Why do you want different negatives? Usually, multiple printings with
gum are done using the same (continous tone) negative with different
exposures. For example, the first printing is the longest exposure and
deposits color everywhere but the highlights. A second layer of gum
emulsion is exposed for somewhat less time and deposits a layer of
color in the midtones and shadows, a third, at the shortest exposure
deposits color only in the shadows. With an average contrast negative
(i.e. prints well on #2 silver paper) three printings should yield a
fairly full scale print. I've done up to six printings for a fairly high
contrast negative.
Remember, that a dichromate emulsion has a straight line response to
exposure and that it can yield only a fraction of the tonal scale that
a silver emulsion can. Hence the need for multiple printings. Of course,
if you are interested in some other effect for which you actually need
different negatives with different contrast ranges (it wasn't clear
from your mail) then ignore the above.
As for film, I always had good luck with the Kodak direct duplicating
film (it was designated SO-015, but that was a long time ago). It tends
to run a bit contrastly, but offers the convenience of going directly
from a negative to a negative. Recently, my wife has had good luck making
enlarged negatives for platinum printing with an Agfa film sold by the
Palladio company in Cambridge, MA (USA). Don't know the specific
designation, but it appears to be very forgiving of exposure and
development variables. However, it does require an intermediate positive
step. You can get somewhat continous tone results from litho film
using continous tone developers, but the result produces an obviously
different sort of print from conventional negative material. That may
or may not suit your images.
Hope this helps
Carson Graves
carson@ileaf.com
>
> Scott
>
> --
> Scott L. Robertson
> Graphics, Visualization and Usability Lab
> Georgia Institute of Technology, Atlanta Georgia, 30332
> slr@cc.gatech.edu
>