Distillation is the main method by which essential oils are extracted from plants. Indeed, according to some authorities, it is the only method that produces essential oils as correctly defined - those obtained by other methods being known as essences or absolutes.
Distillation involves heating the plant material, either by placing it in water, which is then brought to the boil, or placing the plant material on a rack or grid and heating the water beneath it, so that steam passes up through it. Leaves, twigs, berries, petals and other parts of the plant may be used. If the plant material is placed in the water, the process is known as direct distillation, and if it is put on a grid and the steam passed through it, the system is known as steam distillation.
In either method, the heat and steam cause the walls of the specialised plant cells in which the plant essence is stored, to break down and release the essence in the form of a vapour. This vapour, together with the steam involved in the distilling process, is gathered into a pipe which passes through cooling tanks, and this causes the mixed vapours to return to liquid form so that they can be collected in vats at the end of the process. The steam condenses into a watery distillate, while the essence from the plant becomes an essential oil. This, being lighter than water, collects in the upper part of the vats and can easily be separated from the watery part. In some cases, the watery distillate is also a valuable product, and is sold as a flower-water or herbal water. In France these distillates are usually described as a hydrolat.
With one or two plants, the amount of essential oil that can be obtained by distilling is insignificant, and is regarded as a byproduct of the production of rosewater or orange-flower water, for example. Other methods, such as enfleurage or solvent extraction are used to obtain the essences from these and other delicate flower petals.
The process of distillation has been known and used for obtaining essential oils since at least the 10th Century A.D. and is thought to have originated in Persia, where the oils were highly prized as perfumes (Shakespeare's 'perfumes of Arabia'). However, recent archaeological digs in Italy have uncovered simple stills which suggest the Romans already knew this technique.
Some of the stills in use today, especially in less developed countries, and at small-scale rural distilleries in Europe, differ very little from the earliest stills known, but in areas where essential oil production is an important industry, they may be very large and complex, though the basic principles of production involved are identical. Stainless steel is often used in the construction of modern stills to avoid any contamination of the distillate, and this may produce better quality oils, though it is non-proven.