WORMWOOD, Artemisia absinthium, is shown in James Sowerby’s 1803 handcolored engraving. Oil of wormwood was extracted from the herb’s leaves, flowers and stem; it gave absinthe a distinctively bitter taste. Wormwood oil includes thujone, which can cause hallucinations, convulsions and permanent damage to the nervous system.

DILUTION of the alcohol concentration of absinthe caused precipitation of a colloidal suspension of terpenes, of which thujone is one. The ritual of presentation involved pouring cold water over a sugar cube placed on a slotted spoon. (Sugar took the edge off the liqueur’s bitterness.) One can get the same visual effect by diluting today’s nontoxic substitute, pastis; the author did this demonstration with Pernod.

Large steam-heated stills such as the pair shown were used In the production of absinthe during the second half of the 19th century. An 1855 recipe from Pontarlier, France, gives the following Instructions for making absinthe: Macerate 2.5 kilograms of dried wormwood, 5 kilograms of anise and 5 kilograms of fennel in 95 liters of 85 percent ethanol by volume. Let the mixture steep for at least 12 hours. In the pot of a double boiler, add 45 liters of water and apply heat; collect 95 liters of distillate. To 40 liters of the distillate, add 1 kilogram of Roman wormwood, 1 kilogram of hyssop and 500 grams of lemon balm, all of which have been dried and finely divided. Extract at a moderate temperature, then siphon off the liquor, filter and reunite it with the remaining 55 liters of distillate. Dilute with water to produce approximately 100 liters of absinthe with a final alcohol concentration of 74 percent by volume.    

COMPOUNDS IN ABSINTHE include several terpenes: thujone (from wormwood), pinocamphone (from hyssop) and fenchone (from fennel). These compounds are ketones and structural isomers of camphor (C10H16O). The aliphatic aldehyde citral (C10H16O), which is also a terpene, comes from melissa. Fennel and anise also contribute an aromatic ether called anethole (C10H12O). The carbon atoms in this illustration are black and the oxygen atoms, red; hydrogen atoms are not shown.

The concentration of ethanol in spirits is customarily expressed as a percentage or a proof value. A 70 percent solution by volume consists of 70 volumes of pure ethanol brought to 100 volumes of solution by the addition of water; because of the interaction between ethanol molecules and water molecules, 33.4 volumes of water, rather than 30, must be added. The most practical means of determining the alcohol concentrations of commercial spirits is the alcoholometer shown here, a device perfected in 1824 by Joseph-Louis Gay-Lussac. The meter measures the specific gravity of a distillate; it is floated in a sample at a prescribed temperature, and the depth to which it sinks indicates the percentage of alcohol in the solution. Percentage or proof value can be read directly from the scales inscribed on its stem. When absinthe was popular, alcohol concentration was expressed in the old French system of degrees, which were numerically equivalent to percentages by volume. The British coined the term “proof"; their proof spirit, which corresponded to 57.27 percent ethanol by volume, was operationally defined as one having the maximum concentration of water that would still allow ignition after admixture with gunpowder. In the U.S. the term was modified; the proof value is exactly double the percent by volume, so that a liquor that is 50 percent ethanol by volume is defined as 100 proof.