Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/325

Rh free access of the air. The time taken in drying varies from three to six weeks.

In the vicinity of London bricks are commonly burnt in clamps; the peculiarity of which process is that, as each brick contains in itself the fuel necessary for its vitrification, the breeze merely serves to ignite the lower tiers, and the heat gradually spreads over the whole. The general structure of a clamp is as follows : A number of walls, or necks, three bricks thick, about sixty long, and twenty-four to thirty high, are built slantingly against a central upright wall, which narrows upwards. The sides and top are cased with burnt bricks. Cinders (or breeze) are distributed in layers between the courses of brick, the thickest strata being at the bottom. A single clamp will contain from 200,000 to 500,000 bricks. For firing the clamps, live holes or flues (9 inches by 7) are left in the centre of the upright, and at every seventh neck or so, extending throughout the thickness of the clamp. These are filled with faggots, and fired by a coal fire at the end of each vent ; and the fire ignites the adjacent breeze. Once the clamp is fairly lighted, the mouths of the live holes are stopped with bricks and plastered over with clay, and the clamp burns till all the breeze is consumed, usually from three to six weeks. After cooling, the bricks are removed, sorted, perhaps, and stacked. (For a fuller account of the most approved methods of building clamps, the reader is referred to Mr Dobson s excellent little treatise on brick- making, to which we are indebted for many of the details given in this paper.)

In the Nottingham district a very hard marl is often used in brickmaking ; and as ordinary weathering and tempering would not make it sufficiently plastic, it is sub jected to grinding between rollers, the hard lumps and pieces of limestone being thus crushed to powder, all pebbles and hard stones having been previously picked out by the hand. The wash-mill is only used in making arch-bricks, and the pug-mill is dispensed with, the tempering of the clay, after grinding, being done by treading and spade labour. Sometimes the clay is kept in damp cellars for a year or more to ripen. Brass moulds (technically called copper moulds) are used, and the moulder, after filling the mould, works over the clay, first on one side then on the other, with a flat implement called the plane. A boy takes the mould with its contents to the floor, where he turns out the brick, and then puts back the mould on the stool, the moulder meanwhile filling another mould. The bricks are sprinkled with sand on the floors, and turned twice at a few hours interval, and are then taken by boys to the hovel or drying-shed, when they are built in hacks. They are burnt in kilns, which are made with four upright walls and a sunken floor. On the two sides of the kiln there are shallow pits with lean-to roofs to protect the fuel and fireman. The doorways are narrow openings at the ends, a step above the ground, and the fire-holes are arched openings opposite each other in the side of the kiln, lined with fire-bricks. Narrow openings or flues are left between the bricks, connecting the opposite fire-holes. Each brick has some free space round it for passage of heat. The fuel employed is coal.

{{ti|1em|In the Staffordshire Potteries it is a common practice to pass the marl through several pairs of rollers, and then mix some three or four marls together, with water, by spade labour. For ordinary bricks the ground marl is mixed with marl that has been tempered but not ground. The pug-mill is employed for tiles and dust bricks, the latter so called from coal dust being used when they are moulded. The bricks are moulded by the slop-moulding process, the mould being dipped in water only before using ; the brick is emptied from the mould on the floor. (The other process is distinguished as that of sand-stock moulding.} The oven used in burning is of circular form, with spherical top, and will contain about 8000 bricks. Red, blue, and drab bricks are produced in the district, besides the dust brick just referred to, which is used for the paving of footways.}}

In Holland, the chief material used for bricks is the Holland, slime deposited in rivers and arms of the sea. It is collected in boats by men with long poles, having a cutting circle of iron at the end, and a bag net with which they lug up the slime. Hard bricks are made of a mixture of this slime with sand from the banks of the Paver Maas. The ingredients are well kneaded together, and the mixture is deposited in heaps. The mode of moulding and drying is similar to that used elsewhere. The kilns are square and will sometimes hold as many as 1,200,000 bricks. Peat turf is used in firing. For an account of brickmaking in India the reader is referred to a paper by Major Falconnet, R.E., in the Pro- fiissional Papers on Indian Engineering, May 1874.

Thus far only brickmaking by hand has been spoken of, but of recent years there has been no little activity in the invention of brickmaking machines, with a view to economy, certainty, and rapidity of production, and improvement in the quality and appearance of the bricks. It is only in brickmaking on a considerable scale, of course, that moulding by machinery can present much advantage over hand moulding, since the cost of moulding bears so small a proportion to the total cost. The various machines that have been offered to the public may be arranged in two classes, those which operate on the clay (with moderate pressure) in a moist and plastic state, and those in which the material used is pulverized and dry, or nearly so. A denser brick, and one less liable to shrinking, is produced in the latter case ; but much care is needed in preparation of the clay, and a much stronger compression is required, to ensure the proper tenacity. The different arrangements of rollers and pug-mills for preparation of the clay, whether plastic or dry, we need not here describe at any length. Rollers and pug-mill are sometimes combined, forming a composite machine. Two or more pairs of rollers are sometimes arranged one set under another, the closest at the bottom ; and opposing rollers are driven at different speeds so as to produce a rub as well as a squeeze, thus promoting disintegration. As to that class of brickmaking machines proper in which plastic clay ia used, we find, in some examples, a continuous length of clay forced out from a vertical or horizontal pug-mill through a suitable mouth piece, and the column divided into bricks by wires or otherwise. Of mouthpieces, some are simple dies, or dies fitted with cores to make hollow bricks ; in others the mould is lubricated by a constant stream of water in others, again, the mouthpiece is made with two or four rollers covered with thick cloth, which are perforated with small holes, and filled with oil to lubricate the faces of the bricks. In cutting, a frame of parallel wires may be moved across, either while the clay is at rest, or while it is in motion, by the wires being moved obliquely at an angle to compensate for the speed at which the clay travels. Or the clay may be cut by radial wires of a wheel, or again by metallic discs. Another variety of treatment of plastic clay is that in which the clay is pressed from the pug-mill into moulds of the form of brick required. In one such machine, a mould-block, with two moulds, moves backwards and forwards under the pug-mill, one mould receiving a charge while the other is having the brick pressed out of it by a piston. In another, the moulds are arranged radially round the border of a circular table, which revolves under the pug-mill. There are piston rods in the mould which ascend an inclined spiral plane, and thus gradually lift the bricks out of the moulds, whence they are taken by a boy and placed on an endless band, which conveys them 