S. Clay intended to be made into bricks ought to be dug out of the earth, and exposed to the air and 'weather for a considerable time before it is employ ed. The longer this'expOsure is continued, so much. the better will it be fitted for making bricks. This exposure answers a variety of purposes. If the stones, by the decomposition, of which the clay has been formed, are not entirely decomposed, this ex posure serves to complete the process, by promoting the disentegrating action of the air and rain. The exposure serves, likewise, to pulverize the clay, which is essential to the making of good bricks. We haie little doubt that the same amelioration in the clay would be produced by simply drying it in the open air, and then grinding it to powder in a mill. By such a process, the quality of the bricks would be prodigiously improved. Nor do we such an addition would greatly enhance the expen ces of the brickmaker, at least in those districts where the mill could be driven by water.
When the clay has been reduced to powder, the next step is to make it into a stiff paste with wa ter. Too much water should not be employed, because it is injurious to the strength of the bricks ; and the utmost care should be taken to mix. the whole of the clay as equally as possible with the water. If some parts of the paste be moister than others, it will occasion an inequality in the texture of the bricks formed of it, w ill render them apt to crack, and will greatly injure both their, strength and their beauty. Hence the great importance of work ing .the clay for a considerable length of time before moulding it into bricks. It is in this part of the process that we believe British brickniakers in gene nil are most defective. As far as we have had an opportunity of witnessing the process of kneading the clay, as conducted either in the of London or Edinburgh, we have always found great sparing of labour. Hence we believe the rea soil why so many of the English bricks appear full of cracks, even when sold to the builder. Such bricks ought never to be purchased, as it is perfectly obvious that they cannot make a durable building.
The kneading of the clay is performed, in some places, by men's feet ; in others, by the feet of horses, and in others by machinery. The last method is undoubtedly the beat; and we conceive likewise that it might be made the cheapest. It would be easy to devise machinery for kneading the clay, upon principles similar to those employed in mashing by the London brewers. And, if such a ma chine were driven by water, we conceive that it would not be nearly so expensive as either or horses.
When the clay is sufficiently kneaded, it iamould ed into the form of a brick, by being put into a very simple wooden mould ; and the upper part of the brick is made smooth and even by cutting off the superfluous part with a wooden knife. The process is very simple, and is conducted by the workmen with great rapidity. A good brickroaker will mould about 5000 bricks in a day. He disen. gages the bricks from the mould by a gentle stroke on the back of the mould ; and the wet bricks are at first arranged in rows upon long boards. When sufficiently.dry to be handled, they are turned, and at last piled up in loose walls, which are thatched with straw to keep off the rain. In this position they are allowed to remain till they have become as dry as they can become in the open air.
In many cases, the clay used for brick-making is destitute of the requisite quantity of sand. If such clay were made into bricks, it would shrink so much in the burning, that the bricks would lose their shape, and would probably crack in every di rection. To prevent this, it is necessary to add a certain quantity of sand. This sand should not bq very fine. It answers best when the particles are of such a size as to be readily distinguished by the naked, eye. Even when as large as coriander seeds, it has been found to answer better than very fine sand. The brickmakers in the neighbourhood of London bring their sand from the bottom of tle Thames near Woolwich, where it is raised by boats ' employed for that purpose, and brought up the river for the use of the brickmakers.
4. No general directions can be given respecting the quantity of sand to be mixed with the clay, because that depends upon the nature of the clay, and upon the uses for which the bricks are intended. The more sand is added, the more accurately do the bricks retain their shape, and the less apt are they to crack during the burning; but, at the same time, their strength is diminished. Chemical lutes are often composed of four parts of sand and one part of clay. Such mixtures do not contract much in burn ing, and, therefore, are not apt to crack and drop off, which is the reason why chemists employ them. But they have not the adhesiveness of brick after be ing burnt, and would not, therefore, answer the pur poses of the brickmaker. In stone-ware, the mixture Consists of about four parts of clay and one of fine sand. It burns to a hard, cohesive substance, capa ble of striking fire with steel. Such a proportion then, in many cases, would answer the purposes of the brickmaker.