Re: (lith) film & pyro

Carl Weese (cjweese@wtco.net)
Fri, 29 May 1998 08:45:46 -0400

Cor,

You explained it correctly: the pyro stain *is* "proportional" to the
silver image. Less silver, less stain, more silver, more stain. The
stain is quite opaque to UV light, and so causes the negative to print
with high contrast in a UV-sensitive process like platinum. The same
negative can usually be printed well in silver too, because silver paper
doesn't "see" the stain as strongly. VC papers have an even more complex
reaction because the stain is roughly the color of a low-contrast VC
filter. So highlights (with lots of stain) print with lower contrast
than midtones, and shadows (with almost no stain) print with higher
contrast than midtones.

Exposure for platinum is dependent on light source to some extent. Pyro
negs seem to need _at least_ a stop more exposure on my UV light source
than the same film processed for platinum in HC110. A couple tests in
sunlight seem
to indicate only a half-stop or so of speed loss.

---Carl