The ol' bloomery vs. tap slag issue is one that archaeometallurgists
have been arguing over for years and will probably be arguing over....
 
Some references to try are:
 
Rostoker, William.
       Pre-industrial iron : its technology and ethnology / by William
         Rostoker and Bennet Bronson.
       Philadelphia, Pa. : Archeomaterials, 1990.
       Archeomaterials monographs ; no. 1
 
The Industrial revolution in metals / edited by Joan Day & R. F.
         Tylecote.
       London : Institute of Metals, 1991.
       Tylecote, R. F.
       Book (Institute of Metals) ; 456.
 
Tylecote, R. F.
       The prehistory of metallurgy in the British Isles / R.F. Tylecote.
       London : Institute of Metals, c1986.
 
 
The other thing you could have is, a bunch of pottery that has been
agglomerated due to a high heating process.  Until a good cut-up
is done to look at the microstructure of the "slag", its really
hard to say conclusively what was done.  Glass making slags, or rubbish heaps
that have glass in them that have been heated also produce "slag" like
materials.
 
Later, Mark Hall  [log in to unmask]
Archaeological Laboratories
UC Berkeley