Testing the Effectiveness of Protocols for Removal of Common Conservation Treatments for Radiocarbon Dating

Abstract

To achieve a reliable radiocarbon (14C) date for an object, any contamination that may be of a different age must be removed prior to dating. Samples that have been conserved with treatments such as adhesives, varnishes or consolidants can pose a particular challenge to 14C dating. At the Oxford Radiocarbon Accelerator Unit (ORAU), common examples of such substances encountered include shellac, the acrylic polymers Paraloid B-67 and B-72, and vinyl acetate-derived polymers (e.g. PVA). Here, a non-carbon-containing absorbent substrate called Chromosorb® was deliberately contaminated with a range of varieties or brands of these conservation treatments, as well as two cellulose nitrate lacquers. A selection of chemical pretreatments was tested for their efficiency at removing them. While the varieties of shellac and Paraloid tested were completely removed with some treatments (water/methanol and acetone/methanol/chloroform sequential washes, respectively), no method was found that was capable of completely removing any of the vinyl acetate-derived materials or the cellulose nitrate lacquers. While Chromosorb is not an exact analog of archaeological wood or bone, for example, this study suggests that it may be possible to remove aged shellac and Paraloid from archaeological specimens with standard organic solvent-acid-base-acid pretreatments, but it may be significantly more difficult to remove vinyl acetate-derived polymers and cellulose nitrate lacquers sufficiently to provide reliable 14C dates. The four categories of conservation treatment studied demonstrate characteristic FTIR spectra, while highlighting subtle chemical and molecular differences between different varieties of shellac, Paraloid and cellulose nitrate lacquers, and significant differences between the vinyl acetate derivatives.

The Hoard of the Rings. “Odd” annular bread-like objects as a case study for cereal-product diversity at the Late Bronze Age hillfort site of Stillfried (Lower Austria)

“…The observed low solubility of the consolidant on sample A may be linked to the ageing of the PVAC, as photochemical degradation leads to cross-linking and chain scission in the material, under release of volatile compounds [76]. Issues regarding the removal of PVAC are, however, not limited to conservation: Especially in the field of radiocarbon dating PVAC can be problematic [77, 78]. As the PVAC used for consolidation on Sample A was not sufficiently removable by solvent treatments but swellable with water, the decision was made to manually remove it from a part of the surface of Sample A after the application of water soaked cellulose fibres.…”

Threading the Weft, Testing the Warp: Population Concepts and the European Upper Paleolithic Chronocultural Framework

“…For charcoal, available pretreatment methods appear to be largely reliable for producing accurate results Higham 2009, Haesaerts et al 2013). However, the isolation of collagen from bone samples for dating is more methodologically challenging than sometimes understood: standard methods such as ABA (acid-base-acid washes), with or without ultrafiltration, cannot be said to reliably remove all non-collagen material from a sample, and the removal of conservation materials, even with the use of additional solvent washes, appears particularly problematic (Brock et al 2013(Brock et al , 2018Marom et al 2013).…”

Amino acid racemization and its relation to geochronology and archaeometry

“…Because amino acids bound within biominerals are expected to be less susceptible to external contamination, the comparison of the radiocarbon dates obtained on a bulk sample and the amino acid fraction of that same sample have been used to provide an indication of the integrity of the sample . Isolating and dating individual compounds as a method to improve dating accuracy has been discussed since the 1960s in consideration of the severe difficulty associated with entirely removing extraneous carbon originating from diagenesis and other types of external contamination by conventional chemical pretreatment for radiocarbon bone dating . This isolated amino acid radiocarbon dating method has also been applied to soil .…”