All the machines mentioned above are used on dry or nearly dry clay, but there are several other types which are employed for wet clays only, and these in addition to breaking up the clay may also be used to mix it. The process is sometimes termed tempering. Soak pits. These are the simplest of the different types of machinery used for tempering, and are employed at a number of common brickyards. Their construction is simple, consisting merely of a pit lined with planking and usually set immediately behind the molding machine. The clay or mixture of clays, with possibly some sand, is dumped into the pit, water poured on, and the whole allowed to soak over night. This process softens the clay, but does no mixing, which is done entirely within the molding machine. The soaked clay is shoveled directly into the machine. Ring pits.-Ring pits (Pl. XXV, Figs. 1 and 2) are employed at many yards where common brick are manufactured and give much better results than soak pits, for the clay receives a more thorough mixing. They are of circular form, 20 to 25 feet in diameter, about 3 feet deep and lined with boards or brick. Revolving in this pit is an iron wheel, 6 feet in diameter and geared so as to travel around the pit, and at the same time move back and forth between the centre and circumference, thus thoroughly mixing the mass. Before starting the wheel, the mixture of clays or clay and sand is dumped into the pit and sufficient water added. The tempering is accomplished usually in 5 or 6 hours, and one pit commonly holds enough clay for 25,000 to 30,000 brick. Two ring pits are often operated in conjunction with one molding machine, so that while the clay is being shoveled out of one pit, the second is tempering the clay for the next day's supply. Ring pits are cheaper than pug mills, but have a lower capacity and require more room. They are operated by either steam or horsepower. Pug mills. These are semicylindrical troughs, varying in length from 3 to 14 feet, with 6 feet as a fair average. In this trough there revolves a horizontal shaft, bearing knives set spirally: around it and having a variable pitch. The clay and water are 15 CL G charged at one end, and the blades on the shaft not only cut up the clay lumps, but mix the mass, at the same time pushing it towards the discharge end. The speed of the clay through the machine varies with the angle of the blades on the shaft. It will be seen at once that the thoroughness of the mixing depends to a large degree on the length of the pug mill, as well as the angle at which the blades are set. For some clays, therefore, it is very inadvisable to use a short mill, one not more than 3 or 4 feet long. Pug mills are thorough and continuous in their action, take up less space than ring pits and do not require much power to operate. They are nearly always used in connection with stiffmud machines, and at the present day are often used with the soft-mud process. Wet pans. These are similar to dry pans, but differ from them in having a solid bottom. The material and water are put into the pan and the clay is crushed and tempered at the same time. Where the clay contains hard lumps of limonite or pyrite nodules, a wet pan is superior to a pug mill or disintegrator, for the charge is crushed and tempered in a few minutes, and can then be replaced by another one. MOLDING. Bricks are molded by one of four methods, viz., soft-mud, stiff-mud, dry-press, and semidry-press, although in reality there is not much difference between the last two methods. Soft-mud process. In this method the clay, or clay and sand, are mixed with water to the consistency of a soft mud or paste and pressed into wooden molds. Since, however, the wet clay is sticky and likely to adhere to a wooden surface, the molds are sanded each time before being filled. Soft-mud bricks, therefore, show five sanded surfaces, and the sixth surface will be somewhat rough, due to the excess of clay being wiped off even with the top of the mold. They are also slightly convex on one side and slightly concave on the other, due to the sides of the soft brick dragging slightly as it is dumped from the mold to the drying floor. Soft-mud bricks are molded either by hand or in machines. In hand molding, the clay is tempered quite soft. A lump more than sufficient to fill the mold is taken and forced into the wooden mold by the molder, who then, by means of a wire, cuts off the excess of clay from the top of the mold box. The latter is then turned over on the drying floor and the brick dumped out (Pl. XXVI, Fig. 1). With such an outfit one man is able to mold about 2,500 bricks in ten hours. Hand molding is in most cases confined to small yards, where the production is small and the capital invested of corresponding size. The hand-molded bricks are usually more porous than the machine-molded variety. The soft-mud machine consists usually of an upright box of wood or iron, in which there revolves a vertical shaft, bearing several blades or arms. Attached to the bottom of the shaft is a curved arm, which forces the clay into the press box. The molds, after being sanded, are shoved underneath the press box from the rear side of the machine. Each mold has six divisions, and as it comes under the press box the plunger descends and forces the soft clay into it. The filled mold is then pushed forward automatically upon the delivery table, while an empty one moves into its place. As soon as the mold is delivered its upper surface is "struck" off by means of an iron scraper. Under favorable conditions soft-mud machines have a capacity of about 40,000 brick per day of ten hours, although they rarely attain this. Four men are commonly required to tend a machine which is operated by steam or horsepower. The molds are sometimes sanded by hand, but more frequently by a machine consisting of a barrel on which the molds are fastened to form the sides. Sand is then put inside and as the barrel revolves on its horizontal axis, the sand falls into the compartments of the molds. As soon as one mold is removed, another requiring sanding is put into its place. The soft-mud process was the first method of molding employed, and is still largely used at many localities. It is adaptable to a wider range of clays than any of the others, and possesses the advantage of not only producing a brick of very homogeneous structure, but one that is rarely affected by frost action. Its cost is small, but the capacity is limited, as compared with a good stiff-mud machine, and a large number of men are also required. It does not produce a product with smooth faces and sharp edges, but this defect can be overcome by re-pressing the product. Stiff-mud process. In this method of molding (Pl. XXVI, Fig. 2), the clay is tempered with less water and consequently is much stiffer. The principle of the process consists in taking the clay thus prepared and forcing it through a die in the form of a rectangular bar, which is then cut up into bricks. The machine now most used is known as the "auger" type, the "plunger" type having nearly disappeared from use. Its general form is that of a cylinder closed at one end, but at the other end tapering off into a rectangular die, whose cross section is the same as either the end or the largest side of a brick. Within this cylinder, which is set in a horizontal position, there is a shaft, carrying blades similar to those of a pug mill, but at the end of the shaft nearest the die there is a tapering screw. The internal shape of the die is variable, depending on the make of the machine. It is heated by steam or lubricated by oil on its inner side, in order to facilitate the flow of the clay through it. The tempered clay is charged into the cylinder at the end farthest from the die, is mixed up by the revolving blades, and at the same time it is moved forward until seized by the screw and pushed through the die. Since this involves considerable power, it results in a marked compression of the clay. With such conditions, there will naturally be more or less friction between the sides of the bar and the interior of the die, causing the centre of the stream of clay to move faster than the outer portion. Much attention has, therefore, been given to the construction of the die, in order to overcome this and facilitate the flow of the clay as much as possible. In case the amount of friction between die surface and clay is greater than the cohesion in the plastic mass, the bar of clay is likely to tear on the edges, producing serrations like the teeth of a saw. The effect of the screw at the end of the shaft, together with the differential velocities within the stream of clay, also produces a laminated structure in the brick, which is often greatest in highly plastic clays, but is sometimes marked in clays of only moderate plasticity when machines |