It seems curious that Bernini, whose sculpture expresses the very essence of the Baroque spirit, should remain relatively conservative in his architecture. Frequently planning on a vast scale and employing striking illusionistic devices, Bernini tends to use the Classical orders in a fairly sober and traditional manner, except, notably, when the architecture is inextricably bound up with sculpture, as in his baldacchino in St. Peter's and the St. Theresa altar in Santa Maria della Vittoria. One might call his architectural style academic, at least in comparison with the unorthodox and quite revolutionary manner of his contemporary Borromini. A new dynamism appears in San Carlino, where Borromini goes well beyond any of his predecessors or contemporaries in the plastic handling of a building. Maderno's fašades of Santa Susanna and St. Peter's are deeply sculptured, but they develop along straight, lateral planes. Borromini, perhaps thinking of Michelangelo's apse wall in St. Peter's, sets his whole fašade in serpentine motion forward and back, making a counterpoint of concave and convex on two levels (note the sway of the cornices), and emphasizes the sculptured effect with deeply recessed niches. This fašade is no longer the traditional, flat frontispiece that defines a building's outer limits; it is a pulsating membrane inserted between interior and exterior space, designed not to separate but to provide a fluid transition between the two. This functional interrelation of the building and its environment is underlined by the curious fact that it has not one but two fašades. The second, a narrow bay crowned with its own small tower, turns away from the main fašade and, following the curve of the street, faces an intersection. (The upper fašade was completed seven years after Borromini's death, and we cannot be sure to what degree the present supplemented and complex structure reflects his original intention.)
The interior of San Carlino is not only an ingenious response to an awkward site, but also a provocative
variation on the theme of the centrally planned church. In plan it looks like a hybrid of a Greek cross and an oval, with a long
axis between entrance and apse. The side walls move in an undulating flow that reverses the motion of the fašade. Vigorously
projecting columns articulate the space into which they protrude just as much as they do the walls to which they are attached.
This moulded interior space is capped by a deeply coffered, oval dome that seems to float on the light entering through windows
hidden in its base. Rich variations on the basic theme of the oval, dynamic ralative to the static circle, create an interior
that appears to flow from entrance to altar, unimpeded by the segmentation so characteristic of Renaissance buildings.
The unification of interior space is carried even further in Borromini's chapel of Sant'Ivo alla Sapienza
in the courtyard of the Palazzo della Sapienza. In his characteristic manner, Borromini plays concave against convex forms on
the upper level of the exterior of his chapel. The lower stories of the court, which frame the bottom fašade, were already there
when Borromini began work. Above the inward curve of the fašade - its design adjusted to the earlier arcades of the court - rises
a convex, drumlike structure that supports the lower parts of the dome. Powerful pilasters restrain the forces that seem to push
the bulging forms outward. Buttresses above the angle pilasters curve upward to brace a tall, plastic lantern topped by a spiral
that seems to fasten the structure, screwlike, to the sky. The striking similarity between Borromini's Sant'Ivo lantern, with
its scalloped entablature and platform, and the 3rd century A.D. Roman temple of Venus at Baalbek - the ruins of which were already
known in the 16th century - underscores the commonality of approach to structural design on the part of the Baroque architects
of both eras.
The centralized plan of Sant'Ivo is that of a star, having rounded-off points and apses on all sides.
Indentations and projections along the angled, curving walls create a highly complex plan, all the elements of which are fully
reflected in the interior elevation. From floor to lantern, the wall panels rise in a continuously tapering sweep that is halted
only momentarily by a single, horizontal cornice. The dome is thus not, as in the Renaissance, a separate unit placed on the
supporting block of a building; rather it is an organic part that evolves out of and shares the qualities of the supporting walls,
from which it cannot be separated. The complex, horizontal motion of the walls is transferred fully into the elevation, creating
a dynamic and cohesive shell that encloses and energetically moulds a scalloped fragment of universal space. Few architects have
matched Borromini's ability to translate extremely complicated designs into such masterfully unified and cohesive structures
as that of Sant'Ivo.