The man who solved the problem was an American chemist, who alsocontributed to the Dictionary, Thomas Sterry Hunt. In 1857, he had beenteaching at Laval University in Quebec and responded to an appeal by thePresident of the Montreal City Bank who was battling counterfeit notes. Huntcreated a special green ink, using chromium sesquioxide, that was pretty wellindestructible. Numerous experiments showed that you couldn’t remove itfrom the banknote without destroying the paper itself – until one chemistsucceeded in doing so, and the Canadians dropped Hunt’s invention. But theAmericans took it up, especially on the back of their banknotes, hence thecolloquial term ‘greenbacks’ for US dollar bills, which was given its own entryin the OED in 1900 and defined as ‘the popular name for one of the legal-tender notes of the U.S., first issued in 1862 and so called from the devicesprinted in green ink on the back’, alongside something rather topical at thetime but now archaic, Greenback Party, defined as ‘a party in U.S. politics,which advocated that “greenbacks” should be made the sole currency of thecountry’, and its various derivatives Greenbacker and Greenbackism.

As long as money hasexisted, the problem of counterfeit currency has too, but it became aparticular problem once printed notes went into general circulation. Ineighteenth-century North America, Benjamin Franklin – who owned a firmthat printed money for several of the colonies – hit on the idea of misspellingPennsylvania on official currency, on the grounds that forgers would spell it