Georgia Tech history professor Macrakis (Seduced by Secrets, 2008) surveys 2,000 years of secret messages penned with an improbable variety of substances and methods: lemon juice, various bodily fluids and other liquids that dry invisibly on the page but turn brown when exposed to heat. Gall-nut extract, for instance, a traditional ink used by George Washington’s agents, turns black when brushed with iron sulfate, and cobalt solutions painted on fire screens blossom into brilliant green foliage when warmed. Some modern inks require four different chemical treatments to develop; an invisible ink invented by Linus Pauling shows up only when treated with a specific antibody. Aside from chemical compounds, microdots can cram pages of documents into a tiny spot the size of the period at the end of this sentence, and digital encoding techniques allow messages to be inserted into Internet porn. Macrakis offers lucid explanations of the chemistry and optics underlying the seemingly magical properties of invisible inks, and she even appends recipes that guide readers in making some of the simpler varieties. The heart of her book, though, is the cat-and-mouse game between spies who conceal their reports in plain sight and counterintelligence officials trying to intercept and detect them, a saga she follows from the subterfuges of Mary Queen of Scots to dueling Cold War intelligence agencies. Her chronicle luxuriates in colorful characters—Jesuits plotting to escape from the Tower of London, James Bond–ian playboy-spies and Mata Hari–like femmes fatales—and intricate cloak-and-dagger featuring inks secreted in fake molars and infused into garments. It’s also a story of government surveillance at a level that would make Edward Snowden cringe: During the world wars, Britain and the U.S. opened millions of letters and tested them for invisible messages. Macrakis unearths a wealth of information, including secret documents she mined from East Germany’s Stasi archives, weaving it together with engaging prose that illuminates a seldom-seen aspect of espionage.
An engrossing study of unseen writing and the picaresque misadventures of those who employ it.