The reasoning behind a flat interpositive is that you don't want to lose any of the detail that the original camera negative contains. I personally do not use lith film for my interpositive. The reasoning for using 100 ISO pan film is that it is a lower contrast material then the lith film. Additionally 100 ISO pan film is easier to control exposure then a 400 ISO material (for instance). I also do not use Dektol for my interpositive. The reasoning is that this is a much more active developer then conventional film developers. The use of these materials at the interpositive stage will hinder producing a flat interpositive in my experience. This by virtue of their inherent characteristics. If you stop and think about it why would one use high contrast film and developer to produce a low contrast (flat) interpositive. That makes no sense to me.
Originally Posted by mobtown_4x5
I have a print of Aggies in which she did an 8X10 enlarged neg of a medium format camera negative. She had to bleach the Azo back extensively to get detail into areas of the print. Azo does require a lot higher DR then conventional negatives for enlarging. But in this particular case apparently the DR of the enlarged negative was beyond the scope of the material's (Azo) characteristics. Azo requires appr .50 log units density increase over a conventional negative designed for purposes of enlarging.
I have no problem using dilute Dektol on lith film in the enlarged negative stage of the process. My normal dilution for Dektol will be on the order of 1-10 rather then the 1-2 or 1-3 that is normally used in developing paper. This dilution will allow APHS film to perform like a continual tone material. This is the typical dilution that is also used in the production of sharp contrast masks.
I really encourage those that want more information to read the material that Bob Herbst has written and posted on Ed Buffaloe's site. His article is excellent and follows what I have written. The platinum printers that have articles posted on the internet also do not go for lith film in the interpositive stage. Some do not even use lith film for the copy neg preferring instead to use other materials that they identify on their sites. As I mentioned earlier a Google search under platinum will turn up some more material.
The beauty of this is that one can photograph a scene with the intention of enlarging the negative. One can then later produce an enlarged negative that will fit the density range of the material or alternative process that is desired.