Masonry Structures

MASONRY STRUCTURES. — Having abandoned the natural shelter of caves and built rudimentary dwellings, man formed them into settlements, which he surrounded with embankments and ditches to protect people and livestock. In time, to make the defence more effective, he employed rough, scattered stone blocks, poorly fitted and irregularly joined. Later, having learned the technique of cutting stone through the use first of flint and then of metal tools, he erected, from the Neolithic age onward, the first megalithic enclosures of Mediterranean civilization (Crete, Malta, Sardinia, the Balearic Islands), formed for the most part by a basal course of large blocks or slabs, laid horizontally or set upright, above which were courses consisting of several superimposed rows of stones of decreasing proportions.

The progressive refinement of these defensive constructions on high ground, around settlements and sacred sites, together with advances in building techniques and in the extraction and shaping of materials, led, throughout the Chalcolithic and Iron Ages, to the erection of those marvellous colossal monuments of astonishing solidity, made of large irregular polygonal stone blocks joined without cement, solely by being superimposed. Such are the walls of Tiryns and of many localities in Greece, Asia Minor and the Iberian Peninsula. Their grandeur is such that, in later periods, it is no longer possible to conceive of so imposing an expenditure of labour for their construction, and their erection was consequently attributed to the fabulous giant Cyclopes (Cyclopean walls). The technique involved in transporting and superimposing such enormous blocks is astonishing, even allowing for the use

of levers, rollers, partial burial, and inclined planes.

I. PELASGIAN WALLS

A subsequent, more advanced phase of these constructions is that in which the blocks are more carefully worked and levelled, so as to make them fit together and connect them more accurately (the walls of Mycenae in the Argolid). A further phase is that in which the tendency toward regularity is more pronounced; the stones are arranged in horizontal courses and are scarcely different from one another. Such are the walls of Argos, Messene (371 B.C.), Athens, and the Piraeus. These constructions, dating from the centuries between the seventh and fourth centuries B.C., are characteristic of the indigenous populations of Greece and Italy and were called Pelasgian because they were attributed to the earliest immigrants from Greece into Italy, called Πέλαγος. In reality, they belong to the indigenous populations of Greece and Italy of Mediterranean origin; in Italy, to the Italic peoples. Such were the walls erected in various periods, from the most remote times to the historical age, to defend the arces and settlements of the Italic peoples of central Italy (Cortona, Rieti, Alba Fucense, Tusculum, Palestrina, Cori, Segni, Norba, Circello, Terracina, Ferentino, Alatri, Arpino, Atina, etc.).

II. OPUS QUADRATUM

While polygonal construction flourished in the Italic cities whose surrounding terrain was predominantly limestone, in Etruria, Rome, and Latium a still more advanced and aesthetically refined form of wall construction had already been adopted, favored by the presence of tuff, a lighter and more easily worked material. The blocks, of greater or lesser size, are of cappellaccio tuff, at first roughly squared and laid in somewhat irregular courses (remains of walls on the Palatine and Aventine, and at Ardea and Veji); they gradually become more regular, as in the so-called Servian Walls of Rome (4th century B.C.). These consist of large blocks of more compact (lithoid) tuff from the quarries of Fidenae and Veji, laid in alternating headers and stretchers and bearing quarrying marks. The horizontal courses are interrupted by arches in order to give the walls greater solidity. Built in opus quadratum are the tholos cisterns of the Germaio, the primitive walls of the Regia in the Forum, the foundations of the temples of Jupiter Capitolinus, Saturn, the Castores, and Concordia, the Rostra vetera, and the ogival cistern of Tusculum (5th–4th centuries B.C.). If the courses are of equal dimensions, the opus quadratum is called isodomum; if unequal, pseudo-isodomum. The primitive temples have their facings in this type of construction, made of tuff, peperino, or another local stone covered with stucco. Some other buildings of the republican period, including the later period, are also in opus quadratum; the use of travertine (lapis Tiburtinus) becomes widespread, as, for example, in the Tabularium.

III. OPUS INCERTUM

This is the name given to that type of Roman masonry consisting in a gradual reduction of polygonal masonry; it is also called opus antiquum. The core consists of a rubble fill (emplecion, opus concretum) faced with small blocks of limestone as they came from the quarries, unhewn and laid irregularly one upon another, bonded with mortar and completed with chips in the interstices (caementa). This type of masonry belongs to the end of the 2nd century and the beginning of the 1st century B.C., and characterizes the remains of buildings erected and rebuilt during the renewal of Rome and the cities of Latium and central Italy, after the destruction caused by the civil wars (88–82 B.C.), during the dictatorship of Sulla (82–78 B.C.); examples include, among others, the remains of the Emporium in Rome and the Sullan constructions at Palestrina, Terracina, etc.

Opus incertum begins the series of properly Roman masonry techniques, whose robustness, which became proverbial, is due to the cohesive strength of the materials employed: lime of excellent quality and pozzolana.

IV. OPUS RETICULATUM

In the first half of the 1st century B.C., a type of masonry structure appears that represents an improvement upon the opus incertum. The facing is formed of regularly squared wedges in the shape of numerous small truncated pyramids, with their bases on the outer face, arranged in oblique lines inclined at 45° to the plane, so as to give the appearance of a net; the corners are reinforced with bonding blocks of tuff. In the first phase of this work, that is, up to the age of Giulio Cesare, there is the opus quasi-reticulatum, in which

the lines are still uncertain and irregular, and the layer of mortar between the tesserae is still visible. In the Augustan period the reticulate pattern becomes more precise and develops into the predominant type of masonry (the house of Livia on the Palatine, remains of houses beneath the palace of the Flavian emperors); under the reign of Nero it reaches perfection.

V. OPUS LATERITIUM

During the feverish building activity of the Augustan age, the manufacture of bricks (lateres) developed, made from brickmaking clay, fired in a kiln, and square, rectangular, or triangular in shape; at first they were used in the construction of minor buildings and private houses. This masonry may in a certain sense be regarded as a practical derivation of opus quadratum. Under Tiberius, the use of such a facing became common and was employed on a large scale for the first time in public buildings, such as the Domus Tiberiana on the Palatine and the Castra Praetoria. Its use subsequently became increasingly widespread for every kind of building; in bath buildings, brickwork was covered with a waterproof layer of crushed pottery (opus signinum). Already before the Flavian age, and even more so during the second half of the 2nd and at the beginning of the 3rd century of the Empire, there was a vogue for the decorative use of exposed curtain walls made of carved, polychrome bricks—yellow, red, and brown (the tomb of Annia Regilla at the Caffarella, the tombs of the Via Latina). Under Hadrian, brickwork achieved complete supremacy and was made more solid by the adoption of courses of large bipedales bricks (59 × 59 cm) at intervals of approximately five feet (Pantheon, Mausoleum of Hadrian, Baths of Caracalla). For the centering of arches, sesquipedales bricks (45 cm on each side) were used, and for bath hypocausts, bessales (22 cm on each side). Each brick very often bears a stamp indicating the clay quarry (praedia) and the factory (figina, officina), together with the names of the respective owners and managers. The date of issue is commonly mentioned; innumerable lateres bear the date of the year 123 (“Paetino et Aproniano coss.”), an indication of the great diffusion of brick material during the reign of Hadrian. In some cases, the intended destination of the material is also indicated (Castra Praetoria, the port of Claudius, and the port of Trajan at Centumceliae). The most frequent form of the stamps is lunate; square and semicircular forms also occur, and after Diocletian, round and rectangular ones. The criterion for dating brickwork is determined in general by the thickness of the layer of lime between the courses of bricks: the thinner it is, the older the work. It must also be borne in mind that the masonry may appear imperfect even when it is of a good period, if it was intended to be covered with plaster. The arches of the aqueducts bringing water to Rome are built largely of brick, with angular reinforcements in stone.

VI. OPUS MIXTUM

At the end of the 1st century of the Empire, the practice began of combining the two systems, the reticulate and the brick, in alternating layers. This type of masonry structure became common in the time of Trajan and Hadrian. A large part of Hadrian’s Tiburtine Villa is built with walls formed of three courses of bricks alternating with broad layers of reticulate work, the whole reinforced at the corners with brick bonding. At the end of the 2nd century, under the Antonines, a characteristic carelessness occurred in the execution of the reticulate work, so that one has the impression that in that period the opus incertum of the Republic had come back into fashion. A reticulate structure deteriorated in this manner, reinforced with reused caementa of varying sizes, is found in walls dating from the end of the 2nd, the 3rd, and also the 4th century, either alone or alternating with courses of bricks, another type of mixed work.

At the end of the 3rd and the beginning of the 4th century, important brick constructions were still being built. A fairly closely set curtain wall is presented by the circuit of Rome, begun by Aurelian (275). Other imposing brick constructions in Rome are the Baths of Diocletian, the Basilica of Maxentius, the Sessorianum, etc. In the Late Empire, a type of masonry work already little used since the end of the 2nd century gained the ascendancy: horizontal bands of fired bricks alternating with layers of squared blocks of tuff or stone (Villa dei Quintili, Circus of Maxentius on the Appian Way, a. 310). Over time, this structure appears increasingly hurried and imperfect. The layer of mortar between the courses becomes progressively thicker; in some cases, as early as the beginning of the 4th century, it is greater in thickness than the bricks themselves. An even more practical and economical system consisted in constructing walls with elongated flakes of tuff or selve, in almost horizontal layers with sharp edges, using little mortar.

The foundations of the basilica of S. Pietro in Vaticano are in banded masonry, as are those of the basilicas of S. Sebastiano, of the martyrs Faustino, Simplicio, and Beatrice on the Via Portuense, and of the title of Vestina (S. Vitale). For the 8th century, there are structures in S. Angelo in Pescheria, ca. 755 (R. Krautheimer, op. cit. in the bibliography, pp. 74–76), and in S. Eusebio, dating from the time of Pope Zaccaria or of Adriano I (772–95; ibid., p. 213, fig. 128).

For the 9th century, there are the structures of the churches of S. Susanna, of SS. Nereo e Achilleo, and of S. Stefano Maggiore in Vaticano, dating from the time of Leone III (795–816) (R. Krautheimer - W. Frank, Recent discoveries in churches in Rome, in American Journal of Archaeology, 43 [1939], p. 388 ff. and fig. 56), of S. Cecilia (R. Krautheimer, V. op. cit. in the bibliography, pp. 106, 108, fig. 70), and of S. Prassede from the period of Pasquale I (817–24). — Vedi tav. CII.