[pure-silver] Re: Temperature Rule of Thumb
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Mon, 28 Jun 2010 20:07:20 -0700
----- Original Message -----
From: "Elias_Roustom" <elroustom@xxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, June 28, 2010 6:11 PM
Subject: [pure-silver] Temperature Rule of Thumb
My darkroom is holding 74 degrees quite nicely these days.
Humidity is such that I don't need a holding bath between
fixing and washing : )
How can I compensate for temperature higher than 68 in my
paper developing? Should I try to cool things down?
Thanks,
Elias
There is a principle in chemistry, the name of which I
have forgotten, for the ratio of temperature to reaction
time, however, because development is not a simple reaction
the time does not have a linear relation to the time. For
one thing the induction time, that is the time between
application of the developer and first appearance of the
image, varies with the nature of the gelatin. In any case,
prints are developed to "completion" that is, to the maximum
contrast the emulsion is capable of rather than to a lesser
contrast is is film. For that reason the development time is
much less critical than for film, essentially you develop
paper until it stops developing. The developer temperature
and time does have some effect on exposure time. In the past
processing temperatures have been limited by the hardness of
the gelatin. For gelatin the term hardness does not mean it
resistance to scratching as in minerals, but rather its
resistance to swelling when wet due to changes in
temperature or the pH of the solution its exposed to. The
harder the gelatin the warmer the processing chemicals can
be without excessive swelling and softening. Old type
emulsions were quite soft, in fact, books up to about the
late 1930's recommended processing at 65F, after about 1938
the emulsions began to be hardened enough in manufacture to
allow raising this to the familiar 68F. Modern color films
and some B&W films are designed to withstand 100F
processing. Paper emulsions, in general, are softer but 75F
is still within the safe limits for most of it. If you find
your development times are too short, you can dilute the
developer a bit. For papers without incorporated developer
the development in most developers should run about two
minutes with four minutes about the limit without risking
fogging. For some fast papers, like the late, lamented
Kodabromide, and for developer incorporated paper the time
runs about one minute. At 75F the times will be somewhat
shorter perhaps as much as 30% but, again, some adjustment
of exposure will usually compensate.
The idea is to expose so that full black is achieved
with your standard developing time. Contrast variation by
adjusting exposure and development of paper is very limited
despite the popular wisdom to the contrary and reducing
contrast will generally result in poor blacks.
Another thing worth mentioning: older literature
suggests matching the temperature of processing baths very
closely to avoid reticulation and other emulsion damage.
This was probably wise at the time that emulsions were very
soft but a couple of degrees variation with modern materials
will have no deleterious effect. It is, for instance, quite
difficult to delibrately reticulate film as a special effect
although it would probably be easier for paper.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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