do your suggested alternatives intensify the shadows before the highlights like vmi does?
As stated above, the only miracle cure for under-exposure is re-exposure: re-shoot that problem neg if you can.
Intensifiers are characterized as proportional, superproportional, and subproportional. The latter are supposed to increase the lower densities or shadow detail more than the higher densities, which represent the highlights. See Grant Haist,
Modern Photographic Processing, New York: Wiley, 1979, vol. 2, p. 4. However the category an intensifier belongs to isnt always clearly stated, perhaps because it may also depend what stage the process is taken to before the treatment is stopped.
L. E. Muehler and J. I. Crabtrees quinine thiosulphate intensifier seems like it might be the best candidate to try as an alternative to mercury. See Kodak Intensifier In-6 in
Processing Chemicals and Formulas for Black-And-White Photography. Rochester, N.Y. : Eastman Kodak Co., 1977. 7th ed., updated; Kodak publication no J-1. Its also discussed in a lot more detail in Haist, p. 31-34. In summary, Muehler and Crabtree claimed a 900% increase in density, more than any other single-solution intensifier, but only with 1940s coarse-grained emulsionsit only had a negligible or toning effect with fine-grained films. It is also not permanent: the estimate was at least five years at 75 degrees F and 60% RH.
See
Processing Chemicals and Formulas for Black-And-White Photography for Kodak Chromium Intensifier In-4 and Kodak Silver Intensifier In-5, which would probably be considered more proportional than subproportional. This edition still had Kodak's Mercury Intensifier In-1 in it, but I doubt Kodak would want to re-publish it now. They subsequently became much more conscious of safety and pollution issues. Even rinse water used after a mercury intensifier probably shouldn't be sent down the drain.
Incidentally, the stability of pyro is probably similar to the brownish image produced by the In-6 quinone-thiosulfate intensifier. When anybody raises this as an issue, all you get is anecdotes: Ive seen some old pyro negs that seem fine. Maybe pyro stained negs will indeed keep much longer in more optimal conditions, but theyre still inevitably going to fade eventually.
Philip Jackson