True essential oils are those that are extracted through steam distillation. This is where the plant material is placed on a grid with hot water underneath. The steam draws out the essential oils from the plant material. These tiny molecules evaporate with the water and then go through a cooling system where they end up back in liquid form. Here the essential oils are either floating on top of the water or settled on the bottom depending on the density of the oil. Then the essential oils are separated from the water, leaving the floral waters, called hydrosols, which still retain some of the therapeutic properties.
Citrus oils are extracted through expression. Here, the rind of the fruit is pressed between two blocks of wood with one side having a sponge. The little sacs are squeezed so that the essential oils saturate the sponge. The essential oil- or essence is then squeezed out of the sponge. Often rind particles find their way into the essential oils and it is not uncommon to see floating bits or sediment. When this process is used, the proper name for the resulting liquid is an essence; however, most people don’t know this and just refer to them as essential oils.
Essences are very vaporous and will evaporate quickly if left uncapped or in the heat.
Resins are the solid or semi-solid substances exuded from the bark of trees or bushes when wounded – also known as sap. The gum-like substance produced does not exist in the tree beforehand, but is produced pathologically, solely as a result of the incision, and hardens on exposure to air. Various solvents can be used to extract the aromatic molecules from the resins; the most frequently used being the hydrocarbons (e.g. benzene, hexane) or alcohols-each extracting different molecules. The solvents are filtered off and afterwards removed by distillation to leave either resinoids (from hydrocarbon solvents) or absolute resins (from alcohol solvents).
The extraction of concretes is similar to that of resinoids; hydrocarbons are used as solvents. For concretes however, plant material (leaves, flowers, roots, etc.) is used instead of resin – this is the main difference. Most concretes are solid wax-like substances and are often used in food flavourings.
An absolute is prepared from a concrete by adding an alcohol to extract the aromatic (alcohol-soluble) molecules. The alcohol is then evaporated off gently under a vacuum, leaving the absolute, a thick, coloured liquid. It is usually rare and very delicate flower petals that this method is used for because steam distillation is too harsh and no essential oils can usually be derived from such delicate petals. Absolute of jasmine, tuberose and vanilla are achieved using this process
Absolutes and resins are much used in the perfumery world, and although they can be useful in some applications of aromatherapy, it must be appreciated that they always retain a small percentage of the solvents used in their reduction. Luckily, very small quantities of the absolute are used for each aromatherapy application so the risk is negligible.