Beer byproducts were popular canvas primers for Danish Golden Age artists

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. Eckersberg's diaries reveal that he often relied on helpers at the Academy to prepare his canvases using "white or off-white grounds," through the late 1830s, when commercially primed canvases, usually imported from Dresden, Germany, became widely available in Copenhagen. The Academy canvases had a single white layer, while the later commercial canvases featured a lower reddish or light brown later, topped by a white or light gray layer. None of the ten painting canvases analyzed for this study—including the three later ones by Købke—showed that telltale double primer layer, so they were not commercially imported canvases. As for acquiring samples, most Danish Golden Age paintings housed in the National Gallery of Denmark underwent conservation treatments in the 1960s, including a preventative measure called wax-resin lining prior to (temporary) transport to other museums. This involves attaching a new canvas to the back of the original with a mix of beeswax and dammar resin, in addition to trimming the edges of the original before applying the adhesive. The National Gallery preserved the trimmings from each painting, and these were the samples used in this latest analysis. The analysis revealed the presence of proteins from baker's yeast and several species of cereals in seven of the ten paintings produced between 1826 and 1833 when both artists were at the academy—the first time such proteins have been identified in an artwork. None were found in the canvases of Købke's three later paintings. The presence of these proteins may be due to environmental contamination, but the authors argue that the concentration is much too high for this to be a viable explanation. Instead, Di Gianvincenzo et al. suggest two likely alternative sources: fermented flour glue and beer brewing byproducts. The former is a paste made by adding flour to boiling water and cooking it until it thickens. The paste can then be fermented, cooked, or dried and pulverized for better preservation, and it's a common component of several ground layer recipes, per the authors. This would explain the cereal proteins detected in the mass spectrometry analysis. However, the authors also created their own mock-up canvas ground layers to test the properties of this material and found that fermented flour glue was prone to poor cohesion and extensive cracking, making it unsuitable for such a purpose. As for the brewing byproducts, "In 19th century Denmark, beer brewing was a paramount part of the culture and economy," the authors wrote. Beer was regularly consumed, given the scarcity of potable water. It was also used to clean and restore paintings, as well as acting as an adhesive, paint binder, and painting lining. The protein composition of barley and lager beer is already known, and many of the same cereal and yeast proteins appeared in the analysis of the canvas samples of this latest study. "Residual yeast could have been bought by the Academy as waste material from the beer production taking place in Copenhagen," the authors wrote. It was unlikely to be actual beer, since beer was a precious commodity at the time, sometimes even used as currency to pay salaries. The authors also considered whether byproducts from brewing aquavit—a common traditional Scandinavian distilled spirit—but ruled it out. "Although aquavit was initially prepared purely using grains, the easier cultivation of potatoes in Scandinavia has made the tuber a worthy substitute since the 18th century," they wrote. And no potato proteins turned up in their analysis. Nor were there any egg proteins in any of the samples, even though a previous 2017 study using gas chromatography-MS detected them. This confirms the authors' concern about false positives using that method. They suggest that for future proteomics studies, both methods should be used, as they are complementary. DOI: Science Advances, 2023. 10.1126/sciadv.ade7686  (About DOIs).