Distillation of perfume ingredients from their natural sources can be done in three ways: dry distillation, steam distillation or hydrodiffusion. Dry distillation involves high temperatures, since heat (or direct flame) is applied to the surface of the vessel containing the plant material. Usually this technique is reserved for the oils of highest boiling point, typically those derived from wood, because the high temperatures are necessary to vaporize their chemical components. Cade and birch tar are the major oils obtained by dry distillation.
In steam distillation, water or steam is added to the still pot and the oils are co-distilled with the steam. The oil is separated from the water by means of a Florentine flask, which separates them based on their differing densities. The presence of water in the pot during steam distillation limits the temperature of the process to 100°C. This means that much less degradation occurs in this process than dry distillation. However, some degradation does occur. For example, tertiary alcohols present in the plant often dehydrate in the pot and distil as the corresponding hydrocarbons.
Hydrodiffusion is a relatively new technique, and is essentially a form of steam distillation. However, it is steam distillation carried out upside down since the steam is introduced at the top of the pot and the water and oil taken off as liquids at the bottom.
Perfume materials obtained this way are referred to as essential oils, e.g. the oil obtained by steam distillation of lavender is known as the essential oil of lavender, or lavender oil.