The term is used to describe the separation of two mutually insoluble liquids, usually oil and water, while removing any solids present at the same time. Separation can be achieved by gravity or by centrifugal force. A liquid mixture in a stationary settling tank clears slowly as a heavier liquid and solids sink to the bottom under influence of gravity. In a rapidly rotating bowl, the force of gravity is replaced by centrifugal force which can be thousands of times greater. Separation is continuous and happens very quickly. The greater the density differences between the two liquids, the easier the separation.