This method is extensively used for concentrating low-grade ores such as sulphides of copper and zinc. The finely crushed ores are thoroughly agitated with the help of a blast of air in presence of water and a little pine or eucalyptus oil or camphor oil. The ore is preferentially wetted by oil and the gangue by water. The gangues (rocky material) are settled down slowly to the bottom. A forth appears at the top of liquids which carries ore particles and is skimmed off. It is worth noticing that froth flotation process is just the reverse of gravity concentration. In forth flotation process heavy ore particles rise to the top instead of sinking as in gravity process.