There will always be a wide difference of opinion as to what kind of machinery is best suited to the purpose of preparing clays and mixtures of clays and rendering them available for the use of the terra-cotta maker.
It ought not to be expected by any practical clay-worker that any one machine should prove itself of universal capacity and fitted to manipulate every description of clay.
There are various and sufficient reasons for this difference of opinion : First, clays differ very much in particular qualities : some are clean, others are stony ; some are tenacious and strong, others are loamy and friable ; some are from deep alluvial de posits and are plastic when dug, while others are from rocky formations and require to be blasted when being mined, because they are in a condition resembling soft stone.
Second,•the demands of the product are as varied as are the qualities of clay ; we are only considering clays for terra-cotta work and brickmaking, exclusive of those used in the making of Parian, porcelain, china, and granite bodies.
Clay that has stones in it may be used by the maker of com mon brick if the stones are not of a limestone formation, in which case they would be changed into lime in the process of burning, and when the air or water got access to them they would burst and spoil the brick. Or, if rollers or crushers are used, they may be so reduced that they become less objection able.
The maker of facing or moulded brick must gtt rid of all stones from his clay, should any exist ; therefore he has to look for a better and cleaner clay. He can expel them by the use of a separator, or if they are of a quartz nature he may use a pulverizer and reduce them to power, in which case they may do more good than harm.
But the terra-cotta maker cannot afford to risk his product by any faults in his crude clays. He must have as the founda- 7 tion of his stock a clean, tough clay, free from stone and loam, so that he may add to it his own proportion of grit or sand, etc. For this reason he seeks for and obtains the clays specially and best suited to his purpose.
This may seem to be an expensive method, but experience will prove that it is the truest economy. It always pays to begin
right. He must have clays of different colors ; some he must have brought to him in any case ; therefore he can afford to pur chase and freight a selection of clays, which will give him the best results after he has expended his labor upon them.
This the brickmaker cannot afford to do, for self-evident reasons.
It requires about four (4) tons of clay to make one thousand brick ; these when burned will weigh about two-and-a-half (2 tons. These, if common brick, will sell for from five to eleven dollars, according to the market and the quality. If the bricks are facing or moulded bricks, they will sell for from fif teen to eighty dollars per thousand, according to the quality, color, and design. Thus, brick clay, when manufactured into common brick, will produce only from two dollars to three dol lars and seventy-five cents per ton, while even the better grades of facing brick will produce only from five to sixteen dollars per ton ; and even the moulded brick will sell only for from sixteen to thirty dollars per ton. But clay, when manufactured into terra-cotta, increases in value far more rapidly in propor tion to its weight. Four tons of clay will produce three tons of terra-cotta, and the value of terra-cotta-ranges fron fifty dollars for the commoner shapes up to two hundred for the higher grades of architectural work, and the average value is about ninety dollars, or more than three times the value of the best grades of moulded bricks. This difference in value in the pro duct permits the above-described selection of clay on the part of the terra-cotta maker, and it at the same time makes his selection of machinery very simple, and reduces the cost or ex pense of plant and motive power.
The machines required to furnish a very effective plant are as follows : 1st. A clod-crusher and stone-separator.
2d. A stamp-mill for breaking grit.
3d. A bur-mill for grinding grit.
4th. A clay mixing pug-mill.
5th, A good set of tempering pug-mills.