[pure-silver] Re: Using a selenium toner solution to test for residual silver
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Sat, 10 Jul 2010 15:52:52 -0700
----- Original Message -----
From: "Tom Kershaw" <tom@xxxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Saturday, July 10, 2010 3:42 AM
Subject: [pure-silver] Re: Using a selenium toner solution
to test for residual silver
Richard,
On Thursday I processed several full sheets (with no
exposure, aside from a '902' type ILFORD safelight) of
different paper varieties through developer, stop, and a
two bath fix, both diluted @ 1:9; then washed for 45
minutes in a Nova printwasher without an HCA stage. My
intention being to test for both residual silver and
residual fixer (with the standard test) over the full
sheet.
The use of selenium toner as a test for residual silver
does bring into question what kind of processing regime
one should use if selenium toning is included. Should one
try and get all residual fixer out of the paper before
toning in KRST or any other Se toner, which to my
knowledge all contain Ammonium Thiosulphate and will
therefore introduce additional compounds that need to be
washed out of the paper; or is a 15 minute wash between
the 2nd fix bath and the Se toner, and then washing for
the full time (e.g. 45 minutes, depending on testing) more
likely to prove an efficient and effective washing regime?
Out of interest, what is your view on the more recent
practice of using a two bath fix regime with both baths
diluted at 1:4, processing for 1 minute in each bath? -
Aside from increased sales for the manufacturer (ILFORD
HYPAM in my case), I'd have thought one would start to get
into issues with excessive silver concentration in the
fixer for fibre base processing, while at the same time
possibly throwing out solutions with remaining high
activity. One user on APUG advocates a single 1:19
dilution fixer for fibre base printing used fresh and
one-shot but this doesn't seem a very time efficient
technique if making a large number of prints.
Tom
There are a couple of parts to this, both have to do
with the mechanism of fixing and washing.
The rate that thiosulfate washes out of _emulsion_ is
affected by the pH of the emulsion and whether there is a
white alum hardener present. Gelatin has on characteristic
pH, it is what is called an amphoretic, that is, it takes on
the pH of the last solution it was in. However, it does have
a sort of preferred pH. This is the pH at which the swelling
is least and is called the isoelectric point. At this pH the
total charge in the gelatin molecules is neutral. For most
photographic gelatin the isoelectric point is slightly on
the acid side of neutral.
Now, the net charge on the molecules is important
because it determines weather other molecules and ions will
be attracted or repelled. When acid, as gelatin is after a
hardening fixing bath, the net charge is such as to attract
the thiosulfate ions and fixer reaction products and bind
them. In order to release these ions its necessary to adjust
the pH to the point where the net charge is such as to repel
these ions. For photographic gelatin this is about neutral
or alkaline. This is why AGFA and others used to recommend
treating film and paper in an alkaline bath before washing.
Now, there is another effect of pH, and that is the
relationship it has to the "mordanting" effect that white
alum (potassium aluminum sulfate, the common hardener) has
on thiosulfate and fixer reaction products. It causes them
to be bound quite tightly to the gelatin. Treating the
emulsion in an alkaline bath will cause this mordanting to
be ineffective and so release the bound ions, however, it
will also undo the hardening. If the gelatin is brought to
about neutral pH the mordanting is made ineffective but the
hardening is still effective. So, there is an advantage to
making the gelatin neutral rather than alkaline.
Kodak used these effects in designing its Hypo Clearing
Agent, which is buffered to neutral pH. It has another
effect, that is its an ion exchanger for thiosulfate. Kodak
found that sulfite has this effect and will greatly
accelerate the diffusion of both thiosulfate and fixer
reaction products from the emulsion. However, it has much
less effect on the thiosufate in the fibers of unprotected
paper support although it accelerates washing there also.
This is because the washing of the support is not by a
simple diffusion process as it is for the emulsion. The
fixer soaks into the paper and is held by frictional forces
rather than chemical forces to the fibers. It takes time and
agitation to remove it although, again, the ion-exchange
effect of sulfite does help.
Ilford made some measurements that determined that the
uptake of fixer into the paper fibers was minimal if fixing
time could be limited to one minute or, preferably, to
thirty seconds. When done there is a minimum of thiosulfate
in the paper fibers. Ilford based their "archival fixing
method" on this effect. They combined very short fixing time
in strong fixer with a long soak in a sulfite wash aid
followed by relatively short washing time. The method works
but has the drawback that many printing papers will not fix
out rapidly enough. The absolute limit is around two minutes
after which there is no advantage whatever to the method.
The measured residual thiosulfate after using Kodak's
conventional processing with a longer fixing time, a three
minute treatment in KHCA and a wash as recommended for the
material, is as low as after the Ilford method and does not
have the risk of under fixed emulsion.
BTW, in the short time needed for the Ilford method the
use of a two bath system is pretty much precluded. It uses
rapid fixer at film strength but even this may need five
minutes or more to fix some materials. Acid rapid fixer is
also an effective bleach for metallic silver so fixing time
must be regulated carefully.
Inadequate fixing can have more serious effects than
lack of washing because the conversion of silver halide to a
soluble form by the thiosulfate is a staged process and
needs enough free thiosulfate ions to reach completion.
Ilford estimates the capacity of a film strength rapid fixer
to achieve archival fixing of paper is only ten sheets of
8x10 paper per _gallon_. A two bath system will have
anywhere from four to ten times this capacity. The use of
KHCA will allow the removal of some incompletely fixed
reaction products thus effectively increasing fixer
capacity.
Insoluble reaction products left in the emulsion will
eventually decompose damaging the image. They begin changing
right away so that after perhaps two weeks re-fixing is not
effective.
For fiber paper archival fixing and washing can be
achieved by using either a conventional or a rapid fixer in
a two-bath system adjusting fixing times so that a sulfide
or KRST test shows no residual silver, the use of KHCA as
recommended, and washing as recommended. Ilford's current
wash aid appears to be identical to KHCA. There is a rumor
that Kodak intends to discontinue KHCA but the formula is
well known and it can be mixed at home although it requires
two sequestering agents which may be difficult to obtain.
BTW, Kodak research found out c.1961, that a very small
residue of thiosulfate left in an emulsion acted as a
stabilizer giving some protection from oxidation of the
silver by airborne peroxides. This was such heresy that
T.H.James of Kodak was reluctant to publish his research
results and held back until Fuji's research labs published
similar results. While the protection is minimal compared to
proper toning it is still significant and some prints given
heroic washing treatments but not toned have become degraded
while those given routine processing have not. In general
the amount of residual thiosulfate is not enough to
interfere with toning.
Note that the washing out of emulsions coated on
non-absorbent support like RC paper or film, wash out by a
pretty much simple diffusion process which is very much
faster than the washing of fiber paper. Also, fiber emulsion
is generally coated on a hard gelatin substrate with a
barium sulfate reflective material in it called Baryta. This
layer also tends to be hard to wash and is another reason
for the longer wash times required by fiber paper even when
a wash aid is used. The Baryta layer can also hold some
silver halide so that it accounts for the long fixing times
recommended for fiber paper. Film needs longer fixing time
than RC paper partly because the emulsion is considerably
thicker and also because film emulsions generally have much
more silver iodide in them which is slower to fix out than
silver bromide or chloride which predominate in paper
(although variable contrast papers have considerable silver
iodide in them).
Toning is Selenium, sulfide, or gold, will increase
image stability very greatly. While the formerly recommended
treatment with highly diluted KRST has been found to provide
inadequate protection due to its not toning low densities
well, toning is a stronger solution will provide full
protection but will result in some change in the appearance
of the image. Kodak Brown toner and other polysulfide
toners, tone all densities uniformly and may be used where a
minimum of change in the appearance of the image is desired.
Gold toner is the "gold standard" for protection. It tends
to shift the image color toward blue and is a mild
intensifier like Selenium. Its main drawback is cost.
Not all sulfiding toners tone evenly, notably the old
hypo-alum toners. These toners and the two step "indirect"
type Sepia toners, should be used where the image is to be
toned to completion, that is for full color change at this
point the image will be very stable and well protected
against further sulfiding or attach by peroxides.
Note that film emulsions may not turn the same color as
print emulsions, for instance, microfilm treated in KBT
shifts toward blue rather than yellow.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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