Brunelleschis Dome (14 page)

Giovanni da Prato’s submission to the Opera del Duomo did not end with complaints about the curvature of the dome. His document returned to his earlier obsession with light in the cathedral, complaining that, lit only by the eight windows in the tambour, as well as by the oculus at the top, the cathedral would be made “murky and gloomy” inside. To prove his point, he drew a section of the cathedral showing how a beam of sunlight entering one of the windows in the drum would be insufficient to illuminate either the cupola above or the crossing below.

The illumination of a church was an important architectural consideration. Gothic builders had sought to fill their churches with plenty of light by designing enormous windows filled with stained glass, but the merits of “light” or “dark” churches were matters of considerable debate during the Renaissance. Alberti, for example, argued that churches should be dark inside, lit only by candles and lamps. But Giovanni’s complaint about the murkiness of Santa Maria del Fiore was to be echoed over a century later in Rome when Michelangelo, taking over the construction of St. Peter’s, criticized the previous
capomaestro
, Antonio da Sangallo, for designing a dome that would render the cathedral so dark inside that nuns would be raped and criminals concealed.

In Giovanni’s view there was only one way to save Santa Maria del Fiore from darkness: he urged the wardens to dust off and reconsider his old, rejected plan in which twenty-four windows were to pierce the base of the cupola and thereby fill the church with glowing light. His tone became hectoring and apocalyptic:“Per dio uogliate pro uederui,” he begged them (“For God’s sake, take care of it”). Once again he sought to absolve himself of any responsibility should things go wrong:“I have written this in order to be blameless if nothing should be done about this problem.”

But Giovanni’s pleas again fell on deaf ears. In the amendments made to the cupola project in January 1426, Filippo wrote, “We make no special suggestions regarding the light because the illumination from the eight windows below seems to be adequate.” He added that if it should be discovered that more light was required, then windows could be incorporated at the top of the dome — a solution already angrily rejected by Giovanni as one that could only be advanced “by a fool of small understanding.” It was evident that the Opera sided with Filippo rather than Giovanni, because construction proceeded on the cupola as before, with the same curvature and without windows at the base; and several weeks later Filippo received his salary increase to 100 florins per year. Giovanni, meanwhile, was paid 10 gold florins for his advice, after which he remained on the periphery of the project.

But this debate was not the last that Filippo would hear of Giovanni da Prato. Soon afterward the
capomaestro
began work on another invention, one that would enjoy far less success and esteem than any of his previous ones, and Giovanni would have his revenge.

D
ESPITE HIS SUCCESSES
in the early years of the dome’s construction, Filippo must have been dogged by the knowledge that once the dome reached a height of 30
braccia,
the wardens would meet again to consider whether or not to continue building without centering. Early in 1426, after the second sandstone chain had been swung into place, that moment arrived. The dome had risen to 70 feet above the drum, and the shells, curving inward, had passed the critical angle of 30 degrees, above which friction alone would no longer keep the masonry in place until the mortar cured.

In contrast to the furor when Filippo first floated his plan, in 1426 the debate over whether to continue building the dome without centering appears to have gone smoothly. The
capomaestro
, now at the height of his powers, carried the day: “We still do not recommend centering,” the amended project reported, citing the difficulties of building the necessary scaffolding. But it remained to be seen whether this feat could successfully be accomplished.

The documents of February 1426 give no more than the merest hint how Filippo’s plan would be implemented. At the same time that it was decided to vault the rest of the dome without centering, another amendment was adopted: in certain parts of the cupola a series of uniquely shaped bricks were to be laid in a special fishbone bond. The twelve-point memorandum of 1420 had decreed that after the two shells reached a height of 24
braccia
, either brick or tufa stone should replace sandstone to lighten the load. Brick was ultimately chosen because tufa was not readily available in the vicinity of Florence and so needed to be imported. The Opera therefore contracted for hundreds of thousands of bricks, and Filippo began designing special wooden molds in order to shape them.

The size of bricks was carefully regulated in Florence.
¹
The basic brick used in the construction industry, the
mattone
, was about 10 inches long and 5 inches wide. All brickyards were required to display the mold for this brick, stamped with an official seal, where it could be consulted both by their customers and by inspectors from the Masons Guild, who would arrive on the premises wearing distinctive blue capes and silver badges.

The cupola, however, called for bricks of more unorthodox designs: rectangular bricks, triangular bricks, dovetailed bricks, bricks with flanges, bricks specially shaped to fit the angles of the octagon. The sizes of these bricks were so various, and the templates used to design them so numerous, that at one point parchment ran short and Filippo was forced to improvise: he resorted to palimpsests, drawing his designs on pages torn from old books specially bought for the purpose.

These templates were sent to a barrel maker who constructed the wooden molds used to shape the bricks. Once the mold was finished, it went to the brickyard. These were normally located in the countryside, not only in order to spare Florence the twin nuisances of fire and pollution, but also to be close to the clay pits as well as the supplies of the timber and brushwood that fueled the kilns. Dug from the pits, the clay was kneaded into a smooth and even consistency by the exertions of barefoot men, who trod it underfoot like grapes. The resulting “pug” was then molded, seasoned, and finally baked. The firing would last for several days, but because the kiln was heated to a temperature of 1,000 degrees Celsius, the brickmaker had to wait almost two weeks for it to cool down enough for the bricks to be unloaded. The average kiln held as many as 20,000 bricks and, fired every three weeks, could bake over 300,000 a year. Even at this rate, however, it would have taken one kiln over thirteen years to produce enough bricks for the dome.

Manetti claims that Filippo himself inspected each and every brick destined for the dome. This is surely an exaggeration, given that as many as 4 million were used. But quality control was obviously a major concern for the
capomaestro
. Bricks often shrank or cracked during firing because the clay had not been properly seasoned, and shipments were rejected if the consignment was not of the quality required. Ideally the clay was dug in the autumn and, after being molded, was buried in sand to avoid frost damage in the winter. In summer the unbaked bricks were excavated and then reburied in beds of moist straw to prevent them from cracking in the heat. Alberti cautions that a brick must be seasoned for two years before being burned, making for a process as time-consuming as treating timber. Nor was the baking of bricks the cleanest of occupations: a joke in Florence claimed that only kilnmen washed their hands
before
visiting the chamberpot.

Equally important to the building of the dome was the quality of the mortar, in which Manetti claims Filippo also took a personal interest. Throughout the Middle Ages mortar was made from mixing sand and water with quicklime (calcium oxide), a substance obtained by heating limestone in a kiln. Making mortar for a structure the size of the dome called for enormous quantities of limestone. Most brickmakers burned limestone as well as bricks in their kilns, using a separate furnace for the operation, which took some three or four days. Lime burning was a noxious process that prejudiced the health of anyone living downwind. It was dangerous for another reason as well, since air pockets in the limestone could cause explosions in the kiln. Air pockets were often the result of fossils, a phenomenon with which stone cutters, for obvious reasons, had become more familiar than anyone else. These petrified remains were objects of great curiosity: Alberti, in some fascination, describes having seen worms with hairy backs and a great number of feet “living” inside blocks of limestone.

As we have already seen, the speed with which a mortar set determined the techniques of construction. Medieval mortar set in two phases. The first took place after a few hours, when the material was no longer plastic, while the second was not complete for a much longer period. This second setting required carbon dioxide absorbed from the atmosphere to convert the calcium hydroxide of the mortar paste back into calcium carbonate, the basic constituent of limestone. This is the same chemical reaction that, with greater leisure, creates stalactites on the roof of a limestone cavern, where carbon dioxide turns the calcareous matter in the dripping water back into its native limestone.

Alberti claims it is possible to tell when this second set occurs because the mortar puts forth “a kind of moss or little flower well known to masons.” It is not easy to know which plant he is referring to, though the most likely candidates are mosses from the
Bryum, Tortula,
or
Grimmia
genera, all of which grow on limestone walls several months after a mortar is applied. It has been speculated that Filippo may have sped up the process by using a quick-drying mortar, possibly even pozzolana, which would have been a truly remarkable innovation, marking the first use of Roman cement for a millennium. But mineralogical tests conducted in the 1970s revealed no material difference between the mortar in the cupola and that elsewhere in the cathedral. In each case, however, sodium carbonate, or soda ash, a mineral used in glassmaking, was present. It would have led, whether intentionally or not, to a fairly rapid stiffening of the mortar.
²

Mortar was always mixed on the site. The process took about a day, since if the quicklime was not well slaked — that is, thoroughly mixed with water — it would damage the brickwork. The mixing was done on the cupola itself because it needed to be applied while still plastic. Lime, sand, and water were all hoisted to the top of the dome, where the lime was slaked and then combined with the sand. Slaking generated great heat and caused the quicklime to expand and then disintegrate into a powder. One of the perils involved in mixing the mortar was burning one’s hands on the quicklime, a corrosive substance otherwise used to hasten the decomposition of corpses and so lessen the stench and danger of disease in churchyards. It was also employed by tanners to scorch the hair off animal hides.

Once properly mixed, the mortar was poured by the masons onto their mortarboards and, in time-honored fashion, spread over the brickwork with trowels. Each of the eight teams of masons laid its bricks on the inside of the wall and worked outward, with the bed joints of successive courses inclining as the structure rose. Both shells were raised simultaneously, with the inner built to an average thickness of six feet, a width of over ten bricks. The outer shell, much thinner, was a third that size.

Work advanced at a slow pace because the eight teams of masons were forced to wait until each ring gained strength before they began a new one. The average rate of construction has been estimated at less than one course per week,
³
meaning the cupola would have risen at a rate of roughly a foot each month. Erecting the dome with a centering would have demanded a much more rapid construction because of the tendency of the wood to deform or “creep” over time, but a structure the size of the cupola could not possibly have been built swiftly enough to avoid this deformation — yet another reason for vaulting without centering.

The adhesion of the bricks was not Filippo’s only worry at this point. These were dangerous times for the teams of masons, who now had to work on walls that leaned inward at an alarming angle. A dome built with a wooden centering had a comforting network of scaffolding to break a fall and obscure the view of the abyss. In this case, however, there was nothing: the masons simply moved around the perimeter of the cupola on
ponti
(narrow platforms made from willow withes and supported on wooden rods inserted into the masonry), while below them yawned the chasm. In order to pacify the nervous masons, Filippo built a
parapetto
, or balcony, on the inside of the vault. This contraption consisted of a series of boards erected on hanging scaffolds projecting from the masonry. A platform much wider than the
ponti
, it served both as a safety net and — even more vital — as a screen. According to the documents, it was intended “to prevent the masters from looking down.”

Other safety measures were also implemented. Masons working high on the walls were given leather safety harnesses, and their wine was to be diluted with a third part of water, a mixture normally reserved for pregnant women. Anyone violating this latter rule was subject to a fine of 10 lire, or the equivalent of eleven days’ work. Workers were also forbidden to transport their tools, lunches, or worse still, themselves in the tubs of the hoist. Nor were they permitted to swing inside the hoisting tubs in order to capture pigeons nesting in the cupola. Nesting pigeons were a perennial nuisance to masons. During the construction of Westminster Abbey, canvas sheeting had to be used to keep them from taking up residence among the stones and beams of the half-finished structure. The pigeons so daringly captured by the masons at Santa Maria del Fiore before the edict took effect were probably destined for the dinner pot. Blackbirds also met this fate, for meat was a rare luxury for workmen, being eaten for the most part only on Sunday.