Rectification: Rectification in its broadest sense means 'putting right' and refers to the removal of impurities or unwanted ingredients. In practice this is often accomplished by distilling the oil a second time (double-distillation). Eucalyptus oil is often rectified in this way. Cade oil is often rectified to remove much of the benzopyrene, which is a known carcinogen.
Folding: This involves re-distilling the oils to remove terpenes. The resulting folded or terpeneless oil has a longer shelf life and is more alcohol-soluble. Folding is a type of rectification.
Blending: Often the aromatherapist favours an oil from a particular country or one which has been extracted from a particular part of a plant. Moroccan chamomile is often substituted for, or blended with the more expensive German chamomile. Clove leaf or stem oil is often blended with clove bud oil. It is also important in the case of oils which are expressed that the first pressing is used. Second and even third pressings can be blended with the first pressing to reduce cost.
Degredation: The chemical composition of the oil can alter during storage. Atmospheric oxygen can combine with some of the components of an essential oil. This oxidation alters the properties of the oil. For example oxidation of lemongrass oil results in loss of its antibacterial properties, while oxidation of pine and terbinth oil, produces chemicals which are skin sensitisers. The process of oxidation is speeded up by heat and light. Failure to store oils in airtight bottles may also result in loss of some of the more volatile components of the oil. Preservatives and anti-oxidants are sometimes added to oils to prevent degredation occurring. These oils should be avoided.