Animal, vegetable or mineral? Characterising shell-lime, maerl-lime and limestone-lime mortar evidence from the Late Norse and Medieval site of Tuquoy, Orkney

Recent examination of an extensive curated assemblage of mortar samples, removed from the Late Norse and Medieval site of Tuquoy (Orkney) during excavation in the 1980s, suggested the collection was associated with distinct groups of compositionally contrasting materials related to discrete constructional events. Subsequent petrographic analysis supported this early interpretation and presented evidence for a remarkable series of phase-specific mortars, bound with a range of different biogenic and geogenic lime source materials - including marine shell, coralline algae (maerl) and limestone. Wider landscape survey highlighted the broad range of exposed calcareous materials in the coastal and sedimentary environments dominating the Northern Isles of Scotland today, and that many of these different potential lime sources were exploited by craftspeople at different times in the Medieval and later period is now clear.

Given the high significance of the Tuquoy mortar study for our understanding of the development of this culturally important site, and as a prelude to more general publication of the wider archaeological project, a further investigation of selected samples from the mortar assemblage is now being undertaken through a range of geoscientific techniques. This paper presents emerging evidence from a comparative petrographic, SEM-EDS and XRD study designed to further characterise these various mortar materials, and challenge those previous interpretations of contrasting building lime sources. Like most environmental archaeological investigations, this study is essentially concerned with interpreting the depositional histories of surviving materials, but with a particular focus on establishing the distinction between (anthropogenic) kiln relict and (natural) added temper mixtures when both contain biogenic and geogenic clasts.