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Divo Adriano
Pantheon
Pantheon architecture
Pantheon: (Italian) bibliography
Pantheon, restaurations
Poseidonion
Pantheon: the dome
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As a covering of the "Rotonda" (Pantheon) cell, there is the dome in concrete, the largest in the world. It measures infact 43.30 m in diametre and half in height, being perfectly hemispheric: ideally, a sphere could be inserted in the temple, as the circular wall on which it rises mesures in height, as much as the radius of the cell. The dome is larger than the one of St. Peter, of 42 m in diametre, and of the one of St. Paul in London which measures hardly 31 m. Interiorly the dome structure begins from the second cornice, but exteriorly it can be seen only at a much greater height.
Actually, at the exterior there are three cornices, of which the second coincides internally with the basis of the dome. Over this second cornice, the brickwork of the tambour continues up to the height of 30.40 m. or at about 10 m. behind the vault.
Over this third cornice, there are seven high steps which are concentric which constitute the counterfort, and finally the curve structure of the dome, apparently very squeezed.
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The thickness of this is not uniform, but little by little it diminuishes as it gets nearer to the central summit : from 5.20 m. from the basis to 1.40m. in correspondence of the opening. Foreseeing so that it have a greater weight in the low part, the architect of the Pantheon suceeded in cancelling the dome tendency to drop : but the most ingenious of its construction rests in the inert materials adoperated in the construction of the opus coementicium of which it is made. The corresponding stratum to the first two fascias of interior lacunars infact, compounded with concrete stratum, alternating with remains of tufa and bricks, while stratums of concrete, with remains of tufa and pomice, constitute the rest of the dome.
In this way it was rightly calculated that the bending moment would be maintained more or less unchanged, all along the section of the structure and that would assure the stability, and thus it was.
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The same meticulous study has been applied at the interior of the edifice, from the foundations. These 4.50 m. deep, are constituted of two concentric rings with a total thickness of 7.30 m. in concrete with remains of travertine ; initially the architect had forseen only one ring of a thickness of hardly one major metre on the raised part. This one, during the raising of the dome or immediatly after, had evident signs of sinking at the two extremities of the main axis of the temple, which required a strengthening of the same by means of the construction of a second ring behind the first and, more painful remedy, to prop the part of the cell with powerful walls parallel to the north-south axis to which had been given the task of strengthening it as a vice so as to avoid phenomena of squashing of the masonery.
In the masonery, in the low portion of the tambour, to the travertine chips, were added chips of a lighter stone, or the tufo and in the high section, just under the dome, the travertine is entirely substituted by the previously said chips of tufo, to which brick chips are added.
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Moreover the thickness of these walls (6 metres), run along many times by arches of discharge, in brick work, it is not complete : the eight niches which enliven the interior of the cella separate eight gigantic pillars, each one of these is, by turn, lightened by as many semicircular rooms, true and proper connection of rooms which open on the exterior, by means of a window. Without these sagacities, the brickwork of the tambour, in addition to using a very long time so that the concrete could dry up and solidifying, would surely have been too heavy to support the dome.
Coming back to that, we can say that it was constructed by horizontal jetties over a wooden curvature, on which already, had been made the reliefs of the 140 lacunars (28 for each of the five concentric rings) around the oculum which measures nearly nine metres (8.92) in diametre.
These are of very involved shape not only because formed in a double curved surface, but also because drawn according to perpective principles. The la- cunars of the first four concentric orders are constituted even of four stratums (as if there were four lacunars, one in the other) while, those of the last order have only three.
The most exterior stratum of any one, that we can call cornice, slips slenting towards the interior, as if it were the base portion of an oblique pyramid having the top externally to the cupola : more over, as they rise, the lacunars diminish in length and deepness. In ancient times, they were adorned with a big bronze rosette, at the centre, anchored to the concrete. Exteriorly, the cupola is entirely covered by semitiles laïd like scales, covered with a stratum of opus signum, 15 centimetres in thickness which makes it impermeable under the lead slabs.
The central oculus around which have remained traces of the ancient bronze covering, it is coated in its thickness with bricks forming many successive flower beds. (De Angelis d'Ossat has observed, with regard to this, that in the period of great diffusion, there existed a quasi constant correlation between the cupola diametre and the one of the skylight, correlation which was about one fifth and one sixth. At the time of Mercurio at Baia, the correlation is equal to 0.169 and, in the Pantheon, to 0,205 nearly an exact one fifth. The octogonal half of the Domus Aurea of Nero (Nerone) instead, distances itself remarkably from this proportion in which the data of the correlation are constituted by 13.48 m. and 5.95 m.)
The 9/10 of the Pantheon are constituted by cement work, this is why what appears at the exterior is only a "coating". Exteriorly infact, one finds himself facing a brick wall run by support arches with double or triple armillaries, distant about 1.50 m. one from another. To a certain extent these also contribute at diminishing the load of the brickwork on the seven radial niches which constitute the main decoration of the interior and on the entrance bay. In the same way are displayed the frequent courses of the large tiles. To interrupt the monotony of the wall, there are three anular cornices placed at different hights : the lowest which ends before the porch, is only of brick ; the intermediary one, with fascias of brickwork and marble consoles, continuing also on the structure of the foreport, it constitutes the horizontal side of it ; the third one, finally, richer than the second, although very similar, goes all around the rotonda and the forepart, forming the dripstone of the roof . On the forepart also, over the tympanum are seen the arches of discharge, even if indirectly, this induces at considering this structure as a whole with the rotonda, and such it is.
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The forepart, the purpose of which is to contain two flights of steps which lead to the higher part of the cell and also of uniting this to the porch, interiorly, it is constituted of the same kind of concrete as the one of the rotonda. The two elements are, in truth, one body which, from the plan, appears as an 'omega" of the greek alphabet. In the Pantheon construction, diverse, highly specialized workers were necessary, as master masons, stone masons, to direct the brick laying, and moreover, obviously an architect, but the specialized craftmen who served for the construction of the marble classic temples were not requested, except for the porch and the interior orders.
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Using structural materials, 'anonimous' in themselves, as mortar, sand and bricks, the workers organization and their relation with the flow of materials and the sense of timing the flow of the concrete, is perfectly consistent with the totally roman predilection for the efficient and clearly fine organization.
There was not any moment, in the construction process, so dramatic in the high medieval construction, as well as in the highest vaults of the naves of a gothic cathedral were decentralized as the wooden moulds were made skinner to reveal a thin cover of stone as the sheet of structures, 15 and more metres deep. The Pantheon does not have the luminous and animated sculptural presence of the great greek temples of the Vth century a.C. But it is this that should have been.
The architect was not trying to make a thing already made in the pass or what other people would have been able to do. This architect, subject to the orders of the emperor, was looking for a kind of ineluctable roman architecture, an architecture which tell of the new faceted culture of the late antiquity world, with the name of the Roman Empire.
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