Steam Distillation is a method used to extract essential oils from plant materials. The process is used to extract oils for perfume, aromatherapy, cooking, scented candles, or concentrated essence of garlic for killing vampires with. The process is rather simple.
- Place whatever plant matter you want to extract the oils from into a pressure vessel, probably on some sort of a rack that allows easy removal of it.
- Pressurized steam, possibly superheated, is passed over the plant matter. This vaporizes the oil, which is then carried along with the steam.
- The steam / oil combination is then run through a condenser. Basically, there's coils with cold water running through them, keeping the coils cold, which causes the hot steam to cool down and return to a liquid.
- The resulting liquid drips into a vat. Since most of the oil will be immiscible with the water, the oil will float on the top1, to be easily separated from the rest of the mix. Some of the oil, however, will remain dissolved in the water, which is then called hydrolat, or floral water (if it was flowers that were used).
These operations range in scale from tiny home based operations to massive industrial ones that can do tons of plant material at a time. They are definitely better suited for larger operations, due to the fact that you generally don't get much oil. The ratio of mass of oil obtained to plant matter distilled ranges from 1:200 to 1:3000. It is the most popular method of obtaining essential oils, and one of the oldest, likely due to it's simplicity. For some examples of some oils that you might want to obtain, check out the aromatherapy node.
1: In some cases, the essential oil will be denser than the water. In this case, the oil will have to be drawn from below the water. It's slightly more complicated, but still works.
Sources:
The English Chamomile Company, "Distillation Frameset," The English Chamomile Company. <www.chamomile.co.uk/distframe.htm> (December 17, 2004).
Fragonard, "Steam Distillation," Fragonard: Perfumer Secrets 2002. <www.fragonard.com/@en-us/3/9/14/article.asp> (December 17, 2004).