Apses Vaulted with Half Domes
Once adopted from Roman architecture as a standard part of the church plan, the construction of the apse was naturally based upon Roman models, and since these were always vaulted with a half dome of masonry, similar half domes were employed by the Christian builders of the early mediaeval period. During the Romanesque era, these half domes were almost always of stone laid in horizontal courses, supported by substantial walls of semicircular or polygonal plan. They opened directly into a transept or a tunnel-vaulted choir. The earliest of these half domes were of semicircular elevation, but the pointed form made its appearance in the late eleventh or early twelfth century in many churches. In both forms, the principles of construction are the same.
Lighting Problems Connected with the Construction of Half Domed Apses
It is a characteristic of the half dome that it exerts a large amount of downward pressure and but little outward thrust, particularly if it be of pointed section. For this reason, such a vault requires a firm support but only a slight amount of buttressing. As long, therefore, as the half dome rested directly upon comparatively low exterior walls, it had plenty of support, and it was even possible to pierce the walls with windows without endangering its stability. But with the increase in height of the more developed Romanesque churches and the introduction of ambulatories, it became difficult to light the sanctuary and still retain the half dome.
Two methods were evolved for overcoming this difficulty. The first, which may be seen in the abbey church at Cunault (Maine-et-Loire) (second half of twelfth century),[339] consisted in the construction of a lofty ambulatory opening into the apse through arches rising to the impost of the half dome, or even slightly above it, and resting upon piers of as slender proportions as possible, so that, although the sanctuary was deprived of all direct light, a certain amount was obtained from windows in the outer wall of the ambulatory or from the radiating chapels, while, at the same time, the vault of this ambulatory aided in the support of the apse and vice versa. Such a system, though structurally correct, was not entirely satisfactory. The sanctuary and choir were the portions of the church most in need of lighting, since they contained the altar and the seats for the monks or clergy by whom the services were chanted, and indirect light was bound to be insufficient.
The second method, which may be seen in the church of Saint Savin (Vienne) (eleventh century),[340] consisted in reducing the height of the ambulatory, even when this involved making it lower than the side aisles, and then placing a clerestory above the ambulatory arches beneath the springing of the half dome. This may be considered as the best type of apsidal termination developed during the purely Romanesque period. It was only when the half dome was discarded that a satisfactory solution was finally reached in the development of the chevet vault. There were, however, two important series of ribbed half domes, the second of which, at least, may have had some bearing upon the evolution of the chevet type.
Apses with Ribbed Half Domes
The first series lies largely in southern France in the Romanesque school of Provence. Here there are a certain number of churches, among them the chapel of Saint Honorat in Les Alyscamps at Arles (Bouches-du-Rhône) (eleventh century?), in which the surface of the half dome is broken out at regular intervals into a number of flat, pilaster-like, radiating strips, forming a part of the actual masonry of the vault. These divide the half dome in much the same manner as true Gothic ribs, but they do not support it in any way and seem to have been used for the decoration which such a change in the surface of the vault produced.[341] As a general rule, these ribs radiate from a point slightly back from the crown of the apse arch and often from a raised masonry ring as in the chapel just cited. They vary, however, both in number, thickness and width, some being comparatively thick and widening out from the central keystone as in the cathedral of Notre Dame-des-Doms at Avignon (Vaucluse), others being but slightly salient and of the same width throughout like those in Saint Honorat at Arles.
Much larger in number and extent is the second series of churches with rib-vaulted apses, though they are in general of later date than those in Provence. Their radiants have a certain structural character, for they are independent of the vault surface and were doubtless erected in most cases as a permanent centering to aid in the construction of the half dome. They do not, however, aid to any extent in its support, for the courses of masonry in the vaults are still horizontal and concentric with the curve of the apse, and the completed half domes would therefore stand just as well were the ribs removed. It may be that they were introduced in order to make the apse correspond more closely in appearance with the ribbed vault which had in many cases been introduced in the naves of the churches in which the ribbed half domes are found. In any event, they mark a stage in apse vaulting between the simple half dome and the developed chevet, which is worthy of careful consideration. Most of these vaults date from the second quarter of the twelfth century and are to be found within the zone of influence of the Ile-de-France, though occasionally an example is found at a long distance from this center as in the case of Sant’ Abondio at Como,[342] Santa Maria di Castello at Corneto-Tarquinia in Italy,[343] and such churches as that of the Monasterio de la Oliva (Navarra) in Spain,[344] (1198). The number of ribs varies considerably, though two is most common particularly in the smaller churches and chapels.[345] Of these, the church at Morienval (Oise) [(Fig. 77)] furnishes a good, though recently reconstructed, example, while Saint Georges-de-Boscherville [(Fig. 61)] may be cited as possessing a large apse of similar character.
The important thing in a comparison of these two vaults is the difference in the lighting of the completed apse. At Boscherville, it was a simple matter to pierce the exterior wall with windows, in this case in two stages, and still keep their crowns practically below the level of the impost of the half dome, since the latter rested directly upon the outer walls. But at Morienval there was an ambulatory, and in order to get a clerestory above its arches, the windows had to be cut into the curved surface of the half dome itself, with the result that they were so deep as to prove of only limited usefulness. Other examples could be cited where this same attempt is made to obtain sufficiently large windows by shoving their heads into the half dome,[346] while at Beaulieu (Corrèze)[347] the windows lie entirely above the impost.