Placer Mining

a method of collecting grains of gold or other precious metals. Placer mining requires an abundance of water and an abundance of sand, gravel, or earth, containing particles of gold. The sand may be shaken in a pan of water or rocked in a miner's cradle until the precious grains settle to the bottom. Millions of dollars of gold have been taken from the banks of streams in this way. Under typical conditions, however, water is obtained from an elevated source--some distant mountain stream--and is allowed to play through a huge nozzle against the goldbearing bank of gravel or sand. The earth melts away before the water and runs off in a muddy stream through a long, slightly inclined flume or trough. Cleats are nailed at intervals across the bottom of the trough. A pool of mercury is poured into the trough above each cleat. The particles of gold, however small, are heavy. They settle to the bottom of the trough and roll along till they reach mercury, with which they form an amalgam. After driving the muddy current through the flume a while, the operator finds a bar of amalgam above each cleat. These bars he removes, replacing them by fresh pools of mercury. No matter how rich the soil may be, this kind of mining is out of the question where water cannot be had. Claims have been put forth for a machine that works dry soil. By grinding, elevating, sifting, and blowing, somewhat after the plan of a fanning mill, it is said the gold particles can be collected in the dry much more completely than by the use of water. According to a test described by a scientific journal, ninety-six particles of gold so fine as to lie within a capital "O" were mixed with 200 pounds of clean sand. The whole was put through the mill. Ninety-five pieces of the gold were recovered. Eight cubic yards of earth may be winnowed per hour. See Gold; Mercury; Amalgam