Pantheon
Behind a temple front of granite columns opens a round space as high as it is wide: 43 metres. The dome is cast in concrete that becomes lighter towards the top and has an open eye, the oculus, at its crown. The Pantheon became a church in the 7th century and has therefore been in continuous use for almost 1,900 years.
The first Pantheon was built by Augustus’ associate Agrippa around 25 BC but burned down. The present building was erected under Hadrian, roughly between AD 113 and 125, as stamps in the bricks among other things show. Hadrian nevertheless kept Agrippa’s name in the inscription above the portico, as if the temple had merely been restored. The architect is unknown. The secret behind the dome is Roman concrete, a mixture of lime mortar, volcanic sand and crushed stone. The builders chose ever lighter aggregate the higher they went: heavy travertine at the bottom, then tuff and brick, and at the top light pumice. The walls also contain hidden relieving arches of brick. The sixteen columns of the portico are granite monoliths shipped from Egypt.
The height of the rotunda equals its diameter, so that an imaginary sphere would fit inside the space. The dome has five rings of 28 coffers each, which both make it lighter and catch the light. The only light enters through the oculus, and the patch of sunlight moves across the walls during the day. When it rains the water falls straight in; the floor slopes gently and has small holes for drainage. The floor is laid in a pattern of circles and squares of coloured marble and porphyry. Along the wall, deep niches alternate with small temple-like altars. Between the rectangular portico and the rotunda lies an intermediate block that clearly shows two forms joined together. The dome is still the world’s largest of unreinforced concrete.
In 609 the Byzantine emperor gave the building to the Pope, who consecrated it as a church of Mary and the martyrs. This saved it from being plundered for stone. Gilded bronze roof tiles, however, were removed as early as the 7th century, and in the 1620s Pope Urban VIII had the bronze of the portico roof melted down. Two bell towers added in the 17th century were demolished in the late 19th century. Raphael is buried here, as are Italy’s first kings. The Pantheon has been the model for countless domed buildings, from Renaissance churches and Palladio’s villas to museums, libraries and Thomas Jefferson’s university in Virginia. Swedish architects such as Nicodemus Tessin the Younger studied the building closely in Rome, and rotundas in the spirit of the Pantheon appeared in Nordic palaces and parks.