Mary Beth,
Here is Paul's response:
"Cast-iron" nails are indeed the end result of pouring molten metal
into nail casts. A short discussion of the process appears in Edward
J. Lenik's article 'A Study of Cast Iron Nails" in Historical
Archaeology v.11 (1977), pp. 45-48. A short discussion also appears
in Lee H. Nelson's "Nail Chronology as an aid to dating old
buildings", History news, v.24. n.11 (1968). Since sending out my
query I have found three or four more lcoal sites where casr-iron
nails were found.
Cheers,
Paul Williams