Historic Mortars from Tuquoy, Westray, Orkney Islands: a SEM-EDS study

The principal aim of this investigation, was to assess the evidence for the composition of the provenance of the binder in three historic mortar specimens from Tuquoy. In addition, general description of the mortar specimens supplied reveals features of interest, particularly fragments of slag in one specimen. These can contain information indicative of the composition of materials calcined for production of the binder, and also conditions of firing, such as temperature and reduction / oxidation during production.
Analysis of the specimens from Tuquoy revealed a variable composition and texture of mortar between samples, characterised in the main by the differences in the non-binder fraction (the aggregate, additives and/or inclusions). Overall, the binder fraction in TWO-4 and 8, was composed of a high-Ca carbonate, with only minor (< a few %) evidence of other elements such as Mg, S or Fe. This was indicative of course, of a similarly composed carbonate source, certainly comparable to the biogenic sources seen as aggregates in the mortars, something also supported by the evidence of pseudomorph textures found in calcined lime inclusions, similar in dimension to the biogenic aggregates. The binder phase in sample TWO-9 has a consistently higher content of Si and Al, possibly indicating either a hydraulic composition or the incorporation of fin grained silicate minerals incorporated as part of the aggregate.