Part 2 (1/2)
The method of grouping three arched windows of the same height is adopted in apse windows, each of them occupying one side of the exterior. As the deep, narrow mullions are set radiating, the arch is narrower inside than outside. But this difficulty was overcome, partly by lowering the inner crowns, so that the arch is conical, partly by winding the surface. In the Pantokrator (p. 238), instead of radiating to the centre of the apse, the side and mullions are placed parallel to the axis of the church, thus obviating all difficulty. Generally the centre to which the mullions radiate is considerably beyond the apse, so that any necessary little adjustment of the arch could easily be made.
Triple windows supported on circular columns are not infrequent in the north and south cross arms. Sometimes the central light is larger than the lateral lights, at other times, as in the Pantepoptes, the three lights are equal. The lower part of these windows was probably filled in with a breastwork of carved slabs, as in S. Sophia, while the upper part was filled by a pierced grille. At present the existing examples of these windows have been built up to the abaci of the capitals, but in the church of S. Mary Diaconissa (p. 186) the columns still show the original form on the inside.
_Vaulting._--All Byzantine churches of any importance are vaulted in brick. The only exception to this rule in Constantinople is the little church known as Monastir Mesjedi (p. 264). The different systems of Byzantine vaulting have been so fully treated by Choisy and other authorities, that in the absence of any large amount of new material it is not necessary to give here more than a few notes on the application of these systems in Constantinople. It should always be kept in view that, as these vaults were constructed with the lightest of centering, the surfaces and curves must have been largely determined by the mason as he built, and would not necessarily follow any definite geometrical development. ”Il serait illusoire,” remarks Choisy, ”d'attribuer a toutes les voutes byzantines un trace geometrique rigoureus.e.m.e.nt defini.”[30]
[Ill.u.s.tration: PLATE II.
(1) THE MYRELAION (SINCE IT WAS BURNED), FROM THE NORTH-WEST.
(2) THE MYRELAION (SINCE IT WAS BURNED), FROM THE SOUTH-EAST.
(By kind permission of H. M. Dwight, Esq.) _To face page 20._]
The vaults commonly found are the barrel vault, the cross-groined vault, and the dome-vault. The first is frequently used over the cross arms and the bema, and sometimes over the narthex in conjunction with the groined vault (Diaconissa). It is the simplest method of covering an oblong s.p.a.ce, but it does not easily admit of side windows above the springing.
A very beautiful form of cross-groined vault is found in S. Sophia and in SS. Sergius and Bacchus, in which the crown is considerably domed, and the groins, accordingly, lose themselves in the vaulting surface.
This form is found in Greek churches of late date, but does not occur in the later churches of Constantinople. A full description of the form and construction is given by Choisy[31] and by Lethaby and Swainson.[32]
The cross-groined vault as found in the Myrelaion and many other churches of the city is level in the crown, with clearly marked groins.
It is sometimes used with transverse arches resting on pilasters, or without these adjuncts.
One of the most interesting of the vault forms is the dome-vault, a shallow dome with continuous pendentives. It is distinguished in appearance from the groined vault, as found in S. Sophia, by the absence of any groin line, and is completely different in construction.
The geometrical construction is that of the pendentives of all domes.
The four supporting arches intersect a hemispherical surface whose diameter is equal to the diagonal of the supporting square. The pendentives produce at the crown line of the arches a circular plan which is filled in by a saucer dome of the same radius as the pendentives, constructed of circular brick rings, the joints of which radiate to the centre. If the s.p.a.ce to be covered is not square the broader arches intersect at a higher level, while the narrow arches are not stilted, but kept down so as to receive the dome surface, and in this case the narrow arches are not semicircular, but segmental. Where the difference in size between the two sides was not great, the difficulty presented was easily overcome by the Byzantine builder, who in the later buildings, at any rate, rarely built anything within four inches of its geometrical position. Where the difference was too great it was frankly accepted, and we find segmental arches at the narrow ends.
[Ill.u.s.tration: FIG. 11.]
The vaulting of the outer narthex of S. Saviour in the Chora ill.u.s.trates this fully (Fig. 11). Though some of the bays of that narthex are oblong and others almost square all are covered with dome vaults. The almost square bays, although their sides vary considerably, are covered precisely as if their sides were exactly equal. But in two of the oblong bays, which are nearly three times as long as they are broad, such a method could not be applied. Longitudinal arches (AA) were accordingly thrown between the transverse arches (CC) and made to rest on their spandrils. The oblong form of the intervening s.p.a.ce was thus very much reduced, and over it flat domes are thrown. Their rings are true circles, and as the s.p.a.ce they cover is still somewhat oblong they descend lower, with additional segments of rings (BB), at the ends than at the sides. In the remaining two oblong bays of the narthex, the result of introducing the longitudinal arches is to convert a decidedly oblong s.p.a.ce in one direction into a slightly oblong s.p.a.ce in the opposite direction, an additional proof, if any were needed, that the exact shape of plan with this form of vault was a matter of comparative indifference to the builder.
In S. Sophia the vault springs from the intrados of the transverse arches, that is, from the lower edge. In SS. Sergius and Bacchus it springs from a point so slightly raised as to be hardly noticeable. In the later vaults, however, the transverse arches, when present, are boldly shown, and the vault springs from the extrados or outer edge (_e.g._ S. Saviour in the Chora, S. Theodore).
_Construction._--Most of the churches of the city are covered with thick coats of plaster and whitewash, both within and without. Only in a few cases, where these coatings have fallen away through neglect, or in some remote corner of a building to which these coatings were never applied, can the construction and the laying of the brickwork be studied. The two-storied chapel, known as Bogdan Serai (p. 283), is almost denuded of plaster, and is therefore of importance in this connection. The bricks of the wall arches on which its dome rests are laid considerably flatter than the true radiating line, leaving a triangular piece to be filled in at the crown. On the other hand, the bricks of the transverse arches under the dome radiate to the centre.
It has been supposed that the method followed in the wall arches was employed in order to economise centering, since bricks could gradually be worked out over the s.p.a.ce, each course simply sticking to the one below. This is undoubtedly the case in some examples. But here centering could not have been of any service in the wall arches, and the transverse arches are laid without flattening of the courses, though that arrangement might have been useful in their case. It is therefore more probable that the flattening of the courses in the wall arches is simply a piece of careless workmans.h.i.+p. The pendentives, like all pendentives that could be examined, were formed of horizontal courses corbelled out to the circle. The dome, bema, and the barrel vault in the lower story (p. 285) seem to be laid with true radiating joints. The springing of the barrel vault is formed of four courses of stone laid horizontally and cut to the circle, and above them the entire barrel is of brick. The dome arches of the Sanjakdar Mesjedi (p. 270) are formed of three distinct rings, not bonded into one another. They radiate to the true centre, and the pendentives are, as usual, in horizontal courses. The transverse arches of the outer narthex in S. Saviour in the Chora are also built with true radiating courses.
The gynecaeum of the side-chapel of the Pammakaristos (p. 153) has never been plastered, and consequently the laying of the brickwork can be seen there to advantage. The little stair leading up to the gallery is covered with a sloping barrel vault built in segments perpendicular to the slope of the stair and could easily have been built without centering. The same remark applies to the cross vault at the head of the stair, which is similarly constructed in 'slices' parallel to each side (p. 154). The arches of the gynecaeum itself, the vaults, and the two little domes, seem to have true radiating joints. The ribs of the domes are formed in the brickwork, and are not structurally separate. In these last examples, and in all door and window openings, in which the joints invariably radiate from the centre, a certain amount of centering was inevitable.
[Ill.u.s.tration: PLATE III.
(1) THE MYRELAION (SINCE IT WAS BURNT). INTERIOR, LOOKING EAST.
(By kind permission of H. M. Dwight, Esq.) (2) SULEIMAN AGA MESJEDI, BESIDE S. SAVIOUR PANTOKRATOR.
_To face page 24._]
On the other hand a little pa.s.sage in S. Saviour in the Chora between the church and the parecclesion (p. 311), is covered with a barrel vault evidently built without centering. The s.p.a.ce is first narrowed by two corbelled courses of stone and, above them, by three projecting courses of brick. From this springs the vault, built from each end in strongly inclined segments. These segments meet in the middle, leaving a diamond-shaped s.p.a.ce filled in with longitudinal courses. Like the stairs in the Pammakaristos, this pa.s.sage is very narrow, some 85 cm., yet the builders thought it necessary to corbel out five courses before venturing to throw a vault without centering.
Near the Pantokrator is an octagonal building, now Suleiman Aga Mesjedi but generally regarded as a Byzantine library, which has on each side a large wall arch strongly elliptical in form (p. 270). Two arches of somewhat similar form and apparently original are found in the south end of the gynecaeum of the Pantokrator (p. 237). These arches may have been built in this manner to economise centering. Still, in the library they are wall arches easily constructed without centering.
Failing the examination of a larger number of buildings in Constantinople we can hardly judge of the later methods of vault and arch construction, but one point may be further noticed. The wall internally is often set back slightly at each spring course, so that with the projection of the course a considerable ledge or shelf is left.
On this ledge centering could easily be supported and would have required no further framework to the ground. Centering seems to have been used for dorm, arches, vaults, and door and window openings. It was not used in small vaults. But it is difficult to imagine any method of constructing such groined vaults as those found in the narthexes of the Pantokrator without a very considerable amount of centering.
_Ties._--As a general rule tie rods or beams were used, either of iron or wood. In the latter case they were painted with leaf or fret ornaments, and were evidently considered as natural features. But large vaults are often found without such ties as in the narthex of the Pantokrator. Many churches have ties to the dome-arches, and none to the main vault; but it is difficult to lay down a fixed rule. The enormous amount of mortar in the walls must have made them yield to a certain degree when newly built, and some of the larger vaults would have been the better for rods.
_Abutments._--The system of abutments in the Byzantine churches of the great period has been carefully studied by M. Choisy.[33] In early examples the dome springs directly from the pendentives on the inside, but is thickened externally over the haunches, producing a double curve and an apparent drum. This is seen very clearly in SS. Sergius and Bacchus. In S. Sophia the numerous windows are cut through this drum, so that it resembles rather a series of small abutments. The object was to support the crown of the dome by adding weight over the haunches. In both these churches the thrust of the dome and its supporting arches is taken by the two-storied galleries, which form, in fact, flying b.u.t.tresses within the buildings, and are adapted to their architectural requirements. The square plan and the enormous size of the dome in S.
Sophia demanded the great b.u.t.tresses on the sides; while in SS. Sergius and Bacchus the eight b.u.t.tresses show only on the outside of the dome and are not carried over the aisles as they are in S. Sophia. Below the roof the arches and piers of the galleries and aisles are arranged so as to carry the thrust to the external walls, and following the tradition of Roman vaulting all b.u.t.tressing is internal. In S. Irene, where the true drum dome first appears, the b.u.t.tresses between the windows of the dome still remain, though much reduced in size. A dome raised on a drum can evidently no longer exercise a thrust against the dome-arches; its thrust must be taken by the drum, and only its weight can rest on the arches.