The manufacturers of clay pots recommend that they always be placedin a cold oven.
 

The manufacturers of clay pots recommend that they always be placed in
a cold oven.
However, in an emergency, I have occasionally put the pot into a pre-
heated 350F oven
and it did not crack nor did the food come to any harm. It is also
possible to reheat food
in a clay pot. Soak the lid in cold water for 10 minutes, cover the
pot and pop it back into
a 350F oven for 30 minutes.