In the distillation of Australian tea tree oil, for instance, a common bush-still consists of a 1,600-litre (115-gallon) capacity tank with a removable or hinged lid capable of being sealed to make the container steam-tight. A grid is fitted in the tank about 30 cm (1 ft) above the bottom to support the closely packed leaf (called the charge) and allow an even passage of steam through it. If steam is generated in the still itself (sometimes it is supplied by a separate boiler or steam generator), a constant level of water is maintained in the bottom and a fire built underneath it. An outlet at the top of the still carries the mixture of steam and oil vapour to the condenser, where the steam condenses back to water, and the oil vapour condenses as well. Because they are not water soluble, the oils separate and collect on the surface of the water when cool, and can be collected quite easily. That 1,600-1itre tank holds half a metric tonne of fresh leaves, takes two to three hours to distil, and yields 7-10 kg (15-22lb) of oil. Extraction by Volatile Solvents
The process is similar to steam distillation, with the basic material placed in racks in a huge tank like a pressure cooker. Volatile solvents are heated and allowed to flow through the racks. The solvents, when saturated with the plant essentials, are evaporated off, leaving certain odiferous molecules and constituents behind, together with some chemical residue. It is a process which many producers and the perfume industry favour because its return in terms of fragrance is so very much higher than that of steam distillation, and with rose, for example, the fragrance obtained is actually stronger. The product extracted by this method is not essential oil, but what is known as a concrete. A concrete should never be used in therapy, since it not only contains chemical residue from the solvents, but because the balance of constituents extracted by the solvents is different to those extracted by steam distillation.
Benzene used to be one of the solvents used in the extraction of essential oils to obtain a concrete, but it is now used less and less as it leaves a residue behind that is known to cause allergies. Legislation has actually recommended that the traces left should be under 10 parts per 1,000. There are also formal restrictions for its use in the perfume industry due to its toxicity for the workers that handle it. The effects of its toxicity have been recognized by distilleries in Grasse as an industrial illness Hexane and chlorure of methylene are other extraction solvents which are even more volatile than benzene. It is estimated that 700 tonnes of chemicals per year disperse in the air around Grasse alone.
To obtain an absolute from the concrete, the concrete is treated with a strong alcohol in which certain constituents dissolve. The alcohol is evaporated off completely, leaving behind the absolute. The absolute has a different balance of constituents to the concrete and the essential oil. Dissolving can also be used for extracting gums and resins of plants and trees such asgalbanum, frankincense and myrrh. The gums and resins are immersed in alcohol in which they dissolve. The alcohol is then evaporated off completely. What is left behind is called a resinoid and it is a heavy sticky substance. This process is cheaper than steam distillation and is widely used in the cosmetic industry. For aromatherapeutic use it is preferable to obtain oils that have been steam distilled.
This is the technique employed for obtaining the oils from the rinds and peels of fruits like oranges, mandarins and lemons. The rinds are pressed or grated, then the oils from the torn cells are collected in a sponge and squeezed out. It was once done by hand, but now it is performed by machine. It used to be the case that workers who handled the sponges full of essential oil for any length of time suffered allergies and problems on their hands. However, as mechanization has taken over, there are fewer problems of this kind.