This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
Essential oils are extracted by pressure from those substances whic - contain them in great quantity, and where these oils are almost on the very surface of the substance. The lemon, orange, and all similar fruits contain the essence in the outer rind, or zeste, which incloses their acid pulp. To obtain the oil, all of the yellow or green portion of the surface of these fruits is rasped off, and the mass is inclosed in a small hair sack, and subjected to the action of a press between sheets or plates of fine tin; it is allowed to clarify, and is then decanted. The volatile oil obtained by this process is more fragrant than that extracted by distillation, but it will not keep so long; besides, it is impure, and is always clouded, because it is charged with mucilage, and a small propor-tion of water which is expressed from the rind. The oils obtained by pressure are yellow, highly odorous, thicken quickly, in time acquire a disagreeable odor, leave a grease spot on cloth, are not entirely soluble in alcohol; while those that are distilled are more fluid, have a less agreeable odor, are more soluble in alcohol, and keep for a long time.