Contact copying film to film

I would like to copy some 4x5 negatives to positive. Everywhere I look for information about copying negatives to film, I find information about taking pictures of negatives with a camera. Using sheet film, it seems that I should be able to put a negative in contact with a sheet of unexposed film and make a positive directly. My goal is to be able to make backlit frames to display B&W and eventually color reversal images. How do I calculate the exposure? Am I trying to go about it the wrong way?

Very common, pretty easy, and a way to "fix" contrast while you're at it. Use any film you want, from standard sheet film to "litho" film. Find exposure just as you would with printing to paper -- test strips or a step wedge. Just expose emulsion-to-emulsion.

You now have a second point to play with exposure and development, moving your image up or down the curve and increasing or decreasing contrast.

You may wish to use an orthochromatic film that is only sensitive to the blue end of the spectrum -- so you can work under a dim safelight, but if you're used to working in the dark, anything will do.

I like making interpositives, they are great to look at. Fortuitously, I'll be posting a paper negative photo today of some of my interpositives taped up to my kitchen window. For my Photo of the Day. It's kind of artsy/crappy on fogged paper, but still.

It can be done for black and white under the enlarger. Set one known lens, lens aperture, head height and filtration (none) and use time to fix exposure. Expose strips of film under a step wedge to figure out what the correct exposure and development is. Ideally you want a film with an upswept curve to keep highlight detail in the transparency.

If you use a colour balancing filter, or if a dichroic enlarger, the right filtration, the enlarger can also be used to duplicate colour material.

Per David's advice, Ortho Litho film, in low contrast developer allows visual development to fine tune and get you close before finessing with time vs temperature.
Litho films can also yield a very striaght line, if not upswept curve.

There is an excellent Kodak book on the issue, which I think is called Copying and Duplicating.

I have some "solarised" (Sabat(t)ier effect) negatives in which the images look better reversed, so duplicating to film in this manner might be a way to get a good printable negative. Can anyone suggest a very general rule of thumb for starting times? Given the same light source, how do the exposure times for, say, 100 ASA film compare to those for a typical paper?

There are a lot of variables, of course; I understand that. I'm just looking for some guidance as to whether to start testing exposures at five seconds or two minutes!

Thanks

-NT

assuming paper is asa 3-10 then you have 3 1/3 to 4 2/3 stops closed on the lens. If you were at F5.6 you'd close to F16 to F32 or whatever. There really is no general rule once you account for reciprocity though. Shorter exposures are better.