About the year 80 B.C. the method known as _opus reticulatum_ was introduced, in which the bricks presented square faces (about 4 inches each way), and were arranged diagonally to form a network pattern (Fig. 187). At Pompeii the _opus reticulatum_ dates from the time of Augustus; it is laid on concrete, and the bricks are small four-sided pyramids with bases 3 to 4 inches square.[2435] This method lasted down to about A.D. 130 in Italy. It should, however, be noted that it was commoner in stone than in brick, the latter material not having come into general use for building at the time when it was employed.[2436] But even when tufa was used for the reticulated work, bricks or tiles were used for quoins at the angles, and for bonding courses through the walls, as well as for arches and vaults (Fig. 188). This combination of _opus reticulatum_ and brickwork is well illustrated in the palace of Caligula.[2437] In the case of vaults, indeed, the use of brick seems to have been general, as in the baths of Caracalla, and many other buildings (cf. Fig. 189). Vitruvius[2438] advises the use of _tegulae bipedales_ to protect the wooden joists over the vaults from being rotted by the steam from the hot bathrooms; they were to be placed over the whole under-surface of the concrete vault, supported on iron girders, which were suspended from the concrete by iron clamps or pins. Over the whole was laid a coating of cement (_opus tectorium_) in which pounded pottery was the chief constituent, and this was stuccoed.[2439] [Illustration: FIG. 189. CONCRETE ARCH; HALF WITH BRICK FACING REMOVED.] The _opus mixtum_ (the term is not classical) prevailed regularly under the later Empire, from the fourth to the sixth century; the earliest example which can be dated is the circus of Maxentius. It is also used in work of the time of Theodoric.[2440] The method of construction is shown in Fig. 190. The reason for the limited use of brick in Rome may have been the scarcity of wood for fuel for the kilns. But in any case the pointed backs of the bricks made a good bonding with concrete, and presented a large surface with a comparatively small amount of clay. The secret of the wonderful durability of Roman buildings is that each wall was one solid coherent mass, owing to the excellence of the concrete. In the Pantheon the concrete of the dome is nearly 20 feet thick, the brick facing only about 5 inches. The character of the brick facing often indicates the date of a wall, the bricks in early work being thick and the joints thin; later, the reverse is the case. But caution must be exercised in dating on this principle, owing to the great variety of methods employed during the same reign, and even in the same building.[2441] [Illustration: From _Blümner_. FIG. 190. DIAGRAM SHOWING CONSTRUCTION OF WALL OF OPUS MIXTUM. ] * * * * *