Refining is the process of purification of a substance. The term is usually use of a natural resource that is almost in a usable form, but which is more useful in its pure form. 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 byproducts. The term is broad, and may include more drastic transformations, such as the reduction of ore to metal (for which see Refining (metallurgy)).