Pantheon is a building of ancient Rome built in origin as a temple dedicated to all gods,
or better to the 7 planetary divinities (Sun, Moon, Venere, Saturno, Giove, Mercury, Mars). In more modern books we will find above all indicated Mars and Venere. The Pantheon word has Greek origin: it is an noun adjective "the gods totality", and in the greater part of the cases it subtends the noun "tempio", therefore from the Greek "the temple of all the gods", it has derived the latin word Pantheon used by the old Plinio in Nat. Hist. 34, 13, that has delivered the word to the Italian language.
The first Pantheon was ordered in the 27-25 a.C. from Marco Vipsanio Agrippa, Augusto’s friend and brother-in-law, in the plan of the monumentalisation of Campo Marzio. The original registration of the building dedication, brought back on the next reconstruction of adrianea age, says: M.AGRIPPA.L.F.COS.TERTIUM.FECIT (translation: "Mark Agrippa, Lucio’s son, consul for the third time, built it"). The third consulate of Agrippa is about the year 27 a.C. however Dione Cassio lists it among the works completed from Agrippa in the Campo Marzio in the 25 a.C..
From the rests recovered to the end of XIX the century it is known that this first temple was of rectangular plan with cross-cell, wider than long (as the temple of Concordia in Foro Romano and the small temple of Veio on Campidoglio), built in travertine blocks covered by marble slabs. The building, destroyed from the fire in the 80 d.C. came restored under Domiziano, but it endured a second destruction under Traiano.
Under Adrian the building came entirely reconstructed. The lateritious bubbles (factory marks on the bricks) belong to years 123-125 and we can think that the temple was inaugurated from the emperor during his permanence in the capital between 125 and 128. According to some plans, written after the destruction of the previous building in traianean age, would be attributable to the architect Apollodoro of Damascus. Regarding the previous building the guideline changed, with points out towards the north.
The dome, with 43 linear metres of diameter, is decorated to the inside from five orders of twenty eight drawers, decreasing measure towards the high, and introduces to the centre an oculus of 8,92 metres of diameter. The oculus had to be encircled by a bronzed frame fixed the dome that perhaps caught up the higher row of drawers. Numerous cavities in the cement allow to assume that also drawers and the intermediate spaces between them, were covered in bronze.
To the outsider part, the dome is hidden inferiorly from a raising of the round’ wall (for 8,40 m), and therefore it is articulated in seven overlapped rings, the inferior of which still maintains the covering in marble slabs. The remaining part was covered by golden bronze tiles, removed by bizantin emperor Costante II, with the exception of those which encircled the oculus, still in situ. The thickness of the masonry diminishes towards the high (from 5,90 m inferiorly to 1,50 m in correspondence to the part around the central oculus). Moreover, to the inside part of the masonry various types of more and more light tiles proceeding towards the high (in the culminating part there are lightest stones) have been used. These solutions have allowed the balance of the dome weight.
The dome is over a masonry ring in lateritious work (cement with vestment in bricks), on which there are openings on three levels (signaled to outside from the frames marcapiano). These openings, used to aesthetic purposes, like the exedras in the internal part, in part empty spaces with mostly structural functions, compose a structure of articulated support, closed in the continuous ring that appears at the view. On the external wall of the round we can see, after the passing of covering plaster, the complex articulation of drainage arches in bipedals (squared bricks of two side feet) put in the masonry from side to side, which unload the weight of the dome on the points of greater resistance of the ring, lightening the weight in correspondence of the empty ones.
The particular roman cement composition technique allows the dome lacking in supports to stand up from nearly twenty centuries. A dome of these dimensions would be in fact difficultly to be built with the modern technologies, because the modern cement doesn’t resist. The determining factor seems to be a particular construction technique: the cement was added in small amounts draining water in excess. This, eliminating air bubbles that normally are formed with the drying, confers to the material a wonderful resistance. Moreover lighter materials were gradually used for caementa mixed with maltha in order to form the cement: from the travertine of the foundations to the volcanic stone of the dome.
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