Buhr

or Burrstone, a variety of quartz distinguished by flinty hardness and numerous small air cavities. Buhr of excellent quality is found in the quarries of western Pennsylvania and adjacent parts of Ohio, but the French buhr is by far the best. Cut into wedge-shaped pieces and bound together by heavy metal hoops, buhr is the material of which millstones are made. Before the invention of the steel roller process the flour and meal of the world was ground between upper and nether millstones of buhr. Many millers have not discarded millstones yet. With use, millstones become too smooth to grind and must be roughed by the use of a hammer or pick with a steel edge shaped like that of a chisel. Picking millstones must be done by a uniform tap-tap and is an art in itself. See Flour