The chemical is thiocarbamide (it has many other names as well, but I don't
have my reference literature at hand). "Thio" is normally organic sulfur,
and most brown toners use sulfur compounds to change the black silver image
to the brown silver sulfide.
Bleach and redevelop toners are just that. Today, the easiest way to make
the necessary solutions is to use color bleach (NOT bleach/fix!) to change
the silver image to a silver halide, wash, then use any of a number of
formulas for the redevelopment. For example, almost any soluble sulfur
compound will form a brown tone. If you're really interested, you might
check out the 1950 American Annual of Photography, which has a comprehensive
article entitled "A Study in Brown Toning" by some guy called Sil Horwitz.
If anyone is interested in specific formulas, please contact me with exactly
what you're looking for, using my personal e-mail address silh@iag.net.
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net