Radiocarbon age of consolidants and adhesives used in archaeological conservation

Abstract

Consolidants and adhesives used to conserve archaeological artifacts must be carefully removed prior to radiocarbon dating of the artifact, otherwise the radiocarbon signature will be compromised. It is therefore paramount to understand how the artifact was conserved and which conservation products were used in order to determine: (1) the best location on the artifact to sample; (2) how to remove the consolidant physically and/or chemically; and subsequently (3) whether or not the consolidant was successfully removed. The first two considerations are a matter of communication between the archaeologist, the conservator, and the radiocarbon laboratory, but the third consideration can be a bit tricky to determine. The archaeologist usually knows the approximate time period of the artifact given the context in which it was found so when the age is not as expected, it is possible the consolidant was not completely removed. However, it can be purely speculation unless the radiocarbon signature of the consolidant (old or young) is known. Here we present results from the radiocarbon and stable isotope analyses of 21 consolidants and adhesives commonly used for archaeological conservation. The consolidants and adhesives cover both natural (animal and fish glues, tree resins, starches) and synthetic materials (acrylics, poly(vinyl acetate), poly(vinyl butyral), polyethylene glycol, glycerol, cellulose ethers, cellulose esters, cyanoacrylates and soluble nylon) and are selected from those commonly in use now, as well as a few that were used historically but are now avoided due to poor aging qualities. This paper will present data that may indicate in which direction – young or old – conservation treatments may skew radiocarbon dates, the importance of knowing the conservation history of older samples and how these results should be interpreted. Stable isotope signatures of the consolidants and glues are also provided as the analysis is inexpensive and complimentary to the radiocarbon analysis and could be used to corroborate suspicions of contamination gleaned from offset radiocarbon ages of archaeological samples. Finally, we present the case study that motivated this project: the analysis of carbonized residuals vacuum impregnated with PVA glue.