Sympathetic Ink

Also known as invisible ink, sympathetic inks are invisible until a chemical or thermal treatment renders them visible.

The use of sympathetic inks dates back to the third century B.C., when Philon of Byzantium first described a method of writing with the extract of nutgalls (excrescences formed on trees), which only became readable after the treatment of the paper with a copper salt containing iron. The Roman poet Ovid (author of The Art of Love') recommended that lovers compose their clandestine missives in milk, which can then be read by dusting the paper with soot. Muslim priests are said to have used invisible inks to inscribe the name of Mohammed on stones which then became visible under the heat of one's palm, a trick not long afterward adopted by magicians and purported mystics. In the Renaissance and afterward, the Vatican as well as various governments used sympathetic inks to convey sensitive diplomatic correspondence, and the British and Americans used such inks during the American Revolution in the late eighteenth century, and the wartime use of sympathetic inks continued until as recently as World War II. Currently, however, invisible inks of nearly all formulations can be detected by scientific means, if one is looking for secret writing.

Scholars of invisible inks disagree about the best way of classifying them—according to substance used, according to the nature of the writer (a prisoner or a free person), according to the color of the ink when developed, or according to the methods required to develop them. In terms of the substances used, these can include body fluids (blood, saliva, perspiration, urine, and others), foods and juices, chemicals (such as acids and bases), as well as various soap solutions, glues, adhesives, and so on. Common materials that are utilized include vinegar (acetic acid), citrus-fruit juices (in particular lemon juice), baking soda, salt, sugar, rice, water, aspirin, gum arabic, boric acid, starch, ammonia, Epsom salts, silver nitrate, and many others. As for detecting sympathetic inks, there are basically four classifications: optical (the use of special lighting such as infrared or ultraviolet, special angles of viewing or special angles of lighting, or viewing through special materials), mechanical (dusting the encoded substrate with a fine powder such as graphite, exposing the substrate to fumes of iodine, or moistening the surface with water, an iodine solution, or a dye solution), thermal (exposing the substrate to heat or a flame), or chemical (exposure to ammonia vapor, immersing in chemical baths, or other procedures reminiscent of photographic developing).

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