It is an intimate mixture of oil and water, generally of a milky or cloudy appearance. Emulsions may be of two types: oil-in-water (where water is the continuous phase) and water-in-oil (where water is the discontinuous phase). Oil-in-water emulsions are used as cutting fluids because of the need for the cooling effect of the water. Water-in-oil emulsions are used where the oil, not the water, must contact a surface - as in rust preventives, non-flammable hydraulic fluids, and compounded steam cylinder oils, such emulsions are sometimes referred to as inverse emulsions. Emulsions are produced by adding an emulsifier. Emulsibility is not a desirable characteristic in certain lubricating oils, such as crankcase or turbine oils, that must separate from water readily. Unwanted emulsification can occur as a result of oxidation products - which are usually polar compounds - or other contaminants in the oil. See illustration of an oil-in-water emulsion at polar compound.