Cutting fluids degrade over time due to contaminants entering the lubrication system. A common type of degradation is the formation of "tramp oil", also known as "sump oil", which is unwanted oil that has mixed with cutting fluid. It originates as lubrication oil that seeps out from the slideways and washes into the coolant mixture, as the protective film with which a steel supplier coats bar stock to prevent rusting, or as hydraulic oil leaks. In extreme cases it can be seen as a film or skin on the surface of the coolant or as floating drops of oil.