Hi Bob:
There are many different flavors (mixtures) of pyro. I was refering to the
Pyro PMK formula developed (invented, refined) by Gordon Hutchings within
the past ten years.
The first pyro formula was probably invented by Thomas Wedgewood in 1802,
but was not used for photographic (as we know it today) purposes until 1851
by Frederick Scott Archer. You can find more info about pyro in Gordon
Huntchings book, "The Book of Pyro" (probably at your library).
Pyro PMK is more stable than past pyro mixtures, however it is still toxic.
Pyro is a great neg developer as it enhances the edge-effects in the
negative and it does not block the shadows or highlights like other
(HC-110) developers.
Hope this helps.
David P. Milton