There is also a brief section on Selenium/Tellurium toning in _Theory of
the Photograpic Process_ revised edition, C.E.K. Mees. He has a brief
explanation of the chemistry of the process and states: "Buffered sloutions
of Sodium Selenite in sulfite solution, if used in teh presence of
thiosulfate, form excellent direct toning baths. Selenosulfate is believed
to be the toning agent."
He also refers to a patent by Rehlšnder, USP 1,493,313 (1924) and USP
1,499,749 (1924). When I have the chance I will check on these and find
out what exactly they are.
BTW, Selenium and Tellurium are not metals, heavy or otherwise.