The Pantheon: From Antiquity to the Present

by Unknown

  As mentioned earlier, Hadrian’s Villa has numerous other domes and half domes. The vestibule of the Piazza d’Oro has curved walls that support a “pumpkin” dome with single and double curving elements alternating much like the dome of the Serapeum. Its plan resembles the Pantheon’s in having alternately rectangular and curved niches, and it, too, had an oculus (as did the dome of the Large Baths at the Villa). This combination of features is more striking than any one feature, and they occur in what is essentially a freestanding building. The method of construction used for the vestibule’s dome was similar to the Pantheon’s in that a ring of concrete was created first by cantilevering inward to decrease the opening that needed to be spanned. Elsewhere at Hadrian’s Villa, cantilevered balconies can be seen on the nearby Barracks of the Vigili and the long building parallel to the pool of the Canopus. Corbeling had been used to similar effect previously, for example in the main hall of Trajan’s Markets, but prior to the Pantheon this structural principle is not known to have been used to create a dome or half dome.45

  (J) “Relieving Arches.”

  Concrete had largely replaced stone for the construction of arches and vaults before the end of the first century AD. The designation “relieving arches” applies not to open arches but to ones filled with masonry underneath; their purpose was often, it seems, to reduce or relieve pressure over an opening lower down. As Martines also notes, this type of arch was used widely in concrete buildings dating from the reigns of Trajan and Hadrian, including Trajan’s Baths, Trajan’s Forum, Trajan’s Markets, and those at Hadrian’s Villa. It is generally assumed that relieving arches were made of brick and mortar rather than concrete;46 however, examples in the exedras of Trajan’s Baths can readily be seen to be made not just of brick but of brick and concrete in combination. Equivalent arches exist in numerous earlier Roman buildings and in the construction of aqueducts; where the internal structure of the arch can be seen, it seems invariably to have been constructed of bricks alternating with portions of concrete shaped like voussoirs.47 Examples of concrete voussoirs that were used before and after the Pantheon can be seen together in a section of the Arcus Neroniana (Caelemontana) adjacent to the Scala Sancta (Fig. 5.12).48 Examples constructed during the Hadrianic period include the Serapeum (Fig. 5.10) and the Small Baths at Hadrian’s Villa. There concrete voussoirs alternated with portions of brick in the same way that the walls of the Pantheon were no doubt constructed, in line with the typical configuration of relieving arches in this period (Fig. 5.13). When the level of the arch was reached, bipedales were placed radially on top of the centering and were separated from one another by a facing made up of half or quarter bipedales, leaving spaces later to be filled by concrete.

  5.12. Detail of brick-faced concrete with concrete voussoirs in the Arcus Neroniana (Caelemontana), built under Nero (AD 54–68) and reinforced during the reign of Septimius Severus (AD 193–211), who inserted a similar arch underneath. (Photo author in 2007)

  5.13. Diagram of brick-faced concrete showing the construction of concrete voussoirs. (Courtesy of Nikolaos Karidis)

  In conformity with the general perception that relieving arches were made of solid brick, the Pantheon’s great arches are usually presumed to have been made of brick and mortar extending entirely through its walls (as in Fig. 5.14).49 The main evidence for this view is the brick barrel vaulting that projects from the center of the south side of the Pantheon (see Fig. 7.5), but this served a different purpose in acting like the flanking buttresses of the main hall in Trajan’s Markets or the buttresses of the Basilica Nova.50 It may be noted how, as a consequence of its radial plan, all of the relieving arches of the rotunda have to taper; this means that the great arches that show on the exterior become considerably smaller on the interior (see Fig. 7.1). Such tapering or conical vaults had long been used in the substructures of theaters, and they were easily constructed of concrete. To have constructed them of brick alone would have required that some bricks be made wedge shaped both vertically and horizontally and/or that mortar joints be wedge shaped.

  5.14. Pantheon, cutaway view. (Josef Dell, “Das Pantheon in Rom,” Zeitschrift für bildenden Kunste, ns. 4, 1892, p. 273)

  Evidence that the great arches of the Pantheon may actually be brick-faced concrete was found in 1882 when a building attached to the back of the rotunda was removed, and a damaged wall surface revealed what was behind its brick facing. A specialist in Roman construction methods, J. Henry Middleton, examined these sections of wall before they were restored, and he stated that “the brick facing, including the arches, only tails into the wall to an average depth of 5 or 6 inches, so that in reality these apparently relieving arches are of little more use (as regards the pressure) than if they were painted on the surface.”51 The illustration he provided makes the same point for general practice. In this volume, Wilson Jones discusses a brick-faced arch he found in the west staircase of the Pantheon that can be seen to be toothed into the concrete. However, only an examination of the concrete core of the building itself could reveal whether its relieving arches were constructed of solid brick and mortar or brick-faced concrete.

  (K) Concrete Vaults with Brick Linings.

  The soffits of vaults in the mid-imperial period were often made of brick linings. These were laid like tiles (not on edge) on a centering to form the intrados of a concrete vault, and they remained attached when the concrete cured. The earliest-known example of this construction method appears in the barrel vaults at Trajan’s Markets and other Trajanic structures, and it had been used experimentally in several different forms. In all known cases where the structure is visible, brick linings face vaults made of concrete, not of brick.52 The intrados of the great arches of the Pantheon’s exedras have a lining of bipedales; even if they could have been attached using mortar – they weigh about 25 kilograms each – they would not have been needed on the intrados of a solid brick arch or vault.

  Conclusion

  The round plan of the Pantheon provided a secure and permanent support for its dome. The almost exclusive use of masonry enabled a fireproof building to be constructed so as to avoid the fate of its predecessor, and the use of concrete enabled an unprecedented span. A rectangular portico was married to the cylinder by means of a transitional block in between, housing the stairs. Although previous buildings provided the elements for its design, none proved to be equally permanent, and no larger dome has ever been made of unreinforced masonry. A new means of construction was required to produce the desired effect.

  While taking its cue from Agrippa’s project, the Pantheon was created by combining design and structural features that had been used for at least six different building types. Baths provided precedents for domes or half domes with coffers and an oculus and an equivalent arrangement for the incorporation of trapezoidal stairs. Tombs provided a precedent for a windowless cylinder that was freestanding. Both tombs and theaters provided an example of a cellular structure and a geometrical method for laying out circular plans. Theaters provided models for conical barrel vaults and multiple levels of cellular structure. Basilicas offered examples of a forecourt, an attached portico, and a clerestory arrangement. Triumphal arches provided exemplars for the form and proportions for the transitional block. Temples provided precedents for a pedimented portico.

  Of the precedents erected in the preceding decades, the octagonal hall of the Domus Aurea was the most innovative, but it differs in many respects from the Pantheon. Its plan is more complex, and its use of light is more sophisticated, but its structure and method of construction are simpler. Compared with the Pantheon, there are other significant differences: the shape (with an octagonal plan and largely octagonal vault rather than a round plan and hemispherical dome), the lighting (with openings at a high level around the perimeter aside from the oculus), the decoration (with a plain intrados rather than coffers), and the structure (part of a larger building rather than freestanding). In addition, its relatively small and thin dome is monolithic. At Bai
ae, the even earlier Temple of Mercury is likewise relatively small and has a thin shell, and it too lacks coffers. This is also true of the more imposing Temple of Diana and the Temple of Venus, although these have windows in their walls, and it is uncertain if either was constructed earlier than the Pantheon. Regardless of their exact dates, none of the three domes at Baiae provides a close parallel with the Pantheon in terms of its design or construction, and all formed part of complexes rather than being freestanding. Tholoi are still less likely to have been directly influential in that none is definitely known to have had a dome, although they had circular peristyles rather than projecting porticoes.

  The domes and half domes most nearly equivalent in scale and materials to the Pantheon had recently been constructed for Trajan’s Baths and Trajan’s Markets, one certainly and the other possibly designed by Apollodorus. Even though only the lower parts of their domes survive, enough remains of the coffering and other details to show just how important an influence the Baths undoubtedly were. As a more complete geometrical form, a dome has inherently more strength than a half dome, and since half domes had been successfully built that were 100 feet across, about two-thirds as wide as the Pantheon interior, these examples could well have suggested the practicality of constructing a still larger dome with the same proportions, as well as suggesting a similar approach to the incorporation of the stairs. There are some similarities in constructional technique, but this apparently did not extend to layers divided by through-courses of brick, of which only occasional use was made in the walls. Like the Pantheon, Trajan’s Markets includes numerous concrete vaults with brick linings, as well as walls with concrete cores separated by layers of bipedales. Its half domes, however, were not constructed this way. Such qualifications aside, the largest number of design elements and constructional features of the Pantheon occur in buildings constructed during Trajan’s reign.

  At Hadrian’s Villa several buildings have features in common with the Pantheon, but without more exact dating, they must be considered parallels rather than sources. The Serapeum of Hadrian’s Villa provides the closest parallel for the way in which the Pantheon’s walls were constructed. Both buildings have cellular walls thick enough to provide internal buttressing. The Roccabruna has step-rings, and the dome of the vestibule of the Piazza d’Oro was cantilevered inward to reduce the span that had to be fully supported by centering during construction.

  Regardless of whether constructional refinements such as step-rings with through-courses were invented specifically to help cope with the enormous span of the Pantheon, they represent the culmination of centuries of development in the creation of space through the use of concrete. As impressive as its technical accomplishments are, though, the principal achievement of the Pantheon lies in its captivating and memorable space. The visual impact of the interior was greatly enhanced thanks to the dramatic chiaroscuro of the exedras, the articulation of the coffering, and the illumination that floods in through the ample oculus. These features and many others were derived from a wide variety of sources that can be identified directly or indirectly, but despite the extent of its multifaceted indebtedness to earlier architectural achievements, the design of the Pantheon exceeded the achievements of its sources and set a new standard for the conception of interior space.

  Mark Wilson Jones provided detailed advice that has improved every aspect of this chapter. When our conclusions differ, mine were reached reluctantly.

  1 Domes with an oculus were used in bath buildings as early as the first century BC (Vitruvius, 5.11.5; see Vitruvius: Ten Books on Architecture, trans. Ingrid D. Rowland, commentary and illustrations by Thomas Noble Howe with additional commentary by Ingrid D. Rowland and Michael J. Dewar, New York 1999. Early examples are discussed in Fikret Yegül, Baths and Bathing in Classical Antiquity, Cambridge 1992, pp. 37–38, where the author provides persuasive evidence that domes “started with bath buildings” (p. 3).

  2 The architect Georges Chedanne established that the existing Pantheon was constructed more than a century after Agrippa’s death; he exhibited a drawing of the key brickstamps that he discovered and publicized his findings, but he never published them. They were, however, summarized by R. Phené Spiers, “Monsieur Chedanne’s Drawings of the Pantheon,” Journal of the Royal Institute of British Architects 2, 1895, pp. 180–182. The results of the excavations in 1892–1893 were published by Luca Beltrami (Il Pantheon: La struttura organica della cupola e del sottostante tamburo, le fondazioni della rotonda, dell’ avancorpo, e del portico, avanzi degli edifici anteriori alle costruzioni adrianee. Relazione delle indagini eseguite dal R. Ministero della Pubblica Istruzione negli anni 1892–93, coi rilievi e disegni dell’ architetto Pier Olinto Armanini, Milan 1898, and Il Pantheon rivendicato ad Adriano 117–138 d.C., Milan 1929); Beltrami reconstructed Agrippa’s Pantheon as a south-facing T-shaped structure. The evidence was limited, however, to a small piece of foundation; see Kjeld De Fine Licht, The Rotunda in Rome: A Study of Hadrian’s Pantheon, Copenhagen 1968, p. 219, and Mark Wilson Jones, Principles of Roman Architecture, New Haven 2000, Fig. 3.3). For the recent excavations in front of the portico, see Paola Virgili and Paola Battistelli, “Indagini in piazza della Rotonda e sulla fronte del Pantheon,” Bullettino della Commissione Archeologica Comunale di Roma 100, 1999, pp. 377–394.

  3 Most of the ancient texts relating to Agrippa’s Pantheon and the existing Pantheon can be conveniently compared in Licht 1968, pp. 180–183. Paul Godfrey and David Hemsoll argued that none of the three Pantheons was a temple. Noting that the present building is only known to have been used as a tribunal, they added that “despite their many uses, the imperial administration of justice was not conducted in temple buildings” (Godfrey and Hemsoll, “The Pantheon: Temple or Rotunda?” Pagan Gods and Shrines of the Roman Empire, ed. Henig et. al., Oxford 1986, p. 202).

  4 Licht (1968, pp. 203–225) mentioned the importance of considering groups of features, but he discussed separately each building that was a possible source, rather than seeking clusters of features. MacDonald’s book on the Pantheon contained his fullest treatment of its sources (William L. MacDonald, The Pantheon: Design, Meaning, and Progeny, London 1976, pp. 44–75); cf. MacDonald, The Architecture of the Roman Empire, vol. 1:An Introductory Study, London 1965; 2nd ed. rev., New Haven 1982, for additional details. For a more detailed discussion of the sources for the Pantheon’s design and construction, see Gene Waddell, Creating the Pantheon: Design, Materials, and Construction, Rome 2008.

  5 Kjeld De Fine Licht, Untersuchungen an den Trajansthermen zu Rom, Copenhagen 1974. Dimensions for the exedras of the Baths of Trajan are given in meters and Roman feet in Wilson Jones 2000, p. 218.

  6 The span of the central space of the Basilica Ulpia was 23.43 meters or 79.15 Roman feet (James Packer, The Forum of Trajan in Rome: A Study of the Monuments, Berkeley 1997, folio 25). Some early reconstructions reproduced by Packer show the exedras of the Basilica Ulpia with half domes like the exedras of Trajan’s Baths, but subsequent archaeological evidence has established that their roof structures were not vaulted. Recent excavations of Trajan’s Forum have been interpreted differently; see Roberto Meneghini, “Il foro Traiano. Ricostruzione architettonica e analisi strutturale,” Mitteilungen des Deutschen Archäologischen Instituts, Römische Abteilung 108, 2001, pp. 245–268, and James Packer, “Templum Divi Traiani Partici et Plotinae: a Debate with R. Meneghini,” Journal of Roman Archaeology 16, 2003, pp. 109–136; some uncertainty remains about how its east and west ends were configured. The dates for Trajanic buildings are from Amanda Claridge, Rome: An Oxford Archaeological Guide, Oxford 1998, pp. 161–164, 170–172, and 288–290. The diameter of the Maritime Theater or “Island Enclosure” is 44.20 meters or 148.5 Roman feet (William L. MacDonald and John Pinto, Hadrian’s Villa and Its Legacy, New Haven 1995, p. 82).

  7 For the construction of Trajan’s Markets, see Lynne Lancaster, “Building Trajan’s Markets,” American Journal of Archaeology 102, 1998, 283–308, and Lynne L
ancaster, “Building Trajan’s Markets 2: The Construction Process,” American Journal of Archaeology 104, 2000, pp. 755–785. Trajan had the end of the Quirinal cut away and terraced to provide room for his forum. On the use of retaining walls by the Romans, cf. Vitruvius 6.8.7. MacDonald (1976, p. 58) argued that “the Market hall is a rectilinear forerunner of the Pantheon.”

  8 MacDonald 1976, pp. 54–55. The width of the Domus Aurea is given by Wilson Jones (2000, p. 218) as 49.49 feet or 14.65 meters measured from corner to corner.

  9 On the bath buildings at Baiae, see Yegül 1992, pp. 107–110, and Amedeo Maiuri, Phlegraean Fields from Virgil’s Tomb to the Grotto of the Cumaean Sybil, Rome 1947, pp. 61–73. The dates assigned for these structures vary widely, being based on constructional comparisons thought to be more securely dated, but it is often uncertain how long methods of construction persisted. For example, Yegül dates the Temple of Mercury from the late Republican to the early Julio-Claudian period, and likewise the “Temple of Apollo” at Lake Avernus, which has a span of 36.6 meters. However, Maiuri dates the latter to the reign of Nero (Maiuri 1947, pp. 139–141), though it has also been dated to the Antonine period (M. E. Blake, Roman Construction in Italy from Nerva through the Antonines, Philadelphia 1973, p. 270).

  10 The Domitianic hall at Albano has a dome with an oculus, but like those at Baiae it formed part of a larger structure. The Forum Baths at Pompeii has an earlier example of an oculus, but this lit a conical vault (Licht 1968, pp. 211–215). The oculus was also used to regulate the temperature by partially closing it (Vitruvius, 5.10.5). No dome of a bath building is known to have had coffers.