After the removal of the turf and surface-soil, which is called encaltowing, the clay is dug out, in the autumn, and placed in heaps or places called kerfs, and there the different kinds are mixed together and other ingredients added, as the nature of the clay requires, for the production of a first-class article. Of course, all stones and other unsuitable substances are picked out, and sometimes, when the clay is hard or uneven, it is ground in a mill to a finer admixture.
This done, the clay remains in these heaps or kerfs during the winter; and, about the end of March, the winter's seasoning having reduced its liability to warp, the clay is turned over, trodden under foot, or ground in a mill, which tempers the clay and renders it ready for conversion into bricks.
Malm, the washed and artificial brick-making earth, is likewise made in the autumn by mixing the newly dug clay with a proportion of chalk, to make up the deficiency in the natural constituents. This is done in a mill, and as soon as the mixture becomes of a creamy nature, it is run through large sieves into what are called backs, large tanks in which the substance becomes hardened, after which a layer of fine screened ashes is sandwiched between two layers of malm, and in this condition the mass remains during the winter to mellow.
The clay or malm being thoroughly ripe, it is ready to be made into bricks, which can be done by machinery or by hand, the latter making the better bricks.
Machine-made bricks are of two kinds, dry or plastic; in the latter case the clay, when still in a soft, sticky condition, is forced by pressure through a rectangular channel of indefinite length but the width of a brick lengthwise and the height of a brick on edge (that is, about 10 in. x 5 in.) on to a smooth greased iron platform. When sufficient has descended on to the platform, the supply through the channel is cut off, and what remains, being 10 in. x 5 in. and of indefinite length, is cut by knives or wires, in a frame worked by machinery, into the height of the brick required - whether it be 2 1/2 in. or 3 in.
This, in the course of a few seconds, cuts a definite number of bricks out of the clay, and these are carried away to a shed or out into the sun to dry gradually.
Machine-made bricks can also be formed from clay which has been dried and ground to a powder, which is put into a mould (as explained hereinafter in treating of hand-made bricks) and subjected to great pressure, so that a consolidated mass is formed of very even surfaces. The resulting brick, being already dry, is ready to be burnt in the kiln.
Hand-made bricks are of two kinds, slop or sand-moulded; the difference being that, for the former, the mould (which is made a little larger than the ultimate size of the brick required, so as to allow for shrinkage as the clay or malm dries) is dipped into water, to prevent the clay from adhering to its sides; whereas for the latter the same end is attained by sprinkling the inside of the mould with sand, fine ashes, or a specially ground powder.
Sand-moulded bricks are considered to be cleaner, more sharp,, regular, and perfect in shape than slop-moulded bricks. In both cases alike, the clay or malm is pressed by hand into the mould on a flat and smooth table, and the superfluous clay is cut off by a straight edge.
The rectangular block of clay is then turned out of the mould and taken to the shed to dry, exactly as with the machine-made bricks.
In some cases a frog or indent is formed on the bed of a brick, either hand-made or made by machine; or the brick may be made by hand and afterwards machine-pressed to give it a closer body and finer finish.
This frog or kick, as it is called, is made by a projection on the stock board, over which the mould is placed and fitted to form a bottom before the clay is pressed home within the mould to form the brick; or in the case of machine-pressed bricks, projections are left on the top and bottom plates of the pressing box.
The soft, half-dry bricks are then dried gradually, in the sun or under sheds, being scintled, that is, stacked diagonally, so that the air can have free access all round them. When they are dried out of doors, they are placed on hacks; these are long narrow banks, raised above the ground which is covered with dry material, such as ashes or brick rubbish, to reduce the moisture to a minimum and to facilitate the drying process.
When the raw bricks are sufficiently dry they are wheeled into the kilns or clamps and stacked in layers, sometimes diagonally, to allow of a perfect circulation of the heat from a bottom of burnt bricks, covered with six inches of breeze, channels, filled with faggots, being left across the bottom to draw the heat. This done, and the clamps covered up or the ends of kilns blocked, the fire is lighted and kept burning for a period of from two to six weeks, after which they are allowed to cool, and the brick is then ready and fit for the builder.
Clamp-made bricks should always have a certain amount of ashes or breeze incorporated in them in the mixing, so that they may take the fire better and burn more readily.
Clamps are chiefly used in country brickyards, and for temporary purposes; they consist of walls, built of dried raw bricks, smeared with clay to hold the heat, and encircling a honeycomb of flues, on which the bricks to be burnt are stacked, on layers of breeze, laid over the flues, in such a way as to leave each brick as far as possible exposed on all sides to the heat during the process of burning; and they are then covered over by a few courses of burnt bricks, after which the fires are lighted, when the whole mass, connected by the flues, commences to bum gradually.
Kilns are more permanent structures, and may be either circular or rectangular in plan, the former being the better of the two; but a detailed description of them is beyond the province of these notes. Glazed bricks are burnt in domed kilns.
As soon as the bricks are cool they are fit for use, though they require some selection; those near the flues being over-burnt and run together, called burrs; and those near the outside being soft and more or less imperfectly burnt, called place bricks, grizzles, or samels.
Chuffs are bricks on which rain has fallen while they are hot, making them full of cracks, and otherwise wholly defective.
A sound, good brick should be hard, well-burnt, square, and regular in shape, with clean sharp arrises. It should give a clear ring when struck; be free from lumps of lime and other impurities, and as non-absorbent as possible.