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Draining tiles are made of burnt clay, like bricks and earthen-ware.
In general terms, the process is as follows:—The clay is mixed with sand, or other substances which give it the proper consistency, and is so wetted as to form a plastic mass, to which may be given any desired form, and which is sufficiently stiff to retain its shape. Properly prepared clay is forced through the aperture of a die of the shape of the outside of the tile, while a plug-held by a support in the rear of the die, -projects through the aperture, and gives the form to the bore of the tile. The shape of the material of the tile, as it comes from the die, corresponds to the open space, between the plug and the edge of the aperture. The clay is forced out in a continuous pipe, which is cut to the desired length by a wire, which is so thin as to pass through the mass without altering the shape of the pipe. The short lengths of pipe are dried in the air as , thoroughly as they can be, and are then burned in a kiln, similar to that used for pottery.
Materials, The range of earths which may be used in the manufacture of tiles is considerable, though clay is the basis of all of them. The best is, probably, the clay which is almost invariably found at the bottom of muck beds, as this is finer and more compact than that which is dug from dry land, and requires but little preparation. There is, also, a peculiar clay, found in some localities, which is almost like quick-sand in its nature, and which is excellent for tile-making, requiring no freezing, or washing to prepare it for the machine. As a general rule, any clay which will make good bricks will make tiles. When first taken from the ground, these clays are not usually adhesive, but become so on being moistened and kneaded.
It is especially important that no limestone pebbles be mixed with the clay, as the burning would change these to quicklime, which, in slaking, would destroy the tiles. The presence of a limey earth, however, mixed through the mass, is a positive advantage, as in this intimate admixture, the lime forms, under the heat of the kiln, a chemical combination with the other ingredients; and, as it melts more readily than some of them, it hastens the burning and makes it more complete. What is known as plastic clay, (one of the purest of the native clays.) is too strong for tile-making, and must be “tempered,” by having other substances mixed with it, to give it a stiffer quality.
The clay which is best for brick-making, contains Silica, and Alumina in about the following proportions:
Silica. . . . . . . . . . . . . . . . . . . . . . . . . 55 to 75 per cent.
Variable quantities of other materials are usually found in connection with the clay, in its native condition. The most common of these are the following: —
Magnesia....1 to 5 per cent.—sometimes 20 to 30 per cent.
“ware, and, therefore, allow its constituent substances to “combine in such a manner as to form a resisting body; “and this is performed with a temperature lower in pro“ portion as the necessary elements are more abundant.”
When the earth of the locality where tiles are to be made is not sufficiently strong for the purpose, and plastic clay can be cheaply obtained from a distance, a small quantity of this may be used to give strength and tenacity to the native material.
The compound must always contain a proper proportion of clay and sand. If too little clay is used, the mass will not be sufficiently tough to retain its compactness as it passes through the die of the tile machine; if too little sand, the moulded tiles will not be strong enough to bear handling, and they will crack and warp in drying and burning. Within the proper limits, the richer earths may be moulded much thinner, and tiles made from them may, consequently, be made lighter for transportation, without being too weak. The best materials for tempering stiff clays are sand, pounded brick or tile, or scoria, from smelting furnaces.
Preparation of Earths, The clay from which tiles are to be made, should be thrown out in the fall, (the upper and lower parts of the beds being well mixed in the operation,) and made into heaps on the surface, not more than about 3 feet square and 3 feet high. In this form, it is left exposed to the freezing and thawing of winter, which will aid very much in modifying its character, making it less lumpy and more easily workable. Any stones which may appear in the digging, should, of course, be removed, and most earths will be improved by being passed through a pair of heavy iron rollers, before they are piled up for the winter. The rollers should be made of cast iron, about 15 inches in diameter, and 30 inches long, and set as close
* Klippar's Land Drainage.
together as they can be, and still be revolved by the power of two horses. The grinding, by means of these rollers, may add 50 cents per thousand to the cost of the tiles, but it will greatly improve their quality. In the spring, the clay should be prepared for tempering, by the removal of such pebbles as it may still contain. The best way to do this is by “washing,” though, if there be only a few coarse pebbles, they may be removed by building the clay into a solid cone 2 or 3 feet high, and then paring it off into thin slices with a long knife having a handle at each end. This paring will discover any pebbles larger than a pea that may have remained in the clay. Washing is the process of mixing the clay with a considerable quantity of water, so as to form a thin paste, in which all stones and gravel will sink to the bottom; the liquid portion is then drawn off into shallow pits or vats, and allowed to settle, the clear water being finally removed by pumping or by evaporation, according to the need for haste. For washing small quantities of clay, a common mortar bed, such as is used by masons, will answer, if it be supplied with a gate for draining off the muddy water after the gravel has settled; but, if the work is at all extensive, a washing mill will be required. It may be made in the form of a circular trough, with scrapers for mixing the clay and water attached to a circular horse-sweep. “Another convenient mixing machine may be constructed “in the following manner: Take a large hollow log, of suit“able length, say five or six feet; hew out the inequalities “with an adz, and close up the ends with pieces of strong “plank, into which bearing have been cut to support a re“volving shaft. This shaft should be sufficiently thick to “permit being transfixed with wooden pins long enough to “reach within an inch or two of the sides of the log or “trough, and they should be so beveled as to form in their “aggregate shape an interrupted screw, having a direction “toward that end of the box where the mixed clay is de“signed to pass out. In order to effect the mixing more “thoroughly, these pins may be placed sufficiently far apart “to permit the interior of the box to be armed with other “pins extending toward the center, between which they “can easily move. The whole is placed either horizontally “ or vertically, and supplied with clay and water in proper “quantities, while the shaft is made to revolve by means of “a sweep, with horse power, running water or steam, as “the case may be. The clay is put into the end farthest “from the outlet, and is carried forward to it and mixed “by the motion, and mutual action and re-action of the pins “in the shaft and in the sides of the box. Iron pins may, “of course, be substituted for the wooden ones, and have “the advantage of greater durability and of greater strength “in proportion to their size, and the number may therefore “be greater in a machine of any given length. The fluid “mass of clay and water may be permitted to fall upon a “sieve or riddle, of heavy wire, and afterward be received “in a settling vat, of suitable size and construction, to drain “off the water and let the clay dry out sufficiently by sub“sequent evaporation. A machine of this construction “may be made of such a size that it may be put in motion “by hand, by means of a crank, and yet be capable of “mixing, if properly supplied, clay enough to mold 800 “or 1000 pieces of drain pipe per day.” Mr. Parkes, in a report to the Royal Agricultural So. ciety of England, in 1843, says: “It is requisite that the clay be well washed and sieved “before pugging, for the manufacture of these tiles, or the “operation of drawing them would be greatly impeded, by “having to remove stones from the small space surround. “ing the die, which determines the thickness of the pipe. “But it results from this necessary washing, that the sub.