Brunelleschis Dome (5 page)

What exactly Filippo sought in these excavations was unknown even to Donatello. Antonio Manetti claims that Filippo, secretive as ever, made his study of the ancient ruins while pretending to be doing something else. He inscribed on strips of parchment a series of cryptic symbols and Arabic numbers: a secret code, that is, like the reversed handwriting that Leonardo da Vinci would later use to describe his own inventions. Before patents or copyrights, scientists frequently resorted to ciphers in order to conceal their discoveries from jealous rivals. Two centuries earlier the Oxford philosopher Roger Bacon, known as “Doctor Mirabilis” for his experiments with telescopes, flying machines, and robots, claimed that no scientist should ever write of his discoveries in plain language but must resort instead to “concealed writing.”
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What was the purpose of Filippo’s cryptic symbols and Arabic numerals, the latter of which the Commune of Florence had banned in 1296?
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Manetti claims he was surveying the antiquities of Rome, measuring their heights and proportions. He fails to record what method Filippo used, but he could have determined the height of columns or buildings with an upright rod. This method would have been familiar to him from Leonardo Fibonacci’s
Practica geometriae
(1220), a work that was studied in the schools of Florence. Or he could have employed a quadrant or, even more simply, a mirror, whose use for mensuration Fibonacci likewise describes. The surveyor placed the mirror on the ground some distance in front of the object to be measured, then moved himself into a position such that the top of the object appeared in the center of the mirror. The height of the building was then calculated by multiplying the distance between the object and the mirror by the height of the observer divided by his own distance from the glass.

Filippo was not the first person to survey the ruins of Rome. As early as 1375 Giovanni de’ Dondi, the famous clockmaker, measured the obelisk of St. Peter’s, a process that he described in his book
Roman Journey
. But the knowledge that Filippo sought to uncover was unique. In calculating the proportions of columns and pediments he determined the measurements specific to the three architectural orders (Doric, Ionic, and Corinthian) that had been invented by the Greeks and then imitated and refined by the Romans. These orders were governed by precise mathematical ratios, a series of proportional rules that regulated aesthetic effect. The height of a Corinthian entablature, for example, is a quarter of the height of the columns on which it stands, while the height of each column is ten times its diameter, and so forth. Numerous examples of these three orders existed in Rome in the early 1400s. The columns in the Baths of Diocletian are Doric, for instance, while those at the Temple of Fortuna Virilis feature the Ionic, and the portico of the Pantheon the Corinthian. The Colosseum makes use of all three: Doric on the lowest level, Ionic on the second, and Corinthian at the top.
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Knowing that a dome was planned for the cathedral in Florence — a dome that no one as yet knew how to build — Filippo must have taken a special interest in the methods of vaulting used by the ancient Romans. In the early years of the fifteenth century any number of domes would have been available for him to scrutinize. After large parts of the city were burned in the fire of
A.D.
64, Nero had established regulations (much like those adopted after the Great Fire of London in 1666) that widened the streets, controlled the water supply, and — most vital from an architectural perspective — restricted the use of inflammable building materials. The Romans therefore started to use concrete, a new invention, in their buildings. The secret of Roman concrete was in its mortar, which contained a volcanic ash made available by active volcanoes such as Vesuvius. Combined with lime mortar, it resulted in a strong, fast-setting cement to which an aggregate of small broken stones was added. Unlike conventional mortars made from quicklime, sand, and water, which set only when the water evaporates, “pozzolana concrete” (as it is known) combines chemically with water so that, like modern Portland cement, it cures swiftly, even underwater. Although various Roman baths had been vaulted in concrete since the first century
B.C.
, extensive and inventive use of concrete arches and domes was made only after the fire of
A.D.
64. The history of domes commences, effectively, with the opportunities created by this great conflagration — one that the Romans believed was either the work of Nero himself or else that of the wrathful gods.

The Domus Aurea, or the Golden House of Nero, begun immediately after the fire by the architects Severus and Celer, shows the confident use of concrete to exploit new architectural shapes. This splendid urban palace stretched from the Palatine to the Esquiline across an area that had been decimated by the fire. Enormously expensive to build, it contained elaborate decorations, including the
Laocoön
(which would be rediscovered there in 1506), and mechanical wonders such as pipes concealed in the ceiling of the dining hall that sprayed perfumes on the emperor’s dinner guests. Its most interesting architectural feature, however, is an octagonal room in the east wing that is roofed by a dome whose span is some 34 feet across. The octagonal shape must have interested Filippo, who would have known, of course, that the dome of Santa Maria del Fiore, though much larger, was also intended to be eight-sided.

Of even more interest to Filippo would have been the Pantheon, the emperor Hadrian’s temple to the gods of all the planets, executed between
A.D.
118 and 128. Unlike the octagonal cupola in the Domus Aurea, the dome of the Pantheon is colossal, spanning 142 feet internally and rising to a height of 143 feet. Almost thirteen centuries after its construction it was still the largest dome ever built, and it had escaped plunder because it was now converted into a church, Santa Maria Rotonda. The modern Romans and pilgrims alike were amazed by the immense dome. With no visible signs of support, it seemed to defy the laws of nature. They called it the “house of devils,” attributing its construction not to the skilled engineers of the ancient world but rather to the sinister forces of demons.

What structural features of this “house of devils” might Filippo have studied? The architects of the Pantheon faced the statical problems encountered by builders of all domes: how to counteract the forces that act on any vault. These forces are separated into “push” and “pull” energies, known respectively as compression and tension. All elements in a building — its columns, arches, walls, roof beams — are subject to one or other of these actions: their stone or timber beams are compressed from above (which causes them to shorten) or pulled from the side (which causes them to stretch). An architect must design a structure that will counteract these pressures by playing them off against each other — a game of action and reaction — and channeling them safely to the ground.

The Pantheon.

The first type of pressure does not create insurmountable problems for an architect. Stone, brick, and concrete all possess such enormous compressive strengths that buildings can be raised to colossal heights without the blocks of stone crushing at the base. The tallest spire in England, that of Salisbury Cathedral, stands 404 feet high, and the two towers of the cathedral in Cologne each rise to 511 feet, or the equivalent of a fifty-story building. At this height they are almost a dozen feet taller than the Great Pyramid at Giza, another edifice whose tremendous size is made possible by the strength of the stone blocks from which it was built. Yet not even these soaring structures come close to exhausting the compressive strength of stone: a column of limestone could be built to a height of 12,000 feet, or over two miles high, before starting to crush under its own weight.

The stones in a dome, however, are not only crushed from above but also thrust outward by the pull energy known as “hoop stress,” in the same way as the rubber of an inflated balloon will bulge outward if one compresses it from above. The problem for architects is that stone and brick do not respond nearly so well to this lateral thrust as they do to compression.

The Romans seem to have possessed some understanding of the structural problems created by tension and compression, and they attempted to solve them by making extensive use of the new pozzolana concrete. Where the horizontal stress is greatest, at the base of the dome, the concrete wall of the Pantheon was built to a massive thickness of 23 feet. It then tapers to only two feet at the top, at which point a round window or “oculus” is left open. Five thousand tons of concrete were poured in horizontal layers on to wooden formwork, but at the top of the dome lightweight aggregates such as pumice and, more inventive still, empty
amphorae
(clay bottles used for shipping olive oil) were added to the concrete in place of stone in order to reduce the load. The inside of the dome was also coffered, which not only lightened the load still further but also added a decorative feature that has since been extensively imitated.

Hoop stress. The dotted line shows how the dome is deformed by weight at the top.

The Pantheon would have presented Filippo with proof that it was possible to span a space as vast as that at Santa Maria del Fiore. Yet Hadrian’s architects were not entirely successful, for a series of cracks are visible along the inside of the dome, running like lightning strokes down the ceiling to the springing line, the point where the dome begins to curve inward. These fractures are the result of the hoop stress that causes the dome to spread at its haunches, stretching the fabric horizontally around the circumference. Filippo could have seen a similar pattern of radial cracks around the base of the semidome in the Baths of Trajan, and indeed such cracks have been an all too common feature of masonry domes. Containment of this horizontal stress — one that it appears not even a concrete wall 23 feet thick could neutralize — was therefore of paramount importance in constructing a stable cupola. For all their ingenuity, not even the Romans, it seemed, could provide the solution to the challenge laid down by Neri di Fioravanti and his committee.

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It is not known exactly how long Filippo remained in Rome or when precisely he left. His stay there was one of the first examples of a new kind of quest. Pilgrims of a different variety soon began arriving in the city, ones seeking relics other than the bones of saints on display in the Christian churches. The image of Rome would be transformed during the Renaissance. Far from being condemned for its pagan associations, the ancient city came to be venerated for its architecture, its statuary, and its learning. The architects Leon Battista Alberti, Antonio Filarete, Francesco di Giorgio, and Michelangelo were all to follow in Filippo’s footsteps, traveling to Rome to take their inspiration from the ruins. No longer was it considered bad luck to unearth pagan remains, either in Rome (where Cicero’s house was excavated) or elsewhere. In 1413, for example, the bones of the Roman historian Livy were exhumed in Padua, causing an outburst of almost religious fervor. The bones were enshrined in Padua’s town hall, and soon afterwards the city fathers received from Alfonso, king of Naples, the urgent request for a femur. An even more spectacular relic was the perfectly preserved body of a young Roman girl excavated from one of the tombs along the Via Appia and exhibited to the populace of Rome in 1485.

Other treasures were also discovered. Manuscripts were disinterred from where they had lain entombed throughout the centuries. The
Annals
of Tacitus, Cicero’s
Orator
and
De oratore
, the poems of Tibullus, Propertius, and Catullus (the lone manuscript of whose work was found stoppering a wine barrel), the
Satyricon
of Petronius, the poems of Lucretius, a complete copy of Quintilian’s
Institutio oratoria
— all of these shards of ancient Rome, lost or unknown for centuries, were recovered in the first decades of the fifteenth century. Like the fragments of stone studied by Filippo, such manuscripts would form a link between the ancient Romans and the artists, philosophers, and architects of the Quattrocento. And it was from these broken stones and faded parchments that the world would now be made afresh.