For baked bricks the processes must have been much the same, with, of course, the addition of the baking in the furnace. Existing Roman bricks are nearly always of well-tempered clay and well baked; but the clay exhibits a great variety of colour—red, yellow, and brown. The paste is remarkably hard, breaking with an almost vitreous fracture, and sometimes shows fragments of red brick (_pozzolana_) ground up with it to bind it together, and prevent warping. This may be seen in the Flavian Palace on the Palatine, and in an archway in the Aurelian Wall near the Porta Latina. As an instance of varieties of brick found in the same building, Nero’s Aurea Domus may be cited.[2417] The durability of Roman tiles is ascribed to their careful preparation and seasoning, which give them a much longer life than modern tiles; hence they were frequently used up again in early mediaeval buildings and in Romanesque churches in England, as at St. Albans, St. Mary-in-Castro, Dover, and St. Botolph’s and Holy Trinity, Colchester.[2418] During the period of the Republic private houses and public buildings alike were built of unburnt brick in Rome, as we learn from the words of Dio Cassius,[2419] Varro,[2420] and Cicero[2421]; Varro speaks of _domus latericiae_, and Cicero of “the brick (_latere_) and concrete of which the city is constructed.” After the Republican period this material was still employed outside Rome with burnt-brick cornices,[2422] but even this was exceptional. Pliny mentions walls of sun-dried bricks at Arretium and Mevania.[2423] Henceforth, then, burnt brick was employed more and more as Rome grew more populous.[2424] In Vitruvius’ time (the beginning of our era) the materials used for building were stone for substructures, burnt brick (_structura testacea_) for the outer walls, concrete for the party-walls, and wood for the roofs and floors. He explains the cessation of the use of unburnt brick as due to the legal regulations of his time, which prohibited party-walls of more than 1½ foot in thickness, and unburnt bricks could only support one story above them in that size.[2425]