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four or five feet high, (which will secure them against further warping,) and left until they are dry enough for burning,—that is, as dry as they can be made by exposure to the air.
Burning.—Tiles are burned in kilns in which, by the effect of flame acting directly upon them, they are raised to a heat sufficient to melt some of their more easily fusible ingredients, and give to them a stone-like hardness.
Kilns are of various construction and of various sizes. As this book is not intended for the instruction of those who are engaged in the general manufacture of tiles, only for those who may find it necessary to establish local works, it will be sufficient to describe a temporary earthen kiln which may be cheaply built, and which will answer an excellent purpose, where only 100,000 or 200,000 tiles per season will be requiredDirections for its construction are set forth in a letter from Mr. T. Law Hodges, of England, to the late Earl Spencer, published in the Journal of the Royal Agricultural Society for the year 1843, as follows:
"The form of the clay-kiln is circular, 11 feet in diame"ter, and 7 feet high. It is wholly built of damp, clayey "earth, rammed firmly together, and plastered, inside and "out, with loam (clay ?). The earth to form the walls is dug "out around the base, leaving a circular trench about four "feet wide and as many deep, into which the fire-holes of "the kiln open. If wood be the fuel used, three fire-holes "will be sufficient; if coal, four will be needed. About "1,200 common brick will be wanted to build these fire"holes and flues; if coal is used, rather fewer bricks will "be wanted, but, then, some iron bars are necessary,— "six bars to each fire-hole.
"The earthen walls are four feet thick at the floor of "the kiln, seven feet high, and tapering to a thickness oi "two feet at the top; this will determine the slope of tut "exterior face of the kiln. The inside of the wall is car* "ried up perpendicularly, and the loam plastering inside "becomes, after the first burning, like a brick wall. The "kiln may be safely erected in March, or whenever the "danger of injury from frost is over. After the summer "use of it, it must be protected, by faggots or litter, "against the wet and frost of winter. A kiln of these "dimensions will contain 32,500 l^iach tiles, * * * "or 12,000 2£-inch tiles. * * *
"In good weather, this kiln can be filled, burnt, and "discharged once in every fortnight, and fifteen kilns "may be obtained in a good season, producing 487,500 "1^-inch tiles, and in proportion for the other sizes.
"It requires 2 tons 5 cwt. of good coals to burn the "above kiln, full of tiles."
A sectional view of this kiln is shown in Fig. 46, in which C, C represent sections of the outer trench; A, one
of the three fire-holes; and B, _B, sections of a circular passage inside of the wall, connected with the fire-holes, and serving as a flue for the flames, which, at suitable intervals, pass through openings into the floor of the kiln. The whole structure should be covered with a roof of rough boards, placed high enough to be out of the reach of the fire. A door in the side of the kiln serves for put
ting in and removing the tiles, and is built up, temporarily, with bricks or clay, during the burning. Mr. Hodges estimates the cost of this kiln, all complete, at less than $25. Concerning its value, he wrote another letter in 1848, from which the following is extracted:
"The experience of four years that have elasped since "my letter to the late Earl Spencer, published in the 5th "volume of the proceedings of the Royal Agricultural "Society, page 57, has thoroughly tested the merits of "the temporary clay-kilns for the burning of draining"pipes described in that letter.
"I am well aware that there were persons, even among "those who came to see it, who pronounced at once upon "the construction and duration of the kiln as unworthy "of attention. How far their expectations have been real"ized, and what value belongs to their judgment, the fol "lowing short statement will exhibit:
"The kiln, in question, was constructed, in 1844, at a u cost of £5.
"It was used four times in that year, burning each "time between 18,000 and 19,000 draining pipes, of lj "inches in diameter.
"In 1845, it was used nine times, or about once a fortw night, burning each time the same quantity of nearly "19,000 pipes.
u In 1846, the same result.
"In 1847, it has- been used twelve times, always burn"ing the same quantity. In the course of the last year a "trifling repair in the bottom of the kiln, costing rather "less than 10 shillings, was necessary, and this is the only "cost for repair since its erection. It is now as good as "ever, and might be worked at least once a fortnight "through the ensuing season.
"The result of this experiment of four years shows not "only the practical value of this cheap kiln, but Mr M Hatcher, who superintends the brick and tile-yard at Ben "enden, where this kiln stands, expresses himself strongly "in favor of this kiln, as always producing better and "more evenly burned pipes than either of his larger and "better built brick-kilns can do."
The floor of the kiln is first covered with bricks, placed Od end, at a little distance from each other, so as to allow the fire to pass between them, and the tiles are placed on end on these. This position will afford the best draft for the flames. After the kiln is packed full, the door-way is built up, and a slow fire is started,—only enough at first to complete the drying of the tiles, and to do this so slowly as not to warp them out of shape. They will be thoroughly dry when the smoke from the top of the kiln loses its dark color and becomes transparent. When the fires are well started, the mouths of the fire-holes may be built up so as to leave only sufficient room to put in fresh fuel, and if the wind is high, the fire-holes, on the side against which it blows, should be sheltered by some sort of screen which will counteract its influence, and keep up an even heat on all sides.
The time required for burning will be from two days and a night to four days and four nights, according to the dryness of the tiles, the state of the weather, and the character of the fuel. The fires should be drawn when the tiles in the hottest part of the kiln are burned to a " ringing" hardness. By leaving two or three holes in the door-way. which can be stopped with loose brick, a rod may be run ii, from time to time, to take out specimen tiles from the hottest part of the kiln, which shall have been so placed as to be easily removed. The best plan, however,—the only rrudent plan, in fact,—will be to employ an intelligent man who is thoroughly experienced in the burning of brick arid pottery, and whose judgment in the management of the fires, and in the cooling off of the kiln, wili save much of the waste that would result from inexperienced management. After the burning is completed, from 40 to 60 hours must be allowed for the cooling of the kiln before it is opened. If the cold air is admitted while it is still very hot, the unequal contraction of the material will cause the tiles to crack, and a large portion of them may be destroyed.
If any of the tiles are too much burned, they will be melted, and may stick together, or, at least, have their shape destroyed. Those which are not sufficiently burned would not withstand the action of the water in the soil, and should not be used. For the first of these accidents there is no remedy; for the latter, reburning will be necessary, and under-done tiles may be left, (or replaced,) in the kiln in the position which they occupied at the first burning, and the second heat will probably prove sufficient. There is less danger of unequal burning in circular than in square kilns. Soft wood is better than hard, as making a better flame. It should be split fine, and well seasoned.
Arrangement of the Tilery.—Such a tilery as is described above should have a drying shed from 60 to 80 feet long, and from 12 to 18 feet wide. This shed may be built in the cheapest and roughest manner, the roof being covered with felting, thatch, or hemlock boards, as economy may suggest. It should have a tier of drying shelves, (made of slats rather than of boards,) running the whole length of each side. A narrow, wooden tram-way, down the middle, to carry a car, by which the green tiles may he taken from the machine to the shelves, and the dry ones from the shelves to the kiln, will greatly lessen the eost of handling.
The pug-mill and tile-machine, as well as the clay pit and the washing-mill, should be at one end of the shed, and the kiln at the other, so that, even in rainy weathei the work may proceed without interruption. A shed of the size named will be sufficient to dry as many tiles of