When Benjamin Franklin moved to Philadelphia in 1723, he got to witness the beginning of a risky new experiment: Pennsylvania had just begun printing words on paper and calling it money.
The first American paper money had hit the market in 1690. Metal coins never stayed in the 13 colonies long, flowing in a ceaseless stream to England and elsewhere, as payment for imported goods. Several colonies began printing bits of paper to stand in for coins, stating that within a certain time period, they could be used locally as currency. The system worked, but haltingly, the colonies soon discovered. Print too many bills, and the money became worthless. And counterfeiters often found the bills easy to copy, devaluing the real stuff with a flood of fakes.
Franklin, who started his career as a printer, was an inveterate inventor who would also create the lightning rod and bifocals, found paper money fascinating. In 1731, he won the contract to print £40,000 for the colony of Pennsylvania, and he applied his penchant for innovation to currency.
During his printing career, Franklin produced a stream of baroque, often beautiful money. He created a copper plate of a sage leaf to print on money to foil counterfeiters: The intricate pattern of veins could not easily be imitated. He influenced a number of other printers and experimented with producing new paper and concocting inks.
Now, in a study published Monday in the Proceedings of the National Academy of Sciences, a team of physicists has revealed new details about the composition of the ink and paper that Franklin used, raising questions about which of his innovations were intended as defenses against counterfeiting and which were simply experiments with new printing techniques.
The study draws on more than 600 artifacts held by the University of Notre Dame, said Khachatur Manukyan, a physicist at that institution and an author of the new paper. He and his colleagues looked at 18th-century American currency using Raman spectroscopy, which uses a laser beam to identify specific substances like silicon or lead based on their vibration. They also used a variety of microscopy techniques to examine the paper on which the money was printed.
Some of what they observed confirms what historians have long known: Franklin’s paper money contains flecks of mica, also known as muscovite or isinglass. These shiny patches were most likely an attempt to combat counterfeiters, who would not have had access to this special paper, said Jessica Linker, a professor of American history at Northeastern University who studies paper money of this era and was not involved in the study. Of course, that didn’t stop them from trying.
“They come up with very good counterfeits, with mica pasted to the surface,” Dr. Linker said.
In the new study, the researchers found that the mica in bills for different colonies seems to have come from the same geological source, suggesting that a single mill produced the paper. The Philadelphia area is notable for its schist, a flaky mineral that contains mica; it’s possible that Franklin or printers and papermakers associated with him collected the substance used in their paper locally, Dr. Manukyan said.
When they examined the black ink on some of the bills, however, the scientists were surprised to find that it appeared to contain graphite. For most printing jobs, Franklin tended to use black ink made from burned vegetable oils, known as lampblack, said James Green, librarian emeritus of the Library Company of Philadelphia. Graphite would have been hard to find, he suspects.
“So Franklin’s use of graphite in money printing is very surprising, and his use on bills printed as early as 1734 is even more surprising,” Mr. Green said in an email.
Could using graphite ink have been a way to differentiate real money from fakes? Differences in color between graphite and lampblack are likely to have been subtle enough to make that a difficult task, Mr. Green said. Instead, we may be looking at another example of Franklin’s creativity.
“It suggests to me that almost from the start he was using his money printing contracts as an opportunity to experiment with an array of new printing techniques,” he said.
To understand more clearly Franklin’s intent, more analyses of printed documents from the era would be helpful, said Joseph Adelman, a professor of history at Framingham State University in Massachusetts.
“The comparison I would most like to see would be Franklin’s other publications,” Dr. Adelman said. “To really test this theory — does Franklin have this separate store of ink?”
In future research, Dr. Manukyan hopes to collaborate with scholars who have access to larger collections of early American paper money. These techniques can be quite valuable in the study of history, Dr. Linker said, if scientists and historians can work together to identify the best questions to answer.
“I have questions about a whole bunch of inks. There’s a really weird green on some of the New Jersey bills,” she said, referring to money printed by a Franklin contemporary. “I would love to know what that green ink was made of.”