Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/324

280 carved with the chisel. From the latter part of last century (1784) till 1850, bricks in this country were subject to taxation. In Holland, where stone is scarce, bricks have been in use from a very early period, both for domestic and public buildings.

The quality of bricks depends primarily on the choice of an earth. There are three principal classes of brick- earths: (1.) Pure clays, consisting chiefly of alumina and silica, in various proportions, and with a small percentage of other salts, iron, lime, magnesia, &amp;lt;fcc. ; (2.) Loams or sandy clays ; (3.) Marls, or earths with a considerable proportion of lime. A paste of pure clay alone (made with water), while it may be easily moulded, will shrink and crack in drying and firing, in proportion to the excess of alumina over silica ; but this can be remedied by mixture with a milder earth or with sand. Loams, again, are often so loose that they require the addition of lime as a flux. The London brickmakers add lime and ashes, or breeze, to their loams and marls, both as a flux and to prevent shrinking ; such admixtures also, as will be seen, affect colour. Brick-earths are very various in composition. The proportion of ingredients in a good earth will be something like the following : silica, three-fifths ; alumina, one-fifth ; iron, lime, magnesia, manganese, soda, and potash forming the other one-fifth. The clays of which fire-bricks for furnaces are made are almost entirely free from lime, magnesia, and like substances, which act as fluxes ; they are found throughout the coal measures, immediately beneath the coal. The best, that of Stonrbridge, will bear the most intense heat that can be produced, without fusion. The Welsh fire-bricks, and those of Windsor, Newcastle, and Glasgow, are other well-known varieties. The Dinas fire-brick consists almost entirely of silica ; to this is added about one per cent, of lime, and the mixture, after moulding, is intensely heated. In Austria, a very refractory siliceous brick is manufactured by M. Khern, the chief ingredient being quartz of the highest possible degree of purity. The colour of bricks is determined by the proportion of hydrated oxide of iron and other ingredients they contain, also by the degree of heat in burning. Where iron is present without lime or such substances, a moderate red heat makes the bricks red, the intensity varying with the proportion of iron ; with more intense heat, the bricks, if slightly fusible, may be vitrified externally, and become greenish blue (e.g., the blue bricks of Staffordshire). The presence of lime changes the red colour produced by iron to a cream brown ; magnesia also arrests the development of red. Clays burning a pale red will burn yellow if mixed with a fusible white sand, such as is often found on heaths. Some clays, as those of Devonshire and Dorsetshire, burn of a clear white. The London malms give a rich brimstone yellow. The art of ornamental polychrome brick-work has of late years been much developed, especially by the German architects. The principal varieties of common bricks made in England are place bricks, grey and red stocks, marl facing bricks, and cutting bricks. The first two are used in ordinary walling. The marl facing bricks, made in the neighbourhood of London, are superior to the stocks, and used in the outsidcs of buildings. Cutting bricks, which are the finest kind of marl and red bricks, are used in arches over windows and doors.

The process of brickmaking varies considerably in different localities. In the following account we shall, in the first instance, confine our attention to methods adopted in the vicinity of London, and thereafter note some of the peculiarities of other systems. The most common mode of preparing the clay, in the London district, is that of malming. Among the varieties of brick-earth found there malm is a substance that can be used for bricks without any addition. But it is now rare, nnd an artificial malm is made by mixing chalk and clay, previously reduced to pulp, and allowing the mixture to consolidate by evapora tion. Bricks of the best quality are made with this alone ; but for the commoner sorts some of the malm is added to the clay or loam, sufficient to make it fit for brick- making.

The earth is dug up in autumn, and placed on a level floor, banked round in order to retain the malm in the process of malming. Exposed during the winter, it is more or less broken up and pulverized by the frost, &c. The machinery for malming consists of two washing-mills, viz., the chalk and clay mills, which are placed together, not far from the brick-earth. The chalk-mill is a circular trough in which chalk is ground in water, by two heavy wheels with spiked tires, drawn round by horses. The pulp thus made passes by a shoot into the clay -mill, another and a larger circular trough, where it gets mixed with clay that is being cut and stirred in water by knives and harrows, also put in motion by horses. The creamy liquid malm passes through a grating into shoots which convey it to the brick-earth, over which it is distributed as equally as pos sible. It is now left to settle for a month or more, the water being drained off at intervals, till the mass is firm enough to bear a man walking over it. A thin layer of ashes, about 3 inches for every spit of earth, is spread over the surface (this process being technically called soiling), and the whole is now ready for the moulding season, which commences generally in April. The mass of earth, malm, and ashes is first tempered, or thoroughly turned over and mixed with the spade, while water is added to give it the proper consistence. The tempered clay is then conveyed to the pug-mill, a conical tub, in which revolves (driven by horses) a vertical shaft with horizontal knives so inclined that the clay is slowly forced down to the narrow end by their motion. Several of these knife-arms are furnished with cross knives, which assist in the cutting and kneading process. The clay comes out laterally at the bottom as a uniform mass, and is ready for moulding.

The moulder stands at a table or stool, on which are placed some of the tempered clay in front of him, a little dry sand to the right and left, a small tub of water with the strike in it, a brick-mould, and a stock-board. Back wards from the stool extends the page, a pair of iron rails, on wood, on which the raw bricks are pushed away by the moulder. The brick-mould is a rectangular case of sheet- iron, without top or bottom, having the two longer sides strengthened with wood. The stock-board, supported on four pins in the moulder s stool, fits easily into the mould : it has often a solid elevation in the middle, for producing a hollow in the brick. The moulder receives from the clot- moulder (usually a woman), standing on his right, a piece of clay somewhat larger than a brick. Having sprinkled sand on the stock-board, and dashed the mould, after mois tening it, in the left sand heap, he places the mould on the stock-board, and dashes the clay into it with force, then pressing it with his fingers so as to fill the angles. With the strike (a short, smooth piece of wood) he strikes oft&quot; the surplus clay ; then he turns the brick out of the mould on a thin board or pallet, rather larger than the brick, and slides it along the page to the taking-off boy, who stands ready to put the bricks on a barrow of special construction ; on this, after sprinkling with sand, they are conveyed to the hack ground. The bricks are each carefully removed from the barrow between two pallets, and built up in hafks, about eight bricks high, and two in width (placed edgewise, and in an angular direction, the hacks being about 11 feet apart, from centre to centre. They are covered with straw or reeds at night or in bad weather. When half dry the bricks are separated somewhat (scintled). to allow 