Dating Medieval Masonry Buildings by Radiocarbon Analysis of Mortar-Entrapped Relict Limekiln Fuels—a Buildings Archaeology

Abstract

This paper considers how the data returned by radiocarbon analysis of wood-charcoal mortar-entrapped relict limekiln fuels (MERLF) relates to other evidence for the construction of medieval northern European masonry buildings. A review of previous studies highlights evidence for probable residuality in the data and reflects on how this has impacted on resultant interpretations. A critical survey of various wood-fired mortar materials and lime-burning techniques is then presented, to highlight evidence suggesting that a broad spectrum of different limekiln fuels has been exploited in different periods and that growth, seasoning, carriage and construction times are variable. It is argued that radiocarbon analysis of MERLF fragments does not date building construction directly and the heterogeneity of the evidence demands our interpretations are informed by sample taphonomy. A framework of Bayesian modelling approaches is then advanced and applied to three Scottish case studies with contrasting medieval MERLF assemblages. Ultimately, these studies demonstrate that radiocarbon analysis of MERLF materials can generate reasonably precise date range estimates for the construction of medieval masonry buildings which are consistent with other archaeological, historical and architectural interpretations. The paper will highlight that these different types of evidence are often complementary and establish that radiocarbon dated building materials can provide an important focus for more holistic multidisciplinary interpretations of the historic environment in various periods.

The Castle of Achanduin, Lismore—a Point of Reference for the Radiocarbon Analysis of Mortar-Entrapped Relict Limekiln Fuels

“…The first "multidisciplinary" models presented by the study (1b, 2b, 3b & 4b) once more demonstrate the complementary character of MERLF 14 C data (from primary phase fabric) and documentary sources (which are often late incidental references to a completed building of some sort) (Thacker 2016(Thacker , 2019(Thacker , 2020. The documentary TAQ associated with Achanduin effectively truncates the positive side of the End Boundary distributions generated by all the "standalone" models (see, for example, Figure 2), whatever their shape, resulting in similar very refined constructional estimates in all "multidisciplinary" models at both 95.4% and 68.2% probability.…”

“…Recent work has highlighted that the principal potential sources of disjuncture between MERLF 14 C measurements and constructional completion of masonry structures are oldwood effect, lime transport and maturation times, and building construction times (Thacker 2020). The first of these has been mitigated in the Achanduin study by the selection of MERLF fragments for 14 C dating with relatively short-lived characteristics.…”

Medieval buildings and environmental change: chronology,ecology and political administration at Castle Sween, Knapdale

“…The methodology follows previously presented protocols (Thacker 2020) and in this study is comprised of four main strands:…”

“…Bayesian modelling techniques are employed within the OxCal programme (Bronk Ramsey 2009a) to generate preliminary date range estimates for the completion of each period of construction, initially using 'standalone' radiocarbon measurements and phasing information only (Thacker 2020). These chronological estimates are then used to situate the castle's constructional evidence within a wider environmental context.…”

“…As also suggested by site survey, the NW Tower and NE Tower distributions are not related to each other directly in the model, and an 'Order' function is included to calculate the probability that the NW Tower precedes the NE Tower on the basis of the radiocarbon data and stratigraphic relationships to phases 1 and 2 alone. The SMCCCP is continuing to refine Bayesian approaches to MERLF radiocarbon data which may be subject to inbuiltage but, following recent work at a number of other medieval castle sites across Scotland (Thacker 2020), each of the dates from Castle Sween has been situated within OxCal 'General' Outlier and 'Charcoal' Outlier models (Bronk Ramsey 2009b) depending on archaeobotanical character. No historical or architectural-typological data is included in the current 'standalone' model but will be discussed below.…”