The building consists of a pronao connected with a wide round cell by means of a rectangular structure intermediate.
The pronaos, ottastilo (with eight columns of gray granite facade), a measure m 34,20 x15, 62 m and was raised by 1.32 m on the level of the square to which it was accessed by means of five steps. The total height is of the order of 14.15 m and 1.48 m in diameter have stems at the base.
On the facade of the frieze bears the inscription of Agrippa in bronze letters, while a second inscription on a restoration under Septimius Severus was later engraved on the lintel. The pediment was to be decorated with figures in bronze, fixed with pins on the bottom: the position of the holes left has assumed the presence of a great eagle with spread wings.
Inside, two rows of four columns divide the interior into three naves, the central one wider leads to the large access door to the cell, while the two side ends of large niches that must have housed the statues of Augustus and Agrippa moved here from Augustan building.
The shafts of the columns were granite gray (eight -sided) or pink (eight, distributed in the two rows behind), from the Egyptian quarries of Aswan, and even the drums were in the arcades of the square gray granite, although smaller in size. The Corinthian capitals, bases and elements of the entablature were white Pentelic marble, from Greece. The last column of the eastern side of the pronaos, missing from the fifteenth century it was replaced by a frame gray granite under Pope Alexander VII and the column at the eastern end of the façade was also replaced under Pope Urban VIII with a stem of red granite: the original alternating colors in the columns was therefore altered over time. The new columns came both from the spa Neroniane.
The eardrum (which is not calibrated according to the proportion canonical Greek) has become smooth for bronze decoration has been lost, but for which you can still see the holes for the media that supported it.
The gabled roof is supported by wooden trusses, supported by block walls with arches resting above the rows of interior columns. The coverage of the bronze wooden truss of the pronaos was removed in 1625 under Pope Urban VIII for the edification of the Canopy of St Peter's, designed by Gian Lorenzo Bernini, and for the construction of 80 guns of Castel Sant'Angelo: for this "recycling" was written the famous lampoon " quod non fecerunt barbarians, fecerunt Barberini."
The portico is paved with slabs of colored marble which are arranged in a geometric design of circles and squares. Even the sides of the porch are clad in marble.
The intermediate structure that connects the porch to the cell is a stem in brickwork (bricks), consisting of two massive pillars that rest at the roundabout, connected by once went seamlessly suspended when the original bronze of the central part the pronaos. The pillars are inserted access stairs to the top of the rotunda. The wall is covered with slabs of Pentelic marble and decorated on the outside and on the sides of the compartment door by an order of pilasters that continues the order of the pronaos. Between the pilasters are inserted decorative panels with garlands and with sacrificial instruments.
Outside the property has the same height of the cylinder of the round like this and had to have a coating of stucco and plaster, and then disappeared.
On the facade of a brick pediment that repeats at a greater height of the pronaos, and relates to the divisions of the cornices present on the roundabout, which continued without interruption on the outer walls of the rectangular structure above the order of pilasters. The pediment, hidden from the porch, still had to be visible from a great distance.
The difference in level between the two gables did hypothesize that the vestibule of the building had been originally expected larger, with stems of column 50 feet (14.80 m) instead of 40 feet (11.84 m), but that the granite quarries in Egypt, already exploited to the stems of the monumental northern gate of the Forum of Trajan, were not able to provide other monolithic shafts of such exceptional size and that the project had to be reduced and therefore changed.
The bronze door of a different aspect ratio of the opening, comes perhaps from another old building.
The exterior of the round
The exterior of the dome hides round for a third, building a cylindrical body which is nothing more than the continuation of the vertical drum. Between the bubble and the outer wall is so large enclosed space where they were obtained a double system of windowed rooms, arranged on a circular corridor, which also serve to lighten the weight of the time.
The outer body of the rotunda, excluding the dome, was not visible in ancient times, as hidden by the presence of other adjacent buildings, which is why no special decorations, except for three frames with shelves to different heights at the entablature of the first order interior, along the line of tax and on the crown of the dome. To each of these three bands also correspond different materials used in the building, gradually lighter.
The interior of the rotunda
"I wanted that this sanctuary of all the gods represented the terrestrial globe and the celestial sphere, a globe within which are contained the seeds of eternal fire, all contained in the hollow sphere »
(Marguerite Yourcenar, Memoirs of Hadrian)
The internal space of the cell is round consists of a cylinder covered by a hemisphere. The cylinder has a height equal to the radius (21.72 m) and the total height of the interior is equal to the diameter (43.44 m).
On the lower level there are six large niches distile (with two columns on the front), either rectangular in plan (actually trapezoid) and semicircular, and the more the niche of the apse. This first level is framed by an architectural order with columns at the opening of the niches and pilasters in the intermediate wall sections, which support an entablature. Only the apse opposite the entrance is flanked by two columns instead of protruding from the wall, with the entablature which runs inside as set in the apse semi-dome.
Between the pilasters, in the spaces between the niches, there are eight small shrines on a high base, with alternating triangular and curved pediments. The walls are covered with slabs of colored marbles.
The higher order, in opus sectile, had an order of pilasters porphyry which framed windows and a coating of colored marble slabs. The windows overlook the first inner annular corridor relief. The original Roman decoration of this belt was replaced with one made in the eighteenth century (probably in the years 1747-1752), which only partially restored the antique look. In the south- western part of the original Roman appearance of this level was restored later, but not entirely accurate.
The floor of the rotunda is slightly convex toward the sides, with the highest part (moved about 2 meters to the north -west of center) is raised about 30 cm, while it is concave in the center to make the rain that falls to interior of the temple through the oculus placed on top of the dome, from flowing toward the 22 drain holes at the center of the rotunda. There are some legends that from the oculus not enter the rain, because of a system of air currents, but are evidently false. The lining is in slabs with a design of squares in which they are enrolled either circles or squares smaller.
The dome has a diameter of 43.44 m, is decorated inside by five orders of twenty-eight chests of drawers, a sliding scale upwards, except in the wide smooth band nearest the central oculus, of 8.92 m diameter. The oculus, which gives light to the dome, is surrounded by a frame of large tiles wrapped in bronze fixed to the dome, which perhaps went inside to the top row of drawers. A Roman tradition has it that in the Pantheon does not get into the rain to the so-called "chimney effect": in fact, it is a legend linked to the past when the myriad of candles that were lit in the church, produced a stream of warm air rising toward up and that meeting with the rain nebulized, thereby suppressing the perception of the entry of water.
The realization was made possible thanks to a series of expedients which contribute to lightening of the structure, use of the drawers, the use of materials gradually more and more light upwards: in the layer closer to the cylindrical drum have layers of concrete with slivers of brick, concrete going up there with slivers of tufa, while at the top, near the oculus are concrete made with conventional aggregates, mixed with volcanic lava ground.
Outside, the dome is hidden below by a raising of the wall of the round, so it is divided into seven overlapping rings, the bottom of which still retains the lining slabs of marble. The remainder was covered with gilded bronze tiles, removed by the Byzantine Emperor Constans II, with the exception of those that surrounded the oculus, which is still in situ. The thickness of the wall tapers upwards (from 5.90 m to 1.50 m at the bottom at the part around the central oculus).
The dome rests on a thick ring of brickwork masonry (concrete with brick facing), on which there are openings on three levels (reported outside the cornices). These openings, in part used for aesthetic purposes, such as indoor exedras, in part blanks with predominantly structural functions, comprising a support structure articulated, incorporated in the ring which appears continuous to the eye. On the outer wall of the rotunda is now visible after the disappearance of the plaster coating, the complex articulation of the arches in the exhaust bipedales (square bricks of two feet from the side) inserted in the walls from side to side, which discharge the weight of the dome on points of greatest resistance ring, lightening the weight in correspondence of the voids.
The special technique of composition of Roman cement allows the dome without reinforcements to stand for nearly twenty centuries. A dome of this size would indeed be difficult with modern building materials, due to the low tensile strength of the concrete modern, without armor. The determining factor appears to be a particular construction technique: the cement was added in small amounts immediately draining excess water. This, in whole or in part by eliminating the air bubbles that are normally formed with the drying, gives the material an exceptional resistance. In addition, materials were used more and more light for caementa mixed with mortar to form the cement: the travertine foundations of the volcanic pumice dome.
The characteristics of the construction
The construction of the Pantheon was a masterpiece of engineering, where the architectural idea was perfectly interpreted with a technical approach empirical (subsidence and cracks occurring shortly after the building were promptly remedied). The spatiality perfectly spherical gives the viewer a feeling of extraordinary harmony, " motionless and enveloping ", thanks to the balanced relationships between the various members, which articulated with effects of light and shade in cassettonature, niches and newsstands.
The insertion of a large round room behind the portico of a classical temple has no precedent in the ancient world, at least judging by the architecture that we have received or that we know from literary sources. There is a precedent in Rome circular building with porch, dating from the late Republican era, though far more modest in scale: the Temples of Largo di Torre Argentina. The merger between a classical model (the columned portico) and a building from the new space typically Roman (the roundabout), was a sort of compromise between the spatiality of Greek architecture (careful essentially outside of buildings) and that of Roman architecture (centered on the interior). This raised a number of criticisms, but it was "an obvious tribute to the dominant classical culture of Rome," which manifested itself permanently in the following centuries.
The model of the circular space and covered by a dome was taken from that of the large spa rooms "imperial" of Baia and Rome, but it was a novelty to use this type of coverage for a temple building.
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Photos: Yumiko Kimura