Expression is the method employed to extract essential oils from the oil sacs contained in the rinds of fruit such as orange, lemon, bergamot, mandarin and tangerine. In times past this was a very labor-intensive process, involving pressing the rind into the sponges, which absorbed the essential oil, and were then squeezed out. Today it is done by machines.
Solvent extraction is often used for gums and resins, but also for other plant materials. It involves covering the plant material in a solvent, which may be a petrochemical, and then extracting the essential oil by filtering the plant material and evaporating the solvent. While this method may be perfectly acceptable to the perfume trade, good aromatherapy companies try to avoid buying essential oils and absolutes produced using petrochemical solvents.
EDITED to remove weblinks.
Solvent extraction is often used for gums and resins, but also for other plant materials. It involves covering the plant material in a solvent, which may be a petrochemical, and then extracting the essential oil by filtering the plant material and evaporating the solvent. While this method may be perfectly acceptable to the perfume trade, good aromatherapy companies try to avoid buying essential oils and absolutes produced using petrochemical solvents.
EDITED to remove weblinks.