Leonardo Da Vinci may have been a genius, but he was also a hot mess. When he died in 1519, he left behind 7,000 pages of undated drawings, scientific observations and personal journals, more or less jumbled up in a box. So when his assistant collected Da Vinci’s papers, he did his best to collate them into journals, or codices, mostly based on subject matter. Ever since, art historians have used all sorts of techniques to make a timeline of the various documents now held in museums and collections across the world.
A new system developed by a University of Wisconsin-Madison engineer could help in that centuries-long effort.
William Sethares, a professor of electrical and computer engineering at UW-Madison, and PhD student Elisa Ou, are using a camera system and sophisticated algorithms to study the actual paper to match the undated drawings and writings to others with established dates. And it’s not just Da Vinci; the two are working on a project dating the works of Rembrandt and believe the system is applicable to any artwork or document on pre-industrial paper.
Here’s why. Prior to the middle of the 19th century, when industrial production began, paper was more or less a handmade product. Paper makers poured a slurry onto large mesh screens to produce large sheets of paper, which they then cut or folded and sold in bundles. Each of those screens was made of vertical chain wires and more delicate and numerous horizontal laid lines. Sometimes paper makers also included little pictures made from fine wire, like animals, flowers or other symbols, called watermarks, to identify their products.
“If you can find two pieces of paper that have the same chain lines and watermarks, then they came from the same mesh molds, and that puts them in proximity in time,” says Sethares. “That’s because these molds only lasted about six months or a year.”
Professor William Sethares examines a drawing made in preparation for Rembrandt’s painting “Return of the Prodigal Son,” which is not definitively dated. Watermarks and lines on the paper may allow art historians to match it to dated artworks from the same batch of paper. Submitted photo.
Using these idiosyncratic markings, grouping artworks from the same batch of paper, known as mold mates, makes it possible to date the works if they are related to another, firmly dated piece of paper.
Seeing these chain lines and watermarks is difficult, however—especially on delicate paper covered by ink, paint or writing by some of the world’s foremost artists. Comparing all those subtle marks in paper is also a tedious, inexact task when done by hand.
That’s why Sethares helped design and develop a hardware and software system called the watermark imaging system (WImSy), detailed in a paper in the journal Heritage. In the system, an artwork is placed on a light plate, which backlights the paper. A camera takes several photos with light coming from different directions to capture detailed images of the artwork and the paper itself.
Next, algorithms de-noise and augment the photos to remove the surface image and extract information about the paper’s internal structure, including chain lines, laid lines and watermarks barely visible to the naked eye. Other algorithms then align and compare this image of the “blank” paper or its watermarks to others in a database to see if there is a match. The team first tested the system on artworks at the Metropolitan Museum of Art in New York and at the Getty Museum in Los Angeles.
The system is already being used for some significant projects. In 2022, for example, art historians used it to isolate watermarks to authenticate a newly discovered drawing by the German Renaissance artist Albrecht Durer.
It’s also part of the Da Vinci project, called LEOcode, which involves researchers from UW-Madison, Cornell University and the Institute of Fine Arts at New York University. While Sethares has not yet had direct access to Da Vinci’s journals, he has used the algorithms to examine high-resolution photos of the Codex Leicester—a collection of Da Vinci’s scientific writings owned by the Bill and Melinda Gates Foundation—to identify and catalogue watermarks.
Some scholars of Rembrandt—another artist who rarely dated his artwork—are also embracing WImSy. Sethares has photographed the Dutch master’s drawings at the Teylers Museum in Haarlem, in the Netherlands; the machine is now at the Boijmans museum in Rotterdam and next goes to the Rijksmuseum in Amsterdam. Other institutions in Europe are also interested in trying out the system.
For the Rembrandt project, the team also plans to photograph papers from the Dutch National Archives—among them, deeds of sale, wills and other paper documents. “The ‘boring stuff’ is almost always dated,” says Sethares. “So if we can go through the archives and match those pieces of paper up with Rembrandt’s, that will at least get us to within a couple of years.”
Eventually, Sethares says he would like to build a second WImSy system and would like see a repository of papers photographed by the device that other researchers could access to date artworks and documents. He’s still figuring out some of the fine print details, however—like who owns the images taken with the system and who should have access. In the meantime, WImSy is making the rounds of other European museums whose researchers are eager to experiment with the system before it returns to museums in the United States.
Featured image caption: Marleen Ram, an art curator at the Teylers Museum in Haarlem, the Netherlands, places a drawing by Rembrandt under the watermark imaging system’s camera along with Rick Johnson, an emeritus professor of electrical engineering at Cornell University (center), and Rob Fucci, an art historian at the University of Amsterdam (right). Credit: William Sethares.