Both in gum printing and in Pt/Pd printing I have found that a long slow
exposure tends to give enhanced contrast and more subtle gradation. In the case
of gum there is also a improvement in the ability of the image to take
manipulation.
There does not appear to be an obvious explanation for this phenomenon but it is
striking that here is another cooking analogy that holds true.
I use the sunbed to print with in class for speed and convenience but for the
reasons I mention in para 2, I use a graphic arts high pressure lamp for my
personal work.
Terry