Usual apologies for crossposting....
 
        First of all, please allow me to thank everyone who responded to the
original posts on bloomery slag.  A special thank-you goes out to Charles
Keller for actually mailing some samples to the Poplar Forest lab.
        In retrospect, the original post was not entirely clear.  Some
clarifications (for those with long memories) follow....
 
1) we already knew what normal blacksmith slag looks like; we have relatively
large (for our site) deposits of black slag concentrated in areas of the
site.  We also have good reason to believe that a blacksmith's shop was in
the general vicinity, although its location remains unknown to us as of this
writing.
2) the material under question (identified by a second party as possible
bloomery slag) is coarse rather than smooth, and a pale grey with occasional
small (unidentified) inclusions.  It is bulbous, but not in the chaotic way
usually associated with "normal" slag.  Also note that it's concentrating
in a very different area from the "normal" slag (alas, the concentration
leads towards a neighbour's back garden).  Finally, when I noted that at
least one piece had "partially vitrified,"  I meant that one fragment had
literally partially turned to glass rather than just appearing glassy.  If
nothing else, this would at least appear to denote a high silica content
(or was that a case of stating the blatently obvious?).
 
        In any case,  we're still working on this.  If readers of HISTARCH
and ARTIFACT would like me to, I could put together a bibliography on the
subject from the various posts I/we received.  Thank-you again to everyone
for the info - many of the leads have proven extremely useful
 
Alasdair M. Brooks
archaeology lab supervisor
Thomas Jefferson's Poplar Forest