Re: pyrogallol & pyrocatechin

From: "Gordon J. Holtslander" <holtsg@duke.usask.ca>
Subject: pyrogallol & pyrocatechin
Date: Fri, 02 Apr 2004 21:15:40 -0600

> I use a low pH phenidone ascorbic acid developer to produce continuous
> tone negatives from ortho lith film. It works well, but suffers from slow
> speed and poor shadow detail - its essential to pre-flash to get any
> shadow detail.

I've found that problem with some films, particularly Freestyle APH
film. Kodalith Ortho 6556 Type 3 or Ektagraphic HC gives better speed
and also higher Dmax at low gamma. I think APH type films need to be
processed in more vigorous developer (such as D-19 or D-72/Dektol) and
accept high contrast.

> I've been intending on experimenting with adding pyrocatechin to the
> developer to see if it resolves some of these issues. pyrocatechin is
> superadditive with phenidone, which may increase the speed.

Catechol (more modern name for pyrocatechin) is superadditive with
phenidone, yes. But what "superadditive" means is that the "rate of
development" of phenidone plus catechol is greater than each agent's
rate of development added together. This may say something about
time-contrast relation in the practical situation, but it doesn't
imply anything about useful speed out of a particular emulsion.

Also very consistent with above, in my experience, effective speed you
get from phenidone developers is not influenced by such addition, as
long as enough superadditive developing agent (in your case ascorbate)
is present.

One virtue of ascorbic acid as seen in fine grain film developers like
my DS-10 and Kodak XTOL is that ascorbic acid is present in monoanion
(active) form even at the neutral pH, due to its low pK1 compared to
hydroxybenzene agents like hydroquinone, catechol, and pyrogallol, all
of which having high pK1. Catechol has slightly lower pK than HQ, but
still it'd be inefficient at pH 8.0 (DS-10) to 8.2 (XTOL) range.

> I've also read (where I don't recall) that
> a pyro was favoured in the past as a developer that exposed shadow detail
> well. I don't remeber if it was pyrogallol or pyrocatechin.

That statement must have come from around 1900. People were discussing
which of pure bromide plates or iodobromide plates were superior for
such and such applications (portraits, usually). Before organic
developing agents like hydroquinone and pyrogallol came to wide use,
ferrous sulfate or later more commonly ferrous oxalate developer was
used. These developers would give much lower speed, and would have
significant difficulty developing plate emulsions containing
iodide. So, pyrogallol was said to give superior shadow details at
that time, but those statements were made in the context of ferrous
salt developers.

> My question is to what extent can I assume pyrogallol and
> pyrocatechin will produce the same results in this situation. I
> should probably assume nothing :(

Pyro-soda developer would be an appropriate thing to use if you are
trying to reproduce the plates from 1900-1920's with homemade
emulsions, but for practical purposes, well formulated phenidone-based
developers would give much superior results, in particular relation to
shadow details.

If you or anyone is interested in history of developing agents and
their evolutions in practical formula, from practical aspects put in
parallel to studies on mechanisms of development, Levenson has
published a couple of excellent review articles from 60's to late 80's
and I can get you the reference info next week. Henn has written some
good ones as well, but these are more chemistry oriented. (No, none
of them is in PDF files.)

--
Ryuji Suzuki
"All the truth in the world adds up to one big lie." (Bob Dylan 2000)