This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
165. Where bricks are made on a large scale the work is now done by machinery, by three different processes, known as the soft-mad, the stiff-mud, and the dry-clay processes.
Soft-Mud Process. The clay, after being dug from the clay bank, is thrown into a pit, usually 6 feet deep, and 8x12 feet in area, lined with plank; water is then turned into the pit, and the clay is soaked for twenty-four hours. It is usual to provide three pits, so that the clay may be soaking in one, while the second is being emptied and the third filled. The clay is thrown out upon an endless chain, which carries it along to the machine, into which it falls. The upper part of the machine contains a revolving shaft on which arms are placed. These arms break up and work the soft clay, after which it falls to the bottom of the machine; here revolving blades force it forward, and a plunger having an up-and-down motion, forces the clay into a mold immediately under the plunger. When the mold is filled, it is either drawn or forced out on a shelf or table; another mold is then placed under the machine, and the filled mold is emptied by hand and the brick taken to the drying yard.
Stiff-Mud Process. The difference between a soft-mud and a stiff-mud process is, that in the latter, the clay is first thoroughly ground, and just enough water is added to make a stiff mud. After this mud goes through the pug mill, it is placed in a machine having a die the exact size of the brick required. The opening in this die is made the size of either the end or side of a brick. The machine forces a continuous bar of clay through this die, and, as it emerges, it is automatically cut in the form of a brick, and then taken to the drying yard. The soft brick are placed in rows in a yard covered by a rough shed with the sides open, where they are sun or air dried for three or four days. When properly dried they resemble somewhat the "adobe" brick, formerly used for constructing houses, and still used in some of the Southwestern states and territories, and also in Mexico and Central America.
Dry-Clay Process. The third process, and one much used, is known as the dry-clay process. The clay is used in this method just as it comes out of the bank, and is apparently perfectly dry. It contains, however, from 7 to 10 per cent. of moisture.
The clay is first mined, either by hand or steam shovel, as circumstances may require. It is then usually stored under cover, in order that a supply may be constantly on hand, and also that it may further dry and disintegrate. In many cases, two or more grades of clay are mixed together in proper proportions, determined by trial, as the clay is thrown in the dry-pan, which is a circular machine about 4 feet in diameter, and 2 feet deep, having a perforated metal bottom. In this pan are two wheels which revolve on horizontal axles. Between these wheels and the bottom of the pan, which also revolves, the clay is ground; it then drops through the holes in the bottom of the pan to a wide belt, which passes above an inclined screen on which the clay falls. Such portions of the clay as are sufficiently ground pass through the screen on to another belt, while the coarser particles pass into the dry-pan to be ground over, and again carried to the screen for sifting.
The belt carries the finely ground clay to a mixing pan, which by constant agitation thoroughly mixes the particles.
The clay falls from the mixing pan into the hopper of the pressing machine, and thence into the molds. The loose clay fills evenly steel boxes the same width and length as the finished brick, but much deeper. Steel plungers, forced under great pressure into these boxes, compress the clay until the requsite thickness is obtained. The pressed brick is then pushed on a table, and from this the bricks are placed on a barrow or car and taken to the kiln.
Molded bricks are made in the same way, the difference being that the box is made to give the shape of the brick required. In most of the dry or pressed clay brick machines, a small jet of steam is admitted into the clay just before it enters the mold, in order to slightly moisten it, and cause the particles to cohere better.
Whenever the term pressed brick is used, it should mean the brick made by the dry process. There are many so called dressed or face brick, however, that are made by repressing soft-mud brick.