Eleanor
We had a structure at Jamestown that was occupied from Ca 1645-1665 that
had two brick hearths in the bottom of an English basement. There was baked
subsoil an average of 6" behind the brick lining. We also made the mistake
of burning a Chimnea (one of those portable clay fireplaces) one evening on
a patch of subsoil at the dig. One fairly short lived fire of a couple of
hours and the heat penetrated the clay bottom of the thing and cooked our
subsoil to a nice salmon red.
Eric Deetz
Jamestown Rediscovery