What is called "sponge stone" or "Pompeian pumice" appears to have been reduced by burning from another kind of stone to the condition of the kind which we see. The kind of sponge-stone taken from this region is not produced everywhere else, but only about Aetna and among the hills of Mysia which the Greeks call the "Burnt District," and in other places of the same peculiar nature.... it seems to be certain that moisture has been extracted from the tufa and earth, by the force of fire, just as it is from limestone in kilns.

Chapter VI, Sec. 2-3 - De architectura (The Ten Books On Architecture) (~ 15BC) - Book II