478 Coroneo
The main reason for sheathing the entire building in stone lies in the necessity
of walls thick and sturdy enough to resist forces bearing down on them, and
to shift their weight to the ground without recourse to counterforts. The barrel
vaults necessitated the modification of the outer walls to withstand powerful
lateral pressure and aggregate weight, and the cupola presented the additional
problem of imposing a hemispheric dome on a square base. The base was a
central cube formed by four piers supporting thick archivolts from which the
barrel vaults of the arms originated. While the segments of the dome that in-
tersected the sides of the square could have rested on these with no difficulty,
those corresponding to the inner corners would have rested on a void, and thus
required some point of juncture.
This problem was usually resolved with squinches (semi-spherical elements
generated from small arches) in late Roman architecture, and with pendentives
(flat triangular elements) in Iranian and Byzantine structures. Both squinches
and pendentives allow a dome to be built on a base, and thus transition from
a square to a circular plan. Of the three Sardinian buildings, Sant’Antioco and
probably San Saturnino achieved the transition from the dome to the base
with squinches, while pendentives occur only at San Giovanni di Sinis.
Figure 18.3 Sinis. San Giovanni di Sinis church, interior.
Photo: R. Coroneo.