The groined vault is formed by the intersection of two barrel-vaults (Fig. 47). When several compartments of groined vaulting are placed together over an oblong plan, a double advantage is secured. Lateral windows can be carried up to the full height of the vaulting instead of being stopped below its springing; and the weight and thrust of the vaulting are concentrated upon a number of isolated points instead of being exerted along the whole extent of the side walls, as with the barrel-vault. The Romans saw that it was sufficient to dispose the masonry at these points in masses at right angles to the length of the hall, to best resist the lateral thrust of the vault. This appears clearly in the plan of the Basilica of Constantine ([Fig. 58]).
The dome was in almost all Roman examples supported on a circular wall built up from the ground, as in the Pantheon ([Fig. 54]). The pendentive dome, sustained by four or eight arches over a square or octagonal plan, is not found in true Roman buildings.
The Romans made of the vault something more than a mere constructive device. It became in their hands an element of interior effect at least equally important with the arch and column. No style of architecture has ever evolved nobler forms of ceiling than the groined vault and the dome. Moreover, the use of vaulting made possible effects of unencumbered spaciousness and amplitude which could never be compassed by any combination of piers and columns. It also assured to the Roman monuments a duration and a freedom from danger of destruction by fire impossible with any wooden-roofed architecture, however noble its form or careful its execution.
CONSTRUCTION. The constructive methods of the Romans varied with the conditions and resources of different provinces, but were everywhere dominated by the same practical spirit. Their vaulted architecture demanded for the support of its enormous weights and for resistance to its disruptive thrusts, piers and buttresses of great mass. To construct these wholly of cut stone appeared preposterous and wasteful to the Roman. Italy abounds in clay, lime, and a volcanic product, pozzolana (from Puteoli or Pozzuoli, where it has always been obtained in large quantities), which makes an admirable hydraulic cement. With these materials it was possible to employ unskilled labor for the great bulk of this massive masonry, and to erect with the greatest rapidity and in the most economical manner those stupendous piles which, even in their ruin, excite the admiration of every beholder.
FIG. 48.—ROMAN WALL MASONRY.
a, Brickwork; b, Tufa ashlar; r, Opus reticulatum; i, Opus incertum.
STONE, CONCRETE, AND BRICK MASONRY. For buildings of an externally decorative character such as temples, arches of triumph, and amphitheatres, as well as in all places where brick and concrete were not easily obtained, stone was employed. The walls were built by laying up the inner and outer faces in ashlar or cut stone, and filling in the intermediate space with rubble (random masonry of uncut stone) laid up in cement, or with concrete of broken stone and cement dumped into the space in successive layers. The cement converted the whole into a conglomerate closely united with the face-masonry. In Syria and Egypt the local preference for stones of enormous size was gratified, and even surpassed, as in Herod’s terrace-walls for the temple at Jerusalem ([p. 41]), and in the splendid structures of Palmyra and Baalbec. In Italy, however, stones of moderate size were preferred, and when blocks of unusual dimensions occur, they are in many cases marked with false joints, dividing them into apparently smaller blocks, lest they should dwarf the building by their large scale. The general use in the Augustan period of marble for a decorative lining or wainscot in interiors led in time to the objectionable practice of coating buildings of concrete with an apparel of sham marble masonry, by carving false joints upon an external veneer of thin slabs of that material. Ordinary concrete walls were frequently faced with small blocks of tufa, called, according to the manner of its application, opus reticulatum, opus incertum, opus spicatum, etc. (Fig. 48). In most cases, however, the facing was of carefully executed brickwork, covered sometimes by a coating of stucco. The bricks were large, measuring from one to two feet square where used for quoins or arches, but triangular where they served only as facings. Bricks were also used in the construction of skeleton ribs for concrete vaults of large span.
VAULTING. Here, as in the wall-masonry, economy and common sense devised methods extremely simple for accomplishing vast designs. While the smaller vaults were, so to speak, cast in concrete upon moulds made of rough boards, the enormous weight of the larger vaults precluded their being supported, while drying or “setting,” upon timber centrings built up from the ground. Accordingly, a skeleton of light ribs was first built on wooden centrings, and these ribs, when firmly “set,” became themselves supports for intermediate centrings on which to cast the concrete fillings between the ribs. The whole vault, once hardened, formed really a monolithic curved lintel, exerting no thrust whatever, so that the extraordinary precautions against lateral disruption practised by the Romans were, in fact, in many cases quite superfluous.