Bleach, redevelopment, like Ian suggested is safe, easy and gives good results.
I use a different bleach than Ian though. Mine is a simple rehalogenating bleach made from potassium ferricyanide and potassium bromide
Try
15g potassium ferricyanide
15g potassium bromide
1 liter of water
or a similar formula with a smaller volume. The proportions are not critical as long as there is enough bleach and bromide to do the job.
Everything can be done in daylight. Toss the neg in the bleach, and watch till all but the very faint stain image is gone. Drain and transfer to the developer. Use your pyrocat or other staining developer. Develop to completion (I use my "Normal" time).
What you are doing is redeveloping the silver (which the bleach changed back to silver bromide) and adding another layer of stain (read contrast) at the same time.
It is possible to repeat this process another time, but the emulsion gets progressively more brittle with repetitions, so try to keep it to a minimum. Each BRD cycle will add another layer of stain and contrast.
I recently salvaged a negative I inadvertently developed at N-2 instead of N+1 with this method.
Edit: I guess I should mention that, as mentioned above, you can often get good prints from an underdeveloped negative. Sometimes I try to print first (if the neg is "on the edge" of being printable by my judgment. If the neg is really too thin, though, I usually just bleach-redevelop during development, when it's easy. Do try to print it first if you think it's possible.
My use for this technique is for salvage reasons, i.e., when I somehow give a negative the wrong development, as in the case above (mysteriously, my filmholders sometimes change positions in the dark all by themselves... either that or the Gremlins switch them...). I also use this technique to simply help get the print I want. Sometimes the extra zip is just what an even well-developed negative needs to make the print sing.
Hope this helps,
Doremus Scudder
www.DoremusScudder.com
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