Brunelleschis Dome (9 page)

The stonemasons eating their breakfast on the tambour that historic August morning would have needed a good deal of courage. Below them they could see the newly completed vault of the south tribune, where just three weeks earlier a stonemason named Donato di Valentino had fallen a hundred feet to his death. Another man had also died in the rush to finish the tribune so that work on the dome could begin in the summer. The Opera had paid for both funerals, but this was the extent of the charity that the men could expect. Anyone injured on the job would face a grim future, as would his family, because neither the Opera nor the Masons Guild made provisions for either disabled workers or the widows and children of dead ones. The only social obligation of the members of the Masons Guild was attendance at one another’s funerals.

Present in the minds of the stonemasons must also have been the awesome and abiding fact that none of them yet knew whether the structure could actually be erected according to Filippo’s plan. Certain details of the cupola’s design had been established, of course, in the twelvepoint building program adopted the previous month. The width of the inner dome, for example, was to taper like that of the Pantheon, diminishing from seven feet at its base to just under five feet at the top. And the exterior shell — added to protect the inner one from the elements as well as to make the entire structure appear
più magnifica e gonfiante,
“larger and more inflated” — was to narrow from a width of slightly over two feet at its base to just over a foot at the oculus. Likewise the eight vertical ribs at each corner of the octagon were to taper as they rose skyward. And while the dead load of the Pantheon had been lessened by the use of pumice stone and empty bottles, in Santa Maria del Fiore the shells were to be built out of stone for the first 46 feet, then from either brick or tufa, the latter being a light, porous stone formed from volcanic ash. The building program also outlined, albeit vaguely, the incorporation of a number of rings of sandstone beams held together with cramps of leaded iron — the chains that Neri di Fioravanti had envisioned encircling the dome’s circumference. These would be embedded in the masonry and therefore hidden from view.

It was the twelfth point that raised the most doubt. The wardens agreed that for the first 30
braccia
of their height — that is, for a distance of about 57 feet above the drum — both shells were to be built without any scaffold-supported centering. Thereafter, from 30
braccia
upward, the dome was to be built “according to what shall then be deemed advisable, because in building only practical experience will teach that which is to be followed.”

This vital condition reveals the reservations of the wardens in the face of Filippo’s daunting plan. Acceptance of it represented a concession on Filippo’s part, a means of appeasing the nervous wardens by committing himself to building only the first fifth of the dome without centering. If he succeeded, he would then have to make his case to raise the rest of the cupola in similar fashion. He must have felt frustrated by the wardens’ continuing lack of faith, but he may also have felt relieved that he was given some time to consider his plans. It is conceivable that even he was unsure of himself at this early stage. Uncertainty about how to execute his audacious plan, and not simply fear of someone stealing his ideas, may have been one of the reasons why he refused to divulge to the incredulous wardens the secret behind the procedure of vaulting without centering. As late as the summer of 1420, for example, he had still to work out the design of the circumferential stone chains. He would not in fact devise a plan for the first one until June of the following year, barely a month before its construction was due to begin. And plans for the second were not completed until 1425, when yet another model had to be made.

Neither Filippo’s brick model of the dome nor the cupolas he built for the two chapels could quite have prepared him for the task ahead. It had long been known that architectural models were poor guides to statics, because what worked structurally in a model could not necessarily be achieved when the proportions were magnified. In the Middle Ages and Renaissance, proportionally identical models behaved differently depending on their respective sizes, and scale models were generally misleadingly strong.
*

Given the experimental nature of Filippo’s plan, the 30-
braccia
limit seems to have been a wise precaution, especially since a sound logic governs the restriction. At a height of 30
braccia
the bed joints of the masonry would have risen to form an angle of 30 degrees to the horizontal, or just inside the critical angle of sliding.
³
Friction alone would keep the stones in place up to an angle of 30 degrees, even when the mortar was green; therefore, no centering would have been required until that point. Above that level, however, each course of masonry would incline more sharply, reaching a maximum angle, near the top, of 60 degrees to the horizontal. No doubt it was impossible for the wardens to imagine how these courses might be held in place without centering of some sort.

Both Filippo and the wardens seemed to be purchasing themselves a little time by deferring the central question of how the dome should be vaulted. All were agreed that, in an unprecedented structure like the dome, any constructional difficulties could be solved, as the 1420 program stated, only by means of “practical experience.” This was perhaps to err on the side of optimism. But just such a process of trial and error was about to begin.

I am accustomed, most of all at night, when the agitation of my soul fills me with cares, and I seek relief from these bitter worries and sad thoughts, to think about and construct in my mind some unheard-of machine to move and carry weights, making it possible to create great and wonderful things.

T
HESE WORDS ARE SPOKEN
by the statesman Agnolo Pandolfini in a philosophical treatise written by one of Filippo’s ablest disciples, the architect and philosopher Leon Battista Alberti.
Della tranquillità dell’animo
(On the tranquillity of the soul) was composed in 1441, a few years after Filippo’s dome had been completed. It features a dialogue between two men who have suffered miserably from changes in fortune: Agnolo, who has retired, disillusioned, from public life, and a younger man, Nicola de’ Medici, whose bank has failed, leaving him destitute. Their conversation takes place inside Santa Maria del Fiore, under the new dome, and concerns the various means of overcoming depression. Agnolo lists a number of traditional remedies for raising the spirits, such as wine, music, women, and sports. But his most effective tactic, he tells Nicola, is to fantasize about the construction of giant hoists and cranes that can be used to create “great and wonderful things” — machines for raising magnificent structures, that is, like the dome that swells above them.

One of the most obvious problems in building the dome of Santa Maria del Fiore — or indeed any large structure — was how to transport heavy building materials such as sandstone beams and slabs of marble several hundred feet above the ground and then place them into position with the accuracy demanded by Filippo’s design. The sandstone beams weighed some 1,700 pounds each, and hundreds of them needed to be raised onto the cupola. To solve this problem Filippo was compelled to imagine “some unheard-of machine” to move and carry tremendous weights to incredible heights. The hoist that he created was to become one of the most celebrated machines of the Renaissance, a device that would be studied and sketched by numerous other architects and engineers, including Leonardo da Vinci. And it was also, no doubt, the inspiration behind Agnolo’s soothing fantasies.

A number of machines were already in use on the building site, of course. Twenty years earlier a
rota magna
, or “great wheel,” had been constructed to raise the heavy stones used in the facade, drum, and tribunes of the cathedral. This machine, still operational in 1420,was a treadmill that winched loads aloft under the motive power of several men who walked around, hamsterlike, inside a large wheel. Such devices had been in use since ancient times. In
De architectura
the Roman architect Vitruvius describes a treadmill turned by “tramping men,” presumably slaves. The treadwheel, a sort of giant spool, either wound or unwound a rope that, in passing through a system of pulleys, raised or lowered the weight attached to its end. The muscular effort involved in powering these winches was not excessive, provided the loads were relatively light and the heights to which they were transported not especially high.

Recognizing that the
rota magna
would be woefully inadequate to the task of lifting heavy stones to the height required by the dome, the Opera del Duomo had specifically called for models of lifting devices in the 1418 competition. But the models submitted in the ensuing months showed only the cupola or its centering rather than the machines for their construction. Two weeks after the appointment of the three
capomaestri
, the Opera was still referring in its documents to an intended plan of using a plain treadmill — possibly the old
rota magna
— for hoisting the materials. Filippo must have found such a lack of enterprise appalling. He eagerly responded to the challenge and, in one of his first acts as
capomaestro
, began designing a machine that would be powered not by men but by the busiest and most prized beast of burden in the Middle Ages, that powerful but placid creature, the ox.

Work on this new hoist started in the summer of 1420. For its parts Filippo contracted with a wide number of artisans, many of them from outside Florence. Several weeks after the celebratory feast on the tambour, the Opera received shipment of an elm tree from which the drums for the new hoist were to be hewn. The tree must have been enormous, because the largest of the three drums was five feet in diameter. Elm was chosen for its resistance to the elements, for clearly the hoist would need to be in service for many years. Other parts for the hoist had also begun arriving: chestnut poles for the building of the supporting frame and a harness and reins for the oxen. A rope was ordered from Pisa, a shipbuilding town where the art of rope making was highly advanced. Still, Filippo’s hoist must have taxed even rope makers accustomed to fitting out the largest galleons, for it required one of the longest and heaviest ropes ever manufactured: 600 feet long and weighing over a thousand pounds.
¹

Construction on the hoist continued throughout the winter of 1420-21. A blacksmith was engaged to make bearings for the pulleys of the hoist and a turner to cut cogs from ash wood for its wheels. Meanwhile a barrel maker began making hoisting tubs to hold the loads of masonry and mortar on their ascents. Finally, two master carpenters were hired to build the frame and assemble the various parts. Each of them spent sixty-seven days on the job.

Work must have proceeded at a furious pace, because in the spring of 1421 the hoist took its place on the floor of the octagon. Or rather it took its place on a 29-foot-long wooden platform specially constructed for the oxen that were to make thousands of revolutions over the next dozen years. Before the dome was complete, the hoist would raise aloft marble, brick, stone, and mortar weighing an estimated 70 million pounds.

A drawing by Taccola of Brunelleschi’s ox-hoist, in this case being driven by a horse. At the bottom, the helical screw that raises and lowers the wheels is clearly depicted.

Filippo’s ox-hoist was remarkable both for its sheer size and power and for the complexity of its design, especially its reversible gear, an important innovation for which there is no known precedent in the history of engineering. In the words of one commentator, the machine was “centuries ahead of the technical understanding of the time.”
²
It consisted of a wooden frame, fifteen feet in height, to which were attached a number of horizontal and vertical shafts or spindles that rotated each other by means of cogged wheels of varying sizes. The machine was set in motion by either one or two oxen yoked to a tiller that turned the vertical shaft. This shaft or rotor was furnished with two cogged wheels, one at the top and another at the bottom, either of which could mesh with a much larger wheel on a horizontal axis. Only one of the wheels on the rotor could be engaged at a time, however: one to raise loads, the other to lower them. The change in gears was effected by a large screw with a helical thread. Turned in either direction, it would lift or drop the rotor several inches, thereby engaging one or other of its pinions with the teeth of a wheel attached to the
sùbbio grosso
, the largest of the three rope drums.

This screw that raised and lowered the rotor was one of the hoist’s most ingenious features. It served as a clutch, connecting or disconnecting the two gears from the wheel of the large drum. This meant that the hoist could be reversed — and loads either raised or lowered — without the driver being forced to unyoke the oxen and turn them around. The oxen, that is, always moved in a clockwise direction. The obvious benefit of this gear change was that a great deal of time was saved between each ascending or descending operation. Oxen were ideal for moving heavy loads, given their stamina and strength. But they could not be made to walk backward more than a few steps, a reluctance that hampered any driver trying to unyoke them from the tiller.