The dome of the Pantheon
Under the emperor Hadrian the Pantheon is built with a concrete dome 43 metres in diameter, with an open eye at the top. The interior is a perfect sphere that fits exactly between floor and ceiling.
The dome showed what concrete and the vault could achieve. It inspired Brunelleschi, Bramante, Palladio and Jefferson.
The first Pantheon was built by Augustus’ associate Marcus Agrippa around 25 BC, but it was damaged by fires and had long since vanished when Hadrian became emperor. The new temple was erected roughly between 118 and 128. Hadrian was unusually interested in architecture, yet he had Agrippa’s name placed in the inscription above the entrance, where it remains to this day – a way of linking himself to Augustus, the founder of the Empire. We do not know for certain who designed the building. The temple was dedicated to all the gods, and the form of the interior, a sphere beneath a vault resembling the vault of heaven, has been interpreted as an image of the cosmos.
In front of the rotunda stands a portico of sixteen columns, each a single granite shaft shipped from Egypt. Behind it opens the circular interior. The dome is cast in concrete, and the builders made it lighter towards the top by mixing in ever lighter stone, such as tuff and pumice, nearest the crown. The coffers in the ceiling reduce the weight further and create a pattern that leads the eye towards the oculus, the open circle at the centre. Through it the light falls as a moving patch across the walls during the day, and rain drains away through small holes in the floor. The dome is still the largest unreinforced concrete dome in the world.
The Pantheon survives because in 609 Pope Boniface IV was given the building by the Byzantine emperor and consecrated it as a church dedicated to the Virgin and the martyrs. It was thus spared the decay that befell so many other temples. The bronze from the portico, however, was removed in the 17th century by Pope Urban VIII, partly to make cannon. Raphael is buried here, as are Italy’s first kings. For Renaissance architects the Pantheon was proof that antiquity had been able to build what no contemporary could. The building’s combination of temple front and rotunda has been copied in countless churches, libraries and museums, from Schinkel’s Altes Museum in Berlin to Thomas Jefferson’s university library in Virginia.