'Since we decided a few weeks ago to adopt the leaf as legal tender, we have, of course, all become immensely rich.'…
'But we have also,' continued the management consultant, 'run into a small inflation problem on account of the high level of leaf availability, which means that, I gather, the current going rate has something like three deciduous forests buying one ship's peanut.'
– Douglas Adams, The Ultimate Hitchhiker's Guide to the Galaxy

Back in 18th-Century America, in the rather well-charted back streets of colonial Philadelphia, printer Ben Franklin had both a wonderful business opportunity and an interesting challenge. He had a licence to print money. Literally. From the colonies of Pennsylvania, Delaware, and New Jersey. That was the opportunity. The challenge was how to defeat the counterfeiters. As it turned out, Franklin – an inventor far ahead of his time – was more than up to the challenge. In fact, it took other scientists 200 years to figure out how the heck he did it.

A Bit of Background

Benjamin Franklin (1706-1790) first arrived in Philadelphia as a youth of 17. He had a loaf of bread under his arm, a dollar in his pocket, and a head full of ideas. He promptly rested his head by falling asleep in the Quaker Meeting in the Arch Street Meeting House. Refreshed, he set about looking for business opportunities.

Then-governor William Keith convinced Franklin to go to London, promising to help him buy a printing press to bring back. This promise turned out to be as hollow as many another politician's campaign slogans. Franklin spent the next several years working in print shops in London, refining his trade. He returned to Philadelphia in 1726 and eventually set up his own shop. He also bought the Pennsylvania Gazette and turned it into a first-rate newspaper.

Franklin recognised the need for colonies to have their own paper currency. In 1729, he published a paper on the subject, called 'The Nature and Necessity of a Paper-Currency'. In 1730, Franklin secured the contract to print money for Pennsylvania. His money-printing enterprise later expanded to include New Jersey and Delaware.

How to Fight Counterfeiters, or: Criminals Are Dumber Than Printers

Counterfeiting was a serious crime because it devalued the currency. This threatened the economic survival of everyone in the colony. The problem was figuring out how to print money that wasn't easy to imitate.

Franklin's first ploy was to deliberately misspell 'Pennsylvania'. This worked because counterfeiters only thought they were cleverer than North America's premier natural philosopher. Assuming the bill they had in front of them was a knock-off by a counterfeiter who spelled worse than they did, they'd correct the spelling. Foiled again!

Fighting Counterfeiters with Nature

Franklin's stroke of genius was adding images of leaves – real leaves, from trees – to the backs of his bills. This rendered his money unique and inimitable, since nature itself had done the live engraving. Nobody else had this feature.

But how was it done? This puzzled researchers for a couple of centuries. After all, there was no photography in the mid-18th Century. Naturally, Franklin didn't leave any notes on the process. It was a trade secret, and it wouldn't do for the counterfeiters to find out. Obviously, at least some of them could read well enough to spell 'Pennsylvania'.

The secret to Franklin's 'nature prints' turns out to have been plaster moulds and metal casting. First, he glued a leaf, face-down, to a piece of cloth, and stuck that to a hard surface. He oiled the leaf to prevent sticking, and made a negative image with plaster reinforced with asbestos, brick dust (lots of that in Philadelphia), and mica. He made a metal relief cast from the plaster mould, set it into a printing chase, and voilà, a unique leaf print on a piece of paper money. Ben Franklin: 2, counterfeiters: 0.

This sort of ingenuity is what US history fans have come to expect from Ben Franklin. He was, after all, famous for his experiments with electricity – he may have been the first person to deliberately electrocute a turkey. (He claimed it was tasty.) He started Philadelphia's public library and founded the national post office system. He introduced fire insurance. Foiling counterfeiters was all in a day's work for the internationally-known author and philosophe.

Still, an academic paper from 2018 gushes a bit when it says:

...this article argues that humble maple, sage, and buttercup leaves, cast and printed on small leaflike bills, awakened a form of period liquid intelligence that linked the spheres of natural philosophy, printing, politics, and economics in a single fluid relation.
– Jennifer L. Roberts, 'The Veins of Pennsylvania: Benjamin Franklin's Nature Print Currency', Grey Room (2018) (69): 50–79.