The London brickmakers make another addition to the clay, which, we believe, is peculiar to them. They add to every three parts of the clay about one the ashes from the fire-places of the city of on. Thee ashes contain some earthy matter ; but they consist, in a great measure, of small coal unburnt, and little altered, which has fallen through the interstices of the grate. The consequence is, that such a mixture, when sufficiently heated, takes fire and burns of itself, though very slowly ; so that the London bricks are burnt, in a great measure, by means of the fuel mixed with the clay of which they are composed.
It is essential to dry the bricks thoroughly, in the open air, before burning them. For when heat is applied to wet clay, the water which it contains being prevented from escaping by the adhesiveness of the mixture, is converted into steam, and cracks and breaks the mass of day to pieces. Indeed, after the bricks are rendered as dry as they can become in the open air, they ought to be exposed, at first, to a gentle heat, which ought to be raised to redness, very slowly, and in proportion as the moisture of the brick is dissipated. Water adheres with such obstinacy to clay, that it is never all driven off by the heat at which bricks are burnt. But the portion which remains is so intimately combined with the clay, as to constitute one solid mass, which has no great tendency to absorb an additional quantity of water.
Bricks are most commonly burnt in a kiln. This is very simple structure, usually about 13 feet long, 101 wide, and 12 feet high. The walls are one foot two inches thick, and incline a little to each other as they ascend. The bricks are placed on flat arches, having holes left in them like lattice work. After the bricks are arranged on the kiln, to the number of about 20,000, they are covered with old bricks or tiles. Some brush-wood is then kindled in the kiln, and a moderate fire kept up till the bricks are rendered as dry as possible. The time required for this is two or three days, and the bricks are known to be dry when the smoke (which is at first black) becomes transparent. The mouth of the kiln is then filled up with pieces of brick and clay, leaving only room to introduce a faggot at a time. This structure at the mouth of the kiln is called a shinlog. The kiln is then supplied with faggots of furze, heath, fern, or whatever vegetable substance can be procured at the cheapest rate, till the arches look white, and the fire appears at the top. The fire is then diminished, and at length al
lowed to out, and the kiln is permitted to cool. This burning process usually lasts about forty-eight hours.
The method of burning bricks in the neighbour hood of London, is very different from this. We do not know whether it be practised any where else. It obviously originated from the difficulty of procur. ing a sufficient quantity of vegetable matter to burn the enormous number of bricks consumed every year in London. if we consider the immense of houses which has taken place in London within the last thirty years,—if we consider that this vast city, containing above 1,000,000 of inhabitants, in almost renewed once every century, we may be able to form some notion of the prodigious quantity of bricks which it must consume. In the country round London there is a particular kind of clay, well known by the name of London Clay. This clay is almost everywhere covered with a bed of gravel, which varies in thickness according to the elevation of the surface. Hence the whole of the country round London is fit for making bricks. Nothing more is necessary than to dig through the surface of gravel, and get to the clay.
We have already mentioned, that about a fourth of the London bricks consists of small coal part kneaded up along with the clay. When the bricks are sufficiently dry, they are piled up on each other in parallelepipedons to the intended height. Be tween each two rows of brick there is strewed a quantity of cinders, amounting to about three inches in thickness. At the distance of about nine feet from each other, perpendicular spaces are left, about a brick wide, which serve the purpose of flues. These are made by arching the bricks over so as to leave a space between each about a brick in width. Over the whole is strewed a pretty thick covering of cinders. The flues are filled likewise with cinders, or. if they cannot be had, with coal. The fireplace is usually at the west end, and is generally three feet high. The fire, when once kindled in the fire place, propagates itself very slowly through the whole clamp, as bricks piled in this manner are called. So very slow is the progress, that bricks in the neigh bourhood of London take about three months in the burning. The heat is very intense, and, as the fuel is mixed up with the clay itself, every part of the brick is sure to be sufficiently burnt.