Researchers have advanced the study of late-medieval mercury treatments by analyzing concentrations in dental calculus instead of bone for the first time.

While the use of mercury might evoke images of the Mad Hatter from Alice in Wonderland, this highly toxic element was also employed in late- and post-medieval Europe as a treatment for venereal syphilis and leprosy.

The study, published in The Journal of Archaeological Science, analyzed the dental plaque of skeletons believed to have suffered from leprosy to test the method and determine if mercury was administered as medicine.

“Ancient dental calculus (calcified plaque) has recently been identified as a medium that can incorporate and preserve organic and inorganic materials. It has become instrumental in tracing ancient diets, occupations, and medical treatments experienced by individuals during their lives. However, it has never been used to investigate mercury as a medical treatment in ancient populations,” explained the study’s authors.

Researchers took samples from skeletons and soil at four late-medieval cemeteries, two of which were known leprosaria. The study authors noted that it was not uncommon for individuals to have been misdiagnosed. By comparing healthy individuals with those who had been diseased, the researchers aimed to determine whether the bodies contained mercury and if dental plaque could serve as an effective research tool.

Dental plaque: the key to medieval methods

The study analyzed dental calculus from 76 individuals and 45 soil samples across four sites. Samples were collected from two medieval leprosaria: St. Leonard’s in Peterborough and St. Thomas d’Aizier in France. In most cases, soil samples were taken directly from the burial sites or their fill.

“We analyzed soil samples taken from the graves to assess whether mercury could have entered the calculus after burial,” said the study authors to Phys.org.

They followed a published protocol for sampling and cleaning dental calculus, which involved photographing the teeth and describing the calculus’s quantity, color, and texture. The samples were then stored in plastic tubes. Mercury concentrations were determined using atomic absorption spectrometry and cold vapor atomic fluorescence spectroscopy.

The results indicated that individuals buried in the leprosaria had significantly higher mercury levels than those from the other cemeteries. The soil samples confirmed that these individuals had been treated with mercury during their lifetimes, rather than absorbing it from the soil. “In addition, there is no evidence of local environmental sources, such as mining, that could explain these patterns,” added Dr. Fiorin to Phys.org.

Individuals buried in the chapel of the leprosarium showed the highest mercury levels, suggesting that their higher social status afforded them greater access to medical treatments. This detail indicates that mercury was an accepted treatment for leprosy, but was less accessible to those with fewer means.

New research approach

The study successfully established that dental calculus can be utilized in the study of medieval medicine and and applied to broader research efforts.

While future research is needed to further validate this emerging method, the authors have contributed to advancements in how this science is conducted, innovating research methods that will help scholars study the past with greater accuracy.