Apologies if I have sent this before - my mail program still doesn't
understand this list.
I used to develop platinum prints in warm potassium oxalate in an enamel tray
on the top of a very low flame on a gas cooker. An electric hotplate would
have been equally satisfactory. My photographic tray warmer is too low power
for the job.
I don't think enamel trays are sold for photographic use nowadays - mine came
from a scientific supplier, but I got it 20 years ago. Enamel is less reactive
than stainless steel and can be used for all photographic solutions.
Potassium oxalate is not to any extent volatile and there were no signs of any
decomposition at the temperatures I used, so there were no dangerous fumes.
Peter Marshall
On Fixing Shadows and elsewhere:
http://faraday.clas.virginia.edu/~ds8s/
The Buildings of London: http://www.spelthorne.ac.uk/pm/index.htm