Effects of consolidants and their removal by polar solvents on the stable isotope compositions of bone

Abstract

Although conservation practices facilitate the preservation of museum collections, procedures used to stabilize ancient materials can alter their chemical and isotopic compositions, impacting the viability of such specimens for life history investigations. In this study, we applied two common consolidants – polyvinyl acetate (PVAc) and acrylic resin (Paraloid B-72™) – followed by two consolidant removal procedures to assess the impacts of these processes on the stable isotope compositions and structural characteristics of a series of subsamples taken from a representative archaeological faunal bone. We examined the collagen (δ13Ccol and δ15Ncol), structural carbonate (δ13Csc and δ18Osc), and phosphate (δ18Op) isotopic compositions of the bone sample before and after consolidation to evaluate consolidant removal procedures. We also measured δ13Csc, δ18Osc, and structural characteristics before and after the pre-treatment commonly used to remove organic matter and secondary carbonate prior to isotopic analyses.Our results produced five main outcomes. First, an acetone treatment shorter than 48 h was sufficient to remove PVAc and acrylic resin from archaeological bone without altering the original δ13Ccol and δ15Ncol. Second, prolonged exposure (>48 h) to polar solvents during the consolidant removal procedure and collagen extraction modified collagen isotopic compositions, lowering its original δ13Ccol by 0.3–0.4 ‰ and increasing its original δ15Ncol by up to 0.9 ‰. Third, the consolidant application and removal procedures altered the original δ13Csc and δ18Osc. Fourth, the change in δ18Osc was likely caused by the bleach (2% NaClO at 20 °C for 72 h)/acetic acid (0.1 M for 4 h at 20 °C) pre-treatment applied prior to isotopic analyses. Fifth, our results confirm and expand on previous studies reporting that δ18Op remains unaltered after consolidant and solvent procedures. These outcomes provide a framework for obtaining reliable stable isotope compositions from archaeological bones that have been stabilized using consolidants.