Tournus Lower Narthex

The lower narthex of Saint-Philibert at Tournus is one of the largest early eleventh-century structures in France (Fig. 32; see Fig. 14).2 The central vessel is covered with square groin vaults and the two aisles with transverse barrel vaults. Unevenly finished stones the shape of small bricks are used for almost every part of the interior, including the walls, transverse arches, wall arcades, round piers, and even the socles and neckings of the piers

To build the groin vaults in the lower narthex, masons followed the Italian method of corbeled and pointed-web construction. In each bay, they first constructed the formerets and transverse arches and then corbeled the lower courses of the groin vault. One can deduce that the

32. Tournus, Saint-Philibert, plan (after Maurice Berry, 1955).

lower portion of the groin is corbeled from the relative position of the stones in the vault in relation to the stones in the formeret and transverse arches. The stones in the lower, corbeled portion of the vault, are in front of, and therefore came after, the stones in the arches behind them (Fig. 33a). At this level of the groin web, then, the extrados of the arches could not have been used to lay lag boards for a true vault.

As in Italian vaults, the corbeled base is straighter and steeper than the true vault above it. At about 25 percent of the height of the groin, the previously built formerets and transverse arches could have been

33. Tournus, Saint-Philibert, interior, lower narthex, western bay, central and northern aisles; (a) corbel vault, overlapping the face of arch; (b) web of true vault resting above the extrados of the arch; (c) pointed groin web.

used to brace the wooden centering and lag boards for the true vault. At this level, the vault reflects these changes in construction, because it abruptly alters its position in relation to the arches. The web rests above the extrados, instead of in front of the face of the voussoirs (Fig. 33b); and the groin lines become more horizontal and irregular than in the corbel vault below.3 Even these true-vault groin lines, however, are straighter than they would be in a typical short-segmented northern groin vault. At the peak of the vault, the angle of the groin lines flattens.

In the lower narthex at Tournus, the pointed lateral webs of the groin vaults serve the same purpose as the pointed groin-vault webs at Lomello (Figs. 33c, 34). Given the geometry of a square bay (see Fig. 32), the line of the diagonal groins is longer, and therefore higher, than the semicircle of the perpendicular transverse arches adjacent to it. To compensate for this difference in size, masons made the groin webs taller on the sides, by pointing the webs where they descend to meet the lower transverse arches.

In the lower narthex at Tournus, masons could easily have stilted the transverse arches, as is the case in the aisles of the later nave and upper narthex. This extra length would have elevated the crown of the webs on the sides of the vault to reach the height of the webs at the crossing of the groin lines. For good structural reasons, however, they chose not to raise the webs by stilting the arches.

In the aisles adjacent to the main vessel, masons rested the edges of the lateral barrel vaults on transverse arches so low that the arches spring below the necking of the piers (see Fig. 14). Every inch gained in this manner by lowering the semicircular barrel vaults could be used to narrow the mass where the webs intersect at the springing of the vaults. Reducing the bulk of the masonry at this springing also allows them to narrow the width of the transverse arch that is needed to support it. A narrower transverse arch, in turn, saves materials, time, and labor, and decreases the heaviness of the structure.

In the aisles, the major advantage of lowering the transverse arches comes from the role they play in buttressing the weight from the groin vaults in the central vessel and transferring it to the outside walls.4 Groin vaults probably were selected for this location because they brought aesthetic benefits: Groin vaults, with point support and pointed webs, allow light and space from the side aisles freely to enter the

34. Tournus, Saint-Philibert, interior, lower narthex, central bay, detail of southern arcade; (a) stones in the shape of bricks, placed horizontally between the extrados of the arcade and the web of the groin vault.

central bays, where there is no clerestory or raised elevation. The structural downside of this type of groin vault is that it produces narrowly focused, stressful points of weight that have to be supported. In the central vessel, above each freestanding round pier, the weight from two converging groin webs passes, at a relatively vertical angle, from above the pier necking to below the pier necking in the side aisle (see Figs. 14, 32, 33). To stabilize this converging and steeply descending weight, masons had to find a way, at the lowest possible level, to buttress the freestanding pier and transfer the thrust from the central vault to the outside wall.

To accomplish these goals, they lowered the springing of each transverse barrel vault below the level of the necking of the adjacent freestanding pier. Beneath this springing, they also dropped the level of the transverse arch that buttresses the freestanding pier. At first glance, the low height of the springing of the transverse barrel vault and transverse arch seems to do nothing but interrupt the interior space by blocking the line of sight down the aisle. Moreover, the transverse arch, for no good reason, seems to interrupt the pier by severing its necking and colliding into its side. In fact, however, the transverse barrel allows light and space to penetrate laterally into the central vessel; and the low height of the transverse arch provides outstanding support for the steeply descending weight from the groin vault.5

A price had to be paid, however, to receive the structural benefits of springing barrel vaults at a low level. Once masons decided not to stilt the semicircular barrels but, instead, to drop the webs and transverse arches as low as possible below the pier necking, they were left with little choice for the size of the semicircular longitudinal arcade. They had to make the circumference of the extrados of these arcades the same dimension as the circumference of the semicircular barrel vaults, whose webs the arcade arches also support.

This relationship, in which the circumference of the semicircular barrel vaults determines the circumference of the semicircular arches in the adjacent arcade, did not pose a problem of aesthetics or construction in the aisles. The problem came in the adjacent central vessel, where the lateral webs of the groin vaults rest on the same longitudinal semicircular arches as the barrel vaults in the aisles (see Fig. 33). The crown where these groin lines cross is substantially higher than the top of the extrados of the longitudinal arcades; therefore, in order to keep the groin webs from drooping where they rest on the arcades, masons had to point the lateral web of the vaults. At Tournus, then, it is in the context of building groin webs - specifically, to make their height align with the crown of the groin - and not in the context of building barrel vaults, transverse arches, or groin lines, that masons introduced the point in vault construction.6 In a similar context in the brick church at Lomello, Lombard masons introduced a point to heighten the webs in relation to the crown of the groin.

In the brick construction at Lomello, masons prepared to lay the pointed groin webs by inserting progressively larger brick slivers above the semicircular formerets. In the first two bays at Tournus, progressively larger filler stones also run between the extrados of the semicircular arches and the peak of the groin webs (see Figs. 33c, 34a). These filler stones serve two objectives, both related to construction. By keeping the longitudinal arches semicircular - and thus lower than the pointed webs of the central-vessel groins - masons could use these arches to support the intentionally depressed, semicircular barrels in the aisles. It was only on the other side of the arcades - the side of the central vessel - that they inserted the row of progressively larger filler stones above the arches. This additional construction serves a second purpose, which is to allow the same round-headed, longitudinal arches to support the higher, pointed groin webs in the central vessel (see Fig. 33).7

The close parallels between the groin vaults at Tournus and Lomel-lo show that masons in Burgundy had mastered the most complicated brick techniques of vaulting. Despite sharing a similar approach to structure, however, masons on opposite sides of the Alps preferred different types of buildings. Italian masons clearly preferred wooden-ceiling basilicas: They relegated barrels to small, low choirs, and assigned groin vaults a subsidiary role in the aisles and crypts.

In contrast, in important churches in Burgundy, the masons who used brick techniques usually preferred an entirely vaulted structure. Pointed groin webs are used in complex, fully vaulted systems that include an important role for the barrel vault. In these systems, masons fully integrated the barrel and groin vaults, and, from all indications, did not see one type of vault as an evolutionary improvement over the other.8 Rather, in buildings like the lower narthex at Tournus, they utilized the unique structural, constructional, and aesthetic properties of each vault type to reinforce the benefits and reduce the deficits of the other. At Tournus, they succeeded in combining the barrel and groin vault to create an open, lighted, fully vaulted central space.9

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