PAVING-BRICKS are coming into use in the United States with great rapidity. Clay for paving-brick should stand vitrification. or. more properly, semi-Nitrification. without losing its shape; but it need not be a fire-clay. Shale is much used for the purpose. Paving-hricks are often repressed. The finished product should lie tough, stand abrasion and impact, and be quite non-absorbent.
SENvEn-Ilittex are ordinary brick, selected for hardness, regular shape, and smooth surface.
After the clay is prop erly worked it must be molded, dried, and burned_ (Clay-working is treated under CLAY, autd burning under molding was formerly done by hand, and this method is still followed at small yards, or for the manufacture of spe cial brick; but machine molding is becoming more and more common. Molding-machines are of the soft-mud, stiff-nrad or wire-cut. and dry clay types. Soft-mud machines are generally used by the smaller makers. Their characteris tie features are an upright receptacle of wood or wise or sidewise, the machines and product being known, accordingly, as end-cut or side-cut. The plunger machines are end-cut. The auger machines tend to give the brick a laminated structure, which is objectionable. Some molding ma•hines combine a vertical pug-mill (see under the article CLAY, paragraph Pug-Mills), with a revolving arm which forces the clay down into molds ranged around the edge of a horizon tal revolving table. Plungers from both above and below compress the clay. Dry-clay machines use a pulverized. screened clay, which is fed from a hopper through canvas tubes or chargers into molds, where the clay is pressed from above and below, somewhat as just described. Brick thus made are very dense, but their particles are not well united. These machines are used chiefly to make front brick. After the bricks are turned out by the machines it is necessary to dry them before burning. This is done in the open air, in covered sheds, or in heated tunnels, from which they are taken to the kilns. For a description of burning see KILNS, and for accounts of the brick industry, both in general and in detail, consult: The several annual issues of The Mineral Indus try (New York, 1S02-1900) ; Wheeler, Vitrified Paring Wick (Indianapolis, 1895) ; Davis, Prac tical Treatise oa the Manufacture of Bricks, Tiles, and Ter•a-Cotta (London, 1SS4).
or BlucK. Throughout antiquity two kinds of brick were used, the crude or sun dricd and the kiln-dried or fired bricks. Brick making was invented in Babylonia, where nature had provided no other building material, there being no quarries or forests; and if, as is assert ed by eminent authorities, the earliest. civiliza tion was formed in Babylonia, bricks may he deemed the earliest material used in permanent construction. as well as the basis for the inven
tion of the arch, dome, tunnel. and other methods of vaulting. Buildings made of bricks have been discovered dating long before B.C. 4000. The same • processes were followed in Egypt, where, how ever, the use of brick was not nearly so' general, as stone-quarries were abundant. Assyria fol lowed Babylonia far more closely, using stone hardly at all, except occasionally for foundations and facings. In the preparation of crude bricks, the clay was first freed of grit, stones, and lumps, then mixed with finely chopped straw, then a fixed proportion of water wa* added, and finally the mass was kneaded with the feet in shallow basins. This material was then thrown into molds varying in size from 14 to 151/2, inches square, and from 2 to 4 inches thick. The Baby lonians allowed the bricks to dry thoroughly before use, but the Assyrians were satisfied with brief and imperfect desiccation: for usually it is possible to detect separate courses and even sepa rate bricks in Babylonian ruins, but in the Assy rian walls all individuality has been lost through the adhesion of moist surfaces.
The kiln-dried bricks were used in much smaller quantities, for facing the masses of crude brick so as to preserve them from disin tegration, especially by water, and for thin and inner walls. These bricks were burned by wood fires in ovens or flare kilns. They also were commonly square, somewhat smaller than the crude brick, about a foot in length and breadth. Usually, one of their flat sides was stamped with the name and titles of the reigning king, and this side was laid downward. These bricks have been the best means of fixing the dates of many temples and palaces in Babylonia and Assyria. Besides the regular-sized burned bricks. others were molded in special shapes and sizes: seg ments of circles for use in columns; wedge shaped. for use in arches; triangular, for cor ners; even convex. Undoubtedly. the Babylo nians deserve the palm for both quality and vari ety. The bricks were laid in liquid clay, lime mortar, or bitumen. According to llerodolus. in building the walls of Babylon the clay from the trenches was used for making the bricks, which were then laid in "hot asphalt for cement, and between every thirtieth course of bricks placing mats of woven reeds." (Cf. Genesis xi. 3, for the building of the Tower of Babel.) A third form of brick used by Babylonians and Assyrians was the enameled brick, in which the body of the brick was only lightly fired so that the enamel could better penetrate.