Counterfeiting is nearly as old as money itself. So are security features to ensure currency’s authenticity. In the 1730s, Benjamin Franklin began innovating in his printing press, devising various methods to outwit counterfeiters. Some of his security measures were clear to see—he printed nature patterns on different denominations and embedded colored fibers into the paper—but some of his methods had been lost to time.
It wasn’t a discovery of Franklin’s missing journal that brought to light his innovations but the application of existing physics techniques. Materials scientist Khachatur Manukyan partnered with physicists and art historians at the University of Notre Dame to study rare bills dating between 1709 and 1790. A significant part of the research work involved proving that their spectroscopy methods would be nondestructive. Because the bills are both fragile and unique, Manukyan’s team had to return them to a special university-library collection at the end of every day to keep them safe.
Using x-ray fluorescence and Raman spectroscopy, the team was able to see the difference between genuine Franklin-printed money and currency produced by other printers, even those who were legitimate suppliers. One difference was in the ink. Although Franklin made and sold his ink, he never distributed the exact type he used for printing currency. Raman spectroscopy revealed different peak positions for the bone-black ink used by other printers and the type used by Franklin. The ink on Franklin’s bills tended to be darker and bleed less, which produced crisper images that even other official currency printers were unable to achieve.
In the x-ray fluorescence images, the difference is even starker. Counterfeit money the researchers examined showed significant amounts of calcium and phosphorus, as seen in the green and purple filters, respectively. The black printing pigments used by Franklin, however, have a negligible amount of those elements. As a result, Franklin’s currency appears under those colored filters to be almost entirely black. That composition is indicative of pigment derived from the burning (pyrolysis) of bones, which contain hydroxyapatite. Franklin, however, did not use bone black. The results suggest that Franklin took the process further and developed a unique graphite-based formulation to make the black ink used in his currency.
Aberration-corrected scanning transmission electron microscopy with novel segmented detectors was also used for perhaps the first time in conservation studies. The technique can be used for studying materials with subnanometer resolution, mapping chemical composition at atomic scales, and even mapping electric fields. Manukyan and colleagues extracted 50-nm-thick specimens from Franklin’s money to analyze under a dispersed electron beam with high magnifications to reduce damage to samples. That revealed specks of muscovite filler particles incorporated into the paper. A letter from Franklin to a contemporary scientist indicates that he was working on developing sturdier paper, but the findings demonstrate what he was experimenting with. It is possible that Franklin even noticed that larger muscovite flakes lent a reflective property to the money—an additional security feature that others were unable to replicate.
The researchers say they are excited to see what else can be discovered when their novel techniques are applied to other historical artifacts. (K. Manukyan et al., Proc. Natl. Acad. Sci. USA 120, e2301856120, 2023.)
