I've done 35mm slides from 35mm negatives, using Kodak 2302 (B&W movie film for release prints) developed to the highest contrast I could (in stock Dektol).
Given that I have a 6x6 projector, I have thought about doing something similar with 120 negatives.
My first try would be using available Efke Print Film (it's for sale at Freestyle). Contact print under a glass sheet, then develop by inspection in stock Dektol.
There are lith films (APHS) available as well. I've done an 8x10 transparency by projecting a negative onto such a film, then developed. I suppose contact printing could also work.
The first two options are rather cheap; after that, you could go with more expensive products such as ortho films: Rollei Ortho 25 in 120 is available, and Ilford Ortho+, as already mentioned, is available in 4x5 and above.
But the key is: finding a film that will get to a very high contrast. If your average film has a gamma around 0.6, and you need a final slide gamma of 1.5, then by a rule of third, your positive film should be able to develop all the way to an inherent gamma of 1.5 / 0.6 = 2.5, which is extremely high.
Using film since before it was hip.
"One of the most singular characters of the hyposulphites, is the property their solutions possess of dissolving muriate of silver and retaining it in considerable quantity in permanent solution" — Sir John Frederick William Herschel, "On the Hyposulphurous Acid and its Compounds." The Edinburgh Philosophical Journal
, Vol. 1 (8 Jan. 1819): 8-29. p. 11
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