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.
Volatile oils, which have become rancid, and although very much deteriorated, entirely deprived of their odor and color, and almost without fluidity, are not lost beyond remedy. They may be restored in all their purity, but the ordinary rectification is insufficient, because they are then deprived of all their odor. Different methods are adopted for their rectification, in order to restore to them all their original properties. The volatile oil which is to be rectified is placed in a still, along with a quantity of the fresh fruit or seed, and a sufficient quantity of water; the distillation is proceeded with. When the volatile oil which has been spoiled by age is rectified, it is saturated anew with the odor of the fruit, and passes over with the volatile oil arising from the fresh plant. In this manner the volatile oil is completely renewed.
When a volatile oil is not altogether changed, but has commenced to lose its color and limpidity, it is sufficient, in order to restore it, that it be poured into a small glass retort placed in a sand-bath over a furnace, the receiver attached, and the distillation proceeded with at a moderate heat, about the temperature of boiling water. The volatile oil which passes over is limpid and almost without color. The distillation is suspended as soon as the drops begin to be colored; that which remains in the retort is thick, and has very much the appearance of a resin. All volatile oils lose considerably by rectification; some about one-third, and others more, according to the state of deterioration in which they are when rectified.
Small quantities of resinified volatile oils, the flavor of which has not been impaired, may be restored after Curieux by agitating them for fifteen or twenty minutes with a magma formed by mixing a solution of borax with animal charcoal, when the resinified portion will unite with the borax, leaving the oil limpid and the odor restored.