The Pantheon: From Antiquity to the Present

by Unknown

  The brick arches, which are such a feature of the external face of the drum, are an important part of the structure of the rotunda; following earlier scholars, these are assumed to pass through the whole thickness of the drum and be made of solid brick, although this has in fact never been proven.7 These and most of the other arches that appear on the face of the structure are made of bipedales, although a few of the smaller ones are of sesquipedales. The bridge above the grottoni rests on a solid brick vault of very similar construction. Also of bipedales are the level courses of bricks (the so-called bonding courses), which run right through the thickness of the fabric at regular intervals. At more than 25 kilograms each, bipedales require careful handling and cannot be laid at the speed of ordinary bricks, a point that also needs to be taken into account in the calculations.

  The construction of both the brick-faced concrete and the arches can be broken down into simple actions carried out by individual masons, and all of the materials that need to be put in place – bricks, rubble aggregate, mortar – are sufficiently small and light to be carried by individuals as well, most probably in baskets. The only exception is the main interior order of the rotunda, which requires a very different range of materials and skills that are more difficult to quantify. Stone working needs specialized labor, and most elements, particularly the monolithic marble shafts weighing more than 20 tonnes each, need special equipment and large teams of men to raise and guide into position.

  The most difficult element for analysis is the dome. The basic fabric, especially up to the top of the step-rings visible on the exterior, is little different from that of the drum of the rotunda, and similar skills and actions were required for putting the concrete in place, although the caementa are larger and lighter so that 4,000 pieces per meter cubed is a better estimate. The real difficulties lie in attempting to reconstruct the formwork and centering used to support the concrete while it cured, which in turn affects the sequence of construction. Since there is no consensus about this among scholars, and to argue the case is beyond the scope of this study, here it is assumed that the lower part of the dome up to the top of the drum is cantilevered out and does not require full centering (see Chapter Seven), but that the upper part of the dome from the top of the level of the drum is erected as a series of rings over a complete centering.

  The Portico

  The same kinds of skills required for the erection of the interior order of the rotunda are also necessary for the traditional masonry construction of the portico (see Fig. 1.1 and Plate I). The demands here are greater, however, since the elements are heavier (the 40-Roman-foot granite column shafts weigh more than 50 tonnes each), and the heavy blocks of the entablature and tympanum need to be raised to greater heights, involving even more specialized equipment and even larger groups of men. The main problem with understanding the portico construction, however, is that the current supports for its roof are not ancient. The roof has been much altered since antiquity, acquiring its present state in the 1920s, leaving many questions still to be resolved.8 Although sufficient drawings of the ancient bronze trusses that were robbed in the 1620s remain for a basic reconstruction,9 the roof may have been part of the restoration program of Septimius Severus,10 leaving open the strong possibility that the bronze trusses are Severan and that the older roof was of a more traditional timber construction.11 Even if the bronze trusses were original, reconstructing the manpower figures for their manufacture and construction, if indeed possible at all, would require a separate study. Thus, the calculations here are based on a traditional timber truss structure similar to the current system but with marble roof tiles.12

  Groundwork, Foundations, and Substructures

  The least straightforward elements of the overall construction to assess are the groundworks and foundations, so that any result can only be a broad approximation. The foundations of the portico incorporated substantial amounts of an earlier ashlar building, presumed to be part of the original Agrippan Pantheon, which were partly reworked to support the front row of columns, while new concrete foundations were created for the side and internal rows.13 In contrast, earlier structures were removed rather than reused from the foundation trenches for the grottoni, as for the rotunda itself, before the concrete was laid (see Fig. 2.6).

  Since the portico of the present Pantheon lies about 1.4 meters higher than its predecessor, and its internal floor was closer to 2 meters higher than before, the earlier building need only have been demolished to roughly the new higher level, not to its foundations. This would have saved time and manpower. On the other hand, allowance has to be made for raising the overall level of the internal floors of the whole building, creating further need for labor. This seems to have happened before the foundation trench for the present rotunda was dug, as the interior face of the foundation appears to have been cast in a trench to just below the current floor level (see Fig. 2.4 and Plate XVIII). This is in contrast to the upper part of the foundation externally, which is built of brick-faced concrete and higher than the external early second-century ground level, forming a low ring podium.

  XIII. Visualization of the sequence of operations in building the Pantheon. (Model Mark Wilson Jones and Robert Grover)

  XIV. Plan of Pantheon (in black) overlaid with reconstruction of Agrippa’s Pantheon (in red), according to Rodolfo Lanciani 1897

  XV. Schematic reconstruction of Augustan Pantheon, according to Gerd Heene 2004.

  XVI. Aerial view of alignment between Pantheon and Mausoleum of Augustus. (P. Vergili, “Scavi in piazza della Rotonda e sulla fronte del Pantheon,” in Grasshoff, Henzelmann, and Wäfler 2009, Fig. 10)

  XVII. Pantheon, reconstruction views, intended (below) and as executed (below). (Model John Burge)

  XVIII. Plan of 1891–1892 excavations. Red arrows point to trenches exposing earlier floor. (Beltrami 1898)

  XIX. Composite section combining information from excavations under portico and rotunda. (Drawing Giovanni Joppolo)

  XX. Section detail of 1891–1892 excavations illustrated in Figure 2.3, looking toward pilaster framing east side of entrance portal. (Pier Olinto Armanini in Beltrami 1898)

  The nature of the foundations raises further problems. Although it is widely presumed that the foundations of the rotunda are about 4.5 meters deep and rest on clay, the situation is in fact more complex. The upper part, forming the podium externally, is about 2 meters above the early second-century ground level, and thus structurally more part of the building than of the foundations, which implies that the foundations proper would be only 2.5 meters deep. It is also clear from Armanini’s summary drawing of the late nineteenth-century excavations of the Pantheon foundations that most of these are cut into a constructional fill, as the natural clay lies slightly below the base of the earlier travertine wall under the portico. From the foundations of other major Roman buildings of similar height, such as the Flavian Amphitheatre (ca. 7 meters into alluvium and bedrock plus ca. 6 meters of substructures), the Baths of Caracalla (6.5 meters into clay plus 8 meters of substructures), or the Domitianic Triclinium on the Palatine,14 we would expect the foundations to be deeper in themselves and bedded further into the underlying natural geology. Even minor buildings such as the Trajanic/Hadrianic insulae at Ostia, at approaching half the height and a fraction of the span of the Pantheon, often have foundations more than 3 meters deep extending into the subsoil.

  These known examples can be compared with the few indications we have in the written sources. Vitruvius gives no rules for determining the depth of foundations except that for temples they should be proportionate to the size of the work, and should extend “into the solid” (de arch. 3.4.1). Palladius (1.8.2), writing in the fourth century in relation to villas, recommends that foundations in solid clay need to be one-fifth to one-sixth of the total height of the building, while those in loose earth need to be dug until solid clay is found, or to a total height of one-quarter of the building. The early medieval Mappae Claviculae, generally argued to be bas
ed on earlier Roman building practice, recommends that the depth of foundations for vaulted structures should equal the height of the walls to the springing of the vault.15 The obelisk of Augustus’s solarium north of the Pantheon in the Campus Martius was reputed to have had foundations equal to its height of 22 meters (Pliny, Naturalis Historia 36.72–73); while this figure cannot be taken as entirely reliable, it does suggest that the foundations were considerable. These figures suggest a possible range for the depth of foundations from 7 to 9 meters up to 22 meters. Given how limited the excavations of the rotunda foundations have been, it is in fact quite likely that they were much deeper than the 4.5 meters generally presumed.16

  For the purpose of this exercise, it will be assumed that the depth of the foundation below the podium is 7 meters, the minimum of the range suggested by these sources, which is also in keeping with figures for the Flavian Amphitheatre and the Baths of Caracalla, and no allowance will be made for labor saved by reusing earlier structures. This will give a very rough estimate only, but might at least provide an order of magnitude. The figures that can be calculated for the foundations are for digging and filling those of the rotunda, the intermediate block, and the grottoni, constructing the external brick face of the podium, and creating the new foundations for the inner columns of the portico.

  Calculating the Manpower: Assumptions, Approximations, and Limitations

  The estimates of manpower given here are based on a volumetric analysis of the main parts of the structure, derived from the dimensions given by Licht, where necessary scaled from plans and sections he supplies,17 while taking into account the factors outlined in the previous section. Supplementary information derives from the recent laser survey conducted by the Karman Center in Bern (Fig. 6.6). In order to avoid excessively complicated calculations (e.g., in relation to precise forms of arches), an idealized geometry has been employed, mainly in the assumptions that all curves are parts of circles, spheres or cones.18 Based on experience from calculations for the Baths of Caracalla, small openings (2 meters wide or less) have been ignored, as the work required to form them in thick walls roughly equates to work saved. This should allow quantities of materials to be calculated to a reasonable degree of accuracy.

  6.6. Laser scan of longitudinal section. (Karman Center, Bern, BDPP0021)

  Nevertheless, several other assumptions, approximations, and limitations have to be made to arrive at manpower estimates, relating both to the fabric of the building and to the conditions of labor. As we have seen, some elements of the foundations, the roofing of the portico, and the centering for the dome structure have had to be excluded, or only a very rough estimate given, because there are simply too many unknowns. Also excluded from the calculations are most preliminary groundworks; the preparation of materials away from the site, including quarrying, lime burning, and brick production; the transport of materials to the site and the removal of construction debris; and any waste of materials, which might have added upward of 10 percent in terms of mortar alone. Minor elements of the main fabric, for example the brick and travertine external cornices, have also been excluded. Apart from the labor required for shaping and setting up of the columnar orders of the rotunda and the portico, which is an essential part of the construction process, decoration and finishing will be discussed only very briefly. On the other hand, aspects of the construction process that are not immediately obvious in the finished fabric often require large amounts of labor and have been included, such as processing materials on site, moving building materials horizontally and vertically to where they are required to be put in place, erecting scaffolding, and formwork. Also taken into account is supervision at rates of 1:10 for most construction, and 1:4 or 5 for complex actions such as raising and setting in place the architectural orders or erecting large formwork.

  Other assumptions affect the operation of the building site and have direct implications for labor requirements. Once the building materials have arrived on site, they need to be stored until needed, with some elements requiring further processing, such as lime slaking and mortar mixing. The most likely areas for this are in the open space to the north, which later became the Pantheon precinct, and the area of the Saepta Iulia alongside the Pantheon to the east, which, according to the evidence of brickstamps, was rebuilt immediately after the Pantheon.19 The internal area of the rotunda may also have been used in the early stages of construction. The availability of such large spaces in the immediate vicinity suggests that horizontal movement of materials on site could be reduced to a minimum, and an average figure of 50 meters is assumed in the calculations. Finally, it is assumed that no extra work was necessary for the furnishing of the construction infrastructure – the cranes, pulleys, ropes, scaffolding, and baskets essential to the construction process – as all of this, including special large cranes and equipment needed for moving and putting in place large architectural elements and formwork, would presumably have been available either in imperial stores or with imperial contractors involved with previous large-scale building works, such as the Baths of Trajan and the Forum of Trajan completed in AD 109 and AD 112/113, respectively.20 Even much of the timber for the formwork should have been readily available from the same sources, although some extra work would be necessary to adapt this to current needs.

  The manpower constants needed to convert volumes of materials into the amount of labor required for construction are derived from historic figures for labor output in the building trades, in particular data from nineteenth-century Italy where traditional methods of construction and building materials comparable to those of the Roman period were still in use.21 The particular figures and formulae used have been chosen to match as closely as possible the process of construction and specific working conditions that can be identified from traces left in the fabric of the Pantheon, or, where this is not possible, from more general observations of contemporary Roman construction in brick-faced concrete. From the consistency of rates for laying bricks from different periods and/or places, for example, it is reasonable to assume that this would have been very similar in the Roman period when similar tools were used, suggesting a figure of 500 bricks per day for a mason, for actually laying the face of the Pantheon, with an assistant to supply the bricks and mortar.22

  A number of assumptions have had to be made about how these relate to ancient working practices, but these are all slanted toward providing minimum figures for labor, on the understanding that the actual figures are likely to have been higher.23 The specific assumptions used here concerning working conditions are those used for the labor estimates relating to the Baths of Caracalla. The historical labor constants are given for an hour’s work, over a day of 10 working hours (12 hours including breaks), and it is assumed that the ancient working day was the same. All labor here has been treated as the same, although it would be possible at a later stage of analysis to divide it into unskilled and a variety of skilled trades (e.g., masons, carpenters, marble workers).

  Since it is not possible to make accurate allowance for all of the uncertainties inherent in the exercise, these calculations provide only approximate and hypothetical figures, which serve as a guide to the scale of operations rather than any kind of precise values for the actual situation in antiquity.24 In addition, the figures as calculated are generally minimum possible figures, while the exigencies normal to building operations in all periods strongly suggest that actual figures would have been higher.

  Since the building divides neatly into four virtually separate elements, it is simplest to present the raw manpower figures as four tables, each divided into the relevant number of construction phases or processes.

  Altogether, the calculations suggest that a minimum of about 400,000 (399,000) mandays were required for the main phases of construction of the Pantheon, comprising 340,000 mandays of general construction labor (rotunda 272,900; intermediate block 30,400; porch 7,300; grottoni 26,000) plus about 60,000 mandays for shaping the architectural elements (see Tables 6.1,
6.2, 6.3, and 6.4). A more useful way of expressing this data in order to assess the historical significance of a building project is as a number of men working over a given time: the figures given here for the Pantheon are the equivalent of 1,100 men working every day for a year, or 110 men for 10 years. If, however, we take into account the likely actual sequence of construction along with attendant practical considerations, it should be possible to estimate more precisely the minimum size of the workforce and the shortest possible period over which it was required.

  Table 6.1. Manpower for building the rotunda in mandays (mdays) of labor

  * * *

  RotundaFoundationsLower zoneMiddle zoneUpper zoneDomeTotal

  Quantities

  Total volume (m³)9,4907,0506,7108,0705,18036,500

  Brick pieces in facing24,000365,000178,000136,000178,000881,000

  ActionsRateTotal mdaysTotal mdaysTotal mdaysTotal mdaysTotal mdaysTotal mdays

  Excavate in rocky ground to 1.6 ma 0.165 mdays/m³ 320 320

  Excavate 1.6 m 0.248 mdays/m³ 782 782

  Remove debris over 50 mb 0.164 mdays/m³ 1240 1,240

  Slake lime for mortarc 1.2 mdays/m³ 771 535 426 586 437 2,760