M. Kouppas also describes a hydraulic cement made of “coarse lime (titanos) slaked by water into powder, sifted and laid in layers with cotton shreds. This was thoroughly mixed, and then olive oil was poured in, and the whole gradually brought to a homogeneous mass.” Andreossy[347] describes a mixture (called lukium) made of a hundred “ocques” of lime, freshly slaked in the form of powder, twenty-five “ocques” of linseed oil of the best quality, and twenty drachms of filaments of cotton. This was reduced to a dough, and then before using fresh oil was added. Strzygowski[348] also speaks of a Turkish cement “of six parts by weight linseed oil, eight parts slaked and powdered lime, and one part of cotton.” He refers to a Roman mixture mentioned by Pliny of “oil and quicklime.”
By far the best and earliest account of the methods used for obtaining lime and making cement at Constantinople is contained in Dr. Covel’s MS. in the British Museum (1670-7). The lime was burnt in a pit dug in the ground, the stone, which was hard and black and like “Plymouth stone,” being piled up in and above it like a beehive hut, an opening being contrived in the side for inserting fuel, and a smaller pit dug in the middle for the ashes; it was fired for three days. Then he describes in detail how a cement was made which recalls what the Anonymous says of the joints of the piers at S. Sophia being made of unslaked lime (asbestos) and oil: “To make good lukium (a strong cement as I may call it) they take the above said calx or burnt stone and slake it with water, and so soon as it is moulded and turned into a meal (even while it is warm) they work it with linseed oil and cotton till it is well saturated and brought to the consistency of plaster, and make present use of it, for it will not rest in its perfection above one day or two at most, and if they use it immediately after it is tempered it is certainly the best. In the works of their Bagnos so soon as it is laid on [as a plastering, understood here] they let the water come to it, which, by tempering the heat of the lime, hinders it from cracking. Cotton is better to be mixed amongst it than hair, it being more tenacious and apt to incorporate.” He again describes a similar cement (“lukium, an excellent mortar”) used in some waterworks. “It is made of unslaked lime and beaten brick most finely powdered and sifted, cotton wool very thinly pulled and strewed on, and then all slaked with linseed oil and mixed together: then they use it whilst it is fresh made, otherwise it hardens immediately.”[349] Such a cement must have had the hardening qualities of gesso; the oil cements or mastics used in England some fifty years ago were closely allied in their composition. Modern mortar has lost much by our neglecting the tradition of using crushed brick.
Eastern builders spared neither labour nor time in preparing and testing their materials. Tavernier tells us the waterproof terraces of the Persian houses were formed of “a layer of lime beaten for eight days, which became hard like marble.” The materials used in Byzantine building were tested by long exposure, slaked lime was sealed up in pits for one or two years; and stones, bricks, and tiles they had found should not be used new, for, as Vitruvius says, “the only way of ascertaining their goodness is to try them through a summer and winter.”
CHAPTER XI
MARBLE MASONRY
§ 1. BUILDING PROCEDURE.
The method and sequence of the building operations as followed by the Byzantines seem to have been very much as follows. After the form of the building had been more or less decided, the first thing necessary was to collect marble monolithic shafts. At S. Sophia the eight verde-antique shafts match one another very closely; they are all of one length, and vary from 7½ to 8 diameters in proportion. The four pairs of porphyry shafts in the exedras differ much more; and, as we have remarked, those in the western exedras seem to be made up of separate drums. The proportions of these vary from less than 7 diameters on one side to 8½ on the other. The great monoliths are the largest known, and of nearly normal classic proportion, so we can readily see that it was necessary to have a certain knowledge where such marbles might be quarried or otherwise obtained, before even the foundations were prepared, for the columns decided the heights and points of support of the building. These once assured, the body of the structure was proceeded with as a brickwork shell without further dependence on the masons, who were only required to prepare bases and capitals, and then the cornices; everything else was completed as a brick “carcase.”
At S. Sophia the main square piers are in fact stone, but this was only for strength, not because they were to be seen finally, any more than the rough brick.
The building completed in this form we must remember was made up of vast masses of thin bricks, of which the mortar occupied probably a half of the aggregate; this had to thoroughly settle down and dry before the rest of the marble masonry was inserted, and the wall casings applied. The marble work, however, was all the while being prepared, and, the building once ready, the windows were inserted as screens in the openings previously left; marble jambs and lintels for the doors were placed in position also, with windows above them filling out to the brick arches. The walls were then sheeted with their marble covering, the vaults were overlaid with mosaic, and the pavement was laid down. In this way, as the bricklayers had not to wait for the masons, the carcase was completed in the shortest possible time; and by reserving the application of the marble until the structure was dry and solid, it was possible to bring together unyielding marble and brickwork that must have settled down very considerably.