An assessment of procedures to remove exogenous Sr before 87Sr/86Sr analysis of wet archaeological wool textiles

Strontium isotope analysis (Sr-87/Sr-86) has been employed as a provenancing tool for archaeological wool textiles. To date, the effect of post-depositional (soil burial environment) contamination on keratin samples, which contain -similar to ppm concentrations of Sr, has not been rigorously investigated. We compared published methods for removing exogenous Sr from keratinous textiles, using either: (1) compressed N-2 gas, (2) HF(aq) solution (with and without a strong oxidising agent to remove dyestuff) or (3) organic solvents. Sr-87/Sr-86 ratios and Sr contents were determined in undyed and madder-dyed/alum-mordanted moieties of the same wool textile, buried for up to three years in contrasting environments (marine sediment/fenland bog), and two archaeological textiles recovered in Iceland (one typical and one atypical of local manufacture). Undyed experimental samples had low Sr contents (0.07-0.29 ppm) that were increased by both dyeing (0.14-8.92 ppm) and soil burial (0.11-15.01 ppm). The efficacy of Sr removal was: HF(aq) oxidising agent > organic solvents > HF(aq) > compressed N-2. Unburied samples showed little variation in Sr-87/Sr-86 ratio between cleaning methods (0.00006-0.00035); buried samples showed greater variation (0.00257-0.00713). Archaeological samples showed Sr contents greater than experimental soil burials (1-118 ppm), and Sr-87/Sr-86 values consistent with Icelandic groundwater (0.70357 0.70540). No cleaning methods retrieved original (unburied and undyed) Sr-87/Sr-86 ratios except treatment with compressed N-2 in undyed samples. Exogenous Sr from the short term soil burial environment is probably mostly present as particulates. We conclude that 87Sr/86Sr ratios of archaeological wool textiles recovered from wet burial environments do not accurately reflect wool provenance even after cleaning with the methods investigated. (C) 2014 Elsevier Ltd. All rights reserved.