[rollei_list] Re: Rolleiflex traveling
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Wed, 5 Oct 2011 17:45:51 -0700
----- Original Message -----
From: "Jerry Lehrer" <glehrer@xxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Wednesday, October 05, 2011 4:36 PM
Subject: [rollei_list] Re: Rolleiflex traveling
Richard,
Don't you mean DK-20, which was known back then to have a
sequestering agent? I found that D-25 or D-"24"
beat it out.
I just looked at some of my 60 year old negatives (Rollei
and Leica developed in Edwal 12 and Edwal 20. To
me, those were the best developers for the films of those
times (I used Plus-X film, mostly)
JERRY
DK-20 was an early extra-fine-grain developer which
relied on sodium thiocyanate as the fine grain agent. It was
replaced later by D-25 because is caused considerable
dichroic fog on many films. Actually D-25 also causes
dichroic fog but less. I've never seen a version of DK-20
with any sort of silver sequestering agent in it. Kodak did
put it up in cans up to I think the late 1940's. D-20 and
D-25 date from 1945. At one time I thought Microdol was
DK-20 but it wasn't.
The 1929 paper describing the properties of D-76
mentions a variation which is very nearly D-20, that is, it
contains only Metol as the developing agent but still has
Borax. D-20 is metol and sodium sulfite with nothing else.
D-25 buffers this to neutral with sodium bisulfite. Metol
will develop even in slightly acid conditions so in a
neutral solution is still active enough to be practical.
Evidently the combination of neutral pH, high salt content,
and the presense of a lot of halide solvent (the sulfite)
results in extra-fine-grain development. Like Microdol-X
D-25 loses about one stop of speed compared to D-76. Also
like Microdol-X when its diluted 1:3 it becomes an acutance
developer and yields full film speed but loses its
extra-fine-grain property.
D-25 also causes dichroic fog. I don't know what
advantage the Microdol formula has over D-25 but evidently
there was something that caused Kodak to adopt it as a
packaged formula.
Previous extra-fine-grain developers mostly relied on
paraphenelynediamine either alone or in combination with
other developers, usually glycin or metol. PPD is itself a
halide solvent. It probably produces finer grain than any
other developer but has very low activity so that when used
alone it looses several stops of speed and had difficulty in
producing reasonably high contrast. PPD derivatives are used
in color developers. PPD is also a strong sensitizer for
alergies. PPD fell out of vogue as finer grained films
became available and as other, less less toxic, developers
became available. The derivatives used in color developers
appear to be less toxic than the original chemical.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
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