What Makes Paint Stick? Identifying Plant Gum Binders
Scientists work with cultural heritage researchers to study how plant gums were used to bind pigments, from antiquity to the present day
Plant gums have been used for centuries to bind and stabilize paints, inks, cosmetics, food, medicines, and many other products. They have been used as binders in paints since the third millennium BCE in ancient Egypt. One of the most widely used plant gums is gum arabic, derived from the sticky exudate of the Acacia tree, which is dried into a powder useful for its high solubility and low viscosity. Gum arabic is derived from two species, A. senegal and A. seyal, yet there are more than a thousand species of Acacia tree, which grow mainly in the Sahel region that extends across the African continent into the Arabian peninsula.
Because of this diversity in plant material, researchers in heritage science and the food industry are interested in developing ways to distinguish among the many different sources of plant gums. Identification using chromatographic and spectroscopic techniques often produces ambiguous results, so researchers have developed a novel methodology based on parietal enzymatic digestion of the samples, followed by matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS). This is the first time a mass spectrometry methodology has been used in this kind of analysis. 
Center researchers are building an extensive database including mass spectra generated from more than 100 reference gum samples, with the aim to characterize and differentiate the plant gums—in some cases at the species level—in samples from art objects from various geographic locations and historic periods.
Making these distinctions helps cultural heritage researchers understand these art objects in many contexts, such as the environments where they were made, trade routes, techniques and processes for artistic production. Well-provenanced samples of gums from Acacia trees were collected from historical and botanical collections from the Kew Royal Botanic Gardens (London), The Field Museum of Natural History (Chicago), and the Faculty of Forestry (Sarajevo). The Acacia samples were supplemented with related gum sources like the the Astragalus (tragacanthgum) and Prunus (fruit tree gums). All samples were partially digested with a specific enzyme that can break down the gum polysaccharide structure into smaller fragments (oligosaccharides) that can be analyzed by the MALDI-MS, creating a characteristic mass spectrum fingerprint.
The same protocol was used to identify micro-samples taken from a painted wooden object dated from the third millennium BCE in ancient Egypt and a 20th-century painting by Georges Braque. Unknown samples may be mixed with other organic and inorganic materials, so researchers created mock-up samples containing gums with different pigments and media, to test how the identification methodology could operate in these conditions.
As a demonstration of ancient plant gum identification, Center researchers used microinvasive analysis to study two painted ancient Egyptian sculptures: Kneeling statue of Amenemophtemhat (Late Period, Saite 664 BCE-610 BCE) from the Metropolitan Museum of Art and the Statue of Osiris (Ptolemaic Period 4th century-1st century BCE) from the Art Institute of Chicago.
According to the Metropolitan Museum of Art records, the Kneeling Statue of Amenemophtemhat figurine was found inside the mummy of a wealthy woman. It represents one of the Four Sons of Horus, who protected the four internal organs that were removed during mummification. It was first examined with Fourier Transform Infrared (FTIR) analysis, which indicted the presence of polysaccharide materials that suggested plant gums and other carbohydrate materials had been applied to the Figurine. With an ancient object, the number of reference samples for historical plant gums is limited, but the initial MALDI-MS fingerprinting suggested several useful possibilities for identifying distinct plant gums, while also providing some areas for caution about what’s knowable with limited historical reference samples.
The same analytical strategy was applied to the study of a painted wooden Statue of Osiris. The statue was previously examined with X-ray fluorescence (XRF), FTIR< and Raman spectroscopy to better understand the painting technique and to obtain informational about the binding medium. FTIR analysis indicated the use of polysaccharide plant gum as the binder for the red and yellow paints, and visual analysis indicted that these area of paint were curiously cracked and flaking away from the surface, showing the white ground layer underneath. MALDI-MS analysis suggested the presence of a relatively less common plant gum A. tortilis, which is less water-soluble than A. Senegal; this solubility may account for the flaking.
In each case, researchers cautioned that these analyses are demonstrating the technology’s possible applications and limitations, which can indicate future directions for study. The initial results from these tests indicate that these mass spectrum fingerprints can be correlated with database results to identify unknown samples taken from artworks.
Read more about uses of the MALDI-MS analysis in identifying the innovative artistic techniques 20th-century artist Georges Braque applied to the surfaces of his paintings in a related project at the Art Institute of Chicago.
Publication:
Granzotto, C., K. Sutherland, J. Arslanoglu, and G.A. Ferguson. 2019. "Discrimination of Acacia gums by MALDI-TOF MS: applications to micro-samples from works of art." Microchemical Journal 144:229–241.