Jordan said:
For ordinary B&W reversal I usually "fog" with an alkaline solution of thiourea. The chemistry is not the same but it is still very consistent. Thiourea is toxic, but easier to handle than hydrazine or dimethylamine-borane. The resulting slides have a pleasing tone (with conventional films -- when I tried TMX the results were sickly-yellow, probably due to the different grain structure).
Alkaline thiourea solution to convert AgX to Ag2S is also a usable technique. One problem is that, because this conversion (toning) process does not involve regular chemical development process, the image forming silver will take the form of sphare-like or tablet-like shapes instead of fine filaments forming steel wool grains. One potential problem here is that, when the grain morphology is sphare-like, the covering power of the grain will be much lower. That is, you may get a lot lower density with the same amount of silver. Also, when the tabular grain has very high aspect ratio, you'll get yellow image. This is probably what you get with TMX. According to a tabular grain expert in the industry (he's now in academia), in tabular grain emulsions, image hue is mostly determined by the grain thickness, not the diameter. Tabular grain emulsions tend to preserve grain shape when developed chemically, and this presents a serious problem when they use tabular grain technology to make document films, because users prefer black images. (Ironically, one of the first t-grain emulsion product was Kodak's microfilm, but at that time, grain wasn't as thin as today's emulsions.) So I can understand that TMX forming yellow image. For document films as well as slow medical films, there are numerous patents that deal with the issue of ensuring good blue-black image with very thin tabular grain emulsions. However, the mechanism of how those work is not understood. (We have ideas of what kind of compounds work, though.)
Another approach is to use a weak solution of alkaline thiourea or any other suitable labile sulfur compound to make Ag2S specks on grains (do a partial conversion) thereby fogging them, and then transfer those fogged films to the second developer bath. By doing this, you are making only small specks of Ag2S on grains, instead of converting the whole grain, but this is sufficient to lower the energy level for the conduction band of the AgX crystal in the neighborhood of the Ag2S speck, where developer can inject electrons, thereby rendering the grain developable in the second developer. This is conceptually similar to purposely overdoing the sulfur sensitization in emulsion making. Similarly, you could do the same with a noble metal toning bath. Au0 specks, for example, does the same to the energy level of the conduction band.
This is getting more academic, but there are dyes that fog emulsions as well. (a lot more expensive approach than other options here.)
In commercial reversal processing, using reducing agents that are a lot stronger than hydroquinone or ascorbate is easier because they can be mixed in a single bath to minimize the number of processing steps. But in a hobbist darkroom, stannous chloride, hydrazine derivatives and DMB are probably more pain in the azz because they are really easily oxidized by air. So thiourea or any suitable labile middle chalcogen compounds could be used in a separate bath prior to second developer. There are a large class of organic selenium compounds useful for this purpose as well (though, again, very toxic).
I've also experimented by mixing a small amount of thiourea in developer. It surely fogs the emulsion and develop them, but I had a bit harder time in obtaining uniform results. I didn't pursue that direction to figure out why.
