This process is especially adapted to clays that contain only about 7 per cent, of moisture as they come from the bank, the clay being apparently perfectly dry. Wet clays are sometimes dried and then submitted to the same process, but the expense of drying materially increases the cost of manufacture.
The various operations generally employed in making brick by this process may be briefly described as follows :
The first step is the mining of the clay, which may be done either by hand or steam shovel, according as circumstances may direct. After being mined the clay is generally stored under cover, so as always to have a supply on hand, and also to permit of further drying and disintegrating. Sometimes, however, the clay is taken directly from the bank to the dry pan.
From the dump the clay is thrown into a dry pan, which is a circular machine about 4 feet in diameter and 2 feet deep, with a perforated metal bottom. In this machine, or pan as it is called, are two wheels, which constantly revolve on a horizontal axis and grind the clay between them and the bottom of the pan, the pan itself revolving at the same time. The clay as it is ground passes through the holes in the bottom of the pan and falls on to a wide belt, which carries it above an inclined screen, on to which it falls. Such portions of the clay as are sufficiently finely ground fall through the screen on to another belt, and the coarser particles roll into the dry-pan, to be again ground and carried on to the screen.
The belt which receives the fine clay from the screen carries it to a mixing pan, which is a machine contrived to thoroughly mix the particles of the clay. From the mixing pan the clay falls into the hopper of the pressing machine, and from the hopper it falls into the moulds, where it is subjected to great pressure, which compresses it to the size of the brick and then pushes the pressed brick on to a table. From the table of the machine the bricks are taken by hand, placed on a barrow, or car, and transferred to the kiln.
Different manufacturers may vary these operations somewhat, but the process, and also the machines, are essentially like the above in manufacturing pressed brick.
The pressing machines are so constructed that the loose clay is made to evenly fill a steel box of the width and length of the intended brick, but much deeper. Into these boxes a plunger is forced, which compresses the clay until the desired thickness is reached, when the plunger stops. If the clay falls more compactly into one box, or mould, than into another, the brick from the first mould will be the denser, as the plunger falls just so far, no matter how much clay is in the mould.
Moulded bricks are made in exactly the same way, the only difference being that the box is made to give the shape of brick desired.
Most of the pressed brick machines admit a small jet of steam into the clay just before it passes into the moulds to slightly moisten it.
Bricks made by this process are very dense, and generally show a high resistance to compression, but the general opinion is that the particles do not adhere as well as when the clay is tempered, and that dry pressed bricks will not prove as enduring as soft mud bricks, although the former are now most extensively used for face bricks.
When the term pressed bricks is used it should refer to bricks made by the dry process, although many so-called pressed bricks, or face bricks, are made by repressing soft mud bricks.