Essential oils are the volatile oily components of aromatic plants, trees and grasses. They are found in tiny glands located in the flowers (neroli), leaves (eucalyptus), roots (calamus), wood (sandal) and resins (frankincense). Essential oils are extracted by four main methods: steam distillation, expression, solvent extraction and efleurage. In the first method the oil is extracted by the action of hot steam and then selectively condensed with water from which it is separated. In the second method the oil is extracted by pressure or centrifugation. In the third method the oil is dissolved in a volatile solvent that when evaporated leaves a heavily natural wax substance called concrete. When separated from the wax, the resulting liquid is called an absolute, the most concentrated from of aroma available. Efleurage is a longer process involving the dissolution of the oils in animal fat and its separation using alcohol. Although essential oils main usage is in cosmetics and perfumery, many of them do have proved therapeutic properties.