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Dio’s text also reveals that in line with Hellenistic tradition, Agrippa had attempted to dedicate a temple to Augustus by presenting him in the immediate context of the Olympian gods. This gesture would not have been a true divinization (which neither the moment nor Roman tradition permitted), but rather an attempt to offer a statue of the princeps (first citizen) to the immortal gods. Worship rendered to the gods would thereby accrue to Augustus, who was thus allied to a god according to formulas established in the Hellenistic period by Alexander the Great and his successors.6 A location within the temple would have been interpreted as an unacceptable sign of divinization before death, yet the potential for divinization was implicit in placing the statues of Agrippa and Augustus in the porch.
Inside the cella, the statue of Julius Caesar stood in the company of gods that included Mars and Venus, progenitors of the Romans and of the gens Iulia, the family line of Caesar and his adopted son. Dio’s text makes it clear that Caesar was placed in the temple both as a god and the adoptive father of Octavian/Augustus.7 Although less blatant, the Hellenistic custom of promoting a ruler cult pervades the entire figurative program of Agrippa’s Pantheon. Had the statue of Augustus been placed in the cella in keeping with Agrippa’s initial idea, the emperor’s quasi-divine status would have been apparent; consistent with this is Dio’s remark that the Pantheon should have been called Augousteion, following the Greek custom of naming a sacred building after the divinity venerated there. In the end, the name Pantheon prevailed (at least from the Neronian period onward) indicating worship of all the gods.8 Among those gods was the divine Caesar, a mortal deified after his assassination; at its doors were statues of his living adopted son and his general, awaiting admission to the cella. These implications were encouraged by the building’s location. Wedged between the Saepta Iulia to the east and the stagnum Agrippae to the west, the Pantheon stood in the palus caprae, or Goats’ Marsh, a place predestined, it seems, for apotheosis. In fact here, according to one tradition, Romulus experienced his consecration and ascent to the heavens.9 The site was thus redolent of kindred associations that could have attached themselves to Augustus and his right-hand man.
The Physical Traces of Agrippa’s Pantheon: The Investigations of Chedanne and Armanini
Excavations conducted in the late nineteenth century by the architects Georges Chedanne and Pier Olinto Armanini and published in incomplete form by Luca Beltrami formed the basis for a modern understanding of Agrippa’s Pantheon, although their implications were not properly understood at the time.10 These excavations revealed the following:
1. The remains of a preexisting podium survive below the podium of the Hadrianic colonnade (see Plate I). Of noticeably greater width (Fig. 2.2), this earlier podium was made of concrete with chips of Monteverde tufa and occasional sections of travertine and brick, and was mostly covered by a massive cap of travertine blocks up to 3 meters deep.11 On the front, where the edges of the pre-Hadrianic and Hadrianic podia coincide, thick load-bearing travertine piers were incorporated into the concrete. By a simple upward extension, they were adapted to support the Hadrianic colonnade (Fig. 2.3a). Inside the portico, the Hadrianic columns rest on a concrete foundation constructed with a reinforcing system inserted into the preceding podium (Figs. 2.2, 2.3b, 2.4, and see Plate XI).12
2. A paved floor once existed in the entrance portico at a depth of approximately 1.50 meters below the current pavement. To judge from the remains of a bed on which impressions of paving slabs are visible, this floor inclined downward from north to south (Figs. 2.3b (arrows) and 2.5, C, D).13
3. Another pavement existed inside the rotunda, at a depth of 2.15 meters below the current one (see Plates XIX, b'–b", and XX, b"). Here again, impressions of paving slabs were preserved. The discovery of two fragments of pavonazzetto (a white marble with purple or yellow veining), along with a third, unidentified one, indicates that these slabs consisted of prized colored marbles.14 The slabs were set along an east–west and north–south alignment, with alternating intervals suggesting some sort of floor pattern (see Plate XVIII, red arrows).15
4. A cohesive mass of concrete survives 1 meter below the preceding pavement and 3.15 meters under the current level. With a thickness of around 1.20 meters, this raft rested on a stratum of alluvial clay belonging to an old bed of the Tiber River (see Plates XIX, e, and XX, g).16
Past readings of this evidence led to erroneous conclusions regarding both the chronology and form of the Agrippan Pantheon, on account of a series of flawed premises. First, it was assumed that the edifice faced or opened to the south, and had a rectangular cella arranged in transverse fashion on the site of the present portico (see Fig.1.3). Second, it was assumed that the colored marble pavement 2.15 meters below the current floor belonged to an area thatwas open to the sky.17 Third, it was assumed that this space constituted a sort of forecourt to the rectangular temple cella. Fourth, it was assumed that the layer of concrete 3.15 meters under the current pavement represented the foundation for the paving of the present edifice, which the investigators considered to be Agrippa’s Pantheon.
2.2. Plan of excavations under portico in 1892–1893. A is edge of pre-Hadrianic podium; B is edge of Hadrianic concrete foundations; E is return to south of pre-Hadrianic foundation. (Pier Olinto Armanini in Beltrami 1898, Fig. XXXV, with new annotations)
2.3. a) Transverse section showing column bases and foundations of portico built over their pre-Hadrianic counterparts, and b) longitudinal section of trench cut in the central aisle of portico, revealed in excavations of 1892–1893; arrows indicate the pre-Hadrianic pavement. (Pier Olinto Armanini in Beltrami 1898, Figs. XIV and XII)
2.4. Section of pilaster framing east side of entrance portal and substructure, revealed in excavations of 1892–1893. a = substructure of present floor; b = substructure of pre-Hadrianic floor. (Pier Olinto Armanini in Beltrami 1898, Fig. X)
2.5. Annotated sketch of structures exposed under portico during excavations of 1892–1893. A = travertine extension of original pier foundations of facade; B, G = supplementary concrete foundations for inner portico columns; C, D = pre-Hadrianic pavement. (Pier Olinto Armanini in Beltrami 1898, Fig. XIII)
It was Rodolfo Lanciani who first posited the existence of a sort of vestibule that was circular in shape (see Plate XIV). He based this conclusion on the presence of a section of wall in reticulated masonry with a curved top – the so-called muro cordonato (encircling wall) discovered during the excavations and attributed to the Augustan building phase (Fig. 2.6, A, and Plate XX).18 According to his theory, in Domitian’s time this space constituted a forecourt which, via a flight of steps (under the intermediate block of the present building), led up to the south-facing oblong building located under the existing portico (see Fig. 1.3). Despite well-founded objections,19 Lanciani’s almost-undisputed authority enabled these ideas to persist until quite recently.20
2.6. Partial plan of 1892–1893 excavations in southeast quadrant of rotunda. The cross-hatching indicates the so-called muro cordonato or encircling wall attributed to the Augustan phase. (Pier Olinto Armanini in Beltrami 1898, Fig. XXV)
The 1996–1997 Excavations and Their Interpretation
Curiously enough, the excavations of Chedanne and Armanini did not in fact corroborate Lanciani’s reconstruction. Nor was it vindicated by the investigations of Paola Virgili and Paola Battistelli along the facade of the Pantheon in 1996–1997, which were published with the support of Giovanni Joppolo’s surveys.21 This recent activity has advanced our knowledge of the early structures in several respects.
The recent archaeological investigation brought to light the staircase at the front of Hadrian’s temple, which is now obliterated by the modern piazza, as well as the paving of the forecourt that lay in front of the building (Fig. 2.7, e).22 The stairs rose 1.30 meters above the Hadrianic paving of the forecourt to the temple’s podium (Fig. 2.7, d). The staircase was composed of seven steps and was adjoined by a fountain at each end. These fountains w
ere composed of Proconnesian marble basins, with brick foundations to support statues from which issued the waters of the Acqua Vergine (Fig. 2.8). The fountains should be connected with the Pantheon as rebuilt after Agrippa’s time, for beneath their remains lay traces of a preexisting building. Importantly, that building had a similar configuration to the one we admire today, except that the staircase, of which traces survive under the present one, was composed of 11 steps (Figs. 2.7 and 2.8).23
2.7. Section reconstruction of two stairs belonging to the (e) present Pantheon and (f) Agrippan Pantheon; a and b denote surrounding level and podium of pre-Hadrianic building; c and d are those for Hadrianic building. (Drawing by Giovanni Joppolo)
2.8. Actual plan of portico (top) compared to Agrippa’s portico (bottom). (Drawing by Giovanni Joppolo)
The evidence recently excavated demonstrates conclusively that the pre-Hadrianic building faced north, as Chedanne had believed and as Heinrich Nissen perceptively conjectured.24 Moreover, the columns of the Hadrianic portico stand on the travertine foundation pillars of the earlier structure, and so both phases must have had comparable intercolumniations and column diameters (Fig. 2.3).25 Because the pre-Hadrianic podium was wider than the later structure, it may have had either a decastyle (10-column) facade or an octastyle facade flanked by two antae (terminal walls) (Fig.2.8).26
The pre-Hadrianic podium slopes appreciably toward the south, as Beltrami and Chedanne detected (see Plates XI, XIX, and XXI). The cause, it would seem, was the weight of the superstructures, and the slope was partially corrected during the construction of the later foundations.27 The difference in level between the pavings 1.50 meters under the portico and 2.15 meters under the rotunda is explained by this southward slope (Fig. 2.3b).28
Although literary sources document a Domitianic reconstruction, no physical trace of it has been found, nor any carbonized debris consistent with destruction by fire.29 In any case, it is unlikely that Domitian’s intervention incorporated fundamental alterations to the structure, given that the level of the Augustan podium remained unchanged until the Hadrianic renewal.
Doris and Gottfried Gruben have hypothesized that the present entrance, with its threshold in marmo africano and great bronze doors flanked by two pilasters (see Figs. 1.8 and 7.14), belonged in modified form to the Augustan Pantheon.30 This thesis suggests that elements of the original building survived successive disasters and were carefully preserved. If true, there is all the more reason to assume that Domitian’s intervention was of little significance.31 Only the fire of AD 110 definitively destroyed Agrippa’s building. Continuity between the Agrippan portico and its Hadrianic successor would also seem to be affirmed by the reprise of the Agrippan dedication carried in the inscription under the pediment. Indeed, the inscription would be much more difficult to reconcile with a total reversal of orientation and change of structure.32
We can now turn to the part where the present rotunda stands. Its exterior face coincides closely with the wall of opus reticulatum (the so-called muro cordonato, or “encircling wall,” that Lanciani attributed to the Augustan phase; Fig. 2.6). This wall is only 61 centimeters thick and its top is rounded, indicating that it could not have supported a vaulted structure. For this reason, William Loerke conjectured an open-air or hypaethral space, perhaps ringed by a barrel-vaulted annular colonnade, an idea that has been visualized in Plate XV. The exedras of the Sanctuary of Fortuna Primigenia at Palestrina offer a parallel, as does the so-called Maritime Theatre at Tivoli (whose internal diameter is fractionally less than that of the Hadrianic Pantheon).33 In Agrippa’s building, an upper order could have masked the vault and incorporated the caryatids of Diogenes of Athens extolled by Pliny.34
Yet this hypothesis has its flaws. For example, so far as we know, spaces that were completely hypaethral – open to the sky – were not paved with colored marbles because of the inevitable weathering and wear to which they would have been subjected.35 The colored marble paving observed during the soundings of 1892–1893 (Plates XI, XIX, b'–b", and XX) appeared to be cut at the edges by the foundations of the rotunda. With patterns oriented east–west, this floor covered the entire interior space and not just the ring-shaped colonnade that would have been the only suitable location for a precious marble paving. Beltrami unfortunately offered limited information in this regard, but he did record slabs of varying sizes, and nothing excludes the possibility of larger slabs of white marble or travertine in the central area, with colored marbles under a perimeter portico. But by the same token, nothing excludes the possibility that the entire area was roofed with the exception of a central space crowned by an opaion, or oculus, analogous to the effect visible today. That traces of supports for such a roof have not been found cannot be considered a conclusive counterargument to this idea, given the limited archaeological surveys carried out in the rotunda.
In any event, the reticulated muro cordonato seems to represent the outermost boundary of the building.36 Its form recalls the enclosures of funerary monuments,37 leading us to wonder if the demarcation of the Augustan building was considered binding for its Hadrianic successor. If so, the foundation of the Augustan cella must have been swallowed up by that of the Hadrianic rotunda, which would explain why no trace of the former has been discovered. In conclusion, the new structure will likely have preserved the dimensions of the preexisting building.
The Relationship between the Agrippan Pantheon and the Hadrianic Pantheon
As we have seen, the Hadrianic rotunda fits neatly inside the Augustan precinct (Plates XIV and XV), while the facade columns sit exactly over their predecessors. The Hadrianic building, like so much Roman architecture, was set out according to an elemental geometry: In the circle locating the centers of the interior columns, a square with sides of about 32 meters can be inscribed. This measurement equals the width of the octastyle colonnade of the portico, enabling us to visualize a second square touching the first (Fig. 1.5 and Plate XII). Since the porticoes of the Augustan and Hadrianic plans substantially coincide, it seems that the mathematical scheme used for the Augustan plan provided the core from which the present Pantheon developed.38 The available documentation indeed suggests that, on the whole, the plan of the Augustan Pantheon – though not the elevation – resembled that of the Hadrianic Pantheon.39
Even if the reconstruction of the Augustan Pantheon produced by the Grubens turns out to be incorrect, their astute suggestion that the portal of the present building could have come from its predecessor adds a further element of continuity between the two structures.40 Such continuity must reflect an underlying conceptual motivation going beyond the symbolic value that the Grubens note was invested in the door since Homer’s day. The possible reuse of elements from the original building and the reiteration of its plan help us make sense of the ostentatious reassertion of Agrippa’s legacy by means of the bold dedicatory inscription under the pediment that was still relevant and even suggestive after a century and a half, even in political and sociocultural conditions very different from those of the Augustan period.
Precedents for Agrippa’s Pantheon and Its Constructive System
The Augustan structure cannot have had a concrete vault. As is generally accepted, the technical conditions necessary to vault such a large space did not yet exist. The Romans’ technical boldness from the time of the late Republican period using concrete for both innovative and structurally complex designs – including those with vaulted roofs and semicircular exedrae – was certainly exceptional. However, no surviving buildings from this period remotely approach the gigantic dimensions of the Pantheon. The so-called Temple of Mercury at Baiae, in reality a bathing hall or an artistically elaborated pool around a thermal spring, still conserves its concrete dome (Fig. 2.9).41 Circular in plan, with a central oculus and large windows at the haunches of the vault, its appearance bears some similarity to the Hadrianic Pantheon. Yet its diameter of approximately 21.60 meters is less than half as big.
2.9. Temple of Mercury, Baia. (Co
urtesy of Luciano Pedicini)
Roman architecture in the Augustan period excelled instead in the construction of trussed roofs, as confirmed by the Diribitorium, the ancient voting hall in the Campus Martius that was famed for the wide span of its roof, some 30 meters in length; similarly, the Basilica Giulia was 101 meters long and 49 meters wide, including the side aisles.42 The exedrae of the Forum of Augustus, comparable in radius to the Pantheon, had wooden roofs covered in stucco.43 Thus, large-span wooden roofs were certainly within the Romans’ technical capability. Behind it lay a Greek inheritance of buildings that were circular in plan and roofed using timber, such as the Arsinoeion of Samothrace (although this had a relatively small diameter).44 The evidence offers sufficient grounds to visualize Agrippa’s Pantheon with a cella paved in colored marble and covered by a wooden roof illuminated by a central oculus.
Such a reconstruction is consistent with the ideology and typological heritage of the monument. The conception of the vaulted space as an imitation of the starry sky and therefore of the cosmos has precedents dated well before the Augustan period.45 Cicero mentions the sphaera constructed by Posidonius “whose single rotations reproduce the motion of the sun, the moon, and the five wandering stars that occur in the sky every day and every night.”46 The frigidarium of the Stabian Baths at Pompeii is a circular structure with a truncated conical roof, central oculus, and walls frescoed with garden motifs.47 The vault was frescoed with gold stars on a blue background in imitation of a night sky.48 At the Pompeian baths, ancient visitors found themselves in an ideal setting, a garden under a starry vault with a central eye permitting a stream of natural light. Although the structure may be tiny by comparison with the Pantheon, the issue of scale does not invalidate the iconographic relationship, nor is it an isolated example. A circular aedicule with a star-studded, domed vault also appears in Pompeii in the third-style wall decoration of one of the rooms of the House of Caecilius Jucundus.49 In the ancient world, vaults often allude to this simple symbolic message: the starry sky is like a canopy, ceiling, or vault.50 In the passage introduced at the beginning of this chapter, Dio describes the dome of the Pantheon as a symbolic representation of the heavens. An echo of such associations remains in late-antique literature as well.51