Mark Wilson Jones

The fame of the Pantheon derives substantially from its wondrous engineering. The immense clear span went unchallenged for thirteen centuries until Brunelleschi raised the dome of Florence’s cathedral, and still the ancient feat is unrivaled as a work of unreinforced concrete. This prompts many questions for the casual visitor and the specialist alike. How was the building constructed? How long did it take to erect? What was the relationship between the various parts? In conjunction with the research of Janet DeLaine, Giangiacomo Martines, and Gene Waddell in this same volume, my aim is to advance these questions to the point of charting and explaining the sequence of building operations that is summarized graphically in Plate XIII.

One way of framing the inquiry is to ponder why the Pantheon has survived intact despite the passage of almost nineteen centuries, bearing in mind that so many other Roman wide-span buildings have not. It is characteristic of this enigmatic monument that the answer is not entirely straightforward. The Pantheon owes its survival to its transformation into a church in the early seventh century, yet doubtless this initiative reflected admiration for the grandeur of the Rotunda in the first place. In any event, the acquired Christian status ensured some remedy for the various injuries suffered down the ages. Most notably, as a replacement for the earlier theft of its original gilded bronze tiles, the dome received a lead covering during the reign of Gregory III in the eighth century. This represented by far the most important single protective measure – who can guess how many other imperial interiors would have survived if they, too, had had their roofs maintained? The front end of the building had a more checkered fortune. Bell towers and the like were added and removed from time to time, while a convent, shops, stores, and hovels latched on to the structure like limpets; each intervention inevitably brought a degree of destruction, contributing to the dilapidation and partial collapse of the east end of the portico.

It is perhaps surprising that more has not collapsed than just a portion of the portico. After all, the interior span comfortably exceeds any other ancient rival; the actual figure of 43.7 meters, measured from wall to wall, was determined by the axial diameter assigned to the ring of columns of 150 Roman feet (44.3 meters). The next largest surviving Roman domes spanned closer to 100 feet, this being the diameter of the misnamed Temple of Diana at Baiae.¹ Other large domes may once have existed of which we have no trace, yet clearly the Pantheon was an exceptionally ambitious undertaking even by the standards of the high imperial period.

The Pantheon’s survival depends most of all on the technical quality of Roman construction, which reached its apogee in the first half of the second century AD. This derived from a long tradition of intelligent experimentation with materials (primarily brick and concrete) and spatial-constructional units (arches, vaults, and the special kind of vaults we call domes).² Illustrative of the attention to technique of the Pantheon’s builders is the grading by density of the aggregate in the concrete, with the heaviest at the bottom and the lightest at the top (see Fig. 1.12). In the upper parts of the dome pumice (or, rather, scoria) was used, a lightweight volcanic material from the region around Vesuvius, which was acquired only with difficulty after the eruption of AD 79 had covered the best deposits under inferior material.³ As Martines relates in Chapter Four, Roman builders also placed much faith in so-called relieving arches, and here too the Pantheon provides the most elaborate known array (Fig. 7.1, and see Fig. 1.13).

7.1. Armature of relieving arches embedded in the Pantheon. (Drawing Mark Wilson Jones and Robert Grover)

The Pantheon stands today, triumphant, yet the grandness of its ambition tested the Roman building machine almost to the point of disaster. It is impossible to say exactly how close failure came; all we can do is witness signs of structural distress and constructional difficulty. These are particularly notable in three respects: the long cracks that fracture the rotunda at intervals; the building complex that butts up against the rotunda to the south, the very existence of which speaks of emergency; and the curious mismarriage between rotunda and portico. While not in itself a structural issue, this, it seems, was bound up with problems in obtaining the massive column shafts originally intended. This essay looks into these matters and reflects on the drama of the Pantheon worksite. As I see it, the project was balanced on a knife edge between success and failure. It was success that prevailed – but at the cost of perfection compromised.

Cracking, Concrete, and Centering

The rotunda displays an array of more or less vertical cracks. Typically, they run from about halfway up the rotunda to halfway up the dome. The mapping of these cracks in the 1930s by Alberto Terenzio during restoration works gives us the best idea of their scale and frequency (Fig. 7.2).⁴ Many can be seen in old photographs, though today most are obscured by surface finishes or by modern repairs. Nonetheless, parts of some can still be seen on the outside of the rotunda, while one can be traced in the staircase on the east side of the entrance. The largest crack coincides with the main axis on the south side, measuring up to 7 centimeters in width where it can be accessed from behind the rotunda.

7.2. Interior elevation of the rotunda, projected flat, showing the principal cracks in the structure. (Wilson Jones 2000, Fig. 9.21a; drawing by Ippolita D’Ayala Valva, after A. Terenzio)

Cracking is important for the way the dome behaves in terms of statics. It performs less like a modern monolithic shell reinforced with steel and more like an array of tapering sections of masonry comparable with segments of an orange. This is consistent with the effect of outward lateral thrust and hoop tension, both being characteristic of unreinforced domes with profiles based on arcs of circles (as opposed to catenary curves, which offer a reduction in tensile forces).⁵ Deformation of the section resulted, with the interior of the dome no longer matching an ideal hemisphere. Recent survey work with laser scanners conducted by the Karman Center reveals that the crown has slumped by around 1½ feet (45 centimeters) with respect to its presumed original hemispherical form. In relative terms, this equates to only about 1 percent of the total height, but in real terms, it still represents a significant shifting of stress and mass. A further cause of structural distress was settlement, for the Pantheon rises not on rock but on clay.⁶ The floor level slopes down by almost 40 centimeters from the front of the portico to the far side of the rotunda (see Plate XXI).⁷ Most of the cracking would have occurred early in the life of the building, though not necessarily all at once when the centering used to shape the dome was dismantled. Some cracking seems to have arisen even before the dome was put in place, as we shall see.⁸

XXI. Section combining information from excavations under portico and rotunda, with sloping floor of Agrippan Pantheon shown in dashed line and XXXX shown in solid line. (Pier Olinto Armanini in Beltrami 1898, Fig. XV)

It would be a mistake to make more than a casual analogy between Roman lime-based concrete and its modern cement-based equivalent, which is typically poured in one go in a relatively liquid state (with formwork initially supporting its entire weight).⁹Modern concrete includes steel to provide tensile strength and combat cracking. By contrast, the Romans addressed performance by varying the density of aggregate, by incorporating relieving arches, and by manipulating the cross section. In fact, in understanding the dome of the Pantheon, it is crucial to distinguish between the upper half, which is a relatively thin shell, and the lower half, which is much thicker and has a quite different profile (see Figs. 1.12 and 6.3). The lower part of the dome is as thick as the drum and contiguous with it; at the crowning cornice of the drum, the section reduces (making space for a perimeter walkway), thereafter diminishing in a series of large steps until the start of the upper shell. None of this can be perceived inside the building, where the visual effect is largely determined by the coffering that follows its own independent geometry.

Construction of the lower part of the dome proceeded by stages, as for the rotunda wall. Ring followed ring, each diminishing in diameter, typically in lifts of 5 feet or so. The concrete was laid relatively dry, in more or less horizontal strata of mortar and aggregate in predominantly fist-sized pieces. Each stage would have been allowed to cure substantially before the next was added. By virtue of closing in on itself, each ring, once complete, could support not only itself but also the next ring slightly smaller in diameter, and so on, creating in effect a kind of corbeling. Higher up, after the top of the step-rings where the section is thinner, the vault was flatter, therefore demanding some kind of support up until the time when the concrete set (or “went off”). Finally, at the very top, the device of an oculus represented a wonderful solution: avoiding construction, lightening the dome while lighting the finished space, besides contributing to its symbolic mystique.

It is theoretically possible to build a lime-based concrete dome without any temporary support.¹⁰ But as regards Roman practice, there is plenty of evidence for the use of formwork; witness the imprints of wooden boarding on the vaulting of many a ruin, including Nero’s Golden House (the Domus Aurea), Trajan’s Markets, and Hadrian’s Villa. It is, unfortunately, impossible to obtain this kind of information for the Pantheon, for the rendered internal surfaces of the dome that we see today are the result of only the latest of a number of restorations, some of which date from a time before it had become customary to document the existing state prior to the commencement of work. The possibility of self-support can be reconciled with the use of formwork if we suppose that a temporary wooden assembly provided structural support for the upper parts alone. For the lower parts, the prime function of the formwork, just as its name suggests, was only to mold the form of the concrete. This was necessary for the geometrical precision of the Pantheon coffering, which creates such a magical dance of chiseled planes of light and shadow.

How was the wooden formwork itself erected? Some authorities opt for a system supported from the ground for the full width of the interior. William MacDonald visualizes “an immense hemispherical wooden form, supported by a forest of timbers and struts.”¹¹Recoiling at the consumption of trees on such a scale, others have imagined centering “flying” across the entire space without vertical supports. Proposals in this vein include those put forward by Eugène Emmanuel Viollet-le-Duc (Fig. 7.3, left) and most recently Rabun Taylor (Fig. 7.3, right).¹²

7.3. Proposals for the centering used to construct the dome. (Left, Viollet-le-Duc 1875, p. 475; right, Taylor 2003, Fig. 120)

Any uniform system of centering, however, seems to be contradicted by the marked difference between the lower and upper halves of the Pantheon dome that has already been highlighted. Accordingly, I visualize a very substantial wooden tower rising from a doughnut-shaped plan, with a ring about 11.5 meters wide (Fig. 7.4).¹³ The upper portion of the dome, being relatively thin, was light enough to have been carried on such a timber structure.¹⁴ Nothing, of course, was needed underneath the void of the oculus, nor indeed for the lower portion of the dome. Since this could be raised by corbeling, all that was required here was non-load-bearing formwork. This part of the wooden assembly alone would have “flown,” spanning a gap of 5 to 6 meters between the drum and the doughnut-shaped tower.¹⁵

7.4. Schematic cross section showing extent (in gray tone) of a hypothetical doughnut-shaped centering tower for constructing the dome, 1:600. (Drawing Mark Wilson Jones)

The quantity of timber consumed must have been considerable, but not beyond the Romans’ capabilities. They had at their disposal extensive forests of oak and sweet chestnut not far from the capital, and are known tohave employed very large timbers, for example, for the trusses spanning the 25-meter-wide nave of Trajan’s majestic Basilica Ulpia. In a treatise on the construction of siege structures, the Poliorcetica, this emperor’s architect, Apollodorus, expounded on the assembly of giant towers using small timber members. The surviving copy, which dates to the Middle Ages, has illustrations that convey an almost naive impression, but the original versions may well have been more precise and technical.¹⁶

By virtue of experience, imperial architects must have been aware that domes pushed outward at the haunches. Three main counterstrategies were adopted in the Pantheon. The first is a very thick supporting wall (thicker than simple vertical loading would require), which, to save materials and weight, was hollowed out by voids in the form of exedras and chambers (see Chapter Four). The second is the carrying up of the drum to a higher level than the springing of the dome, thus creating a mass of weight resistant to lateral movement. The third is the most obscure in its functioning: the vaults and arches embedded within the concrete known as relieving arches, which were made using tile-shaped bricks, mostly 2-foot-square bipedales with some 1.5-foot-squaresesquipedales. As already noted, the Pantheon boasts the most elaborate known arrangement of relieving arches (Fig. 7.1). In part, they served to direct loadpaths to points of greatest strength, the eight “piers” of the rotunda plan. They also facilitated constructional processes, an important consideration for Roman builders. Since brick and mortar cured faster than concrete, the use of relieving arches enabled work to proceed upward faster than would otherwise have been the case.¹⁷ Although it is hard to know the full range of the ways in which they work – or were thought to work – we can still judge them, almost two thousand years later, wonderfully efficacious.

The Grottoni

In spite of these strategies, the stability of the Pantheon was not a foregone conclusion – indeed, it was evidently a matter of great concern for the builders. This is demonstrated by the annex of structures sandwiched between the rotunda and the adjacent basilica to the south (Fig. 7.5, and see Fig. 6.5). Parallel walls and associated floors and vaulting delimit a series of spaces on two levels that are collectively known in Italian, rather suggestively, as the grottoni. Above them, on the main axis, a solid brick arch supported a kind of bridge connecting the basilica with the rotunda. This whole complex constituted, in effect, a gigantic buttress, as may be deduced from the lack of any obvious ceremonial or utilitarian purpose, along with the crude fashion in which it butts up against the rotunda.¹⁸ Indeed, it is plain to see that the lower parts of the grottoni are not bonded with the rotunda.

7.5. Rotunda viewed from the south, above the grottoni. Note the scarring (particularly evident at and above the level of the three openings visible in the middle of the photograph), which testifies to the presence of a lost connection or “bridge” with the basilica to the rear (south) of the grottoni. (Photo Gene Waddell)

It is generally assumed that the grottoni were created after the completion of the rotunda, as an improvised post facto countermeasure to resist its outward pressure. However, my own observations suggest that work on the grottoni began relatively early. The key here is the connection between the rotunda and the structure overhead. Instead of casually butting up to the rotunda as occurs at low level, the “bridge” has a cornice that meets the middle cornice of the rotunda at a bonded miter, or, in other words, in a premeditated relationship.¹⁹ The springing of the arches of bipedales is integral with the rotunda, as shown by photographs taken at the time when parts of the grottoni were rebuilt, and as is still observable at high level (Fig. 7.5). Where fabric associated with the bridge has been lost, the drum is not smooth, as we should expect had an extraneous construction simply fallen away. Instead, material integral to the rotunda has been pulled away.²⁰ Relieving arches belonging to the bridge once thrust into the rotunda, hence the faces of bipedales that are now exposed to view. Despite the lack of bond below, then, at high level construction is all of a piece.

This rather extraordinary state of affairs suggests, firstly, that the grottoni were initiated after the drum had risen to around a third of its height, and, secondly, that they were built speedily so as to catch up with the drum. This occurred before the dome was begun (or, at any rate, before it curved inward to a significant extent). All this suggests that the grottoni were built very fast. A rapid pace of work is attested at Trajan’s Baths by dates inscribed in red pigment on brick-faced concrete walls of broadly comparable width with those of the grottoni; the dates indicate that over a period of around two and a half months, one wall rose by an astonishing 15 meters. The vast substructures of the baths, comprising many other walls equally tall, were probably executed in a single season.²¹ We can only speculate how fast the grottoni were built, yet bearing this comparison in mind, a couple of years or less is not out of the question.

It seems, therefore, that the grottoni respond to a problem that occurred early, before the addition of the dome. The nature of the problem is suggested by the huge crack, already mentioned, that fractures the rotunda approximately on the main axis, where the wall defining the apse is at its thinnest. Unlike other cracks, which tend to peter out earlier, this one reached floor level.²² The cause could be settlement of the foundations, although this cannot be proved without a geotechnical investigation. In short, the grottoni were built so as to minimize the further movement expected when the thrust of the dome came into play. The intervention can be judged a success; despite the alarm it registers the Pantheon stands.

The Connection between Rotunda and Transitional Block

Different problems affected the north end of the Pantheon where its three main parts meet: rotunda, portico, and the structure in between. This is known in Italian as the avancorpo, and, rather less elegantly, as the “transitional block” or “intermediate block.” As noted in the Introduction (Chapter One), over the centuries the relationship among these three parts has provoked markedly contrasting interpretations. Traditionally, the explanation was thought to lie in (various different) phasing sequences, with the rotunda usually being presumed to have been built before the rest.²³ Bound up as it is with perceived compositional shortcomings, the debate has an inevitable subjective component, and so it makes sense to address objective constructional realities first.

The junction between rotunda and transitional block can best be observed in the two staircases on either side of the entrance. Unlike other parts of the building, there is no marble revetment here to hinder inspection, while the stairs facilitate access for the entire height – an enormous practical advantage for the purposes of study. Achille Leclère, one of the long line of prize-winning architects awarded a period of residence at the French Academy in Rome, included a small-scale survey as part of his envoi of 1813 on the Pantheon.²⁴ Otherwise this part of the building has been neglected, leading me to make a new survey in 2005 and 2006 of the east stair, the better preserved of the two, yielding the drawings illustrated in Figures 7.6, 7.7, and 7.8.²⁵ These also help locate pertinent details, such as the soundings, or saggi, made by previous investigators where the rotunda wall (labeled A) meets the side walls (B and D) of the transitional block.

7.6. Pantheon, east stair, section. (Drawing Mark Wilson Jones and Robert Grover)

7.7. Pantheon, east stair. (Drawing Mark Wilson Jones and Robert Grover)

7.8. Pantheon, east stair, plan. (Drawing Mark Wilson Jones and Roberta Zaccara)

Today, the east stair is entered from one of the two great apses of the portico, the ancient doorway on the flank having been blocked up. The stairs have suffered reconfiguration at the top and bottom, but otherwise remain essentially unchanged. The trapezoidal plan makes six full turns plus an extra seventh flight against the curved wall of the rotunda. They afford access to several different parts of the building: to the semicircular chambers in the drum on three levels, to the suite of rooms fronting the transitional block occupied by the Virtuosi of the Pantheon, to the entablature of the portico, to the middle cornice of the rotunda, and finally to the roof (Figs. 7.6, 7.7, 7.8, 7.9).

7.9. Junction of the rotunda, transitional block, and portico on the east side, at high level. (Photo Mark Wilson Jones)

Inspection of the staircases shows that the rotunda and the transitional block are united at low level, but disunited at high level. It seems that both rose as one until somewhere in the region of 12 to 14 meters from the floor of the portico. From then on, work evidently proceeded on the rotunda alone, pending the completion of the transitional block.

At high level, the disjunction is obvious to the untrained eye. Wherever the rotunda is exposed to view it presents finished surfaces that can only have existed if it were built first (Plate XXII). Since the Pantheon stands intact and not exposed for study like a ruin, the unity of the lower parts is less glaringly evident, yet nonetheless inescapable. A key piece of evidence is a sounding, or saggio, located on the second short landing of the west stair, at the junction between the rotunda and the transitional block (see Plate XXIII). The ample view it offers into the “guts” of the fabric (the sounding reaches 63 centimeters deep) reveals no gap, crack, or joint, and the mortar traverses uninterrupted. In addition, there is a course of bipedales that passes unbroken from one part to the other, including a whole bipedalis right where they meet. It would have been quite impossible to insert so large and brittle an element after the original construction.²⁶ So both the rotunda and the transitional block rose together at low level, although about halfway up, construction advanced on the rotunda while that of the transitional block was held back.

XXII. Pantheon, east stair, sounding “S7” near the top of the rotunda. (Photo Mark Wilson Jones)

XXIII. West stair, detail of sounding on level 2. This shows the “gut” of the construction at the junction between the rotunda and transitional block. Note the continuity of mortar and aggregate, as well as a whole bipedalis (indicated by arrow) that traverses the junction. (Photo Mark Wilson Jones)

XXIV. Manfredo Manfredi, permanent tomb of Vittorio Emanuele II, lateral niche of the Pantheon, begun 1884. (Photo Robin B. Williams)

These observations effectively eliminate all previous proposals that would claim that the main parts of the Pantheon were built completely separately. That the rotunda was never planned to stand on its own is further confirmed by the connections between the staircase and the chambers encased within the drum (Figs. 7.7, 7.8). These connections, being perfectly intact, were part of the original construction. This confirms what has become evident given the other considerations mentioned: the stairs were anticipated from the outset. And if the stairs were envisaged, so too must have been the transitional block as a whole.

Inspection of the brickwork surfaces and of the courses of bipedales running around the stair offers further clarification of the relationship between parts of the fabric. The courses of bipedales are particularly instructive, for they traverse at intervals the entire thickness of construction, like layers in a layer cake (Fig. 7.7 and Plate XXIII). In the lower half of the staircase, the bipedales, save for a few exceptions, run at the same level around all four walls, which suggests that these were coeval. This coordination is less pronounced higher up, but one bipedalis course on the sixth turn of the stairs runs right around the staircase and all the way to the dome. That this occurs in spite of the separation between the rotunda and the transitional block points to the temporal proximity of the entire complex, suggesting that work on the latter only suffered a short-lived hiatus. Operations must have resumed on the upper half of the transitional block quite quickly, probably within a year or two.²⁷

The Connection between Transitional Block and Portico

The excavations of the 1890s supervised by Luca Beltrami indicated that the foundations of the existing portico and those of the transitional block were made at the same time. This was later confirmed by A. M. Colini and Italo Gismondi in their study.²⁸ They also reinforced Leclère’s observation of the continuity displayed by the entablature running longitudinally, noting that the blocks incorporating the capitals of the pilasters in the portico are embedded into the fabric of the transitional block too deeply to have been inserted in a separate epoch.²⁹ (Part of the rough marble of a capital block can be seen where it disappears into the masonry in Figure 7.10, right, though it is impossible to gauge the full extent of its penetration; this is indeed considerable, as I have been able to observe from close quarters when there was scaffolding in place.) Colini and Gismondi also noted that the brickwork facing the transitional block has a single inclined line of bipedales tracking just above the portico roof, but not at exactly the same angle. This suggests perhaps that the roof was contemplated when this part of the transitional block was built, though some adjustments came to be made in the course of execution.³⁰

7.10. Pantheon, vestibule, and transitional block at the junction with the portico. Note the unusual grouping of pilasters, and in particular the conjunction of one that forms part of the transitional block with the three-pilaster-faced anta. All four of the antae in the portico have sides toward the great niches that are wider than the other faces. This creates a “leftover” rough portion on each of the capital blocks, since the capitals proper are maintained the same width throughout. The result may be seen on the far right. This and other capital blocks are embedded into the fabric of the transitional block. (Photo Mark Wilson Jones)

The portico and transitional block were, then, planned together and their joint foundations implemented together. The portico cannot have been added in a completely separate campaign. This does not mean, however, that both marched exactly in step. In fact, the raising of columnar structures was normally carried out after completion of any associated masonry structure (see Chapter Six), while there were also reasons that led to a greater delay than normal in this particular instance, as we shall see.

To summarize our examination so far, the fabric of the Pantheon reveals the following:

·         All three main parts of the Pantheon, rotunda, transitional block, and portico, belong to a unitary initial conception.

·         At the south end, the grottoni were not part of the original project; they were added after the commencement of the rotunda, but were built so quickly as to catch up and become united with it before the dome was far advanced.

·         At the north end, the rotunda and transitional block are bonded at the bottom, but about halfway up the elevation the procedure changed. Work was next carried forward on the rotunda alone, with the rest of the transitional block following on soon afterwards.

·         A portico was planned as part of the project from the outset, but it was the last major part of the edifice to be implemented.

The curious phasing of the grottoni can be explained as a response to concerns about the stability of the rotunda, while building the portico last made practical sense. But how can we explain the interruption of work on the transitional block?

Can there be an explanation of a structural kind, for example differential settlement between the rotunda and the transitional block? However, there are no signs of such. Those lesions that are present in the staircases respond to the general pattern of cracks affecting the rotunda as a whole, and they tend to peter out before ground level. There is no cracking visible in the sounding in the west stair where the rotunda and the transitional block intersect (Plate XXIII). Nor are there any significant lesions in the side walls of the staircase (those that run north–south). What explains, then, the hiatus in building the latter? This is an issue, I contend, that cannot be resolved by focusing on construction alone. It is now time to turn to issues of design that might bear on the same puzzle.

The Front of the Pantheon and the “Compromise Hypothesis”

While the structure of the Pantheon solicits both wonder and alarm, its design has historically provoked just as varied responses. Alternating between praise and criticism, the paradoxical fortuna of the monument has been charted lucidly by Tilmann Buddensieg,³¹while surfacing in the Introduction to this book, and in some chapters in the second half. Criticism of the interior was mainly directed at the attic level, and especially the pilasters for being too small and for not aligning with either the main order below or the coffering above. This can be understood as a misplaced faith in academicism, which tended to dominate from the time of the Renaissance, and in particular the “law” of vertically aligning like with like. Instead, we may delight in a coherent scheme that spurns a predictable radial solution for the sake of a genuinely dynamic experience. The “push and pull” effect of the openings and exedras on the eight main axes is accentuated by compositional alignments avoided elsewhere.³² (The proof that all of this was deliberate lies in the way the floor pattern meets the rotunda, being similarly synchronized exclusively on the eight main axes.) Because it is not so obviously tied visually to the drum, the dome appears to hover with an indefinable magical quality.³³

Criticism of the exterior has concerned the difficult marriage of the rotunda, transitional, block and portico, as exemplified by the abrupt termination of the entablature where the circular and orthogonal geometries meet (Figs. 7.9, and 7.11, and see Fig. 1.9). Along with various “solecisms” – offenses to the classical “grammar” of the orders – this lack of unity used to be seen as the legacy of separate phases. Giorgio Vasari related how many artists of his time, “Michelangelo among them, are of the opinion that the Rotunda was built by three architects, the first carrying it up to the cornice above the columns, the second doing from the cornice upwards.... [T]he third is believed to have done the beautiful portico.”³⁴ As late as the 1930s, Giuseppe Cozzo, a specialist of Roman construction who should have known better, continued to maintain that the rotunda was built first (in the time of Agrippa), and the rest later (in the reign of the Severan emperors).³⁵ This was an attempt to reconcile the main Agrippa inscription emblazoned on the frieze of the portico with the secondary longer inscription in smaller letters on the architrave.³⁶

7.11. Junction of the rotunda, transitional block, and portico on the west side. (Photo Mark Wilson Jones)

But the enigma of the Pantheon is not to be solved in this way. Following the work of Georges Chedanne, Luca Beltrami, and Heinrich Dressel in the 1890s, scholars had to accept the implications of brickstamp studies (see Chapter Three). Leaving aside for a moment the precise dates implied, these showed that save for later repairs and alterations, the whole edifice was erected more or less in one go. What explains, then, the character of the design? The inept collision of rotunda, transitional block, and portico continued to elude a positive interpretation, representing something of an embarrassment to be sidestepped as deftly as possible by anyone writing about the Pantheon in the course of the twentieth century.³⁷

Paul Davies, David Hemsoll, and I attempted an explanation of a quite different kind in 1987, arguing that the front of the Pantheon is not what was originally intended, but rather the outcome of compromises induced by circumstances beyond the architect’s control. The “compromise hypothesis” proposes that the portico was originally planned to have a roof at the level of the existing upper pediment, a roof supported on columns incorporating monolithic shafts of Egyptian granite 50 feet in length and 100 tons in weight (Fig. 7.12, Plate XVII).³⁸ For some reason unknown – perhaps because a consignment of the intended shafts had sunk at sea en route between Alexandria and Rome – only after work had started on site was the decision made to employ 40-foot shafts instead.³⁹ It is important to note that Roman monolithic column shafts tended to be standardized in multiples of 4- and/or 5-foot lengths, and that they were typically worked up to near-finished form in major quarries across the Mediterranean world. Large examples can still be seen where they were abandoned in several quarries, including those of Mons Claudianus, the source of the shafts for the front file at the Pantheon. In the context of grand imperial monuments, 40 footers represented the next major step down from 50 feet, and it was this smaller size that was actually used. Forty footers were also much more common (and they weighed only half as much, though this was probably not the key factor).⁴⁰

7.12. Pantheon plans and elevations, intended and as executed. (Wilson Jones 2000, Fig. 10.12)

Although it should perhaps not be admitted in the politely serious domain of scholarly discourse, our article of 1987 was conceived by chance in a London pub after a day studying other things in the Warburg Institute, while sketching from memory. But what started as a bit of speculative amusement came to take on substance upon further research. Calculation showed that 50-foot shafts were perfectly commensurate with an order rising to the cornice running around the rotunda and the start of the upper pediment. Meanwhile, scrutiny brought into focus the solecisms that had worried so many past commentators, while revealing some previously unnoticed ones. In effect, there is quite a tally of features that are sufficiently unusual or perverse as to raise the question of whether they were really intended in the first instance. Here follows the list of points as they stood in the year 2000:⁴¹

i. The transitional block is faced with an accessory pediment that is partially cut off by the main roof (Fig. 7.9). No known earlier building has a comparable arrangement save for the Propylaea of the Athenian Acropolis.

ii. The entablature of the portico terminates abruptly at the rotunda, failing to align with the moldings of the latter (Figs. 7.9, 7.11).

iii. The portico pediment is exceptionally tall in relation to the height of the order (Fig. 7.12, and see Plate I), to judge by the proportions of other Roman buildings of similar size, such as Augustus’s temple of Mars Ultor.

iv. The cornice brackets or modillions of the portico pediment are smaller and are spaced at more frequent intervals than those of the upper pediment, despite the fact that both pediments are the same size (Fig. 7.9).

v. The gaps between the columns, or intercolumnations, are unusually large relative to the column diameter when compared with most other monumental imperial colonnades (although widely spaced rhythms did also exist).

vi. The antae in the portico are oddly unbalanced. The sides facing the great niches are wider than the rest, an arrangement that gave rise to an unsatisfactory resolution of the capitals overhead (Figs. 7.10, right; 7.13, partial plan)

vii. The central aisle of the portico becomes narrower where it enters the transitional block; here is a peculiar grouping of pilasters, as if the ones nearer to the entrance door were added after the others were already in place (Figs. 7.10, 7.14).

viii. Where the portico meets the transitional block the entablature steps out by a small amount, one neither so small as to be insignificant nor so big as to constitute a positive feature (Fig. 7.11).

ix. The transitional block is only bonded with the rotunda in the lower levels of the building. In the upper parts, it merely runs up against the rotunda, as has just been confirmed in the preceding discussion (see Fig. 1.9).

All such solecisms and curiosities would simply not have existed in the hypothetical original project (Plate XVII). Scholarly responses to the compromise hypothesis have been favorable, though of course not everyone is convinced.⁴² Lothar Haselberger, the author of important publications on the building, hastaken issue with such an approach, highlighting the danger of presuming that we can know what ancient architects intended, along with specific objections to some of the points just outlined.⁴³ For him, the juxtaposition of exacting details with various “misfits” could reflect an approach to making buildings that we have yet to fully appreciate, in this as in other imperial monuments.⁴⁴ His is wise council, but yet the juxtapositions he notes (precise-imprecise, regular-irregular, and so on) seem to me not so much to detract as perhaps to add to the argument, for they remain easier to understand in a situation where a design had to be compromised, delayed, and possibly rushed as a consequence. In any event, not one of the previous points has been definitely disproved. And no one has yet been able to show how and why we should positively celebrate the front of the Pantheon in the same way as we certainly can the interior.

7.13. The portico as built (top) and as intended (bottom). Transverse section through the portico, with the transitional block and rotunda seen in elevation, 1: 400, with part-section top right and part-plans in the middle. (Drawing Mark Wilson Jones)

7.14. The vestibule and door, seen on axis with view through to the rotunda beyond. Originally a bronze, suspended, vaulted ceiling would have abutted the reveal of the masonry barrel vault over the vestibule. (Photo Maxim Atayants)

Other responses to the compromise hypothesis take the form of qualified support, such as that published by Rabun Taylor in his book Roman Builders; he runs with the idea of a compromised Pantheon portico, but adapts it in favor of hypothetical columns that were taller still.⁴⁵ Yet Taylor’s 55-foot shaft size is not able to appeal to evidence of the same kind that favors 50 footers. One such is a letter on papyrus from an Egyptian contractor dating to Hadrian’s reign. It calls urgently for fodder for animals involved in transporting overland a single 50-foot granite shaft from the quarries at Mons Claudianus to the Nile, and thence to Alexandria, from where it would in all certainty have been bound for Rome.⁴⁶ Another coincidence concerns the working drawing template of the Pantheon pediment inscribed on the pavement in front of the Mausoleum of Augustus (see the Introduction). The pediment crosses over the plan of a capital that is too big for the actual building, but which freakily happens to be precisely the size that the hypothetical project predicts.⁴⁷ A much damaged 50-foot granite shaft lies under the sea off the ancient port of Alexandria, while the possibility of 50 footers arriving in Rome is underlined by the recent discovery of stumps in the imperial fora.⁴⁸ The find-spot points to their use in the focal cult building of the vast precinct known as the Temple of Peace, begun in the early 70s AD. This adds to the very rare instances of 50-foot monolithic shafts already known in the city: the broken one lying by Trajan’s Column and companions under the nearby palazzo Valentini, which belonged to the Temple of Trajan, as well as the shaft of the Antonine column, which was also of Egyptian granite and extracted in the time of Trajan. Trajan’s Baths is another candidate.⁴⁹ The quarrying, transportation to Rome, and erection of such 100-ton monster stones was thus a feasible imperial ambition, albeit one so audacious that it stretched the Roman building machine to the limit.⁵⁰

It is possible to marshal further fresh evidence in favor of the compromise hypothesis. My inspection of the staircases has established once and for all that the rotunda and transitional block are united at low level, and so part of a unified project. The compromise hypothesis offers an explanation for the interruption of work on the transitional block; furthermore, it fits neatly with a hiatus of relatively short duration during which the design was argued over and revised.

It is also worth scrutinizing once more the relationship between the transitional block and the portico. While Colini and Gismondi’s observations, discussed earlier, concerned the actual portico, certain constructional details fit a hypothetical taller one. The original sequence of nine numbered points embraced by the compromise hypothesis can now be extended with reference to a sectional elevation of the transitional block in both its actual and intended form (Fig. 7.13).⁵¹

x. The 10-foot-wide concrete strip foundations under the portico are unusually wide for the columns they carry, and would have been adequate for larger columns (Fig. 7.13, A).⁵²

xi. At high level, the front face of the transitional block presents some unsightly projecting blocks (Fig. 7.13, B and C). These facilitated construction in some way or other, though exactly how rather baffled Colini and Gismondi. With a hypothetical taller portico, all such blocks would have been hidden from view between the suspended ceiling and the roof.

xii. The profile of the transitional block sets back where the cornice demarcates the high-level register, just as occurs on the rotunda (Fig. 7.13, D). This set-back follows the classical principle of recession, in tune with structural logic (walls high up in a building need not be as thick as those below). On the front of the transitional block, moreover, the set-back tracks the upper pediment, which was therefore an integral feature of the composition. This arrangement makes most sense if a roof had been planned to arrive here – that is, that of the hypothetical taller portico.⁵³ (Contrariwise, there is no such set-back at the level of the existing portico roof.)

xiii. The ancient bronze trusses that once spanned the portico displayed oddities of configuration, as is clear from surveys made before 1625, including one by Borromini at the time this singular assembly was taken down (see Fig. 10.1).⁵⁴ In particular, the tie beams over the central aisle did not reach far enough to be seated over the colonnades, and were instead supported by raking struts (Fig. 7.13, E). As Louise Rice has observed, the design need not have been so complicated had the portico achieved its intended height.⁵⁵ The other puzzle she highlights is the lack of space in the side aisles for semicircular vaults; such a configuration would have conflicted with the trusses (Fig. 7.13, F), and so the ceilings here must have been either flatter than semicircular, or completely flat.⁵⁶ It is significant that the original project offers a majesty and simplicity entirely in tune with the rotunda and dome. The ideal portico could have accommodated semicircular barrel vaults for all three aisles, and not just over the central aisle.⁵⁷ Considering the impact of the one section of barrel vault that remains in place, the brick and concrete portion over the vestibule (Fig. 7.14), the effect of the original tripartite and even loftier arrangement would surely have been astounding. Crucially, the main aisle would have been spanned using simple trusses supported over the columns.

At the same time, the original design was consistent with the following advantages: a total height, measured to the peak of the pediment/roof would have been 100 feet (more or less), an eminently satisfying round dimension that echoed other key dimensions (e.g., the 150-foot diameter of the ring of interior columns, the 75-foot datum for the entablature and middle cornice of the rotunda, the 60-foot height of the columns, and the 50-foot height of their shafts). The relatively steep pitch of the pediment is now explained; this particular rake was a necessary ingredient for sweetly resolving these various conditions and intentions.

These last points, especially xi and xii, suggest that when work resumed on the transitional block, there was possibly still the intention to achieve the taller portico. But other features suit the actual portico, including the inclined line of bipedales just above the roof that Colini and Gismondi observed, and the embedded capital blocks already mentioned. As regards the latter, it is noticeable that these are not neatly encased in the masonry as would befit work made all of a piece; there is a slight gap to the sides that would be consistent with their having been lowered and levered into a seating that was fashioned at a later stage to the initial building of the masonry in this area.⁵⁸ Following the nonappearance of the desired 50-foot shafts, it seems that there was an uncertain phase when both options – to use 50 or 40 footers – were kept open pending a definitive decision.⁵⁹ The choice of the latter was probably put off until the last possible moment, inducing a hastiness that contributed to the messiness of the final outcome.

The compromise hypothesis, then, can potentially account for most, if not all, of the design puzzles that the Pantheon presents on its entrance side. It also concurs with the relative phasing of construction. But can we be more precise and pin down the specific dates involved?

Brickstamps

The practice of imprinting bricks and other Roman building products of fired clay with the identification marks of individual production units (officinae) and their parent brickyards (figlinae) happens to have been particularly prevalent in the years spanning Trajan’s and Hadrian’s rule. Usefully, for study purposes these stamps can be dated either roughly or in some cases to a particular year.⁶⁰ This assigns any building in which they are found a terminus post quem; the building must have been erected after the bricks were made, although it is hard to say precisely how much later.⁶¹

On the basis of the prewar studies of Herbert Bloch and Julien Guey, no less than 115 of the 120 stamps observed in situ in the Pantheon belong to the late Trajanic or early Hadrianic period.⁶² It is significant that similar stamps are dispersed in different parts of the building.⁶³ Thus, all of the brick and concrete parts must be roughly contemporary, including not only the grottoni but also the upper parts of the transitional block. This evidence confirms that the former was built quickly, and that the interruption of work on the latter was brief.

Establishing exactly when works on site began is controversial. As Lise Hetland shows in Chapter Three, the brickstamps that can be dated precisely, or relatively precisely, are mainly late Trajanic. Bloch argued that the Trajanic shipments were stockpiled, not to be taken up until Hadrian instigated the project after coming to power in the middle of 117. Exposing a certain circularity in Bloch’s position, Hetland argues more straightforwardly that the project was Trajan’s, in line with Wolf-Dieter Heilmeyer’s ideas of the 1970s based on stylistic comparisons.⁶⁴ And is it not more logical, asks Hetland, that Trajan commissioned a replacement Pantheon sooner rather than later after the fire of 110 that ruined its predecessor? In short, a start date between 112 and 115 is more likely than one around 118.

The key consideration for the end date is that in AD 123, a higher than usual proportion of bipedales were produced bearing brickstamps, often with the names of the then-reigning consuls Apronianus and Paetinus. The absence of such stamps in the superstructure of the Pantheon shows that it must have been completed by this time or soon after, in other words by around 124.

It is revelatory to focus on a single stamp that does not fit the general pattern. This is the sole example from the whole building that is unambiguously Hadrianic, one recorded by Rodolfo Lanciani and datable to AD 123. Bloch was struck by the anomalous character of this find, in effect adding another enigma to the building that Lanciani called the “Sphinx of the Campus Martius.” Bloch knew that the rigors of his discipline were unassailable; no structure can be earlier than the latest stamp present (provided it is not connected with out-of-sequence working or repairs). Having been found close to ground level, did not this one stamp postpone the start of construction to later than 123? Bloch resolved this dilemma by supposing that Lanciani had simply been mistaken.⁶⁵

Lanciani’s record, however, sounds as if it were accurate: “read by myself on the 25th of April on a piece [scaglia] of brick extracted from a sounding made by the north east corner of the brick front, behind the marble pilaster.”⁶⁶ Rather than doubt his word, there is a way of reconciling it with the evidence of all the other brickstamps. The key is the find-spot, just behind one of the marble pilasters, that is to say exactly where the columnar system of the portico meets the transitional block. In all likelihood, Lanciani’sscaglia would have formed part of fill material that accompanied the positioning of the pilasters and the erection of the columns.⁶⁷ Given that the portico could have been put up relatively rapidly (thanks to the prefabricated bases, shafts, and capitals), this evidence suits the dedication of the new Pantheon sometime between 125 and 127. Since Hadrian returned to Rome in the summer of 125 after his first tour of the empire,⁶⁸ it is tempting to assume that he would have presided over the ceremonies in person.

The Progress of Works on Site

There are thus two main possibilities for the duration of the project from conception to completion: either a period of seven or so years (ca. 118/119 to ca. 125/126), if we give credence to Bloch, or one roughly five years longer (ca. 113/114 to ca. 125/126), if we give credence to Heilmeyer and Hetland, which on balance I think we must. It may also be noted that the papyrus cited earlier that concerns a 50-foot shaft in transit across the eastern Egyptian desert dates to the third year of Hadrian’s reign, specifically the winter months of 119/120.⁶⁹ If the shaft were indeed intended for the Pantheon, the timing seems too early for a Hadrianic commission; on the other hand, it fits neatly with a start under Trajan.⁷⁰

In the normal course of events, as DeLaine demonstrates in Chapter Six, a total construction period of six or seven years would be feasible for the Pantheon. But from what we have seen, events at the site were far from normal. Delays were generated by the improvised erection of the grottoni. Delays are also implicit in the interruption of the transitional block caused by the nonappearance of the intended shafts for the portico. (It remains difficult to say whether these delays ran separately or concurrently.)

The combined evidence of the sources, brickstamps, worksite logistics, and the present examination of the fabric thus allows the sequence of operations and chronology of the project to be reconstructed as shown in Plate XIII, which is to say along the following lines:

110	Previous Pantheon burns	Trajan reigns
112–114	Conception of the new Pantheon; scheme design
114–116	Site preparation and foundations
116–119	Progress on brick and concrete superstructure	(117) Hadrian’s accession
118–121	Rotunda suffers cracking; progress interrupted; improvisation of the grottoni; nonappearance of 50-foot shafts for the portico	(118) Hadrian returns to Rome
120–123	Grottoni completed; work begins on the dome; work on the transitional block interrupted	(121) Hadrian leaves Rome
122–124	Completion of the dome; completion of the transitional block; decision to use 40-foot shafts for the portico
124–125	Completion of the portico; installation of statuary and fittings; finishing and inspections	(125) Hadrian returns
125–127	Dedication of the Pantheon
128		Hadrian leaves Rome

Apollodorus and Hadrian

Inception under Trajan as opposed to Hadrian makes it more likely that the Pantheon was designed by the architect-engineer Apollodorus of Damascus, who was Trajan’s preferred designer but apparently at odds with Hadrian. Certainly Apollodorus is the more credible author of the Pantheon than Hadrian himself, who has also been proposed.⁷¹ As we have seen in the Introduction, ancient sources credit Apollodorus with Trajan’s Forum and Baths, both quite exceptional projects.⁷² Attribution of the Pantheon to him, too, makes sense on several levels. There are shared stylistic traits in the marble decoration of the Pantheon and Apollodorus’s Forum of Trajan.⁷³ The open-air half rotundas of Trajan’s Baths offer points of similarity with the Pantheon rotunda in respect of both proportions (see Chapter Five), and the form of coffering (see Figs. 5.2, 5.3). It is especially significant that in elevation, these spaces present a comparable syncopation to that manifest inside the Pantheon. In two out of three of the surviving exedras in the Baths, the niches in the walls are synchronized with the coffers of the vault on the main and diagonal axes, but nowhere else.⁷⁴ As a result of the present study, it is possible to identify further parallels between both projects as regards staircases. The unusual trapezoidal configuration of the Pantheon stairs is in fact closely anticipated in the staircases behind the half rotundas of the Baths (see Fig. 5.2). Individual flights of stairs also share constructional similarities, for example, in the disposition of bipedales.

It is well, furthermore, to recall discussion about the centering used to build the dome. This would have been a considerable work of engineering in its own right, and Apollodorus was evidently a master architect-engineer with extensive expertise in the erection of giant timber structures, as attested by his authorship of the Poliorcetica. Ancient sources also credit him with a pertinent technological feat, a huge wooden bridge over the Danube, which apparently approached 170 Roman feet or 55 meters in span (though probably less in reality). This sensational structure, which is represented in compact form on Trajan’s Column, was destroyed on Hadrian’s orders out of fear that it would provide a conduit for barbarian invasion. (Some of its stone and concrete piers still survive.)⁷⁵ The bridge was the subject of another treatise by Apollodorus, a work which, though since lost, was referred to by the sixth-century historian Procopius in such a way as to suggest that it was still well known in his own day.⁷⁶ Apollodorus, then, was in a singularly good position to have mastered the erection of large timber superstructures, and so too the centering of the Pantheon’s dome. Against the background of the prevalently anonymous history of Roman architectural practice, this completes as good a case for attribution as can ever be made for an architect of the period on the basis of design approach and circumstantial evidence.

It is curious, too, that the persons of Apollodorus and Hadrian come into conflict, according to the testimony of the third-century senator and historian Dio Cassius.⁷⁷ Apparently, the emperor first banished and later put to death the architect on account of bad feeling that began long before, when Trajan was consulting Apollodorus, who tactlessly put down one of Hadrian’s interruptions with the remark: “be off and draw your pumpkins, you don’t understand any of these matters.” Later, after becoming emperor, Hadrian sent his own design of the Temple of Venus and Rome to Apollodorus, only to receive intolerable criticisms. The divine statues had been made too tall for the height of the cella, so much so that “if the goddesses wish to get up and go out, they will be unable to do so.”

The disparaging reference to pumpkins, or gourds, was most likely an allusion to the scalloped vaults that Hadrian and his architects used to such effect at his villa at Tivoli.⁷⁸ It is tempting to wonder if the story about the Temple of Venus and Rome was a corruption of a text in which the Pantheon was the real focus of dispute.⁷⁹ Dio could hardly have endorsed this possibility since, after all, presumably on the basis of the inscription on the portico, he believed the Pantheon to have been built by Agrippa, as is clear from a passage discussed in Chapter Two. Bad feeling between Apollodorus and Hadrian may have been further fueled by the demise of the former’s bridge on the latter’s orders. Be that as it may, the fact is that Dio reports antagonism between the two men, and rivalry that revolved around contrasting approaches to design. If there were even a kernel of truth to this, and if Apollodorus were indeed the designer of the Pantheon as I argue he was, it is easy to imagine the two men taking up opposing positions over this project and how to resolve the misfortune that had befallen it.

Presumably, Apollodorus held out for the taller portico and its majestic 50-foot shafts, while the emperor sought to prevent further embarrassing delays by resorting to compromise. From his knowledge of Athens, Hadrian may have been aware that the Propylaea of the Acropolis had two separate pedimented roofs, and that when seen from a distance, one might look as if it were superimposed on the other. Was it he who imposed the double pediment solution, while commandeering a batch of 40-foot columns from another project under way in the capital?

Leaving aside such conjecture, the building site of the Pantheon was eventful, to say the least. Improvisation at the south end suggests that the dome was thought to be in jeopardy. Then there was the dilemma caused by the nonappearance of the intended column shafts at the north end. The architect, whoever he was, no doubt had to shoulder the consequences and perhaps the blame for them, too, even if unfairly so. Remembering all the while that design represents a team effort, the architect(s) of the Pantheon can stake a claim to one of the most sublime architectural experiences of all time. As the product of a rare genius and extraordinary technical audacity, it must have given its author immense satisfaction, yet by this interpretation, the building of it was harrowing in its uncertainty and immensely frustrating. The awesome magnificence of the interior should have been matched on the exterior, but instead the designer saw his vision spoiled. Much of his efforts must have been directed at artfully minimizing the negative impact of circumstances that could not be avoided. But compromise is part and parcel of an architect’s business. Building the Pantheon was a dream that turned nightmarish, though in the end it sends all who enter into reveries.

1 The diameter of the Temple of Diana, part of a thermal complex, is fractionally greater than 29.5 meters, or 100 Roman feet. The so-called Temple of Apollo, also at Baiae, apparently measures about 35 meters (ca. 120 ft) in diameter, but too little is known about this structure to be sure that it once supported a dome. In Rome, the caldarium of the Baths of Caracalla, originally domed, spanned about 35 meters too.

2 Selected studies of imperial construction include G. Lugli, La tecnica edilizia romana, Rome 1957; William L. MacDonald, The Architecture of the Roman Empire, vol. 1: An Introductory Study, London 1965, 2nd ed. rev. New Haven 1982; M. E. Blake,Roman Construction in Italy from Nerva through the Antonines, Philadelphia 1973; Jean-Pierre Adam, La construction romaine. Matériaux et techniques, Paris 1984, translated as Roman Building: Materials and Techniques, Bloomington 1994; F. C. Giuliani,L’edilizia nell’antichità, Rome 1990; Janet DeLaine, The Baths of Caracalla in Rome: A Study in the Design, Construction, and Economics of Large-Scale Building Projects in Imperial Rome (Journal of Roman Archaeology, Supplement 25), Portsmouth, RI, 1997; Larry F.Ball, The Domus Aurea and the Roman Architecture Revolution, Cambridge 2003; Rabun Taylor, Roman Builders, Cambridge 2003; Lynne Lancaster, Concrete Vaulted Construction in Imperial Rome: Innovation in Context, Cambridge 2005.

3 Lynne Lancaster, “The Lightweight Volcanic Scoria in the Concrete Vaults of Imperial Rome: Some Evidence for the Trade and Economy of Building Materials,” Proceedings of the XVIth International Congress of Classical Archaeology, Boston, 2003: pp. 212–216. Cf. Lancaster 2005, p. 64. Pumice is used in general literature to refer to both red-to-brown scoria of the type used in the Pantheon (750–850 kg/m³) and dark-light gray pumice (600–700 kg/m³), which tended to be used in the late third and fourth centuries.

4 Alberto Terenzio, “La Restauration du Panthéon de Rome,” Museion 20, 1932, pp. 52–57; on p. 52 he promised a detailed publication of his findings relating to the restoration works of 1930–1931, but this resource remains unpublished.

5 Rowland J. Mainstone, Development in Structural Form, London 1975, pp. 116–117; Robert Mark and Paul Hutchinson, “The Structure of the Roman Pantheon,” Art Bulletin 78, 1986, pp. 24–34; Robert Mark, Light, Wind, and Structure, Cambridge 1990, p. 60 ff.; David Moore, The Roman Pantheon: The Triumph of Concrete, Wyoming 1995, p. 2; Jacques Heyman, “Poleni’s Problem,” Proceedings of the Institution of Civil Engineers 1, no. 84, 1988, pp. 737–759.

6 Giorgio Croci, The Conservation and Structural Restoration of Architectural Heritage, Boston 1998, p. 125; Croci, “Il comportamento strutturale del Pantheon,” in Giovanni Belardi, Il Pantheon: storia, tecnica, e restauro, Viterbo 2006, pp. 263–310, esp. 266–267, 282–283, 285–287. Cf. Kjeld De Fine Licht, The Rotunda in Rome: A Study of Hadrian’s Pantheon, Copenhagen 1968, pp. 89–93.

7 Differential settlement affected Agrippa’s Pantheon to a greater extent, to judge by sloping levels observable in the foundations that survive under the portico of the existing building; see Chapter Two.

8 That the cracking occurred during or soon after construction is suggested by the use of bricks of similar date to those in the rest of the Pantheon for repairing and filling the cracks; see Licht 1968, p. 288, n. 40. Giuseppe Cozzo (Ingegneria Romana: maestranze romane; strutture preromane, strutture romane, le costruzioni dell’anfiteatro flavio, del Pantheon, dell’emissario del Fucino, Rome 1928) made much of the structural problems affecting the southern sector of the rotunda, although his interpretation is fantastical. Alberto Terenzio, s.v. “Pantheon,” Enciclopedia Italiana 26, 1949, pp. 212–214; p. 213 uses the term accidentata to characterize the progress of the works; cf. Terenzio 1932, p. 54. See also Moore 1995, 11–13.

9 On Roman concrete, see Adam 1984 (1994 translation), pp. 73–79, 177–91; Heinz-Otto Lamprecht, Opus Caementitium. Bautechnik der Römer, Düsseldorf 1987; Moore 1995; G. R. H. Wright, Ancient Building Technology, vol. 2: Materials, part 1, Boston 2005, Chap. 6; Lancaster 2005, esp. Chap. 3.

10 S. Huerta, Arcos, bóvedas y cúpulas. Geometria y equilibrio en el cálculo tradicional de estructuras de fábrica, Madrid 2004.

11 William L. MacDonald, The Pantheon: Design, Meaning, and Progeny, London 1976, p. 38.

12 Eugène-Emmanuel Viollet-le-Duc, s.v. “Voute,” Dictionnaire raisonné de l’architecture française du XIe au XVIe siècle, vol. 9, Paris 1875, pp. 471–474, with Fig. B on p. 475; Taylor 2003, pp. 194–211; cf. Adam 1984, Fig. 443. Taylor assigns a major role to ropes, but these would stretch variably according to humidity and temperature and so perhaps not be capable of providing dimensional stability.

13 I thank Dina D’Ayala for generously lending her engineering expertise to vet initial proposals.

14 The structural behavior is composite in nature, meaning that part of the load was resisted by the lower part of the dome (once the concrete had hardened sufficiently).

15 For opinion favoring some kind of central timber tower, see Jürgen Rasch, “Zur Konstruktion spätantiker Kuppeln vom 3 bis 6 Jahrhundert,” Jahrbuch des deutsches Archäologischen Instituts 106, 1991, pp. 311–383; pp. 369–370; Gerd Heene, Baustelle Pantheon: Plannung, Konstruktion; Logistik, Düsseldorf 2004; Lancaster 2005, pp. 44–45.

16 Adriano La Regina, ed., L’arte dell’assedio di Apollodoro di Damasco, Rome 1999.

17 Relieving arches could provide support for higher levels to be initiated without the fabric of the walls enclosed by the arches, this following on later, as convenient. For further discussion, see Heene 2004; Lancaster 2005, pp. 94–95; and Chapter Four in this volume.

18 Terenzio 1932, 54; J. Guey, “Devrai-on dire: Le Panthéon de Septime Sévère? A propos des estampilles sur briques recueillies dans ce monument, notamment en 1930 ou en 1931 et depuis,” Mélanges d’Archéologie et d’Histoire (Ecole Française de Rome)53, 1936, pp. 198–249; p. 237, n.5. For the complex as a whole, see Licht 1968, 157–171. For a proposal that the grottoni could have been used as archival storage, see Amanda Claridge, “Hadrian’s Lost Temple of Trajan,” Journal of Roman Archaeology 20, 2007, esp. p. 79.

19 Licht 1968, p. 163, Fig. 179.

20 Cozzo 1928, pp. 283–285.

21 Rita Volpe, “Un antico giornale di cantiere delle terme di Traiano,” Mitteilungen des Deutschen Archäologischen Instituts, Römische Abteilung 109, 2002, pp. 377–394.

22 Today the crack may be inspected on the second level of the grottoni. Its presence at floor level, though now covered over, is attested by photographs, including one in the Archivio Fotografico, Soprintendenza per i Beni Architettonici e per il Paesaggio di Roma, neg. 2967.

23 For a reasoned summary of preceding opinion, see Licht 1968, pp. 85–88.

24 For Leclère’s survey, see Roma antiqua. “Envois” degli architetti francesi (1786–1901), Grandi edifici pubblici, exhib. cat., Rome 1992, pp. 100–123.

25 I am grateful to many for their kind help with this project: to Giovanni Belardi, the director responsible for the Pantheon of the Soprintendenza per i Beni Architettonici e per il Paesaggio di Roma, for permission; to Cinzia Conti and her students Roberta Zaccara, Tomaso De Pasquale, and Mariangela Perrota for surveying; to Robert Grover for drawing up the results; to Cinzia Conti and Giangiacomo Martines for precious observations in loco.

26 Here, I find myself conscious of a debt to Giovanni Belardi for authorization to study the stairs, yet we disagree over interpretation. He believes the rotunda to precede the transitional block, but to me, the saggio disproves this.

27 Such is the similarity in technique between the upper and lower halves of the staircase in general that their construction may have been supervised by the same people, as remarked to me by Cinzia Conti.

28 Antonio Maria Colini and Italo Gismondi, “Contributo allo studio del Pantheon: La parte frontale dell’avancorpo e la data del portico,” Bullettino della Commissione Archeologica Comunale di Roma 44, 1926, pp. 67–92; especially 83, 87–92; they did not, however, set out sufficient evidence to resolve the question definitively. 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 1892–83, coi rilievi e disegni dell’ architetto Pier Olinto Armanini, Milan 1898, pp. 41–46; Cozzo 1928, pp. 281–282, Fig. 192; Licht 1968, pp. 59–63, 189; Mark Wilson Jones, “The Pantheon and the Phasing of its Construction,” in Gerd Grasshoff, Michael Heinzelmann, and Markus Wäfler, eds., The Pantheon in Rome: Contributions to the Conference, Bern, November 9–12, 2006, Bern 2009, pp. 69–87, Fig. 13.

29 Colini and Gismondi 1926, pp. 70–73. Leclère’s earlier inspections had led him to conclude that the transitional block cannot have been added in a separate phase, and in particular that there did not exist an earlier Pantheon with its front formed by theavancorpo alone, as Carlo Fontana had proposed. If Fontana were right, the horizontal cornice of the upper pediment would once have continued right across the face of the building, but there is no sign of such.

30 Colini and Gismondi 1926, pp. 75–77. Colini speculated that the inclined bipedales would originally have projected slightly beyond the face of the wall so as to resist rainwater ingress at the junction with the roof. Unfortunately, the area where these bipedales would be was covered by a metal flashing during the refurbishment of the roof covering in the autumn of 2010 before my own visit.

31 Tilmann Buddensieg, “Criticism and Praise of the Pantheon in the Middle Ages and the Renaissance,” Classical Influences on European Culture A.D. 500–1500: Proceedings of an International Conference Held at Kings College, Cambridge, April 1969, ed. R. R. Bolgar, Cambridge 1971, pp. 259–267; Paul Davies, David Hemsoll, and Mark Wilson Jones, “The Pantheon: Triumph of Rome or Triumph of Compromise?” Art History, 10, 1987, pp. 133–153; Tod A. Marder, “Bernini and Alexander VII: Criticism and Praise of the Pantheon in the Seventeenth Century,” Art Bulletin 71, 1989 pp. 628–645; Susanna Pasquali, Il Pantheon: architettura e antiquaria nel Settecento a Roma, Modena, 1996, Chaps. 5 and 6; Mark Wilson Jones, Principles of Roman Architecture, New Haven 2000, pp. 187–191, 199–202.

32 Wilson Jones 2000, pp. 191–196.

33 Heinz Kähler, “Das Pantheon in Rom,” Meilensteine europäischer Kunst, ed. E. Steingräber, Munich 1965, pp. 45–75; pp. 58–65; Marder 1989; William C. Loerke, “A Rereading of the Interior Elevation of Hadrian’s Rotunda,” Journal of the Society of Architectural Historians 49, 1990, pp. 32–43, esp. p. 30 ff.; Wilson Jones 2000, pp. 191–196. Cf. MacDonald 1976, pp. 70–72.

34 Adapted from Giorgio Vasari, The Lives of the Painters, Sculptors, and Architects, Florence 1550, trans. A. B. Hinds, repr. London 1963, pp. 275–276. A seventeenth-century source (Cod.Barb.Lat. 4309, f.11v) also attributed to Michelangelo the judgment that the first of the three phases was so good as to be “the product of angels,” which implies that the other phases were not so good. Cf. Buddensieg 1971, p. 265.

35 Cozzo 1928. For a critique of Cozzo’s theories in relation to brickstamps, see Herbert Bloch, “I bolli laterizi e la storia edilizia romana,” Bullettino della Commissione Archeologica Comunale di Roma 64, 1937–1938; Bloch, I bolli laterizi e la storia edilizia romana. Contributi all’archeologia e alla storia romana (1936–1938), Rome 1947. A variant of Cozzo’s (and Carlo Fea’s) ideas on the Pantheon continues to be championed in Giovanni Belardi 2006. Gene Waddell (Creating the Pantheon: Design, Materials, and Construction, Rome 2008, esp. pp. 124–138, 149–150) advocates a substantial Severan restoration of the portico, but too substantial in my view.

36 On these inscriptions and their interpretation, see Adam Ziolkowski, “Prolegomena to Any Future Methaphysics [sic] on Agrippa’s Pantheon,” in “Res bene gestae”: Ricerche di storia urbana su Roma antica in onore di Eva Margareta Steinby, ed. A. Leone, D. Palombi, and S. Walker, Rome 2007, pp. 465–475, esp. 466–468; Ziolkowski, “What Did Agrippa’s Pantheon Look Like? New Answers to an Old Question,” in Grasshoff, Heinzelmann, and Wäfler 2009, pp. 29–39, esp. 38–39; C. Simpson, “The Pantheon’s Inscription, CIL 6.896: Its Date of Composition, Cultural Context, and ‘Message,’” Athenaeum 97, 2009, pp. 149–157; Mary T. Boatwright, “Hadrian and the Agrippa Inscription of the Pantheon,” Hadrian: Art, Politics and Economy, ed. Thorsten Opper, British Museum Research Publications 175, London 2013, pp. 19–30.

37 It has been pointed out, for example, that that even if the junction of rotunda and portico might be judged unsatisfactory, this could not be seen in antiquity from the forum-like “forecourt,” not forgetting that the ground level was at least two meters lower than at present. See MacDonald 1982, pp. 111–113; cf. Wilson Jones 2000, p. 202.

38 The key proportional rule for the Corinthian order set the height of the shaft as 5/6 that of the complete column (including base and capital), and so 50-foot shafts imply columns 60 feet tall; both dimensions harmonize well with 75- and 150-foot measures elsewhere in the whole project. For the design of the Corinthian column, see Wilson Jones, “Designing the Roman Corinthian Order,” Journal of Roman Archaeology 2, 1989, pp. 35–69; Wilson Jones 2000, Chap. 7.

39 Davies, Hemsoll, and Wilson Jones 1987; Wilson Jones 2000, Ch. 10. For shipwrecked cargoes of ancient marbles see P. Pensabene, Il fenomeno del marmo nel mondo romano, in I marmi colorati della Roma imperiale, Rome 2002, pp. 3–67, esp. 34–46. Alternatively, Claridge (2007, p. 94) advances a plausible scenario by which the 50-foot shafts were diverted to the Temple of Trajan. Supplies of such huge stones were evidently inadequate for both projects within acceptable timescales.

40 Wilson Jones 2000, p. 148, p. 155, and Appendix B; Pensabene 2002, pp. 24–25; Paolo Barresi, “Il ruolo delle colonne nel costo degli edifici pubblici,” in I marmi colorati della Roma imperiale, ed. Marilda De Nuccio and Lucrezia Ungaro, Rome 2002, pp. 69–81. This last study puts emphasis on multiples of 4 ft. It is true that the popular sizes of 16, 20, 24, and 40 ft are multiples of 4 ft, but since 20 and 40 ft are multiples of 5 ft as well, and since 5 ft also divides into the other popular sizes of 15, 25, 30, and 50 ft, it is best to speak of multiples of 4 and/or 5 ft, with 5 ft being the dominant of the two.

41 Wilson Jones 2000, p. 203.

42 Theodore Peña, “P. Giss. 69: Evidence for the Supplying of Stone Transport Operations in Roman Egypt and the Production of Fifty-Foot Monolithic Column Shafts,” Journal of Roman Archaeology 2, 1989, pp. 126–132, esp. p. 131; Richard Tomlinson,From Mycenae to Constantinople. The Evolution of the Ancient City, London 1992, p. 163; Jürgen J. Rasch, Das Mausoleum bei Tor de’ Schiavi in Rom, Mainz 1993, p. 54, n. 331; Edmund Thomas, “The Architectural History of the Pantheon in Rome from Agrippa to Septimus Severus via Hadrian,” Hephaistos 15, 1997, pp. 163–186; esp. 179–180; Adam Ziolkowski, s.v.“Pantheon” in E. M. Steinby, Lexicon Topographicum Urbis Romae, 5 vols., 1995–1999; vol. 4, 1999, p. 58; Fikret Yegül, review of Wilson Jones 2000,Journal of the Society of Architectural Historians 60, 2001, pp. 500–504; Paolo Barresi, review of Wilson Jones 2000, Archeologia Classica 53, 2002, pp. 593–598; James Packer, review of Wilson Jones 2000, American Journal of Archaeology 106, no. 2, 2002, pp. 344–345; Thomas N. Howe, review of Wilson Jones 2000, Bryn Mawr Classical Review, April 15, 2002, pp. 469–472, http://ccat.sas.upenn.edu/bmcr/; Alessandro Viscogliosi, “Il Pantheon e Apollodoro di Damasco,” Tra Damasco e Roma: L’architettura di Apollodoro nella cultura classica, ed. Festa Farina et al., Rome 2001, pp. 156–161, esp. p. 159; Taylor 2003, pp. 129–132; Rabun Taylor, “Hadrian’s Serapeum in Rome,” American Journal of Archaeology 108, 2004, pp. 223–266, esp. 244–254 (with a different proposal for the cause); Heene 2004; Eugenio La Rocca, “Templum Traiani et columna cochlis,” Mitteilungen des Deutschen Archäologischen Instituts, Römische Abteilung 111, 2004, pp. 193–238; p. 211. Waddell (2008, esp. p. 135) accepts the change of column size, but thinks settlement prompted a Severan rebuilding of the portico. For a more neutral reception (noting the hypothesis with caution), see Martin Maischberger, Marmor in Rom, Ph.D. diss., Freie Universität, Berlin 1997, pp. 145–146; and for concerns regarding the composition of the building that bear indirectly on this problem, see P. Gros, L’Architecture Romain, du début du III siècle av J.-C. à la fin du Haut Empire, vol. 1: Les monuments publics, Paris 1996, pp. 175–176. For outright hostility, see C. Tiberi, “Saggio introduttivo,” in G. Ortolani, Il padiglione di Afrodite Cnidia a Villa Adriana: Progetto e significato, Rome 1998, pp. 9–16, esp. p. 14. Belardi (2006) advances completely different explanations for the anomalies of the Pantheon.

43 In November 2006, Haselberger presented objections at the conference at the Karman Center in Bern that may be summarized as follows:

·         The Propylaea of the Athenian Acropolis offer a precedent for the upper pediment (as Tiberi observed), which thus could have been intended from the outset (cf. my point i);

·         Other buildings exist with similarly tall/heavy pediments (iii);

·         The spacing of the modillions varies considerably, and so on this basis, it is hard to sustain arguments about intentions (iv);

·         Other buildings exist with similarly wide intercolumnations (v);

·         A capital inside the rotunda is not axially aligned with its pilaster, and so similar misalignments in the portico need not reflect a change of project (vi).

I concede that points iii, iv, and v are relatively subjective, and that they cannot furnish conclusive arguments either way. Point i calls into question a major plank of the compromise hypothesis, yet it does not necessarily negate it, since the idea of a second pediment, perhaps inspired by the Athenian Propylaea, may only have arisen after the Pantheon project ran into problems. As for the misalignment of the capitals (vi), there is a difference between an isolated case in the interior and the systematic occurrence of a more severe misalignment on all four antae in the portico. In short, none of these criticisms is fatal, while the other points (ii, vii, viii, ix) remain unchallenged.

44 Lothar Haselberger, “The Pantheon: Nagging Questions to No End,” in Grasshoff, Heinzelmann, and Wäfler 2009, pp. 171–186. In this article, he includes most but not all of the arguments presented at Bern and covered in the preceding note. I stand corrected in stating that as regards the double pediment, “no ancient building copies this arrangement.” The Temple of Zeus Asklepios in Pergamon, built during the 120s–130s AD on the model of the Pantheon, did adopt a comparable solution, albeit better resolved; see O. Ziegenaus, Das Asklepieion, Altertümer von Pergamon XI,3, Berlin 1981, Taf. 85.

45 Taylor 2003, pp. 129–131, and 2004, esp. pp. 244–251. Taylor’s portico implies a floor level lower than that of the existing portico. Yet the top of the concrete strip foundations lies above the bottom of his hypothetical column bases, rendering them improbable; see Beltrami 1898, Fig. xi.

46 Peña 1989.

47 Wilson Jones 2000, pp. 206–207; cf. Lothar Haselberger, “Ein Giebelriss der Vorhalle des Pantheon. Die Werkrisse vor dem Augustusmausoleum,” Mitteilungen des Deutschen Archäologischen Instituts, Römische Abteilung 101, 1994, pp. 279–308. Although less likely, it is possible that this design was for the Temple of Trajan or the Temple of Venus and Rome, both of which had 50-foot shafts (the former monolithic, the latter not). Cf. Carlo Inglese, Progetti sulla pietra: Strumenti del Dottorato di Ricerca in Rilievo e Rappresentazione dell’Architettura e dell’Ambiente, vol. 3, Rome 2000, pp. 47–50. It should be noted, however, that an Augustan date for the paving and drawing is hypothetically possible.

48 Stefania Fogagnolo, “Scoperta di frammenti di colonne colossali dal foro della pace,” in I marmi colorati della Roma imperiale, ed. Marilda De Nuccio and Lucrezia Ungaro, Rome 2002, pp. 136–137; La Rocca 2004, p. 209, n.56.

49 For a list of 50 footers, see Peña 1989, p. 130. For the Temple of Trajan, see James Packer, The Forum of Trajan in Rome: A Study of the Monuments, Berkeley 1997, p. 457 ff. See also R. Meneghini, “Il foro Traiano. Ricostruzione architettonica e analisi strutturale,” Römische Mitteilungen des Deutschen Archäologischen Instituts, Römische Abteilung 108, 2001, pp. 245–268; and see La Rocca 2004, pp. 208–212, for the theory that the shafts belonged instead to a monumental gateway, countered by Claridge2007, with further detail on the shafts on pp. 63–66. For the Antonine Column, see J. B. Ward-Perkins, Marble Antiquity: Collected Papers of J. B. Ward Perkins, ed. H. Dodge and J. B. Ward-Perkins, Rome 1992. For the shafts of Trajan’s Baths, see La Rocca2004, pp. 209–210, Fig. 10, with further references. I have not myself located on site anything bigger than pieces commensurate with 40 footers; however, Rita Volpe has measured fragments she judges consistent with 50 footers, while Simone Gianolio in his forthcoming doctoral thesis uses evidence from standing walls to deduce their presence.

50 On standardization in the service of the Roman “building machine,” see Wilson Jones 2000, p. 155.

51 This drawing is based on those of Leclère and Colini, supplemented by my measurements of the plan, and aspects of the main order that I was able to check from openings in the staircase. The trusses were reconstructed on the basis of Borromini’s survey and sixteenth-century drawings. Further features were observed and photographed from nearby scaffolding in November 2010.

52 The 10-foot width of the foundations relates to the 5-foot column diameter as 2:1. By contrast, Vitruvius recommends a ratio of around 3:2 (1.5:1), a value more or less consistent with monumental imperial practice. The substructures under monumental colonnades typically project approximately in line with the plinths of the columns, implying a thickness about 1.4 times the column diameter. A ratio in the range of 1.4–1.5 recurs at the temples of Castor, of Vespasian, and of Antoninus and Faustina, as well as on the foundation blocks of travertine and peperino supporting colonnades in the Forum of Trajan. For a generic illustration of a concrete foundation only slightly wider than the plinths of the columns it supports, see Giuliani 1990, Fig. 5.3. In the intended portico, the ratio of foundations to column diameter would have been 1.6:1 (10:6¼), that is to say, still on the safe side.

53 The set-back could also have been intended to seat elements of the roof construction, as Gene Waddell has drawn to my attention.

54 For this drawing of Borromini, see Heinrich Thelen, Francesco Borromini. Die Handzeichnungen, vol. 1, Graz, 1967, cat. no. 25, pp. 32–33 (there is also a second drawing, cat. no. 26); Licht 1968, pp. 50–58; Louise Rice, “Urbano VIII e il dilemma del portico del Pantheon,” Bollettino d’arte 143, 2008, pp. 93–110; Rice, “Bernini and the Pantheon Bronze,” in Sankt Peter in Rom 1506–2006, ed. Georg Satzinger and Sebastian Schütze, Munich 2008, pp. 337–352.

55 Rice 2008a (“Urbano VIII”), pp. 95–96. As Rice discusses, the unusual cross-section of the trusses over the central aisle made space for the semicircular barrel-vaulted ceiling that was in all likelihood suspended from the trusses. This need arose, she argues, due to the revisions to the portico as a whole. The substitution of smaller columns dictated vaults that were 5/4 ft wider and 5/8 ft taller. Meanwhile, the usable height was significantly reduced, since in the revised design, the entablature – and hence the roof space – would have been about 3 ft shorter than the original. In effect, then, more than a meter was subtracted from the height that would otherwise have been available for the vaulting.

56 Rice 2008a, pp. 95–96. This is curious given the semicircular profile of the projecting ledge, over a foot deep, formed by the relieving arches placed at the right height to accept a barrel vault, save that the trusses impeded such a solution (Fig. 7.13, G), hence, the probable implementation of a flat ceiling. I was able to observe this detail thanks to scaffolding and a tour of operations conducted by Giovanni Belardi. The extra meter or so available in the original design could have accommodated a fully semicircular form, though this would have been at a higher level; see Fig. 7.13.

57 The stone blocks projecting from the upper part of the transitional block may have facilitated constructional operations, but there is also the possibility that they were intended to provide some kind of connection with the trusses of the abandoned project (Fig. 7.13, B and C).

58 I have no particular opinion on the three rough blocks immediately above the architrave that runs on top of the capitals, though they may have participated in anchoring the bronze assembly associated with the ceiling of the side aisles.

59 As regards the original project, it is also impossible to know how the transitional block should have looked. It could have terminated more or less as it does today, or it could have been capped by a continuation of the (higher) portico roof; see Davies, Hemsoll, and Wilson Jones 1987, Figs. 7 and 8.

60 On brickstamps and their interpretation, see Heinrich Dressel, Inscriptiones urbis Romae latinae, Berlin 1891; Bloch 1947; E. M. Steinby, “La cronologia delle figliane doliare urbane dalla fine dell’età repubblicana fino all’inizio del III sec.,” Bullettino di archeologia cristiana 84, 1977, pp. 7–113; T. Helen, Organisation of Roman Brick Production in the First and Second Century AD, Helsinki 1975; Janet DeLaine, “Building Activity in Ostia in the Second Century AD,” Acta Instituti Romani Finlandiae 26, 2002, pp. 41–102, and Chapter Three in the present volume. I am grateful to John Bodel for expert guidance on the finer points involved.

61 A typical lag of a few months twixt production and use would be understandable, in part because stamps were imprinted in wet clay, which had to dry before firing, in part for any flaws that might develop to make themselves evident. At times, bricks may have been rushed to market, or they may have been set aside for later use. Note divergent views on this and the implications for Trajan’s Markets, where Domitianic brickstamps may indicate a Domitianic inception (E. Bianchi, “I bolli laterizi dei Mercati Traiani,”Bullettino di archeologia cristiana 104, 2003, pp. 329–352), or as stockpiled supplies consistent with a Trajanic date (J. C. Anderson, Jr., “The Date of the Thermae Traiani and the Topography of the Oppius Mons,” American Journal of Archaeology 89, 1985, pp. 499–509; Lynne Lancaster, “The Date of Trajan’s Markets: An Assessment in Light of Some Unpublished Brick Stamps,” Papers of the British School at Rome 63, 1995, pp. 25–44).

62 Guey 1936; Bloch 1947, pp. 14–19, 102–117, esp. 112. Cf. MacDonald 1982, p. 96; Licht 1968, pp. 180–190, esp. 186–187. A few Severan stamps result from repairs of that period.

63 Guey 1936, esp. p. 233; Bloch 1947, esp. p. 112, who quoted Guey’s expression for their distribution: “un peu partout dans la bâtisse.”

64 Wolf-Dieter Heilmeyer, “Apollodorus von Damaskus – der Architekt des Pantheon,” Jahrbuch des Deutschen Archäologischen Instituts 90, 1975, pp. 316–347. Cf. Haselberger 1994, pp. 296–298.

65 Bloch 1947, p. 114.

66 “... da me letto il giorno 25 aprile su d’una scaglia di mattone, cavata dal tasto fatto presso lo spigolo N-E. della fronte laterizia, dietro il pilastro marmoreo del portico” (Rodolfo Lanciani, Pagan and Christian Rome, Boston 1892, p. 153, cited by Bloch 1947, p. 107). The incomplete text of this stamp may match a frequently attested brickstamp (CIL 549a-d) of the year 123, but in any case the letters PAETI point to the consul Paetinus and, hence, the same date.

67 Bloch (1947, p. 114) judged the stamp to have belonged not to a bipedalis embedded into the structure but one of the semilateres of the lining.

68 Bloch 1947, p. 117. Cf. Licht 1968, p. 186; William L. MacDonald and John Pinto, Hadrian’s Villa and Its Legacy, New Haven 1995, pp. 17–19; Anthony R. Birley, Hadrian: The Restless Emperor, London 1997; Wilson Jones 2000, pp. 177, 210–211.

69 Peña 1989.

70 A date of 119/120 also seems too early for the Temple of Trajan, presuming its design not to have begun before his death in the summer of 117.

71 For recent affirmation of Hadrian acting in effect as an architect, see E. Salza Prina Ricotti, Villa Adriano: il sogno di un imperatore, Rome 2001, pp. 19–25. For collected opinion and a more critical appraisal, see Chapter Three in this volume.

72 Scriptores Historiae Augustae, S.H.A. Hadrian 19.2–13; Procopius of Caesarea, On Buildings, 4.6.12–13. See also MacDonald 1982, p. 130. On the career of Apollodorus, see C. Leon, Apollodorus von Damaskus und die trajanische Architektur, Innsbruck 1961; MacDonald 1982, pp. 129–134; La Regina 1999; Wilson Jones 2000, pp. 21–24; F. Festa Farina, G. Calcani, C. Meucci, and M. Conforto, eds., Tra Damasco e Roma: l’architettura di Apollodoro nella cultura classica, Rome 2001.

73 See Wolf-Dieter Heilmeyer, “Korinthische Normalkapitelle: Studien zur Geschichte der römischen Architekturdekoration,” Mitteilungen des Deutschen Archäologischen Instituts, Römische Abteilung, Supplement 16, 1970, pp. 158–161, on the capitals. Following a hint by Bloch (1947, p. 116), the attribution of the Pantheon was argued in depth by Heilmeyer (1975). For amplification, see Wilson Jones 2000, pp. 192–193; Viscogliosi 2001, pp. 158–159; Heene 2004; Wilson Jones, “Who Built the Pantheon? Agrippa, Apollodorus, Hadrian and Trajan,” Hadrian: Art, Politics and Economy, ed. Thorsten Opper, British Museum Research Publications 175, London 2013, pp. 31–49.

74 Wilson Jones 2000, p. 192. Cf. Kjeld de Fine Licht, Untersuchungen an den Trajansthermen zu Rom, Copenhagen 1974.

75 Piers from the bridge are to be found at Turnu-Severin in Romania. See A. Barcacila, “Les piliers du pont Trajan sur la rive gauche du Danube et la scène CI de Colonne Trajan,” Studi su Cercetari de Istorie Veche 17, 1966, pp. 645–663; ColinO’Connor,Roman Bridges, Cambridge 1993, pp. 142–145; J. Coulston, “Transport and Travel on Trajan’s Column,” in Travel and Geography in the Roman Empire, ed. C. Adams and R. Laurence, London 2001, pp. 106–137, esp. 124–125.

76 By referring his readers to Apollodorus’s treatise, Procopius kept brief his own mention (De Aedificiis, 4.6.11–16). For a fuller account see Dio Cassius, 68.13.1–6.

77 Dio Cassius, 69.4. For the passage in full, see MacDonald 1965, pp. 131–132; Wilson Jones 2000, pp. 23–24.

78 F. E. Brown, “Hadrianic Architecture,” Essays in Memory of Karl Lehmann, ed. L. F. Sandler, New York 1964, pp. 55–58; MacDonald 1982, p. 135.

79 Wilson Jones 2000, pp. 192–193, 212–213; Wilson Jones 2013.