Carlos Rodriguez-Navarro, Luis Monasterio-Guillot, Miguel Burgos-Ruiz et al.
Science Advances, 19.04.2023
Vol 9, Issue 16
Materials and Methods
Excerpts
Collection and analysis of Copan plasters
Lime plaster samples were collected at the Classic Maya center of Copan, a World Heritage Site (declared in 1992), located in the northwest of Honduras (36° 53′ 51.286" N; 95° 55' 33.934" W), close to the border with Guatemala. This city state was the premier site in the south-eastern border of ancient Mesoamerica (22). Following the first written accounts of its existence by the Spaniard Diego García de Palacio back in 1576, the site was “rediscovered” in the 19th century, drawing international attention after the expedition by Stephens and Catherwood in 1841 (22). Since then, continuous archaeological research, site reconstruction, and conservation works have taken place, uncovering the magnificent structures of the so-called “Copan Main Group”, also known as the Acropolis, including Structure 10L-26 that displays the famous Hieroglyphic Stairway, the longest written record of the ancient Maya world known to date (22). As a result of these extensive archaeological works, it is now possible to access the remains of lime plasters at the site, both in the different structures and in the tunnels opened within them, such as tunnel 74 accessing “Rosalila,” the substructure enclosed in what is now Structure 10L-16 of the Copan Main Group (22), as well as the plaster collection at the Copan Museum of Sculptures. Note that, here, the term plaster is broadly used to refer to lime plasters, stuccoes, and mortars: i.e., lime-based structural (mortar), finishing (plaster), and decorative/sculptural (stucco) materials (3).
Samples of Copan plasters (N = 6) were collected from different areas of the archaeological site (Fig. 1 and Table 1). Plasters’ textural features were studied using PLM (Olympus V) with transmitted and reflected light, using uncovered polished thin sections. The mineralogy was determined by XRD on a PANalytical X’Pert Pro equipped with Ni filter (measurement parameters: Cu Kα radiation λ = 1.5405 Å, 45 kV, 40 mA, 3° to 70° 2θ exploration range, steps of 0.001° 2θ, and goniometer speed of 0.01° 2θ s−1). Powder samples were deposited on zero-background Si sample holders for analysis. Mineral phases were identified by comparison with International Centre for Diffraction Data powder spectra and quantified by means of the reference intensity ratio method. The bulk chemical composition of representative Copan plasters was determined by x-ray fluorescence (XRF) on a PANalytical Zetium. TG-DSC was performed on a Mettler-Toledo TGA/DSC1 to determine the amount of organics and carbonates. The historic plaster samples were also analyzed by attenuated total reflectance (ATR)–FTIR (Jasco) in the spectral range of 4000 to 400 cm−1 with a step size of 0.48 cm−1. Additional microstructural and compositional features of Copan plaster samples were determined by FESEM (Zeiss AURIGA) equipped with EDS microanalysis. Samples were carbon-coated before FESEM observations. Ultrastructural features and detailed chemistry of phases in the historic samples were studied using TEM on a Titan and a Talos (FEI) with acceleration voltage of 300 and 200 kV, respectively. Both TEMs are equipped with HAADF detector for Z-contrast imaging and EDS for microanalysis (performed in STEM mode).
The porosity and pore size distribution of the plaster samples were determined using mercury intrusion porosimetry on an Autopore 9800 porosimeter (Micromeritics). Pieces with a mass of ~1 g were cut from the plaster samples and dried in an oven (24 hours at 80°C) before analysis.
Extraction and characterization of Chukum and Jlots aqueous extract
The sap of the barks of Chukum (B. simaruba) and Jiote (H. albicans) collected at Copan (March 2018) was extracted following immersion of freshly collected bark pieces (~2 cm by 2 cm by 0.4 cm in size) in MilliQ water at room T (20°C) for 48 hours. The solid content after drying of the mucilaginous extract was, in both cases, ~5 g liter−1. The ATR-FTIR spectra of the solid residues were very similar and show that the bark extracts of Chukum and Jiote are basically polysaccharides with abundant carboxylic functional groups (see details of the FTIR characterization in Supplementary Text and fig. S11A). Previous studies have shown that the aqueous extract of B. simaruba includes polysaccharides with abundant (poly)carboxylic acids, along with xylose and tannin (responsible for its reddish coloration) (45). Both Jiote and Chukum aqueous extracts have been reported to include galactose, arabinose, and rhamnose residues (8). TG/DSC analysis (in air atmosphere) of both plant extracts showed a broad exothermic band at ~200° to 400°C, with an associated mass loss corresponding to the thermal decomposition in air of the polysaccharides (fig. S11B).
«Materials and Methods»
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