Building Techniques and Materials | Roman Architecture | Second edition | (Part-02)
Vitruvius notes that the best opus testaceum was made out of old roofing tiles (de Arch. 2.8.19). When tiles like these were used for walls the flanges were removed and the tile cut into four triangles. Roof tiles were rarely more than 3.5 centimetres thick and were of very fine grain, and bright red because they were baked very hard to make them waterproof. Examples of tile facings are found at Pompeii from 80 bc, the Praetorian camp in Rome built by Tiberius (ad 14–37),13 and the Domus Tiberiana on the Palatine. Baked bricks, which appear as early as 13 bc in the Theatre of Marcellus, were more yellowish because they were not baked for so long; they were 3.5–4.5 centimetres thick, and more porous to absorb the mortar and give a better bond. Not only could bricks be more easily handled than the somewhat clumsy pyramidical-shaped stones used to face opus reticulatum, but they were convenient to manufacture and to transport. Brick-faced concrete also offered considerable advantages over opus reticulatum in terms of speed and convenience of construction. The bricks came in three main sizes: bessales, 19.7 centimetres square; sesquipedales, 44.4 centimetres square; and bipedales, 59.2 centimetres square. The bricks were cut into triangles for wall facings and rectangles for arches. Bessales were cut into two triangles with sides approximately 26 × 19 × 19 centimetres. They were used especially at the time of Claudius, Nero, Vespasian, Titus, Trajan and Antoninus Pius. Sesquipedales were cut into eight triangles, 31 × 22 × 22 centimetres. They were used especially under Domitian and Hadrian. Bipedales, cut into 18 triangles, measuring 28 × 19 × 19 centimetres, were used only under Domitian. The resultant triangles were of very similar dimensions to those of bessales and can be recognised only by the two cut sides, instead of one. Various cutting methods were used. They could be scored and then broken, in which case the visible surface was uneven. From the time of Claudius up to the time of Hadrian the edges were often smoothed. They could also be sawn into two, a more accurate method of cutting which was mainly used under Domitian and Hadrian. In any kind of cutting much brick was lost, but it was an economical material in that any waste could be used in the mortar fill.
Bricks were produced in vast quantities in factories throughout Italy, as brick-stamps attest. Recently the factory was found of the two brothers, Tullus and Lucanus Domitius, whose stamp appears on bricks in the Colosseum and the Pantheon. In it were two furnaces and thousands of bricks, as well as dolia (large earthenware containers for oil or wine) which were exported all over the Mediterranean. The factory was located near Bomarzo, 80 kilometres north of Rome, not far from the Tiber, allowing the bricks to be easily transported by barge to Rome and Ostia. Bricks can often be dated by the stamp on them. Tiles and bessales were stamped as early as the first half of the first century bc. Bigger bricks were stamped from the time of Claudius. Stamps became more frequent in Flavian times and very frequent under Hadrian; as many as one in two or three were stamped in some cases. More bricks have been found from the year ad 123 than any other. The earliest stamps were rectangular with a one-line inscription, giving the name of the figulus (brick manufacturer); later they extended to two lines, adding the name of the factory and perhaps the names of the consuls of the year.
Semicircular stamps appeared at the time of Claudius. Under Vespasian the shape became a half-moon, with a very wide internal circle (Figure 4.9). This internal circle became smaller and smaller until by the beginning of the third century ad it sometimes disappeared entirely. The inscription could be one line running around the circle of the stamp, or two or even three. By the time of Diocletian (ad 284–305) stamps could be octagonal or circular. Under Theodosius I (ad 379–395) stamps were circular or rectangular with the name of the emperor
Figure 4.9 Roman brick-stamps. From left to right: time of Vespasian (ad 69–79); Hadrian (ad 123); Severan (ad 193–211).
and his titles. A painting in the tomb of Trebius Justus shows a scene at a building site with masons at work erecting a wall of brick-faced concrete (Figure 4.10). The brick facing has reached about 3 metres in height and two masons are at work, standing on a scaffolding, each building one face of the wall, while a third brings up the mortared rubble to be deposited in the core of the wall. He is climbing a ladder carrying it in a split amphora, while another follows behind. A fifth is mixing a heap of mortar.
The normal procedure seems to have been for a pair of masons to lay a few courses of facing bricks followed by the fill, which consisted of mortar and fist-sized pieces of stone
Figure 4.10 Rome, painting from the tomb of Trebius Justus showing Roman builders at work. (By courtesy of the German Archaeological Institute, Rome).
(caementa). When about 25 courses of brick facing had been laid the wall was capped with a bonding course of bipedales (two-foot bricks) which extended through the entire thickness of the wall. In Domitian’s palace on the Palatine there are between 25 and 28 courses of bricks between bipedales courses with the holes for the next level of scaffolding immediately above. The bricks are on average 4 centimetres high and the mortar joints between 1.30 and 1.40 centimetres thick. Therefore the scaffolding levels were about 1.33–1.50 metres above each other, presumably a comfortable working height for the average mason. The caementa used in the fill vary in density depending on where they were used. The drum and dome of the Pantheon provide a particularly instructive example (Figure 8.9). The fill in the lower part of the drum, which was 6.15 metres wide, was travertine and tufa, and higher up tufa and brick. The fill of the dome also changed as it rose, from brick to brick and pumice, and finally tufa and volcanic material near the oculus. At the same time the envelope of the dome diminished in thickness until it was only 1.50 metres thick near the oculus.
Although baked bricks and tiles were used as early as the first century bc, they did not oust opus reticulatum as the principal method of facing concrete walls until the time of Nero (ad 54–68). The main factors in this change were Nero’s great haste to complete the Golden House and the fire of Rome of ad 64, which created an urgent demand for a cheap, fireproof building material. Brick columns became increasingly common, although they were used as early as the first century bc in Pompeii. Some brick façades were designed to be seen and were not covered with veneer. The upper parts of the hemicycle of Trajan’s Markets, built in the first decade of the second century ad, are a case in point. The Tomb of Annia Regilla, built in the middle of the second century ad, is a particularly splendid example of polychrome brickwork and there are numerous examples at Ostia, notably the Horrea Epagathiana (Figure 4.11). Under Hadrian there was a partial return to opus reticulatum,
Figure 4.11 Ostia, Horrea Epagathiana.
often interlaced with bands of brick and called opus mixtum. The term opus vittatum or opus listatum is given to facings of squared blocks of tufa or limestone, varying in size from 10 to 16 × 22 to 29 centimetres, laid in regular horizontal bands. The technique was commonly used instead of opus reticulatum in the cities of central and northern Italy, for example in the Amphitheatre at Pola (Figure 10.17), the Baths at Fiesole, and the Basilica at Trieste, and in theatres such as those at Iguvium (Gubbio), Volterra, Verona and Saepinum (Sepino). In later work the blocks alternated with bands of brick. Examples include the Palace and Circus of Maxentius in Rome.
Of the various types of vault used by the Romans the simplest is the barrel- or tunnelvault, which is a continuous vault of semicircular section (Figure 4.12a). A cross-vault is produced by the intersection at right angles of two barrel-vaults (Figure 4.12b). The cloister or pavilion vault is also the product of the intersection of two barrel-vaults, but in this case the two barrel-vaults rest on the sides of the square which defines the plan (Figure 4.12c). This type of vault can also be described as a four-sided domical vault. Cloister vaults are used in the so-called Tabularium at Rome. A dome is a vault of semicircular section erected upon a circular base, for example the dome of the Pantheon. A shallower dome of segmental section is called a calotte or saucer dome. If the base is square an intermediate member, a squinch or pendentive, must be inserted to effect the transition between square and circle. A squinch is an architectural member inserted across the four corners of the square to create an octagon on which the dome rests, for example in the Arch of Marcus Aurelius at Oea (Tripoli) (Figure 9.17). A pendentive is a spherical triangle whose curvature is that of a dome whose diameter is the diagonal of the square on which it rests. A sail vault (Figure 4.12d) is related to the pendentive. It is an incomplete dome whose diameter is the diagonal of the square on which it rests. The first true pendentives occur in very late Roman and in Byzantine work, although there is a rough approximation to a pendentive in one of the octagonal rooms on the perimeter of the Baths of Caracalla in Rome. A domical vault is not a dome strictly speaking. Its webs
Figure 4.12 Roman vaults and domes: (a) barrel- or tunnel-vault; (b) cross-vault; (c) cloister vault or four-sided domical vault; (d) sail vault; (e) octagonal dome or eight-sided domical vault; (f) umbrella dome.
rise from a polygonal base and are separated by groins (Figure 4.12e), as in the lower part of the domical vault covering the octagonal room in Nero’s Domus Aurea (Figures 5.8 and 5.9). An umbrella dome is divided into webs which are convex in section (Figure 4.12f). There are many examples in Hadrian’s villa and in some, such as the dome of the Serapeum, convex webs alternate with flat ones.
Roman architects had a remarkable understanding of engineering principles when it came to building arches and vaults. They became aware that stone has great strength in compression, but is not strong in tension. Therefore a horizontal lintel, which puts stone into tension, cannot span great distances, whereas an arch, which puts stone into compression, is capable of far wider spans. A stone arch is composed of separate wedge-shaped blocks, termed voussoirs, struck from a common centre (Figures 1.11 and 1.12). The fact that each voussoir is wider at the top than the bottom prevents it from falling vertically under the action of gravity, and forces it to transmit its thrust to its nearest neighbour. Provided that the foundations are sound, the voussoirs are of compatible stone and lateral thrust has been contained, the arch should not fail. Thus a series of arches, as in an aqueduct, can buttress each other and will need consolidation only at the ends of the series. An arch can be flat or nearly flat and will still stay up because of the shape of its elements. Flat or nearly flat stone lintel arches occur, for example, in the Colosseum and although the thrust is almost totally horizontal these arches will remain stable provided there is adequate consolidation at the sides. However, an arch is made more stable by its curve and the larger the curve the stronger the vertical component of the thrust. An arch has to be supported until the last voussoir (the keystone) is in place, and therefore in Roman times arches could not be erected without the use of wooden centring. In the case of a concrete vault the centring had to be capable of meeting two separate demands. One is the need for a continuous surface to give the vault its shape, and the other is the construction of scaffolding sufficiently strong to support the formwork and the weight of the vault above. In the case of the first problem, a continuous surface which corresponds to every curve of a complex vault would have required highly skilled carpentry.
It is often said that when Roman concrete set it formed a ‘monolithic mass’, but this does not mean it was resistant to tensile stresses which can cause cracking. Many writers, including some recent ones, discuss Roman concrete as if it were ‘devoid of any lateral thrust, and covered its space with the rigidity of a metal lid’.14 On the contrary, in creating concrete an artificial building stone has been produced with the attendant problems of weakness in tension. As Mark points out15: “Roman pozzolana concrete, despite its outstanding properties, could not be counted on to exhibit tensile strength.” This could be overcome in Roman times only by using a curved surface. Because of its double curvature a dome is subject to two types of stress. One is meridional or longitudinal stress of the type encountered in arches, which is mainly compressive and increases towards the base. The resultant lateral thrusts can be countered by consolidating the haunches. The supporting drum of a domed room had to be correspondingly substantial; for example, the drum of the Pantheon is 6.15 metres thick. The other stress is circumferential hoop stress, which nearer the crown is compressive and nearer the haunches tensile. The change theoretically occurs at 51.8o from the crown, but much will depend upon the thickness of the envelope of the dome and the arc of embrasure (i.e., whether the dome is perfectly semicircular or not). Domes will develop cracks near the haunches when they are no longer able to resist these tensile forces. This happened in the case of the Pantheon where eight fine vertical cracks, which developed as a result of hoop stress, have been detected.16 The envelope could not be too thin or bending stress would result. It has now been established that if a semicircular arch spans a distance greater than
17.6 times its thickness it will fail.17 Buckling was also a problem, especially near the crown where the envelope approaches the horizontal. This may be the reason the crown was omitted in earlier domes and the oculus was the preferred method of lighting. For the same reason architects at first avoided inserting large windows in the drum. Sometimes a dome of double curvature was used to counteract buckling. Examples of this can be seen in the Mausoleum of Galerius at Thessalonike (before ad 311) and in the baptistery at Nocera (fourth century ad).
In the case of a coffered concrete dome like that of the Pantheon, the curved dome shape and the coffers had to be reproduced in wood. This process would have consumed prodigious amounts of time as well as timber. A simple method later adopted in an attempt to cut down the amount of timber was to line the vault with brick tiles. Rows of bessales, or later, bipedales were laid on the timber scaffolding instead of a full timber planking (Figure 4.13). The concrete was then laid on top of the bricks which remained in place when the timber supports were removed. Brick lining would also have permitted reuse of the timber because the formwork would no longer adhere to the concrete. Brick-lined vaults first appeared in Trajan’s markets, and became common up to the time of Caracalla. Good examples of them can be seen in the Severan structures on the Palatine and the Baths of Caracalla (ad 212–216). They were not used in the dome of the Pantheon, although the small half-domed rooms in the drum are lined with bipedales.
Ribs in vaults derive from Republican use of relieving arches in walls, a technique which continued for many centuries. Ribs of bipedales as reinforcement occur for the first time in Colosseum and they appear to have been used in the Domitianic vestibule in the Forum as stiffening to counteract bending stress. Brick ribs, unlike brick linings, were a structural element in buildings, although they may also have acted as constructional aids. The use of ribs along the groin of a cross-vault first appeared in the Villa at Sette Bassi on the Via Latina which belongs to the second century ad. These ribs did not act in the same way as the ribs in medieval vaulting, as used to be asserted. By the second century ad the ladder rib, with mortar and caementa-filled compartments between the bipedales, became common. By the early third century ad solid bipedales ribbing was being superseded by lattice ribbing which
Figure 4.13 Diagram to illustrate tile-clad vaulting.
Figure 4.14 Diagram to illustrate lattice ribbing. (After G. Lugli, La Tecnica edilizia romana, Rome 1957, p. 667).
consisted of a series of ladder ribs next to each other (Figure 4.14). Found in the Baths of Diocletian (ad 298–306) and the dome of the ‘Temple of Minerva Medica’ (early fourth century ad), it seems that the main purpose of lattice ribbing was to distribute load evenly within the vault (Figure 4.15). The insertion of amphoras into a concrete fill seems to have been a method of cutting down weight, as in the early fourth century ad octagonal hall of the ‘Villa of the Gordians’. Although they are occasionally found in buildings of the second century ad, amphoras were not frequently incorporated into vaults and domes until the late third/early fourth centuries ad. Enormous numbers were used in the vaults of the Circus of Maxentius on the Via Appia. They are also found in the dome of the ‘Minerva Medica’, where they were placed above the windows presumably to channel weight away from them. They were used in combination with lattice ribbing and pumice in the upper part of the dome (Figure 4.15).
Wooden roofing was employed by the Romans up to the end of the Empire, and is described by Vitruvius (de Arch. 4.2.1). An early example of a building roofed in wood is the theatrum tectum at Pompeii (c. 70 bc) which is 26.25 metres wide internally, suggesting that the principle of the truss may have been known to the builders. The truss is a system of binding roofing timbers together in a triangle or series of triangles so as to obtain a rigid, selfsupporting structure. Tie beams more than 27 metres long may have been used, although the timbers do not have to be of the full length; shorter pieces can be spliced and fitted together. The widest nave of any Roman basilica is that of the early fourth century ad basilica at Trier, which had a clear internal span of 27.2 metres, while both the Basilica Ulpia and Old St. Peter’s at Rome had a nave c. 25 metres wide. Documents exist giving the length of the tiebeams in St. Paul’s Outside the Walls as 24.25 metres. The wood used was fir. The widths of the Aula Regia and the triclinium of Domitian’s palace were even greater; the dimensions of the Aula Regia are 31.44 × 38 metres and of the triclinium 29.05 × 31.64 metres. In both
Figure 4.15 Rome, ‘Temple of Minerva Medica’, showing the brick ribs in the dome
Figure 4.16 Diagram to illustrate a king-post truss.
cases internal architectural features would have reduced the clear span. Wood of this length was available to the Romans. The largest tree ever seen in Rome was a larch beam, 120 Roman feet (35.52 metres) long and 2 feet thick, exhibited by Tiberius in the naumachia at Rome (Pliny, Nat.Hist. 16.76.201). The Diribitorium, where the votes were counted, finished by Augustus in 7 bc, was the largest building with a timber roof and one of the marvels of Rome (Pliny, Nat.Hist. 36.24.102). A beam from it, left by Agrippa in the portico, was 100 Roman feet (29.6 metres) long and one and a half feet thick.
A word should be said about the stones and marbles traded and used throughout the Roman period.18 Alban stone or peperino was one of the oldest stones used for squared masonry (opus quadratum). It is a grey stone whose colour and softness make it unsuitable for subtle carved detail. Although easily worked it became friable if exposed. Gabine stone had a long history as a building material. Found at Gabii, about 10 kilometres from Rome, it is both lighter in colour and denser than peperino, and fireproof, as Tacitus affirms (Ann. 15.43). It is perhaps for this reason that it was used for the offices on the SW side of the Forum Julium and in combination with peperino for the back wall of the Forum Augustum. Cappellaccio is the term commonly given to the grey volcanic stone which is composed of ash from the earliest volcanic activity in the Rome region. It is a poor crumbly stone used monumentally only in Rome’s early period, for example in the substructures of the Capitoline temple. Later it was seldom used for anything above ground, its main use being in foundations and sewers. Tufa or tuff is a soft volcanic rock, easily worked, but weak under concentrated loads. Of the various tufas, that from Fidenae was one of the earliest to be used by the Romans, the quarries being opened after the fall of Fidenae in 426 bc. It is of a dark yellowish colour and contains ugly inclusions. The more attractive greyish-yellow tufa of Grotta Oscura was clearly preferred by the Romans who began to exploit it shortly after the fall of Veii (396 bc) in whose territory the quarries lay. An early example of the use of Grotta Oscura tufa is the socalled Servian wall (Figure 1.5). It was one of the commonest of all building stones until the late Republic. By the Augustan period the finely grained lithoidal tufa from the Anio region was used almost exclusively. It was used in conjunction with travertine in the platforms of temples, such as that of Deified Julius Caesar, Apollo Sosianus and Apollo Palatinus. In the platform of the Temple of Castor travertine piers supported the columns and the casing was in Anio tufa. In the second century bc the first travertine quarries were opened in the plains below Tivoli. Travertine is a sedimentary limestone, very hard with a creamy texture and recognisable by its lightly pitted surface. It was used a great deal during the late Republic, especially to carry heavy loads. It was also used decoratively, particularly on the façades of buildings like theatres and amphitheatres, such the Theatre of Marcellus (Figure 3.9) and the Colosseum (Figure 7.1), where durability was important. From the time of Augustus travertine took second place to marble as a decorative material. Its main disadvantages were that it calcinated in fire and tended to split when set vertically. It was also expensive to quarry.
The finest decorative stone as well as the strongest in tension is marble. The first marble temple of Rome was that of Jupiter Stator (146 bc). Another early marble temple, dating to the late second century bc, was the circular Temple of Hercules Victor in the Forum Boarium (Figure 1.16). Although there was much criticism of such luxury, architectural sculptures began to be imported by wealthy individuals during the first century bc, for example Lucullus, who even had a type of marble (africano) named after him. The Dictator, Sulla, brought Pentelic marble columns from the Temple of Olympian Zeus at Athens (Figure 11.2) for use on the Capitoline Hill (Pliny, Nat.Hist. 36.4.45). By 48 bc the marble quarries of Carrara in northern Italy began to be exploited. These marbles were at first landed along with other commodities at the Emporium near the Aventine. By the time of Augustus marbles of every kind became a common sight in Rome and a special wharf was built for them near the later Pons Aelius. There is evidence that the whole area of the Campus Martius was filled with workshops of stone masons and sculptors. The poet Tibullus (2.3.43–4) comments upon the din and bustle of that part of Rome. Of the white marbles Carrara was the most commonly used in Rome. Its colour is pure white, sometimes tending to bluish. Its crystalline structure is extremely compact which gives it a somewhat duller appearance than the Greek white marbles. It was mainly its cheapness which made it popular and its popularity lasted throughout Roman history. Of the Greek white marbles Pentelic is first found in Rome in the Temple of Hercules Victor by the Tiber (Figure 1.16). Later it was used in the Arch of Titus (Figure 7.9). It has the somewhat looser crystalline structure typical of Greek marbles. When chipped the micaceous particles of its structure flash and glow in the light. The iron in its composition makes it weather to a soft golden tone, as can be seen in the Parthenon at Athens. Parian is a pure white marble, composed of large crystalline particles. Architecturally its use was largely confined to roof ornaments, perhaps because of its translucent quality.
Coloured marbles came into use in the Hellenistic period, as shown by wall paintings at Delos and a little later by walls in Pompeii decorated in the First Style, which imitated walls encrusted in polychrome marble. In Rome coloured marble was rare until the time of Augustus. The Forum Augustum made extensive use of Carystian marble from Euboea (cipollino). As its Italian name implies it has something of the texture of an onion because of its strong veining. It is off-white or pale green in colour and is heavily striated with mica. It tends to split easily along the veining. In Augustan times and throughout the Empire it was commonly used for columns, for example in the exedras of the Forum Augustum and in the Temple of Antoninus and Faustina in the Roman Forum. It was used in sculpture for a crocodile at Hadrian’s Villa in Tivoli. The greenish hue of the marble and its strong veining make it a particularly appropriate stone from which to carve the creature. Hadrian seems to have had a taste for sculptures in strongly-coloured marbles, examples of which, such as the two centaurs carved out of black Tunisian marble and the satyr in red marble from Laconia, can be seen in the Capitoline Museum, Rome. Numidian marble (giallo antico), a yellow marble with red or dark veining from Tunisia, was frequently used in inlay work. The red stone quarried in the Peloponnese near Cape Matapan is called rosso antico. Of the breccias or variegated conglomerate stones pavonazzetto was commonly used for decorative purposes, flooring and sometimes columns. Found in Phrygia, it has a violet base with irregular white limestone inclusions. Other breccias are portasanta from Chios with red or yellow patches on a soft grey or pinkish ground, and africano from Asia Minor, with black, grey and bright red patches.
Unfluted monolithic columns of granite became more common as the Empire progressed (Figure 9.15). Granite is an extremely hard granular crystalline rock, and both the pink and the grey types used in Roman construction were quarried in Egypt. Porphyry is a very hard igneous rock with an extremely compact crystalline structure. It came from the Red Sea area of Egypt. It was used for inlay, flooring and columns, although generally for smaller ones than those made of granite. In the late Empire and into Byzantine times it was used for sculpture and for sarcophagi, such as those of Helena and Constantina in the Vatican Museum. Its deep maroon colour, near to purple, gave it imperial connotations in the late Empire. A Byzantine emperor, born in a room in the imperial palace veneered with porphyry was called Porphyrogenitus (‘born in the purple’). Green porphyry or serpentino is a bright green stone speckled with light green crystals. Quarried near Sparta, it was used in conjunction with red porphyry and other stones to produce a cut-stone wall or floor inlay (opus sectile).
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