Page:The American Cyclopædia (1879) Volume III.djvu/273

BRICK 267 covered with boards, as they are in danger of being washed away. The drying should be thorough, or the bricks will be likely to crack in baking. After depositing the bricks on the drying floor, the moulds are taken back, are dipped in water, and then into sand, and are ready to be refilled. When well dried the bricks are removed to be baked. This is effect- ed in England sometimes in permanent kilns, which hold as many as 20,000 bricks, and are filled and emptied like those for baking earthen ware, the burning being completed in about 48 hours. The method in common use in this country of piling the green bricks upon one another to make their own kiln is also adopted there ; but the arrangement is called a clamp instead of a kiln. By this method halt' a mil- lion bricks, or even a million, are burned in one operation. A central double wall is built lengthwise along the kiln, its lower por- tion of bricks already baked, and on each side are parallel longitudinal fire flues built of un- burnt brick, laid very open ; over them the great body of brick is piled after an exact sys- tem, vertical flues ascending to the top, and the whole work being laid in an open manner for the free circulation of the gases. The fires are made in one end of the flues, and the heat is increased by the combustion of the small coal which was scattered throughout the heap as it was built up. The top and sides of these clamps are usually built of bricks that have been already once baked. The underbaked bricks of previ- ous firings may thus be conveniently finished. Over the whole a covering of loam is some- times laid to prevent the fire from burning too rapidly ; and screens of wood or other ma- terial are used to protect portions against the wind. The time required to burn a kiln varies with its extent and the manner in which it is fired. The English accounts state : "So very slow is the progress that bricks in the neigh- borhood of London take about three months in the burning." The time formerly required on the Hudson river for burning the great clamps of from 300,000 to 1,000,000 bricks was about two weeks, and the consumption of oak wood was about 40 cords to 100,000 bricks. The quantity usually regarded as sufficient is, how- ever, 33^ cords. After the introduction of anthracite dust in the clay, the time of burning was reduced to three or four days, and the con- sumption of fuel to 16 cords to 100,000 bricks. The mere expenses of burning this number of bricks are rated at $80 for 16 cords of wood, S3 for 75 bushels of anthracite dust, and $6 for 4 days' attendance ; total, $89. The prep- aration of the clay, moulding, drying, build- ing up of the kiln, waste, &c., make all together a larger amount than the burning. As the bricks in a clamp are exposed to great differ- ences of temperature, they are found of various qualities when the process of burning is com- pleted. Those near the flues are partially vit- rified and melted together. Many are slightly fused on the surface,-and baked to a stony hard- ness. These are called clinker bricks, and are used in situations where they will be exposed to the weather or to rough wear. The soft bricks are selected to be laid for work in sheltered sit- uations. The very slackest baked are returned to the next kiln. The annular furnace for burning brick, invented by Friedrich Hoffmann of Berlin, is extensively used in Europe, and is said to possess great efficiency and economy. The following description and illustration are taken from Dr. Barnard's report of the Paris exposition of 1867. A large annular chamber, with openings at the sides for the reception of the bricks, is constructed with a central chim- ney, and with removable divisions for separa- FIB. 1. Ilofftaann's Annular Brick Furnace. ting the annulus into different parts. When the furnace is filled with unburnt bricks heat is applied to one division, the smoke and hot air escaping into the adjoining one, which is the next to be burnt, the air for maintaining the combustion being received through the compartment last burned, whereby the bricks in it are cooled. Each compartment of bricks or other articles is thus burned in turn, the waste heat of the burning compartment contin- ually drying the compartment before it, and tak- ing all the heat of the one behind. The letters a a, fig. 1, mark the circular arched furnace, furnished with doors, 5 J. Flues, c e, lead to the circular chamber e e, surrounding the chimney,