THE DOME OF FLORENCE
The great dome of the cathedral of Florence marks the beginning of
the Renaissance movement in architecture, though in its general form
and structural character it has no likeness to ancient domes, and has
few details drawn from the Roman classic source. It exhibits a wide
departure from any previous forms of dome construction, and is an
expression of the creative genius of a remarkably gifted man of great
independence, working under inspiration drawn in part from ancient
sources, in part from mediæval building traditions, and in still larger
part from the new motives that were beginning to animate the artistic
ambitions of the fifteenth century.
The dome of the Pantheon and the dome of St. Sophia, the two greatest
domes of former times, had been built on principles that did not admit
of much external effect, and the numerous smaller ones of the Middle
Ages, in western Europe, had been equally inconspicuous externally, if
not entirely hidden from view, in consequence of rising from within
a drum which reached far above the springing level. In most cases
the whole construction was covered with a timber roof, so that from
the outside the existence of a dome would not be suspected. This was
a secure mode of construction, and one that for stability could not
be improved; but it did not give the imposing external effect that
Brunelleschi sought.
[Illustration: FIG. 1.--Hagia Theotokos.]
[Illustration: FIG. 2.--Aachen.]
Attempts to make the dome a conspicuous external feature had indeed
been made before Brunelleschi’s time. The later Byzantine builders
had raised small domes on drums resting on pendentives, and rising
above the main roof of the building, but they had still carried these
drums up somewhat above the springing of the dome, and had further
fortified them with buttresses built over the supporting piers, as in
Hagia Theotokos of Constantinople (Fig. 1). Thus in such designs the
dome still remains partly hidden from view, the drum being the most
conspicuous part of the composition. Among the early domes of western
Europe is that of Aachen (Fig. 2). In this case the drum is carried
up far beyond the springing, and is covered with a timber roof which
completely hides the dome from external view. The same adjustment of
the dome to its drum is, with minor variations of form (the dome being
in some cases polygonal on plan, as at Aachen, and in some cases
hemispherical) found in most other mediæval domes, and the timber roof
over all is likewise common. But in a few cases a different scheme
was adopted in which the dome is set on the top of the drum instead
of within it. In such cases, however, the drum is low, not rising
above the ridge of the timber roof of the nave, and the dome, being
unprovided with abutment, is insecure except in so far as it may have a
form that is self-sustaining as to thrusts (which removes it from the
true dome shape, or may be secured by some kind of binding chain.[7]
An example of such a dome occurs on a small scale over the crossing
of the cathedral of Pisa (Fig. 3). This dome is not hemispherical,
its sides rise steeply, and with such moderate curvature as to render
it measurably self-sustaining as to thrust.[8] Another instance of a
similar scheme, and on a larger scale, is that which appears to have
formed a part of Arnolfo’s design for the cathedral of Florence. This
dome was never executed, and our knowledge of it is derived from the
well-known fresco in the Spanish chapel of Santa Maria Novella.[9]
Here both the dome and the drum are octagonal in conformity with the
plan of the part of the building which it covers. The outline (Fig. 4)
is slightly pointed, but the sides are nevertheless so much curved in
elevation that a structure of this form would not stand without strong
cinctures. It is, however, not unlikely that the fresco painter has
given it a more bulging shape than Arnolfo intended. But domes of this
character were exceptional in the Middle Ages. The builders of that
epoch confined their practice for the most part to the safer form in
which the vault is made to spring from within the drum, and is thus
necessarily, either in part or entirely, hidden from external view.
[Illustration: FIG. 3.--Dome of Pisa.]
[Illustration: FIG. 4.--Dome of Arnolfo.]
[Illustration: FIG. 5.--Section of Baptistery.]
A remarkable dome of this latter class is that of the Baptistery of
Florence, which, though the building has undergone various superficial
transformations since its original construction at an early, though
uncertain, epoch, has come down to us in essential integrity. This
building on plan is in the form of an octagon, and the dome is of
corresponding shape, and sprung from a level far below the top of the
enclosing walls. In elevation the dome (Fig. 5) has a pointed outline,
and is covered by a pyramidal roof of stone the upper part of which
is incorporated with the dome itself, while beneath the lower portion
is a void between the dome and the enclosing wall. The structure has
an internal anatomy that is both ingenious and admirable. The span is
about 25 metres, and the wall at the level of the springing is over 3
metres thick. Above this the wall (_a_, Fig. 5) rises to a height
of about 8 metres. The dome at its base is about 1 metre thick, and
its extrados rises vertically to a height of about 2½ metres, leaving
an open space between it and the wall of the enclosing drum of 1.26
metres in width. Above this vertical portion the extrados is stepped
by several courses of masonry, somewhat after the manner of the dome
of the Pantheon. From the reëntrant angles of the octagon (_a_,
Fig. 6) solid abutments are built up against the salient angles of
the vault, and, between these, two secondary abutments (_b_) are
carried up against each of its sides. These buttresses are in the
form of cross walls dividing the space on each side of the octagon
into three compartments, and over each of these compartments a barrel
vault, on an axis inclined in conformity with the slope of the roof,
is turned. The upper ends of these vaults intersect on the surface of
the dome, as shown in Figures 5 and 6. The voids between the crowns
of these vaults and the buttresses are filled in with masonry so as
to form the sloping planes of the roof below where it is incorporated
with the dome, and on these are laid the slabs that form the external
covering. With such an effective buttress system as is here provided
it is hard to find a reason for the chain of timbers which is inserted
at the haunch of the dome. The constructive principle embodied in this
monument is altogether sound,[10] and its architectural character is in
keeping with the construction.[11]
[Illustration: FIG. 6.--Dissection of the vault of the
Baptistery.]
Such were the models of mediæval dome building accessible to
Brunelleschi when he was forming his great scheme for the covering of
the octagon of the cathedral of Florence. But the idea of a low dome,
or a hidden dome, could not meet the wishes of the Florentines of the
fifteenth century. Their civic pride and large resources called for an
imposing design which should make the dome a dominant architectural
feature of their city. It was decided that it should be raised upon the
top of a high drum, and the task to which Brunelleschi applied himself
was to fulfil this requirement.
Of the vast and soaring dome which he succeeded in erecting many
opinions have been held, but all beholders are impressed with its
grandeur. It has been common to speak as if the master had been chiefly
inspired by the ancient monuments of Rome, and had taken the Pantheon
as his principal model.[12] But although he came to his task fresh
from the study of the ancient Roman monuments, and undoubtedly had
the Pantheon much in mind, yet the dome which he produced has little
in common with that great achievement of imperial Roman constructive
skill. In general it follows, though with great improvements as to
outline and proportions, the scheme of Arnolfo as illustrated in
the fresco of the Spanish Chapel; but the model to which it most
closely conforms, notwithstanding the obvious and essential points
of difference, is that of the Baptistery just described. There can,
I think, be little question that this monument supplied the chief
inspiration and guidance to both Arnolfo and Brunelleschi. A comparison
will show that the dome of the cathedral, with its supporting drum,
is, in fact, little other than a reproduction of the Baptistery of San
Giovanni in a modified form, and enlarged proportions, raised over the
crossing.
[Illustration:
Plate I
DOME OF BRUNELLESCHI
Florence]
[Illustration: FIG. 7.--System of the dome.]
But while taking the scheme of the Baptistery as the basis of his own
scheme, Brunelleschi was obliged to make some daring changes in order
to give his design the external character which he sought. This great
dome (Plate I), like that of the Baptistery, is octagonal in plan and
pointed in elevation. It rises from the top of the octagonal drum, and
consists of two nearly concentric shells of masonry, with an interval
between them. Eight vast ribs of stone rise from the angles of the drum
and converge on the curb of an opening at the crown. These ribs
extend in depth through the whole thickness of the double vault and
unite its two shells. Between each pair of these great ribs two lesser
ones are inserted within the interval that divides the two shells, and
nine arches of masonry, lying in planes normal to the curve, are sprung
between the great ribs and pass through the lesser ones on each side of
the polygon (Figs. 7 and 8), while a chain of heavy timbers (_a_,
Fig. 8, and Fig. 9), in twenty-four sections clamped together at the
ends with plates of iron, binds the whole system between the haunch and
the springing. So much of the internal structure can be seen in the
monument itself, but further details are described in Brunelleschi’s
own account of what he intended to do.[13] From this we learn that
the base of the dome, which was to be built solid to the height of 5¼
braccia, was to consist of six courses of long blocks of hard stone
(_macigno_) clamped with tinned iron and upon this were to be
chains of iron.[14] Mention is also made of a chain of iron over the
timber chain (“in su dette quercie una catena di ferro”); but no
such chain is visible in the monument, and if it exists, it must be
embedded in the masonry of the vault, like the chains at the base.
[Illustration: FIG. 8.--Section.]
[Illustration: FIG. 9.--Part Plan.]
It will thus be seen that while Brunelleschi’s scheme is essentially
different from that of the Baptistery, its structural system is little
more than an ingenious modification of it. The parts of the one answer
to those of the other with singular completeness. The attic wall and
pyramidal roof of San Giovanni are transformed into the external shell
of the cathedral dome, the angle buttresses of the older monument
become the great angle ribs of Brunelleschi’s vault. The intermediate
abutments of the Baptistery are changed into the intermediate ribs of
the great dome, and the inclined barrel vaults of the Baptistery scheme
are represented in the cathedral dome by the arches sprung between the
great angle ribs.
It has been thought by some writers that the rib system of the dome
of Florence gives the structure a somewhat Gothic character, and it
is sometimes called a Gothic dome.[15] But there can be no such thing
as a Gothic dome. It is impossible for a dome of any kind to have
the character of a Gothic vault. The difference between the two is
fundamental. A Gothic vault is a vault of concentrated thrusts, and
it requires effective concentrated abutments. A dome is a vault of
continuous thrust, and for sound construction it requires continuous
abutments, as in the Pantheon. Whatever use the ribs of Brunelleschi’s
vault may have, they do not, and cannot, perform the function of the
ribs in Gothic vaulting. Their use is to strengthen the angles of the
dome, and to augment its power of resistance to the weight of the
lantern which crowns it. They do not support the vault as the ribs
of a Gothic vault do. Being composed of very deep voussoirs, they
have more strength to withstand thrusts, as well as to bear crushing
weight, than the enclosing shells have, and thus to some extent they
may hold these shells in. But it appears plain that the architect did
not feel confidence in their power to perform this function without
reënforcement by a chain, or chains, which, in his own words, “bind
the ribs and hold the vault in” (che leghino i detti sproni e cingano
la volta dentro). However this may be, the ribs of a dome cannot
have any function like that of the ribs in Gothic vaulting. The
shell of a Gothic vault is not held in by the ribs, nor is it in any
way incorporated with them. Both shell and ribs are held in by the
buttresses. This point will be considered further in connection with
the dome of St. Peter’s.
The whole scheme of this dome was a daring innovation of one man, and
in this it differs from former architectural innovations, which were
the comparatively slow outcome of corporate endeavour, progressive
changes being so gradual that no wide or sudden departures from
habitual modes of building were made at any one time, or by any one
person.
It was a prodigious undertaking. The span of the dome is nearly
a hundred and forty feet, the springing level is a hundred and
seventy-five feet above the pavement, and the height of the dome
itself, exclusive of the lantern, is about a hundred and twenty feet.
Such a project might well appall the most courageous of building
committees, and we need not wonder that the Board of Works drew back in
dismay when it was first laid before them.[16]
The successful accomplishment of the work, and the stability which it
has thus far maintained, show that the architect was a constructor
of great ability,[17] and the fact that he managed to raise the vast
fabric without the use of the ponderous and costly kind of centring
that had been commonly employed in vaulting, makes the achievement
still more remarkable. The precise manner in which he did this is not
clear, but of the fact there appears no question.[18]
The dome of Florence is indeed a remarkable piece of construction,
and it is no less remarkable as a work of art. In beauty of outline
it has not, I think, been approached by any of the later elevated
domes of which it is the parent. Yet with all of its mechanical and
artistic merit, the scheme is fundamentally false in principle, since
it involves a departure from sound methods of dome construction. A
bulging thin shell of masonry on a large scale cannot be made secure
without abutment, much less can such a shell sustain the weight of a
heavy stone structure like the lantern of this monument, without resort
to the extraneous means of binding chains. A builder having proper
regard for true principles of construction in stone masonry would not
undertake such a work. For although it may be possible to give the
dome a shape that will be measurably self-sustaining as to thrusts, as
Brunelleschi clearly strove to do,[19] it is not possible to make it
entirely so, and therefore if deprived of abutment it must be bound
with chains. But a structure of masonry which depends for stability
on binding chains is one of inherent weakness, and thus of false
character.[20]
From these considerations it appears to me that Brunelleschi led
the way in a wrong direction, notwithstanding the nobility of his
achievement from many points of view. And in following his example
modern designers of elevated domes have wandered still farther, as we
shall see, from the true path of monumental art.
Moreover, when we consider that a dome set within its drum is not only
stronger, but that it is also much better for interior effect, the dome
of the Pantheon still remaining the grandest and most impressive arched
ceiling of its kind in the world, the unbuttressed modern domes, with
their manifold extraneous and hidden devices for security, appear still
less defensible.
But in the architectural thought of the Renaissance little heed was
given to structural propriety or structural expression, and the Italian
writers, who have largely shaped our modern architectural ideas, have
not only failed to recognize the inherent weakness of such a building
as the dome of Florence, but have even considered the work praiseworthy
on account of those very characteristics which make it weak. Thus
Sgrilli lauds Brunelleschi for having had the “hardihood to raise to
such a height the greatest cupola which until its time had ever been
seen, upon a base without any abutments, a thing that had not before
been done by any one.”[21] And Milizia says, “It is worthy of special
notice that in the construction of this cupola there are no visible
abutments.”[22]
As to the permanent stability of this dome various opinions have been
held by the experts among the older writers.[23] Its form is, as we
have seen, as favourable to stability as it would be possible to make
that of any vault which could be properly called a dome. It appears
to the inexperienced eye as stable as a crest of the Apennines. Every
precaution as to material and careful workmanship seems to have been
taken to make it secure. The wall of the drum on which it rests is five
metres in thickness, and the solid base of the dome itself is built, if
the architect’s scheme was carried out as he had stated it before the
Board of Works, of large blocks of hard stone, thoroughly bonded and
clamped with iron. The lower system is sufficiently strong, and appears
to rest on a solid foundation. But nevertheless there are ruptures in
various parts of the structure which have caused apprehensions of
danger,[24] and its future duration must be regarded as uncertain.
The writers who have maintained that it is secure have argued on the
assumption that the parts of a dome all tend toward the centre.[25]
These writers overlook the fact that the force of gravity above,
especially when the dome is heavily weighed by a lantern, neutralizes
the inward tendency of the lower parts and causes a tendency in those
parts to movement in the opposite direction. This neutralizing force is
lessened by giving the dome a pointed form, as Brunelleschi has done,
but, as before remarked (p. 22), it can hardly be overcome entirely so
long as any real dome shape is preserved.[26]
It may be thought that the object which Brunelleschi had in view,
of producing a vast dome that should be an imposing feature of the
cathedral externally, justifies the unsound method of construction
to which he resorted (the only method by which the effect that he
sought could be attained). But structural integrity is, I think,
so fundamental a prerequisite of good architecture that in so far
as this gifted Florentine was obliged to ignore sound principles
of construction in order to attain an end not compatible with such
principles, the result cannot be properly considered as an entirely
noble and exemplary work of art, however much beauty and impressiveness
it may have.
The example set by Brunelleschi was, in point of construction, a
pernicious one, and bore fruit of a still more objectionable character
in the works of other gifted men less scrupulous than he, and less
endowed with mechanical ingenuity, as we shall see farther on.
Though there is nothing whatever of classic Roman character in this
great dome, the lantern which crowns it, built from Brunelleschi’s
design after his death, has classic details curiously mingled with
mediæval forms. Its eight piers are adorned with fluted Corinthian
pilasters surmounted by an entablature, while the jambs of the openings
have engaged columns carrying arches beneath the entablature in ancient
Roman fashion. From the entablature rises a low spire with finials set
about its base, and flying buttresses, adorned with classic details,
are set against the piers. None of the classic details have any true
classic character, nor has the ornamental carving, with which the
composition is enriched, any particular excellence either of design or
execution. But these details are invisible from the ground, and in its
general form and proportions the lantern makes an admirable crowning
feature of this finest of Renaissance domes.