The advantage of Thiourea over Sodium Sulfide as a toning agent
is the freedom from 'rotten egg' odor, which is damaging both
to undeveloped photo materials and to YOU. The disadvantage is that
Thiourea is itself not a very nice chemical, being on the list of
suspected carcinogens. This does not mean it is deadly, but merely that
it should be handlesd with caution and respect, as one would any other lab
chemical. Another supposed advantage is that it produces a browner-
looking sepia. But this is entirely dependent on the paper used,
I would guess, since all toners act differently on each variety
of paper.
Hope this helps.
Edward M. Lukacs, LRPS
Miami, Florida, USA