Sulfiding of silver images

... I should also respond to Loren's question about sulfur toning versus
"natural" sulfiding.  Ironically this is one of the questions that I
gave my students this past quarter.

The first thing that must be understood is the process of silver
deterioration.  The easiest thing to do is show it in a diagram.

                                           Oxidation
                      Ag+ (silver ion)  <---------------- Reduction to
                            |  |                           red spots,
                            |  V      Reduction            mirroring,
                            |  ----------------------->  orange colloidal
                            V                            silver. (Ag metal)
                      Precipitation as                         ^
                      silver oxide, silver sulfide, <-----\    |
                      silver selenide, etc.                \   |
                                                             \  |
            oxidation                             migration  \ |
Ag metal  ------------------->  Ag+ (silver ion) ----------->  Ag+
  ^         (humidity,          |                           (migrated)
  |          oxidizing gases)   |--------------->|
  |                             |                |
  |                             |                |
  |                             |                |
  |                             |                |
  |                             V                V
  |<-------------------------Re-reduction      Precipitation
                              to Ag metal      as silver oxide,
                                               silver selenide,
                                               silver sulfide, etc.

The diagram shows that before any silver deterioration can occur, silver
*must* be oxidized into silver ion.  Even air and moisture can act as a
strong enough oxidizing combination to cause damage.  I should also note
that there is always a small amount of ionic silver in equilibrium with
with silver metal in a photographic image.  Reducing agents that will
convert this small amount of ionic silver into silver metal can also
cause silver deterioration because La Chatelier's principle (silver
metal tends to be oxidized to restore equilibrium).  Free silver ion is
then reduced somewhere (where the image isn't supposed to be) and image
silver disappears.

Anyway, I digress.  The silver metal of image (on the left center of the
diagram) is first oxidized into an invisible silver ion. It may then be
re-reduced back into silver, precipitated as an insoluble (and therefore
immobile) silver compound, or may migrate.  The re-reduced metal is back
to step one, where it may be reoxidized.  Note that the precipitation
route is a dead end.  Once silver goes this direction, it is unlikely to
change any further.  Migrated silver ion may also go through the same
possibilities and un-migrated silver ion.  It may re-reduce back into
metallic silver. As silver ion, however, there is not any silver
necessarily in the area to join with and therefore, the silver generally
forms as colloidal silver (orangish in color), as a silver mirror (on
the top surface of the emulsion) or as redox blemishes (red spots)--all
are forms of colloidal silver possibly with other things.  The reduced
silver may again be oxidized (in fact colloidal silver should be
generally more susceptible to oxidation than the larger silver image
material because of the larger surface to volume ratio in colloidal
silver.)  The migrated silver ion (and re-oxidized colloidal silver) may
also bond with oxygen, sulfur or selenium to form a stable visible
silver compound.  The oxidized colloidal silver may also re-reduce back
into colloidal silver.   Ultimately the only dead-end is the formation
of stable silver oxide, silver sulfide, silver selenide (etc).  Most
deterioration that we see can be directly explained by the diagram.

What about faded prints.  We see that the Dmin (white) areas tend to get
yellow and darker while the Dmax (black) areas turn brown and get
lighter. The dark areas of silver oxidize and migrate.  Silver that is
re-reduced in these areas tend to be smaller (more colloidal-like) and
therefor warmer (browner).  Silver oxide is also browner and less dense
both because of optical properties and because two silver metal atoms
are needed to make one molecule of silver oxide (reduced covering power
of silver.)  Fine detail especially (wrinkles in white clothing, stray
strands of hair and facial detail) show loss of information.  The
migrated silver wanders off into the Dmin areas and tends to form into
either colloidal silver or visible silver compounds thus raising the
density (darkness) of the Dmin areas and also making them yellow.

Ultimately, you should be able to see the difference between natural
sulfiding (say from unwashed prints) and sulfur toning.  In the case of
sulfur toning we will either use a one step process (such as
polysulfide) or add a halide source into a bleach (such as bromide in
the bleach step of sepia toning.)  The reason is that we need the
oxidized silver particle to remain where it is until it can be turned
into immobile silver sulfide.  In the one step process, the silver ion
is immediately (relatively speaking) converted into silver sulfide
before it has a chance to move anywhere.  In the two step process, we
oxidize the silver and precipitate it into yellowish or white silver
halide (usually chloride or bromide).  These silver halides are
relatively insoluble and therefore don't allow the silver ion to move
anywhere.  In the sulfiding step, since the silver sulfide is so much
less soluble than any of the silver halides, the silver halide
preferentially dumps the halide in favor of the sulfur (sounds like a
soap opera).  In either case, the silver image is converted into silver
sulfide right where it is.  In naturally sulfided materials, there is so
much more time for things to happen that much of the silver is oxidized
first and then has lots of time to migrate before converting into silver
sulfide.  Colloidal silver sulfide has even less covering power than
colloidal silver and therefore we see it as fading.  In fact, if you
take a printed out image (gelatin POP, albumen or a salt print) and
heavily sulfur tone it, you will find that it disappears.  In addition,
the silver has migrated and therefore, the original image has now been
"smeared" all over the photograph (negative or print) and therefore is
also virtually invisible.   In bleach and redevelopment experiments
performed at the National Archives of Canada, it was found that there
was a limit to how badly faded a photograph could be before it was no
longer possible to reconstitute it.  Once the filamentary silver bundles
were smaller than a certain size, the colloidal silver could no long
rejoin on and the print disappeared into oblivion.  Similarly, the
Buffalo program was making neutron activation autoradiographic copy
negatives from severely faded albumen prints.  The radioactive silver
will expose a piece of x-ray film pressed firmly in contact with it
regardless of the form of the silver (silver ion, visible silver
compound or silver metal).  What they found was that no matter how much
pressure they put on the x-ray/albumen sandwich, very badly faded prints
made out of focus negatives--again because the sharp silver image has
migrated (wandered around) making the "image" blur.

So the answer to the question about natural sulfiding versus toning is
that with natural sulfiding, the silver has the opportunity and the time
to migrate and therefore make the image disappear.

-Doug
Image Permanence Institute

                                  ***
                  Conservation DistList Instance 7:13
                 Distributed: Wednesday, July 21, 1993
                        Message Id: cdl-7-13-002
                                  ***