Black and white vs color photography : temperature requirements for developing.
Nathan Martz
I am interested in the chemical differences between black and white and
color film/print developing. Black and white film chemistry can be kept and
used at around 68 degrees C, but color film requires temperatures around
97.8 degrees C. What is responsible for the different temperature
requirements? Is it a kinetic or a thermodynamic issue? Or something
completely different?
Thank you,
Nathan Martz
William R. Penrose
The "developers" (sic) of the color process could probably have
optimized the chemistry for any temperature they chose. Since color
development is much more critical of temperature than B&W, they had to
choose a temperature that, once chosen, would have to be maintained
exactly by users of the process.
1. It is easier to build a temperature-controlled water bath that
operates somewhat above ambient, than one that operates very close to
ambient. The latter would need a refrigeration unit as well as a
heater.
2. By choosing a warm temperature, they would speed up the development
process, a big help to commercial labs where time is money.
3. If the bath was too hot, there would be evaporation and changes in
concentration, as well as decomposition and oxidation of some of the
quinone reagents.
So 97.8 C seems like a good compromise.
Bill
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Richard Flynn
Its been a while since I did any photo processing but I seem to remember that B&W
developing solutions were around 70 F and color solutions were around 100 F. 70
or 100 C would probably cause the film to start to melt.
In all seriousness, color processing requires highly precise temperature control
or the color balance will be off. Before the days of 1 hour processing you could
tell cheap processing labs by the off color prints that you got back. Bad color
balance was caused by (among other things) poor temperature control. With the
advent of the automated processing labs you were able to do away with most of the
temperature control problems.
For the very highest quality black and white photos temperature control is
critical. You can effect the apparent film speed in black and white film by
controling both development time and temperature.
Rick Flynn
J Bauer
Richard Flynn wrote:
> Its been a while since I did any photo processing but I seem to remember that B&W
> developing solutions were around 70 F and color solutions were around 100 F. 70
> or 100 C would probably cause the film to start to melt.
Quite correct. I process professionally at 72 to 75 degrees F (~ 22 to 24 C) for
black and white (depending upon film/developer type and speed) and at 100 degrees F
(~ 38 C) for color. 68 degrees F was the old standby for black and white, and still
used by many. Modern films will take the slightly higher temps.
> For the very highest quality black and white photos temperature control is
> critical. You can effect the apparent film speed in black and white film by
> controling both development time and temperature.
Also true. Agitation is also very important.
Remco Viëtor
William R. Penrose wrote:
>
> Nathan Martz wrote:
> >
> > I am interested in the chemical differences between black and white and
> > color film/print developing. Black and white film chemistry can be kept and
> > used at around 68 degrees C, but color film requires temperatures around
> > 97.8 degrees C. What is responsible for the different temperature
> > requirements? Is it a kinetic or a thermodynamic issue? Or something
> > completely different?
>
> The "developers" (sic) of the color process could probably have
> optimized the chemistry for any temperature they chose. Since color
> development is much more critical of temperature than B&W, they had to
> choose a temperature that, once chosen, would have to be maintained
> exactly by users of the process.
>
> 1. It is easier to build a temperature-controlled water bath that
> operates somewhat above ambient, than one that operates very close to
> ambient. The latter would need a refrigeration unit as well as a
> heater.
agreed, the same reason why chemical data are often quoted at 25 C.
[snip]
>
> So 97.8 C seems like a good compromise.
Shall we make that degrees Fahrenheit? Most films would melt
or dissolve at 97.8 C (probably even at 68 C for that matter,
the emulsion is made of gelatin). This is much more of a
limitation than evaporation and instability of the reactants.
Wet film is already very sensitive to mechanical damage at
room temp., so at higher t, scratches on the film would
be unavoidable.
And the reason that colour film is much more sensitive to
temperature variations is, that the 3 layers react differently
to such variations, leading to colour imbalance. You can get
an idea of the effects (if you have a graphics program
that handles colour corrections) by playing a bit with
the R/G/B balance (or gamma for 1 of the colours).
--
Remco Viëtor Department of Chemistry
re...@chem.gla.ac.uk J. Black Building
University of Glasgow
Glasgow G12 8QQ
U.K.