From the importance attached to wood ties or girdles built into the small domes of Mount Athos, we may be certain that some system of chaining was applied to the great dome of S. Sophia. Choisy gives an example of the former, and also a dome constructed by interlocking semicircular bricks, “two courses of which make a circlet absolutely inextensible.” See B in [Fig. 45]. The dome of S. Vitale at Ravenna is built of layers of earthenware pots or tapering tubes, the end of one fitting into the next and rising in a continuous spiral course, round and round from the bottom to the crown of the dome.
The question of dome construction without centring is of the greatest interest, and much might doubtless be gathered of the traditional methods still followed in modern Greece, Egypt, Persia, and S. Italy. Our [Fig. 43] represents modern domes in Persia, the upper diagram being an ordinary type of exterior from a photograph of Koum. The dome beneath, [Fig. 44], is from a sketch made in a Persian caravanserai by Mr. Wm. Simpson,[344] who describes it as built of burnt brick, square below, round above. “As I was told that centring was never used in Persia I presume this one was constructed without it.” This beautiful form may be considered as four conical squinches penetrating a hemisphere as at A, or as a gradual transition from square to round, B. Ancient Persian domes of substantially the same form, in which a hemisphere penetrates a pyramid, are shown by Dieulafoy.[345]
Figs. 43 and 44.—Modern Domes built without Centring.
Chainage and Walling.—In the East the frequency of severe earthquakes necessitated a manner of construction which should resist disruption. The massive walls of stone of the Classic period are cramped together with metal. The stone Byzantine church at Ezra has a course of interlocking stones forming a chain around the octagon beneath the dome ([Fig. 45] a). At S. Sophia the continuous courses of stone some feet above the floor, mentioned by Salzenberg, are almost certainly converted into a chain by cramps; and the stone course at the springing of the great arches probably has the same function. In brickwork lateral cohesion was usually obtained by a system of continuous wood ties, which is described by Choisy as built into the wall at every five or six feet of height. According to the Greek architect, M. Kouppas, ties of bond timbers were used in this way in the construction of the cisterns, “laid not only along the outside walls but also in parallel rows beneath the lines of pillars and arches;” other rows of timber were built in either as ties or struts in continuous lines at the springing of the vaults.
At S. Sophia there was doubtless a large use made of temporary ties of this kind during the construction. In many places at the springing of the gynaeceum vaults the ends of such provisional ties, which have been sawn away, appear. Besides these there is a series of wood beams which from the first were intended to be permanent, for they are richly carved (C in [Fig. 45]); these are shown by double lines on the right-hand side of Figs. [5] and [6], the single lines showing the iron ties. These carved beams, as Choisy points out, are struts rather than ties. If we take one of the columns standing in an angle in the aisles, an impost of marble connects it with the wall to which it is nearest, and a carved wood beam forms a strut to the other wall. The beam across the central bay of secondary order ([Fig. 5]) forms a rigid strut to the two wider arches (see [Fig. 38], where, however, by oversight the beam has been omitted; it is at the springing of narrow arch high above iron tie). Choisy asserts that “the architect intended to preserve only the struts, all the ties subject to extension were removed, but their suppression was disastrous, and they had hastily to replace them by bars of iron which were fixed with difficulty.” We do not know what reason Choisy had for supposing the system of iron ties to be an afterthought, unless it is because in some cases they appear directly above the ends of the removed wooden ties. Now we believe they occur equally above the carved beams in the openings from the gallery to the nave, and there is no sign of wood ties having been removed from the ground-floor vaults, where the iron bars fulfil such an important function. It is certain that the iron bars to all the nave arches are original, for the marble casing shows no sign of alteration, and they are evidently threaded continuously through the imposts. The important iron ties across the aisles are shown in [Fig. 45]: d is the attachment to the column of great order, e to impost of secondary order behind it, f is a king rod. Across the west gallery the span is lessened by stone corbels beneath the ties g.
With a view of binding the vaults and walls together into a homogeneous mass, the arched vaulting of the interior was carried through the thickness of the walls: in some cases these arches were left open, to be afterwards filled with a screen of windows. The walling of the sides of the church is built independently of the great piers, as straight joints on the exterior show, and Choisy remarks that the independence of masonry unequally charged was a leading idea in Byzantine construction; indeed it is obviously necessary where the quantity of mortar is so great that the brick at times becomes secondary to the joints.
Fig. 45.—Methods of Chainage.
Mortar and Cement.—The mortar used by the Byzantine builders was called Keramotos, from the crushed pottery or tiles which was used in its composition. In an article in the Transactions of the Philological Society of Constantinople M. Kouppas[346] enters fully into the methods which have been traditionally followed in cistern building, and describes this mortar as formed of powdered unslaked lime (asbestos), crushed pottery, coarse sand, and tow or hair, fully a third being lime, another third the crushed pottery, about a fifth the coarse sand, and the rest or 10 per cent. of hair or tow. These were then mixed together in water.