1466. Special Directions for Distilling Essential Oils

1466. Special Directions for Distilling Essential Oils. Substances yielding volatile oils are generally distilled with water, the proportion of which varies with each article, but under all circumstances must be sufficient to prevent the substance from burning before the whole of the oil has passed over. To prevent the risk of burning, it has been recommended to suspend the substance to be distilled in a basket, or a bag of wire-work, in the water, so as not to touch the bottom or sides of the alembic; or to place the substance on a perforated shelf in the upper part of the alembic above the surface of the water. Some substances, such as mustard, bitter almonds, etc., which are mixed to a paste with water, are distilled by the action of a current of steam heated to the necessary degree and admitted into the bottom of the alembic. An excess above what is necessary acts injuriously by holding some of the oil in solution after the mixed vapors are condensed; on the other hand, if too small a quantity be employed, besides the danger of burning, the whole of the oil will not be distilled. Dried plants require more water than the fresh and succulent.

The form of the alembic has an influence over the quantity of water distilled, which depends more upon the extent of surface than the amount of liquid ; by employing a high and narrow vessel the disadvantage of an excess of water is much obviated.

The temperature should bo equable, and regulated so as not to exceed the required degree of heat; and, as some oils are more volatile than others, an appropriate temperature must be obtained and sustained; the use of a higher temperature than is necessary being injurious. Any degree of heat can be steadily applied by the use of a bath, either of water or of some solution (weaker or stronger as required) of which the boiling point is known. (See No. 7 (Table showing the Boiling Heat of various Saturated Solutions).)

The more volatile oils pass freely with the steam into the neck of the receiver, but some that are less volatile arc apt to condense in the head, and return into the body of the still; for these a still should be employed with a large and low head, having a rim or gutter inside, in which the oil may be received as it condenses, and thence led into the neck of the condensing tube (see No. 1077 (Soubeiran's Steam Apparatus for Distilled Waters)), which is better straight than coiled, for convenience in cleaning, as the alembic and all its appurtenances must be perfectly clean before distilling each kind of essential oil.

Certain flowers, such as orange flowers and roses, yield little or no oil when dry, and must be preserved fresh, either with salt, or by means of glycerine, to keep them in condition for distilling their oils. (See No. 1340 (Piesse's Fumigating Pastils).)

The most of the aromatic herbs are usually distilled while fresh, although it is thought by some that they yield a larger product when moderately dried. Dried substances require, previous to distillation, to be thoroughly macerated with water; and to facilitate this end, should bo prepared by slicing, rasping, bruising, or other appropriate means. Sometimes the proportion of oil in the substance employed is so small that it is wholly dissolved in the water distilled, even though the smallest necessary quantity of water has been employed in the alembic. In this case the distilled solution must be redistilled several times with fresh quantities of the substance, until more oil passes over than the

I water will dissolve. This process is called cohobation.