Buhrstone

Buhrstone, a variety of Quartz (q.v.), containing many small empty cells, which give it a peculiar roughness of surface, particularly adapting it for millstones. The name is given without reference to geological relations, but it is vein quartz, rather than true quartz rock, which ordinarily assumes the character of buhrstone. There are different varieties of buhrstone, some of which are more compact, or have smaller cells than others; and those in which the cells are small and very regularly distributed, about equal in diameter to the spaces between them, the stone being also as hard as rock-crystal, are most esteemed. Good buhrstone is found at Conway in Wales, and at several places in Scotland, as well as near Cologne in Germany. Buhrstones are quarried in New York, Pennsylvania, and North Carolina. But the finest millstones are obtained from the quarries of La Ferté-sous-Jouarre, in the department of Seine-et-Marne, near Paris. It is not unusual to form millstones of pieces of buhrstone cut into parallelopipeds, like great wedges of soap, and bound together by iron hoops. The stone is found in beds or in detached masses, and the mode of quarrying is peculiar. When the mass is large, it is cut out into the form of a huge cylinder; around this grooves are cut, at distances of about 18 inches, the intended thickness of the millstones; into these grooves wooden wedges are driven, and water is thrown upon the wedges, which, causing the wood to swell, splits the cylinder into the slices required.—Millstones are sometimes made of siliceous grit-stones, of sandstone, and even of granite (see MILL). Buhrstone millstones are extremely durable.