Page:EB1911 - Volume 23.djvu/618

THE ANCIENT CITY] The use of mortar with opus quadratum is a sign of a comparatively early date. It occurs, e.g. in the “Servian” wall on the Aventine and in the Tabularium. Under the Empire massive blocks, whether of tufa, travertine or marble, are set without any

mortar. It must, however, be observed that in these early instances the “mortar” is but a thin stratum of lime, little thicker than stout paper, used not as a cement to bind the blocks together, but simply to give the joints a smoothly fitting surface. The actual binding together was done by clamps and dowels, as well as by the mass and weight of the great blocks used. Except in the

earliest masonry, each block was very carefully fastened, not only to the next blocks on the same course, which was done with double dove-tailed dowels of wood, but also to those above and below with stout iron clamps, run with lead (Vitr. ii. 8). In more ornamental marble work bronze clamps were often used. Concrete is rarely found in connexion with opus quadratum; part of the “Servian” wall on the Aventine received a backing of concrete at a relatively late period. Up to the 1st century it was faced with opus incertum—small irregularly shaped blocks of tufa, 3 to 6 in. across, with pointed ends riven into the concrete while it was soft, and worked smooth on the face only (see fig. 2). From the beginning

of the 1st century opus reticulatum, formed of rectangular tufa prisms laid in a regular pattern like a net (whence the name), is found. It is very neat in appearance, and is often fitted with great care, though it was generally covered with stucco. The so-called “house of Livia” on the Palatine is a good example of the earlier sort, when the quoins were made of small rectangular blocks of tufa. Under the Empire brick quoins came into use (as may be seen, e.g. in the so-called palace of Caligula). Though in Rome opus reticulatum was almost always made of tufa, in the neighbourhood of the city it was sometimes of peperino or even lava, where these materials were found on the spot.

Of concrete walls faced with burnt bricks no dated example earlier than the middle of the 1st century is The facing consisted at first of triangular fragments of tiles (tegulae), broken for the purpose and more or less irregular in shape and size, but from the latter part of the 1st century

onwards triangular bricks were specially manufactured for wall-facings. This shape was adopted in order to present a large surface on the face with little expenditure of brick, and also to improve the bond with the concrete behind (see fig. 4). Even party walls of small rooms are not built solid, but have a concrete core faced with brick triangles about 3 in. long. In order to support the facing until the concrete was set, the Roman builders used a wooden framing covered with planks on the inside. Sometimes the planks were nailed outside the wooden uprights, as was done with unfaced concrete walls, and then a series of grooves appear in the face of the brickwork. Walls faced with opus reticulatum must have been supported temporarily in the same way.

The character of the brick facing is a great help towards determining the date of Roman buildings. In early work the bricks are thick and the joints thin, while in later times the reverse is the case, so that brickwork of the time of Severus and later has more bricks to the foot than that of the Flavian period.

The length of the bricks as it appears on the face is no guide to the date, since one or more of the sharp points of the brick triangles were frequently broken off before they were used. Moreover,

varieties both in quality of workmanship and size of the bricks often occur in work of the same date. In the remains of Nero's Golden House great varieties appear, and some of the walls in the inferior rooms are faced with very irregular and careless brickwork. Special care and neatness were employed in the rare cases when the wall was not to be covered with stucco, which in the absence of marble was usually spread over both inside and outside walls. All these circumstances make great caution necessary in judging of dates; fortunately after the 1st century, and in some cases even earlier, stamps impressed on bricks, and especially on the large tiles used for arches, give clearer indications. The reason of the almost universal use of smooth facings either of opus reticulatum or of brick over concrete walls is a very puzzling question; for concrete itself forms an excellent ground for the stucco coating or backing to the marble slabs, while the stucco adheres with difficulty to a smooth facing, and is very liable to fall away. The modern practice of raking out the joints to form a key was not employed by the Romans, but before the mortar was hard they studded the face of the wall with marble plugs and iron or bronze nails driven into the joints, so as to give a hold for the stucco—a great waste both of labour and material. The quality of the mortar varies according to its date: during the 1st and 2nd centuries it is of remarkable hardness—made of lime with a mixture of coarse pozzolana of a bright red colour; in the 3rd century it began to be inferior in quality; and the pozzolana used under the later Empire is brown instead of red.

Concrete was at first always made of lumps of tufa; then travertine, lava, broken bricks and even marble were used, in fact all the chips and fragments of the mason's yard. Under the Empire the concrete used was made with travertine or lava for foundations, with tufa or broken bricks for

walls, and with tufa or pumice-stone (for the sake of lightness) for vaults. Massive walls were cast in a mould; upright timbers, about 6 by 7 in. thick and 10 to 14 ft. long, were set in rows on each face of the future wall; planks 9 to 10 in. wide were nailed to them, so as to form a case, into which the semi-fluid mass of stones, lime and pozzolana was poured. When this was set the timbers were removed and refixed on the top of the concrete wall; then fresh concrete was poured in; and this process was repeated till the wall was raised to the required height. Usually such cast-work was only used for foundations and cella walls, the upper parts being faced with brick; but in some cases the whole wall to the top was cast in this way and the brick facing omitted. In strength and durability no masonry, however hard the stone or large the blocks, could ever equal these walls of concrete when made with hard lava or travertine, for each wall was one perfectly coherent mass, and could only be destroyed by a laborious process like that of quarrying hard stone from its native bed. Owing to this method of building the progress of the work from day to day can often be traced by a change in the look of the concrete. About 3 ft. appears to have been the average amount of wall raised in a day.

Marble linings were fixed very firmly to the walls with long clamps of metal, hooked at the end so as to hold in a hole made in the marble slab. Fig. 4 gives an example, of the time of Marble Augustus, fixed against a stone wall. The blocks were usually marked in the quarry with a number, and often

with the names of the reigning emperor and the overseer of the quarry. These quarry-marks are often of great value as indications of the date of a building or statue. Metropolitan