Palladium (but not platinum) will solarize with overexposure, yielding the
muddy brown/bronze tone to which you refer. The best way to avoid this is of
course not to overexpose the shadows. It sounds as if the DMax of 2.1 in
your negatives is too high; try using a negative which isn't so contrasty.
I have found that while a DMax of 2.1 is good for the "typical" palladium
print, the final "contrast grade" ( to use gelatin silver paper terminology)
for palladium varies considerably with the paper. So, I would suggest that
you make exposures with a step tablet to see the most suitable density range
to give you good highlights and adquate shadow detail with your
palladium/paper combination.
If you feel you must overexpose your shadow areas, there is a way to minimize
the solarization in the overexposed areas: after you've coated and dried your
paper, you can humidify it just prior to exposing it. The technique will
also increase the DMax and contrast of your palladium prints, which is the
main reason to do it. I learned about this technique from the instruction
manual accompanying Rob and Sura Steinberg's Palladio paper. The theoretical
basis for it is discussed in Mike Ware's excellent 1986 paper (The Journal of
Photographic Science 34:13-25, 1986). From his paper, it looks as if this
technique will not work on platinum; however, I have not tried the method
myself with platinum chemistry.
After your paper is coated and completely dried, you can use either an
ultrasonic humidifier or a pot of simmering (not boiling) water as a source
of humidity; I prefer the latter. Hold the unexposed paper a few inches
above the vapor source, moving the paper surface evenly over the mist. I do
this about 45 seconds for a small (4"x5") print and about twice that for a
large (8"x10") print. You do not want the paper to become wet; it simply
should lose its stiff feel and become somewhat limp. After humidifying the
paper, wait 4 or 5 minutes before making the print.
Beware of humidifying the paper too much!! Over-humidified paper will stick
to the negative during the print, stain the negative emulsion, and ruin your
negative. I have ruined a couple of good negatives this way. To prevent
negative damage while you're still experimenting with the technique, you can
place a layer of Saran wrap between the humidified paper and the negative
during print exposure.
I do this routinely for all my palladium prints. It gives a subtle but
definite (and aesthetically pleasing) increase in density in the DMax of the
print.
I would be interested to hear from other more experienced workers, such as
Dick Sullivan and Mike Ware, regarding the issues of solarization with
palladium and humidification of coated paper.
Charlie Palmer