How To Make Essential Oils
There are a number of different methods that can be used to make essential oils. Each method has its advantages and disadvantages - and due to the varied nature of essential oils, certain methods will only work effectively with certain oils. It's important to understand which process should be used as this will differ from plant to plant.
Some of the methods used to make essential oils are simple and can be performed successfully by many people with a little learning and careful actions. Other methods may require expensive equipment, special training, and the knowledge and correct execution of strict safety procedures.
It's worth noting also that good storage of essential oils and other plant oils is important. Typically, essential oils are stored in dark, closed bottles in cool, dry locations - because they may be gradually denatured over time by light, heat or moisture. Oils also have a shelf life, which varies depending on the type of oil - and in general, "the fresher the better" - although some oils may still be good for even a few years, depending on the application.
Steam distillation is considered the "classic" and most popular way to make essential oils. It is fairly simple, has been used for many hundreds of years and is attributed to the great Arab physician Ibn Sina (Avicenna) (980-1037 AD). Many of the essential oils one sees for sale in the stores, were created by steam distillation. Steam distillation has the advantage of needing no special solvents - and the basic equipment required should not be of great expense.
Steam has the interesting effect of lowering the boiling point of volatile compounds in the oils, allowing many essential oils to be extracted at temperatures below the point at which they would normally evaporate. This is important, as essential oils which would be damaged by higher temperatures are thus able to be extracted undamaged.
In steam distillation, steam is passed through the plant matter from which the oils are to be extracted. The mixture of vapors is then passed through a condenser, which cools the mixture - typically by passing it through pipes or tubes which are surrounded by cold water. This results in the collection of oil and water, which are easily separated and the water is removed, leaving essential oil! The water is often fragrant, and in commercial essential oil production this "floral water" is sometimes used for other products.
Most essential oils extracted via steam distillation will require a single process - however ylang-ylang requires a more complex fractional distillation. 
It is important that both the temperature and the length of time the plant matter is exposed to the steam are optimized, so as to prevent either damage to the oils or the inclusion of undesired molecules in the final product. Distillation may take less than an hour to a few hours, but will vary with the plant as well as the operating conditions.
In the case of certain oils, particularly flower petal oils of many varieties, steam distillation should not be used. This is because these oils are too "delicate" and would be significantly denatured (i.e. damaged) even by the moderate heat of steam - leading to an inferior, degraded or possibly worthless product. Essential oil experts learn to detect the smell of certain fragrant "markers" and can recognize a high quality oil by scent.
Steam distillers can be purchased for as low as a couple of hundred dollars. Higher quality units are often made from copper or food grade stainless steel, and may even cost a few thousand dollars. Another item to look out for is the essencier, which is used to separate the condensed oil from the water. Some fabulous high quality units, plus full instructions on how to use them, can be observed at the Essential Oil Company web site.
If an essential oil has been extracted through steam distillation, it is sometimes referred to as an otto or an attar - as in "Rose attar", which is essential oil of rose petals extracted through steam distillation.
A more advanced form of distillation is vacuum distillation. In this process, pressure is reduced, which has the effect of lowering the boiling point of the oils and solvents. This means that more temperature-sensitive materials can be extracted undamaged. A wide variety of solvent extraction processes employ vacuum distillation - from extraction using steam, water, solvents or even carbon dioxide extraction. Vacuum distillation requires much more sophisticated equipment and this needs to be made correctly, due to the potential implosion hazards associated with the low pressures in the distillation vessel.  Vacuum distillation may sometimes be used in conjunction with other distillation methods, such as solvent extraction (see below) and it can aid the process, lowering temperatures required and / or speeding up processing.
Cold pressing is widely considered to be a great method for extracting oils - in particular food oils extracted from seeds and nuts; but certain essential oils may be cold pressed - such as citrus peel oils.
One advantage of cold pressing is that there is no heating involved, so the oil is not denatured (i.e. changed) by high temperatures. Also, there are no solvents (see "solvent extraction" below) and therefore none of the problems that can be associated with solvent extraction.
Cold pressing is widely used for the extraction of oils from seeds and nuts, as these give up their oil readily when crushed - and "cold pressed" is seen as a marker of quality for many food oils and massage base oils. Cold pressed oils commonly seen include olive, almond, peanut, coconut, flax, sesame, pumpkin seed and safflower. Many of these oils are widely regarded as having great health benefits - however cold pressed oils vary widely in quality depending on storage conditions, freshness, cleanliness and other processes which may be used.
When used as a descriptive term in product manufacturing, "cold pressed" typically means "that the oil was not heated over a certain temperature (usually 80ºF (27ºC)) during processing, thus retaining more nutrients and undergoing less degradation." The term "first cold pressed" means that the oil came form the first pressing - typically regarded as better quality. The exact standards for cold pressed food oils are complex and in the European Union are very stringent. 
The technology required for cold pressing can vary from small, hand-powered presses that can be worked by an individual, through to commercial power-driven pressing equipment. A typical technology used for smaller-scale seed and nut oil pressing is the expeller press. This uses a "screw thread" driven press to crush the materials under high pressure and extracts the oils in a single process. 
Oils that are cold pressed may typically still contain fine pulp, water and resins, and may require "clarifying". If the oil is allowed to sit for a few days, the water layer will separate and this can be removed. The resulting oil is typically also filtered, and possibly heated in order to destroy microorganisms. 
Most flower petals contain too little oil to be cold pressed, and have delicate aromas which would be too easily denatured by steam distillation. For the extraction of these oils, other methods are required - and one such method is solvent extraction.
A simple form of solvent extraction which can actually be used for a wide variety of plant materials, uses high quality ethanol (alcohol), such as organic grain alcohol. Ethanol is often used, together with distilled water, to make the dark-colored "liquid herbal extracts" one sees for sale in the stores. However, to make essential oil, the alcohol needs to be removed. The simplest way would be to allow it to evaporate (in a well-ventilated place!), but a more sophisticated way involves the use of vacuum technology - (see Vacuum Distillation) - which speeds the process and enables low temperatures to be used more effectively. It's important to note that any impurities in the solvent would be left behind in the product when the alcohol evaporates. Great care and correct safety procedures are also required when using solvents, especially if heating is involved, as many solvents are potentially highly flammable.
Solvent extraction using hexane, has been widely used on an industrial scale in modern times for the extraction of the essences of certain flowers. The flowers are put in a container with hexane and then the container is agitated. Aromatic compounds are drawn out into the solvent, together with pigments and waxes. The solvent is then removed by vacuum processing, leaving a concrete - which is a waxy mass. This is then treated with alcohol, which separates the aromatics from the wax. The alcohol is then evaporated.
Essential oils extracted by solvent extraction are called absolutes and if you see this on the labeling, for example "Jasmine Absolute", then you know it has been extracted using solvent extraction. However, in addition to being potentially hazardous due to the risks involved with solvent use (such as flammability and inhalation), hexane extraction has come under much criticism. In theory, the solvent dissolves the essential oils from the plant, and then evaporates to yield a pure oil. However, in practice, industrially-produced solvents such as hexane are typically not totally pure, and some residue may be left behind in the oil, and perhaps even some solvent - leading to toxicity concerns. It is widely agreed upon that absolutes should not be used in aromatherapy and absolutes are more typically used for scented products such as soaps, candles and perfumery.
A more advanced form of solvent extraction uses carbon dioxide at high pressure (see Carbon Dioxide Extraction, below).
Enfleurage was originally an ancient process which used animal fat to absorb the fragrance of flowers such as jasmine or tuberose. It was once the only method of extracting the fragrance from these flowers. Enfleurage performed in this manner is very labour-intensive indeed and is now largely obsolete.
In traditional enfleurage, a large framed plate of glass was coated with a thin layer of animal fat (such as lard). The flower petals were then placed on the plate and left for a few days. This process was repeated until the desired degree of fragrance has been absorbed by the fat. This product, known as pomade, would then be soaked in pure alcohol, which would absorb the fragrance. The alcohol would then be allowed to evaporate, leaving the essential oil. The remaining fat was then used to make soap, as it still contained some fragrance. 
Fortunately, a simple version of enfleurage has become popular, which is very inexpensive. It uses olive oil instead of fat. Plant matter is soaked in olive oil, and the plant matter is replaced every so often until the desired fragrance is achieved. The fragrant oil can then be filtered, bottled and used as scent.
It's important to note that oils produced using olive oil in this manner are not strictly essential oils, they are fragrant oils that contain some essential oil! Essential oils that one sees for sale are not extracted in this manner - for the simple reason that in order for an oil to be a "pure essential oil", it would have to be pure, with no olive oil! Many "homegrown" sources giving instructions for "how to make essential oil" describe this simple enfleurage method - which can make nice scented oils, but they are not, strictly speaking, essential oil.
A variation on the enfleurage method, called maceration, involves heating the oil. The process is similar, only the container which contains the oil and plant matter is placed in warm / hot water in order to speed up the processing. Maceration is similar in some ways to solvent extraction, only with oil replacing the solvent.
A fascinating, advanced method of extraction uses carbon dioxide under high pressure as a solvent to extract essential oils. The botanicals to be processed are put in a vessel designed to withstand very high pressure, and the CO2 is pumped through at a controlled pressure and temperature. At pressure, CO2 acts as a solvent, and a concrete (a mix of oils and waxes) is extracted. Then liquid carbon dioxide is used to separate the essential oils from the waxes.
There are two main types of CO2 extraction: "Supercritical" and "low pressure cold extraction". Supercritical carbon dioxide is carbon dioxide that is at a temperature and pressure exceeding 31 degrees C and 73 atmospheres pressure. This substance has both liquid and gas-like properties. By controlling precisely the temperature and pressure, the solvent quality can be "tuned" in order to imitate the solubility characteristics of a wide range of solvents. In other words, a very complete extraction of all fractions from a wide range of plants can be achieved. 
The other form of carbon dioxide extraction is low pressure cold carbon dioxide extraction. This involves cooling the CO2 to 35-55ºF and using lower pressure than supercritical extraction, typically around 800-1,500 psi instead of 6,000-10,000 psi. Low pressure CO2 extraction has the advantage over supercritical CO2 extraction in that there is less risk of high pressure damage to molecular ring structures of the aromatic compounds. 
CO2 extraction is widely regarded to be among the best methods available for producing high quality essential oil extracts. It has the benefit that at the end of the process, the carbon dioxide simply evaporates - cleanly, readily and rapidly without leaving the chemical residues that may be left by regular solvents. Also the process does not use high heat and therefore does not cause thermal degradation of the oils.  Supercritical Carbon Dioxide Extraction has been approved by the Soil Association for use with organic products. This method also has potential environmental benefits in that hazardous or toxic solvents are not required.
Supercritical CO2 extraction equipment can be rather expensive but is coming to the forefront of extraction technologies. It is widely regarded to be a professional solution for those serious about commercial production of very high quality essential oils.
It is possible to use other gases instead of CO2 - such as butane or dimethyl ether - however these have other complications such as flammability and require much greater safety procedures and equipment.
Supercritical carbon dioxide extraction-derived essential oil may be seen marketed as an absolute - and so in choosing essential oils for therapeutic purposes it is considered important to know the process that was used to obtain the oil (see Solvent Extraction.)
Some essential oils can be toxic. Essential oils should not be taken internally, and should not be applied undiluted to the skin. It is important to research and be aware of any hazards involved with particular essential oils, and to observe safe procedures.
Essential oils can damage some plastics and rubbers, and so these materials should typically not be used in their preparation or storage. Glass (i.e. pyrex if heat is being used), or stainless steel are suggested materials for essential oil work. Oils should be stored in dark glass bottles and kept in a cool, dry place.
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