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A number of later rotundas with porticoes show the direct influence of the Pantheon. The Tor de’ Schiavi was an imperial tomb with a pedimented portico, and the “Pantheon” at Ostia was a rotunda with an unpedimented portico. However, no domed building earlier than the Pantheon is definitely known to have had an attached portico.16
(B) Transitional Block with Stairs.
The front of the transitional block of the Pantheon, despite the columns of the portico that screen it, resembles a triumphal arch, while in its plan and function it is equivalent to the stair towers built behind some of the exedras of Trajan’s Baths (Fig. 5.2). The main functional purpose of the transitional block is as a container for stairs, which had been included in some Greek temples and which were used more widely in such Roman buildings as baths, triumphal arches, and triumphal columns. As Mark Wilson Jones notes in this volume, the nearest parallels for trapezoidal stairs fitted into a block that mediates between an orthogonal envelope and a circle are those associated with the exedras at Trajan’s Baths.
(C) Cylindrical Drum without Windows.
Windowless walls of concrete faced with brick were fireproof, and they have proven to be one of the most permanent types of construction ever used. In form and scale, the windowless exterior of the freestanding Pantheon most closely resembles the earlier Tomb of Caecilia Metella and the later Tomb of Hadrian (Castel Sant’Angelo), both of which were constructed primarily of concrete, but with stone rather than brick exterior facings. The former has a small corbeled dome with an oculus in the center, and the latter has a small dome in its central tower. Both of these cylinders have square bases. The Pantheon’s walls were hollowed out and given a cellular wall structure similar in plan to the outer ring of the Mausoleum of Augustus (Fig. 5.1). Adoption of the circular form of tombs facilitated support of the largest dome that had ever been created, but there needed to be a way to distinguish the Pantheon from a tomb, and as has been noted, this was accomplished primarily through the addition of a portico.
The Pantheon’s exterior has three cornices that reflect three levels of small chambers within its walls, rather in the way that cornices align with vaulted spaces on the facades of Roman theaters. This building type is unlike the Pantheon in having open arches for circulation, yet there is an affinity inasmuch as theaters were usually freestanding and constructed primarily of concrete, and they were among the few types of Greek or Roman buildings that made use of circular geometry.
(D) Dome with Coffers and an Oculus.
The Pantheon’s dome is by far its most impressive element both visually and structurally, and although there are close precedents for its design, there are also significant differences. In addition to having the same 1:1 ratio between width and height, the exedra on the east side of Trajan’s Baths have coffers that are closest in design of any known precedent to those of the Pantheon (Fig. 5.7).17 Each coffer consists of three successively smaller and more deeply recessed squares that closely resemble the uppermost row of coffers in the Pantheon; the other four rows have coffers with four successively smaller squares. In both buildings, the squares are inflected upward, which enables every square to be visible from the center of the interior.18
5.7. East exedra of Trajan’s Baths. (Photo author in 2007)
Parts of the west dome of Trajan’s Baths have survived, and its east dome is depicted on a surviving fragment of the Marble Plan of Rome, but it is not known if these domes had coffers. In the eighteenth century, Charles Cameron was able to determine that these domes were very close to half the Pantheon’s diameter.19 The Marble Plan depicts a circular room within windowless square walls, suggesting that it was illuminated by an oculus. It had large niches corresponding to the four corners.20 The arrangement of niches was similar to that of the semicircular exedras in the Pantheon, except that these occur on the cross axes rather than the diagonal axes.21
(E) Other Design Features.
Many Roman buildings orchestrate space by means of axes subordinate to the main one, and the Pantheon is a particularly rich example of this principle. The main axis of the interior is made immediately apparent by the arched entrance and apse. The cross axis is emphasized by exedras that align with the square pattern of the floor and coffers.22 A closely similar floor pattern had been used for the apsidal ends of the Basilica Ulpia. Indeed, the patterns of both the floor and walls consisted of basic geometric forms; this design feature and the restraint with which the forms were used are characteristic of Trajanic architecture in Rome. The lavish use of color and of exotic materials characterized imperial architecture generally, but color was usually subordinated to form.
Monolithic shafts of granite were more difficult to quarry and transport than drums of marble, but a granite shaft in one piece was considered worth the extra effort and expense. Granite was one of many types of exotic colored stones that were imported from newly acquired provinces to embellish Rome in the imperial period. Large granite monoliths had been used as early as Vespasian’s Forum of Peace, which was dedicated in AD 75.23 They had been used extensively in the Forum of Trajan for the main floor of the interior of the Basilica Ulpia, which had columns with shafts 30 Roman feet tall, or three-quarters as tall as the 40-foot granite shafts of the Pantheon’s portico.24 Here, the shafts are of two different colors (gray and red), and the monolithic shafts of marble fronting the exedras in the interior are also of two colors (although this is not obvious today due to staining). The use of different colors of marble for the same elements was not unusual, and it was done symmetrically. What is unusual is for a portico with columns to have square column-like piers, or antae, that are different in material (marble rather than granite), in color (white rather than gray or red), and in construction (drums rather than monoliths).
The attic of the Pantheon’s interior resembles the clerestories of basilicas and other buildings, except that its six framed openings or “windows” allow the light passing through the oculus to illuminate the exedras. This arrangement is the reverse of the usual way that a clerestory admits light into a space. Blind windows centered above the adedicules and eight structural piers maintain the pattern and rhythm of the attic. The orders of the main floor and attic story have similar patterns, each with a wider central intercolumniation.25 Although these orders differ in scale and alignment, their stylistic affinity, the similar intercolumniations, and the same materials ensure compositional unity.
The architect Apollodorus had recently used clerestories for the Basilica Ulpia and for Trajan’s Baths. The hemicycle of Trajan’s Markets has alternating triangular and segmental pediments, which prefigure those over the aedicules of the Pantheon. The windows of the Markets are located on an upper floor that resembles a clerestory, and they admit light to the spaces behind them. The pilasters supporting these pediments rest on a running pedestal in an arrangement that is similar to the attic of the Pantheon. As with Roman theaters generally, the uppermost level of the Colosseum exterior has a similar running pedestal, with windows and pilasters rather than an arcade, and its pedestal serves as a facing for barrel vaults in a way that is equivalent to the attic of the Pantheon’s interior.26
The main story of the Pantheon’s interior has exedras fronted by two columns with flanking pilasters, much like the in antis (between the walls) arrangement that had often been used in Greek architecture. The wider intercolumniation in the center was characteristic of Greek Ionic and of Roman architecture generally, and it was used in both the portico and the interior of the Pantheon. Indeed, the A-B-A compositional principle was in fact used throughout the project. Other examples include the way that the portico is flanked by the drum and the way that niches within the portico flank the wider arch of the entrance, a proportional arrangement that closely resembles the openings of a triumphal arch with a similar A-B-A rhythm as on the interior. (A triumphal arch was located opposite the entrance of the Pantheon in the large open space in front.)
The orthogonal floor pattern of the Pantheon most closely resembl
es the pattern of the Basilica Ulpia, which had circles and squares alternately within a grid of squares. In addition, the way in which this rectilinear pattern was cut through abruptly on the perimeter of its semicircular exedras compares directly with the edge condition of the Pantheon’s floor. In turn, the floor pattern of the Basilica Ulpia is similar to that of the adjacent Forum of Augustus, which has squares of similar size separated by broad bands of marble.
The pattern of pilasters on the attic story of the Pantheon’s interior was cut off above the apse and entrance. Since the floor pattern is cut through around its perimeter, cutting through the pilasters can be considered a solution rather than a solecism, as was presumed by Renaissance critics. Despite the relative independence of the patterns of its orders, the interior was given unity through axial coordination and the use of the same materials and colors for the walls and floor, as well as through the use of similar shapes for the walls, floor, and dome.
(F) Simple Proportions.
As Martines discusses in Chapter Four, the proportions of the Pantheon are straightforward and characteristic of Roman architecture generally. All three surviving exedras of Trajan’s Baths have overall heights that are approximately equal to their widths, just as in the Pantheon’s interior.27 The 1:1 proportion seems also to have been used as a basis for the design of the transitional block, which is nearly square like some triumphal arches with triple openings (see Fig. 7.13).28
The Basilica Ulpia had semicircular ends or exedras about 150 Roman feet in diameter, or approximately the same diameter as the Pantheon, as the Maritime Theater at Hadrian’s Villa, and as the hemicycles of Augustus’s Forum.29 This was also the height of the Mausoleum of Augustus, which was 150 feet in radius. The use of 150 feet as a standard measurement was thus well established before it was used for all three dimensions of the Pantheon’s interior.30 The Pantheon’s width on both the main and cross axis as well as its height relate to one another as 1:1:1, and since all three dimensions are so close to 150 Roman feet, the use of this dimension was undoubtedly intended.31
Since the Pantheon’s exedras are the same width as the piers in between them, they relate to one another as 1:1. Its apse was made somewhat wider and the entrance somewhat narrower for visual and structural reasons, respectively. The ratio of the attic story, or upper part of the interior wall, to the main or lower part is 2:3, another simple proportion that was widely employed.32
The Corinthian columns of the Pantheon, both exterior and interior, conform to the orthodox pattern of imperial practice that took hold around Augustus’s time.33 In accordance with these principles, the heights of shafts are multiples of 5 feet (40 feet in the portico, 30 feet in the interior), while the heights of the complete columns (base, shaft, and capital) are 6/5 greater. A ratio was also probably used to determine the thickness of the walls of the rotunda in relation to the distance that needed to be spanned, since proportion represented a convenient way to record what previously had proven to provide adequate structural support. The Pantheon’s wall thickness in relation to its span is 1:7.3, somewhere between the extremes of one-eighth to one-fifth encountered in other examples.34
In many cases, the same buildings that could have influenced the design of the Pantheon also present similar structural elements. The combination of design and structural elements increases the probability of a direct source.
(G) Dome with Its Base Constructed of Rings of Concrete (Step-Rings) Separated by Layers of Brick.
Through-courses of brick (sometimes called bonding courses) are layers of brick a single brick thick that subdivide sections of concrete at intervals. A through-course of bipedales (bricks two feet square) served the purpose of covering the top of each section of concrete and prevented it from drying too fast while it cured (Fig. 5.8). This shows that buildings of the Trajanic and Hadrianic periods were usually constructed one horizontal section at a time. On the exterior and in the staircases of the Pantheon, layers of bipedales are conspicuous at intervals of about 1.2 meters, being thicker as well as wider than the regular facing of bricks that toothed into the concrete and provided a permanent formwork (see Plate XXIII).35
5.8. Typical Roman brick-faced concrete wall with triangular pieces of bessales separated by layers of bipedales. (Middleton 1892, vol. 1, p. 57, Fig. 8)
A concrete half dome at the north end of the hemicycle of Trajan’s Markets resembles the Pantheon’s dome, but was constructed differently. As Lancaster pointed out, its relatively thin shell was constructed first, and a thick layer of opus signinum (a highly waterproof hydraulic mortar made with ground potsherds) was applied on top of the upper surface before a surcharge of concrete was added.36 This surcharge takes a stepped profile, but was not created as an integral part of the dome and does not contain through-courses of bipedales, as is the case with the step-rings of the Pantheon. A thick layer of opus signinum was also used to coat the dome of the Pantheon, but this was added after the step-rings had been created. This exceptionally waterproof layer contributed to the permanence of the building, particularly in the period between the theft of the bronze roof tiles and their replacement by lead roofing.
The Pantheon’s dome was constructed one step-ring at a time, and as each step-ring was added, it was cantilevered inward (see Fig. 1.12).37 Roman domes were usually constructed as monoliths, but up to the top of the step-rings, the Pantheon’s dome was constructed in much the same way as its walls. Martines and Wilson Jones note that in part for this reason, the need for centering was minimized. After setting up, the top step-ring provided a firm base on which to cast a saucer dome, and the step-rings contained the thrust of the saucer dome while it cured. This part of the Pantheon’s dome is a true dome, and it spans about one-third of the width of the interior (not counting the 30-foot oculus).
Step-rings occur in the Serapeum at Hadrian’s Villa, a building similar to the Pantheon in having curving walls with a sequence of piers that radiates like spokes and with a cellular wall on three levels (Fig. 5.9).38 The Serapeum’s step-rings are conspicuous when viewed from the back (Fig. 5.10). Since the building contains bricks that are datable to AD 123, it must be later than the Pantheon, and possibly influenced by it. However, mutual influence cannot be ruled out, nor a lost precedent that influenced both buildings.39
5.9. Plan of Serapeum, Hadrian’s Villa, Tivoli. (Aurigemma 1961, Fig. 84)
5.10. View of east side of Serapeum, Hadrian’s Villa, Tivoli, showing step-rings, concrete voussoirs, and openings to two of three levels of empty chambers in the walls. (Photo author in 2005)
The Roccabruna at Hadrian’s Villa had two domes, one above the other. Its lower dome survives intact, but is less comparable to the Pantheon by being part of a square in plan. The upper part of the Roccabruna, which has been variously reconstructed, had a dome with step-rings, as indicated by large fragments located nearby.
Turning to later examples, the dome of the nymphaeum called the “Temple of Minerva Medica” (Fig. 5.11) is closest to that of the Pantheon in having both step-rings and through-courses that align with one another. Unlike the Pantheon, it has brick ribs or lattices that could easily have misled FrancescoPiranesi into thinking that a similar method was used for the Pantheon.40 The half dome of the northeast exedra of Diocletian’s Baths also has through-courses.
5.11. Section of Temple of Minerva Medica, Rome. (Julien Guadet, Eléments et théorie de l’architecture cours professé à l’école nationale et special des beaux-arts, Paris 1915, Fig. 943)
(H) Cellular Wall on Multiple Levels.
Like the Pantheon, the Serapeum has thick walls that include three levels of wall cells disposed radially and with openings to the outside (Figs. 5.9; 5.10).41 The earlier building most similar in structure was the Mausoleum of Augustus (Fig. 5.1, A).42 This cellular or diaphragm structure confers the benefit of great rigidity that could resist, in the case of the Mausoleum, the thrust of a mound of earth and, in the case of the Pantheon and the Serapeum, the thrust of their domes. The wall c
ells of the Serapeum and Pantheon were too small and poorly lighted to be useful for much other than storage, and most were inaccessible. Being open to the outside, they enabled the escape of the heat generated chemically as the concrete cured. The largely hollow walls that resulted are also less likely than solid walls to crack while curing. Like the Serapeum, the Pantheon’s three levels of wall cells include an upper set at the level of the base of its dome (Fig. 5.10, and see Plate IV).
(I) Concrete with Horizontal Layers of Graded Aggregate and Through-Courses.
We have already seen that from the Trajanic period, concrete was often laid in layers separated by through-courses of bipedales (Fig. 5.8). As an initially independent development, as early as the Colosseum, heavier aggregates had been used in lower layers of concrete and lighter aggregates in higher layers.43 At Trajan’s Baths, both techniques come together, though not systematically, and here half domes have heavier tufa at lower levels and lighter tufa in the upper part. The Pantheon followed a similar principle, but with more gradations of density in between successive through-courses. Alberto Terenzio established that the heaviest aggregate (travertine) was used for the foundation and the lightest (tufa and pumice) for the saucer dome (see Fig. 1.12).44
The damaged front of the Serapeum at Hadrian’s Villa provides a good illustration of how the layers of concrete were constructed for most Roman domes. The exposed surfaces of its intrados show that the concrete was built up with fist-sized aggregate set into place individually in horizontal layers. Horizontal layers of aggregate show clearly in photographs of the Pantheon’s intrados when stucco was removed in the 1930s.