The Romans divided the manufacture of objects in clay into two classes: _opus figlinum_ or fine ware, made from _argilla_ or _creta figularis_; and _opus doliare_, for tiles and common earthenware.[2395] We begin, then, as in the chapter on the Greek uses of clay, with the latter division, including the use of this material in Roman architecture, and primarily in the making of =bricks and tiles=. It must be borne in mind, however, that the structural use of bricks of clay, such as we employ at the present day, was unknown to the Romans; they only used what we should call tiles, and even these were only employed structurally, as a facing to walls and vaults of concrete; no walls were ever built of solid brick, and even in those of seven inches thickness the bricks are built on a core of concrete. Nor were the bricks allowed to appear on the outer face of the building, at least before the second century of the Empire; they were always faced with a coating of marble or stucco. Nevertheless, the general use of bricks or tiles was most extensive, and they were employed as tiles for roofing houses, as bricks for walls and vaults, and even for columns, as slabs for pavements, for furnaces and for covering graves, and in tube form for conveying water or hot air; they are found in temples, theatres, and baths, and are used for cisterns and fountains, and in aqueducts and military fortifications. They were called _lateres_, because, says Isidorus, “they were broad, and made by placing round them four boards.”[2396] The kilns were called _laterariae_, and the makers _laterarii_; to make bricks was _lateres ducere_, _fingere_,[2397] or (with reference to the baking only) _coquere_. The word _later_ seems to be employed indiscriminately for sun-dried (_crudi_) and baked bricks (_coctiles_),[2398] without the qualifying epithet, but _testa_ is also used when burnt brick is intended.[2399] The sun-dried bricks were the earlier and simpler form, used for building walls and cemented together with clay or mud.[2400] Vitruvius in his account of brick-making (ii. 3) only refers to this kind, and apparently never mentions baked bricks except in passing allusions. He describes three kinds, to which he says the Greeks gave the respective names of _genus Lydium_, _pentadoron_, and _tetradoron_ (see Vol. I. p. 95). The two latter are exclusively Greek, but the first-named, 1½ by 1 foot in dimensions, answers to the Roman _tegula sesquipedalis_.[2401] A frequent arrangement, he says, was to employ half-bricks in alternate courses with the ordinary sizes, which served to bind the walls together and present an effective as well as a stable appearance. This information is repeated by Pliny, copying almost word for word.[2402]