Strange Marks Began to Appear on a 600-Year-Old Leonardo da Vinci Codex. Now, Scientists Have an Answer.

Leonardo da Vinci’s Codex Atlanticus is a sprawling 12-volume collection of the Italian polymath’s work, with sections devoted to crossbows, parachutes, waterwheels, folklore, and all manner of other topics. 

Given its enormous value as a historical artifact, the Codex has been carefully preserved over the past 600 years. But in 2006, researchers were alarmed to discover unexplained black spots appearing on some of its pages, even though the document is housed in a strictly controlled and secured microclimatic environment at the Biblioteca Ambrosiana in Milan. 

Fortunately, these dark stains have only been observed on “passepartout” paper that was attached to the Codex during 20th century restoration efforts; the original folios that da Vinci scribbled on have not been stained. Still, the origin of the marks remained a mystery for more than a decade, leaving scholars baffled. 

Now, scientists from the Polytechnic University of Milan (POLIMI) think they may have finally cracked the case after analyzing a single page of the Codex, known as Folio 843, with sophisticated and non-invasive techniques. This approach revealed that the stained sections are contaminated with metacinnabar, a compound containing mercury and sulfide that may be linked to glue used in past restoration efforts, along with ambient air pollution. 

“This blackening phenomenon of the passepartout, observed on approximately 210 pages of the Codex starting from folio 600 onward, raised great concern,” the POLIMI team said in a recent study published in Scientific Reports. “Microbiological studies have been conducted since 2008, which allowed to rule out any type of microbiological attack on the Codex (both passepartout and folio) as the cause of the black stains.”

“The goal of this multi-analytical study is to characterize the materials and the deterioration patterns of Folio 843 and propose a hypothesis on the formation of the black stains,” the researchers added.

After the stains were discovered, the Codex was unbound and its 1,119 leaves were stored individually on new passepartout mounts in acid-free boxes. Prior work on the document had established the presence of mercury in the stains, but the POLIMI researchers were able to hone in on the nature of the blackening phenomenon on Folio 843 in much more detail. 

The researchers studied the folio using hyperspectral photoluminescence imaging and UV fluorescence imaging, which exposed the presence of various glues on the margins. These glues were used to connect the folios to the passepartout papers as part of a project conducted by the Laboratory for the Restoration of Ancient Books at the Abbey of Grottaferrata between 1962 and 1972. 

In addition, the team detected small particles in the stains that measured about 100 to 200 nanometers. An analysis performed at the European Synchrotron Radiation Facility in Grenoble, France, revealed that these particles are made of the special crystalline compound metacinnabar, which is made from mercury and sulfur. 

The glue used by the Grottaferrata restorers may have contained a white mercury salt that was added to prevent microbiological infestations, according to the study. The sulfur component may have also been introduced through adhesives, though the POLIMI researchers propose that air pollution could have been a factor, given that Milan had high levels of sulfur dioxide in its air until the late 20th century. 

It’s still unclear exactly how metacinnabar arose from these components, but the new study provides the strongest explanation for the worrisome marks. 

“Further research steps are needed to assess the chemical process that led to the formation of metacinnabar in the conservation condition of the Codex Atlanticus,” the team concluded. “It has been hypothesized the arrival of sulfur from the environment as a pollutant (SO2) or from additives used in the glue, which could lead to the reaction with mercury salts and the formation of black metacinnabar particles, responsible for the black stains.”