Not exactly a great citation, but the Wikipedia article on the history of heat suggests that their understanding was not very good.

For what it is worth, marginally related ideas seemed to be going around at least by the end of the sixth century since I seem to recall an argument in the Talmud about whether or not transfer of heat by itself from a non-kosher thing to a kosher thing could make the kosher thing non-kosher. But it is possible that I was reading that with a too modern perspective. I'll try to track down the section and see what it says.