In the states of Maxilua and Callet, in Further Spain, as well as in Pitane in Asia Minor, there are bricks which, when finished and dried, will float on being thrown into water. The reason why they can float seems to be that the clay of which they are made is like pumice-stone. So it is light, and also it does not, after being hardened by exposure to the air, take up or absorb liquid.... They have therefore great advantages; for they are not heavy to use in building and, once made, they are not spoiled by bad weather.

Chapter III, Sec. 4 - De architectura (The Ten Books On Architecture) (~ 15BC) - Book II