b) migrating everywhere in the print. When you
redeveloped the image (if you could -- and it's possible that you
wouldn't have enough of a nucleation center to effectively develope out
an image) you would have a faded blurred image. The silver salts
produced during bleaching are not necessarily greatly soluble, but the
halide is added to that the silver will "precipitate" in the emulsion as
silver halide (in this case, bromide) which is pretty insoluble. Since
it's not very soluble, it won't tend to go anywhere while the print is
wet.

We run into a similar problem with deteriorating images. The silver
oxidizes in the presence of poor quality enclosures and atmospheric
pollutants as well as humidity. The silver salts formed tend to be
relatively soluble and there is enough moisture in gelatin that the
silver salts migrate. The higher the humidity, the more the salts tend
to migrate and this is one reason why we recommend that prints be kept
in a relatively dry environment. (Also air and moisture themselves are a
relatively good oxidizing agent.) When I worked at the National Archives
of Canada we stripped some prints down to the baryta layer (something
that Edith Weyde had done back in the 1930s at Agfa) and there was a
ghost image on the surface of the baryta (a demonstration that the
silver had migrated downwards.) Mirroring is also a demonstration of
migration (in this case, the silver migrates to the top surface of the
gelatin.) Electron micrographs also clearly show the migration of silver
from the filamentary bundles from which they started. As the particles
get smaller, two things happen. 1) the color changes and the silver
starts getting warmer (looking orangish red). 2) Eventually, the
covering power of silver is lost and the print simply looks like it's
starting to stain slightly (as the silver migrates into the dmin areas.)
I remember at Buffalo, they tried to use neutron activation
autoradiography to make copy negatives. In this process the image is put
into a nuclear reactor and bombarded with neutrons which makes the
silver radioactive. Since this is a nuclear reaction, it doesn't matter
what chemical form the silver is in (salts or metal.) The print is then
sandwiched under pressure against a piece of x-ray film and the
radioactive silver exposes the film. The problem that they called me
about was that all of their images were blurred even when they increased
the pressure to get good contact and of course the problem was that the
image silver had migrated. (Whew. Long winded stories.) Anyway, this is
the reason for adding the bromide.

Herein lies the problems. The silver bromide is light sensitive and you
will eventually print out an "image." However, the metallic silver may
be too fine to have an appreciable covering power and the image may
simply fade away. In addition, if there is sufficient bromide present,
other photo-chemical processes may occur causing fading (See Mike Ware's
book on historic print fading.)

>1. Are there any suggestions for achieving this kind of effect which will be
>permanent? For example, using bleach from a 2-part sepia toning? Would the
>print need to be fixed after bleaching?

This bleach is similar to the above solution, but with only one
bleaching agent (potassium ferricyanide.) The problem with leaving
ferricyanide (or ferrocyanide) in the print is that contamination may
cause it to start changing color (iron, for example may turn it spotty
blue.) What you might do is use a "thin" print to start with and then
sepia tone it. The covering power of the silver sulfide will be less
than metallic silver and you'll be left with a lighter brownish tone.
The silver sulfide is very insoluble and quite stable.

>2. What happens to the silver as it is bleached? I know that it is converted
>to another metal in the bleaching process and that it can be re-converted to
>the original silver through re-development. But what exactly is the bleached
>silver?

Right. I guess I covered this above. The silver doesn't turn into
another metal, but simply into silver ion (which may be associated with
some counter ion such as bromide.)

>3. Can anyone suggest a good reference book in which I can look up these and
>other similar questions. One of my concerns is to learn enough about the
>chemical reactions taking place to know whether a non-standard use of standard
>chemicals will lead to an archival print. (assuming proper paper, storage,
>etc.)
>

I'm not aware of any books on this particular topic. You need to learn
photographic chemistry though for a start. George Eaton used to have a
fairly decent book to get you started (published by Morgan and Morgan
and sold by Kodak), but it's been out of print for a few years. (You may
find a copy in a used bookstore though.) There was another fairly decent
book that I found, but I lent it out to a student and I can't remember
the title off the top of my had.It was something like Chemistry for
Photographers (a small white paperback worth about $15.) I'll have to
get back to you about the title. There is nothing much written about the
topic of permanence and stability and what there is tends to go out of
date pretty fast. (I have to update notes for my class each year as we
discover new things.)

Good luck.
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