It was, Kodak made printing paper that was very similar to the
later RC quite early on. It was made for quick processing. In
general, RC paper does not need wash aid because the emulsions
are so thin (Bob Shanebrook please correct if this is in error)
and the support does not absorb hypo. A paper from Fuji labs and
a later one from Kodak showed that emulsions can be overwashed.
That is that if ALL the residual thiosulfate is removed the image
silver becomes very vulnerable to peroxides in the atmosphere
while a very small residue of the thiosulfate results in slight
sulfiding of the silver which protects it from further sulfiding
and oxidation. Kodak actually discovered this first but was
reluctant to publish it since it was heresy, until the Fuji paper
was published. It may take me a while to find the references. In
any case, Kodak revised their specifications for residual
thiosulfate for archival storage. The instructions for HCA take
this into account.
Note that HCA is most effective for the emulsion. It is less
effective on the baryta layer of conventional (non RC) paper and
on the paper support. The fibers of the paper tend to bind
residual chemicals by frictional forces which is why paper
requires fairly long washes despite the use of the HCA.
The selenium is a puzzle: the use of dilute HCA for a short
period became a standard method of protecting images until one of
the archives discovered it was not working on their microfilm.
The Image Permanence Institute found that something had changed
and the toner no longer provided adequate protection. They never
discovered what had changed. IPI then found that a treatment in
polysulfide toner would provide protection with the same virtues
as the selenium treatment, that is, no visible change in image
color or structure. The latter is important for very high
resolution images such as microfilm. The problem with selenium
is that it does not tone uniformly. The diluted toner will
provide some protection to the low density parts of the image but
not the higher density parts unless toning is carried out to the
point where there is a change in image color or structure. So,
images fully toned in Selenium are archival but it may not be
suitable for microfilm.
I had not realized the suggestion of using adding selenium
to HCA occurred in the patent.
Also note that a great deal of research was done and
published on the subject so the patent may not really have given
much away.
I have somewhere (if its not lost) a booklet published by
Kodak Labs, I think on its anniversary, which talks about the
protection of industrial knowledge at Kodak. It says that while
they had depended strongly on trade secrets in the past the
evolution of processes had speeded up to the point that they
began to rely increasingly on patents. The article talks about
how secretive the different departments were. Manufacturing would
make changes in processes gotten from the research department and
would not say what they had done so research was crippled in
keeping track of results and performance.
I wish I could find this book. I have three storage garages
filled with stuff including many boxes of books. I have recently
been going through some and finding things I forgot I had. I am
sure others on this list have the same experience.
On 3/2/2021 5:54 AM, Bill Riley wrote:
Very interesting.
Thanks for posting the patent.
It read like a scientific paper rather than a typical patient and divulged way
too much information, IMO, for the claim.
One item of interest was the mention of a waterproof medium in example three. I
wonder if that was a hint at what is now known as RC paper which I believe
Kodak introduced later in the sixties.
Regards,
Bill Riley
On Mar 2, 2021, at 12:14 AM, `Richard Knoppow <dickburk@xxxxxxxxxxxxx> wrote:==========================================================================================================To
FWIW, this is the patent for Kodak Hypo Clearing Agent. The PDF is not too
large so I thought I would post it instead of just the patent number.
--
Richard Knoppow
dickburk@xxxxxxxxxxxxx
WB6KBL
<PHOTOGRAPHIC_WASHING_ACCELERATORS.pdf>
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