36 Refining Used Lubricating Oils
crystal structure becomes sufciently complete, the oil will no longer ow under the
conditions of the test. However, because mechanical agitation can break up the wax
structure, it is possible to have an oil ow at temperatures considerably below its
pour point. Cooling rates also affect wax crystallization; it is possible to cool an oil
rapidly to a temperature below its pour point and still have it ow.
Although the pour point of most oils is related to the crystallization of wax, cer-
tain oils, which are essentially wax-free, have viscosity-limited pour points. In these
oils, the viscosity becomes progressively higher as the temperature is lowered until
at some temperature no ow can be observed. The pour ...