Different temperature coefficients for different developing agents
Since I begun developing my own films I've used the tables and graphs published by Kodak, Agfa or Ilford, to find the correct developing time if my temperature differed from the standard 20 deg C. Last week I stumbled upon a very comprehensive, but old book (1956) on photo chemistry. (A swedish book called "Fotografisk kemi" by Artur Boström.)
To the interesting part: The author doesn't give a lot of explanation on this, but there seem to be a difference in how various developing agents react to a change in temperature. This was quite a revelation to me. Not that it doesn't make sense, because it does. It's just that noone have really pointed this out before (to me that is). I havn't really had any problem with this either, but on the other hand, I didn't use Pyro and Pyrocat developers until the last 3-4 years.
The facts from the book are (roughly translated from swedish):
"In general you state the temperature dependency by giving the temperature coefficent, i.e. that number which tells how much quicker the process is at 10 deg (C) rise of temperature. The more temperature dependent a developing agent is the larger the temperature coefficent. This is for:
(A table feature in the editor would be nice. :) )
Anyhow, it's the last line in the table which makes me react. Even though e.g. Pyrocat HD uses a mix of Hydroquinone and Catechin, the table above gives me a hint that Pyrocat developers are more sensitive to a change in temperature than e.g. D-76. Also, Pyrocat-HD should more sensitive than Pyrocat-MC.
Do I have any question? Yes, as I first don't really get how to use the coefficent in practice. D-76 should be a standard Metol-hydroquinone developer with a coefficent of 1.9, but looking at e.g. the Ilford table they seem to use a higher coefficent (like 2.3 or so). Now you might say that I got my facts wrong, which is possible, but given the quality of the rest of the book, I recon that the author (Mr. Boström) does have the facts right.
It would be nice if some of our knowledgeable experts on the subject of developing agents would like to cast some more light on this issue.