As an ore comes from the mine it is desirable to eliminate at once the gangue. This may be done more or less successfully with some ores by simple hand-picking methods, while other ores must be crushed in a stone breaker or stamp mill, and the gangue eliminated from the finely crushed mass by dressing, which consists of passing running water over the mass in a succession of boxes so that the current will carry the lighter particles to the lower boxes while the heavier particles settle in the upper boxes. There are ores from which the gangue cannot be removed except in furnace processes, and some ores have too little gangue to need preliminary separation.
Some ores are subjected to weathering. They are heaped in the open air and left exposed to sun and rain for weeks or months.
This disintegrates the lumps and washes away soluble salts and much powdered gangue.