The standard method used by the U.S. Bureau of Mines samplers is as follows: A space of 5 ft (1.52 m) in width should be cleared of dirt and powder from top to bottom of the seam being sampled. Down the center of this cleared space, a zone 1 ft (0.3 m) wide is cut to a depth of at least 1 in (2.54 cm) in order to get perfectly clean coal. A cut is then made up the center of this zone to a depth of 2 in (5.2 cm) and a width of 6 in (15.2 cm); or, if the coal is soft, to a depth of 3 in (7.6 cm) and a width of 4 in (10.2 cm). Approx. 5 to 6 lb (2.3 to 2.7 kg) of coal will be obtained for each foot (0.3 m) of thickness of the seam. This should include all bony coal included in the mining operation and exclude all slate or partings, which are thrown out during the operation. The sample obtained should be collected on a waterproof cloth 6 ft by 7 ft (1.83 m by 2.13 m) and then screened, the lumps being broken in a mortar, and all passed through a 1/2-in (12.7-mm) screen. Any 644 impurities, such as slate or pyrite, are crushed to 1/4 in (6.4 mm) or finer and thoroughly mixed with the coal. The coarser materials should be evenly distributed, the sample being then quartered, remixed, and requartered. When the mixing is complete, the sample should be placed in a can with the capacity of 3 lb (1.35 kg) and the top screwed on and sealed with adhesive tape. The can should be labeled with the name of the collector, the location, the date, and any other information necessary for the analysis.