Sluices have their maximum discharge when set straight. Increased grade may be given below any unavoidable curves with advantage, and the outer side of the sluice must always be raised. Steps or "drops" in the sluices help in the recovery of the gold. In general, a grade of 6-6-1/2 inches to the 12-foot box is found best; this is equal to a 4-4-1/2 per cent. grade. Exceptional instances are on record, however, where grades ran from 1-1/2 per cent. to 8 per cent. In a 4 to 7 per cent. grade the water in the sluice should be 10 inches deep at least. The following table gives useful details: Sluice. Grade. Water. 6 ft. × 36 in. 4 to 5 p.c. 2,000 to 3,500 m. in. 4 ft. × 30 in. 4 p.c. 1,800 to 2,000 m. in. 3 ft. × 30 in. 1-1/2 p.c. 600 to 1,000 m. in. "The longer the better," is the sluice-builder's motto. The best "riffles" are made of blocks of pine 8 to 13 inches deep, wedged into the bottom of the sluices. They are laid in rows separated by a space of an inch or an inch and a half. Riffle strips keep them in position, these latter being laid crosswise on the bottom. When worn down to five inches, the blocks should be replaced. This amount of wear will probably require six months. Stone and longitudinal riffles running lengthwise of the box are often preferred. An undercurrent is a broad sluice set at a heavy grade below the level of the main sluice. The fine stuff drops through a grating, while the coarse gravel continues on down the sluice. Refuse material from quartz, hydraulic or other mines is known as tailings. Tailings are deposited on a dump, which in the case of a hydraulic claim must be sufficiently spacious to receive the thousands of yards of debris deposited on it each day. When available a narrow, deep canyon, or a tunnel, may take the places of dumps. Quicksilver is used in the sluices, 14 to 18 flasks being used every fortnight in a long sluice. It is not placed in the last 300 or 400 feet. In working, keep the face of the bank "square." Washing should be carried on continuously. Watches must be set over the sluices, or gold is likely to be missed. As an extra precaution, the sluices should be run full of gravel before shutting off the water. There is no fixed custom regulating "clean ups." Some managers do so every 20 days, others run two or three months, others again clean up but once in a season. In large operations, the first 2,000 feet of sluice are cleaned up every fortnight; the remaining boxes once a year. Sluices are cleaned from the head downward, the blocks being taken up for that purpose. The amalgam of gold and quicksilver is collected in sheet iron buckets. The final step is reached when the amalgam is retorted and melted in a graphite crucible.