The custom of paving buildings with plates of baked clay of greater or less thickness, began with the use of brick; that is to say, it dates from the earliest historic period. The Romans, to whom we must always revert when speaking of architecture, used squares of baked clay for their pavements; at Lillebonne squares measuring 16 inches each side have been found; and others of as much as nearly 2 feet have been discovered at Laudunum. These pavements were intended for halls in which many people assembled; special rooms were paved with those magnificent marble mosaics of which we still possess such fine specimens.
Under Roman dominion, the Gaul preserved the customs of his conquerors; but with the decadence of the Empire, and the great troubles caused by the invasion of the Barbarians, other habits prevailed, and the expensive marble was finally abandoned in favour of pottery. Elsewhere will be found the history of those encaustic tiles from which makers in the Middle Ages obtained such beautiful effects. We are only concerned now with red clay squares, which were always manufactured with more or less care in every country. In France, the most admired specimens come from Beauvais, Burgundy, and Provence.
As regards method of manufacture we must distinguish between:
1. Ordinary clay quarries, called native.
2. Choice quarries, of cleaned clay, called Beauvais or Marseilles squares.
These are made like bricks: by hand or machine; by hand, the clay after being blended, is moulded in square or hexagonal moulds of the required dimensions; then, after having undergone some hardening, the quarries are carefully stamped, returned to dry, and fired in ordinary brick-kilns. With machinery, the process of manufacture resembles that for bricks, except that the stamping must be more carefully carried out, as we shall see in the case of the quarries now to be described.
The preparation and choice of clays are of great importance in the manufacture of these squares. Clays which are too poor cannot be used alone and a certain quantity of rich clay must be added. The cleaning is performed under the same conditions as that of the kaolins and with the machinery represented in Fig. 24. The clay thus purified is transmitted to the pug-mill, and then to the expression machine, the die of which has the dimensions of the quarry to be manufactured (Fig. 602).
Fig. 602. Expression Machine for Quarries.
Fig. 603. Special Cutting-table for Quarries.
If the quarties are to be hexagonal, the prism of clay is cut with crossed wires (Fig. 603); the slabs are then piled together, and when sufficiently hardened, are ready for the final work of the press.
The presses used are similar to tile presses; they are worked by hand and provided with a special system of demoulding. The moulds can be easily changed according to the dimensions of the quarries to be stamped. Sometimes jointed levers (Fig. 606) are substituted for the compression screw. The presses may also be worked by steam (Figs. 607, 608); they are provided with two or three moulds which can be emptied and filled during compression.
Fig. 604. Quarry Press (Jager).
Fig. 605. Quarry Press (Lacis et Cie.).
Fig. 606. With Jointed Levers (Bernhardi Sohn).
Fig. 607. Worked by Steam (Jager).
Fig. 608. Worked by Steam (Bernhardi Sohn).
When stamped, the quarries are dried again, and afterwards undergo polishing by friction between two rollers, one of which is of polished bronze, the other of cast - iron and fluted (Fig. 610).
The upper roller smooths the upper surface of the quarry, while the lower one produces on the other side hollows which will help to increase its adhesion to the mortar. In order to give absolutely uniform dimensions to the quarries, they are cut with hand callipers, or more quickly with a cutting machine(Fig. 609), which consists of four or six flexible blades according to the shape required. The section is conical and without seams.
Fig. 609. Cutting Machine for Quarries.
Fig. 610. Polishing Machine for Quarries.
The quarries are placed in pairs with their polished faces against one another, and piles are formed so as to prevent the surfaces from warping, which would certtainly occur with quarries of slight thickness.
The firing usually takes place in intermittent kilns. Continuous kilns do not seem to be suitable for this process, which requires a very slow and careful enfuntagt; this must be specially watched on account of the shape of the quarries, and of the contraction which they undergo in baking.
The colour of these paving bricks is usually red, but white ones can be made by a suitable choice of clays, and even black ones can be produced by admixture of oxide of manganese, or more economically by a deposit of coal on the squares when still red ("encastage" in presence of powdered coal).