Processing RC papers

I heard the list call my name. (Apparently so did several other
people who brought these two messages to my attention.)

Darius Himes <dhimes<-a t->midas< . >bwc< . >org> writes

> I would like to know if anyone has had experience with testing fixer
> residue (sodium thiosulphate *or* ammonium thiosulphate) levels in
> Resin Coated photographic papers.
>
> I am concerned about the level of fixer present in RC papers which
> have been run through a black and white print processor (a process
> which takes 1 minute). Our institution is currently using such a
> processor, and these prints are destined for our permanent
> collection of photographs. Establishing just how much fixer residue
> these prints retain seems paramount. Are there any suggestions?

Many people on a few lists have heard me preaching about residual
fixer is not the root of all evil. ANSI has as yet not been able to
establish limits for photographic paper, but film will tolerate a
surprisingly high level of residual hypo. If I may digress to film
here for a moment, the current ANSI standard (IT9.1-1996) sets a
limit of 0.02 grams per square meter of thiosulfate for an LE-500
radiographic film. The LE-500 designation means that it is expected
that the information will be readable for at least 500 years when
stored in accordance to the standards. For microfilms and "other
films" the limit is 0.014 grams per square meter. The standard
refers to research which shows that radiographic films, even with
0.30 grams per square meter (more than 10 times the limit) will take
about 2000 years at 23 C/ 60% RH to increase by 0.05 density units
(status A blue) in the dmin which should be just noticeable to a
diagnostician. Similarly, several tests were run with microfilm up
to 0.15 grams per square meter. Only the lowest level of residual
hypo changed enough to make an Arrhenius prediction (greater than
1500 years to reach 0.1 increase in dmin.)

In our collective experience with residual hypo, we have learned
that a small amount of residual hypo is beneficial to the stability
of the image. Kodak first discovered this back in the mid-1960's,
but didn't know how to tell the public to wash well, but not too
well. Their position was that it was probably better to wash too
much than too little (which I think was a responsible position for
the time.) Fuji also found the same thing and presented it at an
SPSE meeting in 1982. We have seen the same effect with both
microfilm and x-ray film--that higher levels of residual hypo make
the image more resistant to oxidation (from poor quality boxes, air
pollutants, high humidity/air, etc.) This is what we call a
Goldilocks effect since too much residual hypo, as everyone knows,
is not good either.

The traditional method for measuring residual thiosulfate levels is
by methylene blue. In this method, one molecule of thiosulfate
converts two molecules of oxidized N,N-dimethyl-p-phenylenediamine
into methylene blue which can be read on a spectrophotometer and
quantified in accordance with Beer's Law. In the past, most RC
papers contained incorporated developers which interfered with the
test so an alternative method was developed. Because of problems
migrating developing agents, companies have apparently removed the
incorporated developers from a number of the RC papers though.

The iodine-amylose method uses iodine in iodide solution which forms
a complex with +-amylose (fractionated linear potato starch) to form
a deep blue color. Thiosulfate reduces the iodine-iodide and thereby
reduces the intensity of blue complex. The concentration of
thiosulfate is determined by spectrophotometry.

Some labs use the silver-densitometric test which uses a mixture of
acetic acid and silver nitrate. The acetic acid breaks down the
residual thiosulfate (as well as polythionates) to form an active
sulfur which in turn reacts with the silver nitrate to form a stain
of silver sulfide. The stain is measured using a densitometer. The
problem with this method is that a calibration curve is needed in
order to properly quantify the residual hypo. The lower limit of
detectability is also higher with this method than with the other
two methods (as you might expect.)

All of these test methods are described in ANSI/NAPM IT9.17-1993 and
(for the international people) ISO 417-1993. Since you will likely
not know whether or not the paper you are using has an incorporated
developer or not, you will probably need to use the iodine-amylose
method.

-Douglas Nishimura
Image Permanence Institute

                                  ***
                  Conservation DistList Instance 11:28
                Distributed: Tuesday, September 23, 1997
                       Message Id: cdl-11-28-003
                                  ***