Learning more about the materials used on historical paintings—paints, pigments, varnishes, and primers used to prepare canvases—is critical to ongoing conservation efforts. Apparently, many artists of the so-called Danish Golden Age used beer byproducts from local breweries to prime their canvases, according to the results of a proteomics analysis described in a recent paper published in the journal Science Advances.

A number of analytical techniques have emerged over the last few decades to create "historical molecular records" (as the authors phrase it) of the culture in which various artworks were created. For instance, studying the microbial species that congregate on works of art may lead to new ways to slow down the deterioration of priceless aging art.

Case in point: scientists analyzed the microbes found on seven of Leonardo da Vinci's drawings in 2020 using a third-generation sequencing method known as Nanopore, which uses protein nanopores embedded in a polymer membrane for sequencing. They combined the Nanopore sequencing with a whole-genome-amplification protocol and found that each drawing had its own unique microbiome.

And just last month, researchers examined mysterious black stains on a folio of Leonardo's Codex Atlanticus and confirmed the presence of starch and vinyl glues in the affected areas. The glues were most likely applied during an earlier restoration effort some 50 years ago. The researchers also identified a likely cause of the dark stains: nanoparticles of a mercury sulfide called metacinnabar in the protective paper holding the folio, although it's unclear how this unusual black crystalline phase might have formed. The mercury salts had been added to protect the codex from mold growth.

Mass spectrometry-based proteomics is a relative newcomer to the field, according to the authors, and is capable of providing a thorough and very detailed characterization of any protein residues present in a given sample, as well as any accumulated damage. The technique is so sensitive that less sample material is needed compared to other methods. And unlike, say, gas chromatography-MS, it's also capable of characterizing all proteins present in a sample (regardless of the complexity of the mixture) rather than being narrowly targeted to predefined proteins.

For instance, the three most common protein-based materials used as the sole standards for other analyses are egg, animal glue, and milk. "The presence of proteins originating from sources outside this group of arbitrarily preselected standard source materials will not be detected, potentially leading to false-positive results and inaccurate interpretations," the authors wrote.

For their own study, Fabiana Di Gianvincenzo of the University of Copenhagen and collaborators selected 10 paintings from two artists prominent during the Danish Golden Age: Christoffer Wilhelm Eckersberg, often deemed the "father of Danish painting" and an instructor at the Royal Danish Academy of Fine Arts, and his student Christen Schiellerup Købke.

The authors selected paintings produced in the 1820s and 1830s, when both were at the academy. "Since the Academy is known to have provided artistic materials for professors [Eckersberg] and students [Købke], this sample set allowed for a direct comparison of canvases that were most likely prepared by the Academy's craftsmen and those prepared outside the institution," the authors wrote. Their analysis included three paintings that Købke completed after leaving the Academy to make that comparison.