Distilling and Alcohol

• Humans have been fermenting fruit and grains for thousands of years, producing wine and beer in varying degrees of strength. Distilling is also an ancient technique, used for thousands of years to concentrate floral and herbal essences for perfumes and medicines. By the late Middle Ages, Europeans had begun to combine the two techniques, creating an alcoholic beverage through fermentation and then distilling it to concentrate its alcohol content. Alcohol evaporates at a lower temperature than water, so if the liquid is heated gently, most of the escaping vapors captured in the still consist of alcohol.

Mellowing the Alcohol

• Extremely pure alcohol straight from the still is useful for dissolving chemicals or providing a clean-burning flame, but it's not very pleasant for drinking. Its vapors are concentrated enough to take the unsuspecting drinker's breath away, and its taste is harsh, oily and fiery. To make the beverage more palatable and less dangerous to consume, it's usually diluted to a predetermined percentage by adding water. Some spirits are aged in oak barrels, giving them an amber color and a milder, more complex flavor. Golden tequilas can be aged this way, although inexpensive brands are simply darkened with caramel coloring.

Alchohol and Proof

• Because alcohol is naturally clear and water is routinely added, unscrupulous distillers could easily cheat their customers by diluting their product unnecessarily. Wary customers such as the British Navy -- and distillers conscious of their reputations -- developed a technique to demonstrate whether the alcohol was at full strength. They used a sample to moisten a small quantity of gunpowder, then lit the powder with a match. If it sputtered, the alcohol was weak, and if it burned freely, the alcohol was too strong. At the correct percentage -- between 55 and 60 percent, or 110 to 120 proof -- the powder would burn steadily but gently.

American Usage

• Most brands of tequila and other beverages sold in the United States are adjusted to 80 proof, or approximately 40 percent alcohol. This is low enough to be a pleasant beverage, but high enough to remain potent. "Overproof" tequilas ranging up to 60 percent alcohol or 120 proof are not unknown, but they're relatively rare specialty products. Whiskey and rum of that potency are often sold as "cask strength," but the term isn't generally used with tequila.