Modelling medieval masonry construction: taxa-specific and habitat-contingent Bayesian techniques for the interpretation of radiocarbon data from Mortar-Entrapped Relict Limekiln Fuels

Mark Thacker

doi:10.1186/s40494-021-00568-3

Abstract

Using data from simulated and actual case studies, this paper assesses the accuracy and precision of Bayesian estimates for the constructional date of medieval masonry buildings, generated from the radiocarbon evidence returned by different assemblages of wood-charcoal mortar-entrapped relict limekiln fuel (MERLF). The results from two theoretical studies demonstrate how Bayesian model specifications can be varied to generate a chronologically continuous spectrum of distributions from radiocarbon datasets subject Inbuilt Age (IA). Further analysis suggests that the potential for these distributions to contain the date of the constructional event depends largely upon the accuracy of the latest radiocarbon determination within each dataset, while precision is predicated on dataset age range, dataset size and model specification. These theoretical studies inform revised approaches to the radiocarbon evidence emerging from six culturally important Scottish medieval masonry buildings, each of which is associated with a wood-charcoal MERLF assemblage of different botanical character. The Bayesian estimates generated from these radiocarbon datasets are remarkably consistent with the historical and archaeological evidence currently associated with these sites, while age range distributions suggest the IA of each MERLF assemblage has been constrained by the taxa-specific and environmentally contingent lifespans and post-mortem durabilities of the limekiln fuel source. These studies provide further evidence that Bayesian techniques can generate consistently accurate chronological estimates for the construction of medieval masonry buildings from MERLF radiocarbon data, whatever the ecological provenance of the limekiln fuel source. Estimate precision is contingent upon source ecology and craft technique but can be increased by a more informed approach to materials analysis and interpretation.

Highlights

Scholars across the world often face significant challenges in ascribing constructional dates to masonry buildings with enough precision to enable meaningful interdisciplinary environmental or historical discourse.The independent evidence returned by radiocarbon analysis of mortar-entrapped relict limekiln fuel (MERLF) fragments can usefully inform these chronological ascriptions, and the sedimentary context within which these materials survive presents some valuable characteristics to facilitate interpretation of that data
This paper describes a re-evaluation of these Bayesian frameworks, with a concern to further characterise statistical relationships which might pertain between the different limekiln fuel resources exploited during the construction of medieval masonry buildings, and the archaeological potential of any surviving MERLF materials
This paper has presented further evidence that a range of different tree taxa were exploited for limekiln fuel in Scotland during the medieval period, and the range of MERLF taxa surviving from this process are generally consistent with regional phytogeographic distributions

Summary

Introduction

Scholars across the world often face significant challenges in ascribing constructional dates to masonry buildings with enough precision to enable meaningful interdisciplinary environmental or historical discourse.The independent evidence returned by radiocarbon analysis of mortar-entrapped relict limekiln fuel (MERLF) fragments can usefully inform these chronological ascriptions, and the sedimentary context within which these materials survive presents some valuable characteristics to facilitate interpretation of that data. The widespread use of wood fuel in pre-industrial limeburning practices has often resulted high concentrations of wood-charcoal MERLF fragment inclusions, and this material has well-recognised radiocarbon dating potential [6, 7]. These characteristics of durability, distinctiveness, relative ubiquity, and radiocarbon dating potential are all underpinned by a rapid post-depositional mortar set, which allows an unequivocal archaeological association between the mortar’s constituent components and the surrounding masonry fabric [8], and precludes infiltration by other materials in later periods. It is crucial for wider interpretation that mortar phasing is correctly understood, and all radiocarbon measurements are accurate [9], but thereafter the determinations returned by all wood-charcoal relict limekiln fuel fragments are expected to calibrate to dates which are no later than the initial deposition of the masonry mortar within which they were entrapped [10,11,12,13]

Methods

Findings

Discussion

Conclusion