John
Just reverse the gum process so that you work from dark to light with body
colour white as the highlights.
The basic silk screen process, or photoscreen works the same way ( but I
prefer to work from light to dark using transparent inks).
Scraper board and similar techniqes would work where the bottom colour is
white.
An ordinary silver gelaltine might also display these qualities under
certain conditions as the gelatine is tanned so that the gelatine will not
be as ready to swell, absorb water, say from the atmosphere, where there
is silver in the print.
Terry
Terry King