Lighting Problems in Naves with Square Bays
Several factors enter into the lack of popularity of the vaulting system just described especially in the more northern countries, but the fundamental one would seem to be the difficulty of properly lighting churches thus covered. If an examination be made of the churches with a single broad nave and no aisles it will be seen that in Italy, where a comparatively small proportion of window space was necessary, the builders were content with a single window in each nave bay as for example, in San Francesco at Assisi. In France, on the other hand, the light thus admitted would have proved inadequate, and in such churches as the cathedral of Angers [(Fig. 19)] and Sainte Radegonde at Poitiers two windows were introduced under each wall rib. This is, however, an awkward arrangement because these windows do not properly fill the wall space, and though this is better accomplished by adding a circular window above the upright pair as was done in La Couture at Le Mans [(Fig. 20)], still the effect even then is not satisfactory and much solid wall which might be utilized for windows is wasted. Moreover, in a church with side aisles, the clerestory arrangement was still more troublesome since important structural difficulties were involved. To raise a great four part vault high above the aisles in order to obtain a large clerestory was no easy task because of the excessive thrust which such a vault exercised at its four points of support. In Italy, where the amount of light required was not great, a very low clerestory with small, circular windows, one to each bay, was all that was essential, and so in such churches as Santa Maria Novella and the cathedral at Florence the nave vault was placed at a point only slightly above the vaults of the aisles, and its thrusts offset by simple ramping walls beneath the side aisle roofs. Such a church in France would have been inadequately lighted, and even if a greater structural skill permitted the French to erect loftier clerestories than those in Italy, there remained the difficulty of arranging the windows to get the maximum of light and the best appearance. A single opening occupying the entire space beneath the wall rib would have been all head and no jamb. One upright window would have admitted too little light for a large nave, and two windows near together not only left a great deal of wall space unused but were most awkwardly placed in churches where one nave bay corresponded to two bays in the aisles as in Le Mans cathedral,[176] because they were not on an axis with the arches of the nave arcade. On the other hand, if placed on this axis, the resulting windows were necessarily of small size like those in such Rhenish churches as the cathedral of Speyer where a second stage of windows has been added one in the center above each lower pair in a far from satisfactory manner since it brings a window above the intermediate pier.
Origin of Sexpartite Vaulting
In view of these facts it is at least a reasonable assumption that the lighting problem had much to do with the discarding by the French builders of the simple square four-part nave vault. As a matter of fact, however, they did not exactly discard it, but evolved from it a vault in six cells, which, while it still retained the old division of the nave into square bays, each corresponding to two bays in the aisle, at the same time permitted the uniform treatment of these in elevation and made possible larger windows,—one to each aisle bay,—symmetrically placed and, in the course of time filling the entire space beneath the wall ribs. This six-part ribbed vaulting would seem to have originated early in the twelfth century, in the French province of Normandy. This province has already been mentioned as the center of a Romanesque school, which extended over the greater part of England after the conquest of 1066, and reached its height during the reign of Duke William, the Conqueror (1035-1087), when a vast number of churches were constructed, many of them of large size. These were in general wooden roofed throughout, though, occasionally, as has been shown,[177] groined vaults were used in the choir or aisles, or both. Toward the beginning of the twelfth century, however, the Norman builders determined to vault the naves of a number of these churches, among them the two abbeys at Caen, and the result of this determination was the evolution of the true and false six-part vault.
Like the Rhine provinces, Normandy had always been strongly influenced by the methods of building developed in Lombardy. Whether this was due to the presence in Normandy of such men as Lanfranc,—who was born in Pavia in 1005 and became successively prior of Bec (1045-1066), abbot of Saint Étienne at Caen (1066), and archbishop of Canterbury (1070-1089), and who may have kept Normandy closely in touch with Lombardy,—or whether there were other more powerful influences, it is impossible to state, but in any event the architectural analogies between the two schools are striking. This is especially true of the type of shafted pier most frequently found in Normandy, and of the alternate system of light and heavy supports, which, while it does not characterize all the churches of the school, is found in many of them. Thus when the Norman builders determined to vault their great churches at Caen, one would naturally expect to find them turning to Lombardy for a method of vault construction, especially since Sant’ Ambrogio at Milan had been successfully completed at least a quarter of a century before their determination was made. And in fact this is probably what they did. But there were certain differences in structure between the churches of the two schools which made it impossible for the Norman builders to adopt unchanged, the heavy square, domed-up, cross-ribbed vaults of Lombardy. The first of these differences lay in the fact that the Norman churches were originally built for wooden roofs,—which may even have been in place, in many cases, when the vaults were begun,—while the Lombard churches were planned from the ground for their vaulting. The second difference was, that the Norman interior system possessed a clerestory window of considerable size centered above each of the arches opening into the side aisles,—that is two in each wall of what would be a square nave bay,—while the Lombard churches either had no clerestory at all, as at Sant’ Ambrogio, or one in which the windows were small and there was no attempt to center them as in San Michele at Pavia.
It was natural that the Norman builders should have preferred to preserve their interior and exterior elevations as nearly as possible as they were when only a wooden roof was used, both to avoid the expense which would be involved in reconstruction and to preserve the large clerestory so essential in a northern country. To vault these churches and at the same time save this clerestory would seem to have been the problem, therefore, which the builders set themselves to solve. That they attempted to use the four-part vault in its solution will be seen from an examination of the seven vaulted churches[178] still remaining in which the old system of square nave bays is found, for in four of these a variant of four-part ribbed vaulting was employed while in the other three a new method was developed out of the four-part type.
A study of the two abbeys at Caen will illustrate this. Of the two, Saint Étienne or the Abbaye-aux-Hommes (cir. 1064-1066) would seem to be the earlier as far as its vaulting is concerned and this would seem to date from about 1135. In its nave [(Fig. 26)] the alternate system of supports is employed, though all the piers are of almost the same section with a single shaft carried up the inner face. The aisles are in two stories and there is a clerestory with a single window in each bay. The nave was originally covered with a wooden roof. With this elevation existing before the church was vaulted it is quite possible to account for the form which this vaulting assumed. The first step must have been to divide the nave into square bays by transverse arches,—assuming that these were not already in place. The springing of these arches must naturally have been governed by that of those which opened into the crossing, and the level of their crowns, by the wooden timbering of the roofs,—which may well have been in place when the vaults were built. The result was that these transverse arches had to rise from a point as low as the clerestory string-course and could only be a slightly stilted semicircle in elevation. If the
