Ben Franklin wove coloured fibres into paper currency to foil counterfeiters

A papermaker in Massachusetts named Zenas Marshall Craneis traditionally credited with being the first to include tiny fibres in the paper pulp used to print currency in 1844. But scientists at the University of Notre Dame have found evidence that Benjamin Franklin was incorporating coloured fibres into his own printed currency much earlier, among other findings, according to a new paper published in the journal Proceedings of the National Academy of Sciences (PNAS).

We first reported on Franklin's ingenious currency innovations—most likely intended to foil counterfeiters (although this is disputedby at least one economist)—in 2021, when Notre Dame nuclear physicist Michael Wiescher gave a talk summarizing his group's early findings. The new paper, co-authored by Weischer, covers those earlier results along with the coloured fibre evidence. As previously reported, the American colonies initially adopted the bartering system of the Native Americans, trading furs and strings of decorative shells known as wampum, as well as crops and imported manufactured items like nails. But the Boston Mint used Spanish silver between 1653 and 1686 for minting coins, adding a little copper or iron to increase their profits (a common practice).

The first paper money appeared in 1690 when the Massachusetts Bay Colony printed paper currency to pay soldiers to fight campaigns against the French in Canada. The other colonies soon followed suit, although there was no uniform system of value for any of the currency. To combat the inevitable counterfeiters, government printers sometimes made indentations in the cut of the bill, which would be matched to government records to redeem the bills for coins. But this method wasn't ideal since paper currency was prone to damage.

By the time he was 23, Franklin was a successful newspaper editor and printer in Philadelphia, publishing The Pennsylvania Gazette and eventually becoming rich as the pseudonymous author of Poor Richard's Almanack. Franklin was a strong advocate of paper currency from the start. For instance, in 1736, he printed a new currency for New Jersey, a service he also provided for Pennsylvania and Delaware. And he designed the first currency of the Continental Congress in 1775, depicting 13 colonies as linked rings forming a circle, within which "We are one" was inscribed. (The reverse inscription read, "Mind your business," because Franklin had a bit of cheek.)

“Benjamin Franklin saw that the Colonies’ financial independence was necessary for their political independence," said co-author Khachatur Manukyan. "Most of the silver and gold coins brought to the British American colonies were rapidly drained away to pay for manufactured goods imported from abroad, leaving the Colonies without sufficient monetary supply to expand their economy.”

Naturally, counterfeiters didn't take long to introduce fake currency, and Franklin and his network constantly generated new ways to distinguish fake bills. Some forms of those techniques are still used to detect forgeries today. For instance, in 1739, Franklin's printed currency for Pennsylvania deliberately misspelled the state's name. The intent was to set a trap for counterfeiters, who presumably would correct the misspellings in their forgeries.

Franklin kept a separate ledger, in addition to his main account book, in which he recorded his dealings with a papermaker named Anthony Newhouse. Franklin purchased "money paper" from Newhouse sometime in the mid- to late 1740s, and likely kept those transactions separate to keep his work on security features confidential, per the authors. “To maintain the notes’ dependability, Franklin had to stay a step ahead of counterfeiters,” said Manukyan. “But the ledger where we know he recorded these printing decisions and methods has been lost to history. Using the techniques of physics, we have been able to restore, in part, some of what that record would have shown.”

The Notre Dame researchers applied a broad range of advanced physics-based instruments and techniques to analyze the structure and composition of the inks, fibers paper, and fillers Franklin's network used to make more than 600 bills between 1709 and 1790, as well as other 18th century currency and known counterfeits. The samples came from the Rare Books and Special Collections of the university's Hesburgh Library. Those methods included Raman spectrometers, transmission electron microscopes (TEM), a 3MV tandem accelerator, handheld X-ray fluorescence (XRF) scanners, micro-XRF scanners, and X-ray diffractometers, among others.

So what did they find? It's known that Franklin made his own inks. The analyses revealed that he used a mercury sulfide red ink, as well as bone black ink (made from carbonized animal bones). His own personal mixed inks seem to have been pure graphite. X-ray analysis of British paper copies showed the phosphorus and calcium signatures of bone black ink, but Franklin's original banknotes don't have that signature.

The image of a leaf on the back of each bill was also a unique feature—akin to a watermark. The intricate structure of the leaves was so detailed one could make out the traces of their veins. This method of "nature printing" was first used by Leonardo da Vinci, per the authors. In the 1960s, historians discovered that Franklin had created the images by making lead casts of actual leaves—"a new way to transfer leaf structures into the printing process," the authors wrote.

Later on, Franklin started introducing slivers of muscovite, or mica (potassium aluminum), into the paper he used to print money, apparently taken from a mound behind his house. That mica had a unique composition, electron microscopy revealed, which would have been extremely difficult for forgers to copy since they didn't have access to his personal pile of ore. And Manukyan et al. found that the size of the muscovite crystals in Franklin's paper increased over time.

Adding the mica may have been initially intended to make the paper more durable, although there are no historical records to corroborate this. The authors do, however, cite a letter from Franklin to British botanist Peter Collinson, mentioning the inclusion of a few sample sheets of paper "made of the asbestos." Franklin apologizes that the paper isn't as strong as an earlier batch that he had made.  "Our results show that Franklin was, indeed, engaged in developing mechanically robust paper types, even though nothing is known about his 'asbestos paper,'" the authors wrote.

The most recent discovery: very thin (between 100–300 microns) indigo-colored blue fibers and threads, found in Franklin's printed currency as early as 1739.  Later bills that Franklin printed in the 1770s incorporated much larger threads and microfibers measuring up to a few centimeters in length. Those blue fibers were not found in either the non-Franklin currency or the known counterfeits. “These [colored fiber] techniques have been used later on in printing federal dollars, and then other currencies all over the world,” Manukyan told New Scientist.

DOI: PNAS, 2023. 10.1073/pnas.2301856120  (About DOIs).