There
are two ways of approaching the use of old processes. Salt prints, platinum,
kallitype and carbon can be used as a straight substitute for modern silver
gelatin printing. This is in fact what they were traditionally meant to
do and they are still a perfectly valid method. However, for present day
workers they may also be employed for more graphic or experimental techniques.
There was a convention at the turn of the century to use gum printing
to overlay images on to platinum or cyanotypes. Steichen and Stieglitz,
amongst others, would use gum to multiple print a negative onto a platinum
image, mainly to reinforce the depth of tone in the shadow areas of the
platinum image. Modern taste now tends to regard this as a waste of good
platinum. You must bear in mind that when you print one process over an
existing image, the negative must be superimposed exactly on top of the
original print when making the second exposure.