Always interesting, Richard.
Thank you,
*Robert Hall*
On Thu, Nov 14, 2019 at 12:52 PM `Richard Knoppow <dickburk@xxxxxxxxxxxxx>
wrote:
I have been reading the old photographic chemistry book by
Crabtee and Mathews, two of Kodak's top researchers. The book was
published in 1938. It is old enough to have the old charts
showing the rate of fixing vs: fixer concentration. These show a
broad dip in fixing time at a specific concentration. It turns
out this was a blunder in the measurement; the film was dry when
put into the fixer. When thoroughly wetted film was used the
fixing rate continued to increase with concentration until
saturation. This probably had some effect on the choice of fixer
concentration in Kodak formulae but maybe not much.
It is also interesting that the hardening properties of a
fixing bath were stressed over its ability to completely fix out
the image. Hardening was important because the emulsions of the
time were quite soft. In fact, if one looks at recommendations of
the time one finds the standard temperature is 65F. It was
increased to 68F later because the emulsions were made harder.
Ammonium thiosulfate fixers are mentioned but ammonium
thiosulfate was expensive at the time and hard to transport. It
is still mostly supplied as a liquid concentrate.
Ammonium thiosulfate fixer (rapid fixer) has the advantage of
dissolving silver iodide much more readily than sodium
thiosulfate. Many emulsions, including some paper emulsions, now
have enough silver iodide to justify the use of rapid fixer since
the fixing times in sodium fixer can be pretty long.
The book has almost nothing about color photography in it.
Color films of the chromogenic type (Agfacolor, Kodachrome) had
been on the market for only a couple of years. Earlier films like
Autochrome and Finley color, are really combinations of various
methods of combining black and white emulsions with built in
filters. Actually except for the early Kodachrome which a
lenticular film using a filter over both taking and projecting lens.
More tidbits as I encounter them. This is still a very
usable book in many ways. A great deal of the stuff was carried
over to other Kodak publications for many years.
We have better films, better developers, better fixing baths
now and a better understanding of how the process works. But the
actual photography and darkroom procedures have not much changed.
--
Richard Knoppow
dickburk@xxxxxxxxxxxxx
WB6KBL
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