Refining is the process of purification of a (1) substance or a (2) form. The term is usually used of a natural resource that is almost in a usable form, but which is more useful in its pure form.1 For instance, most types of natural petroleum will burn straight from the ground, but it will burn poorly and quickly clog an engine with residues and by-products. The term is broad, and may include more drastic transformations, such as the reduction of ore to metal (for which see Refining (metallurgy)).
The refining of liquids is often accomplished by distillation or fractionation; this process is useful, for example, for isolating different fractions of petroleum. Gases can be refined in this way as well, by being cooled and/or compressed until they liquefy. Gases and liquids can also be refined by extraction with a selective solvent that dissolves away either the substance of interest, or the unwanted impurities.
Many can be refined by growing in a of the impure material; the regular structure of the crystal tends to favor the desired material and exclude other kinds of particles.
