Mortar Analysis

Date 2018

Event ID 1089983

Category Recording

Type Mortar Analysis

Permalink http://canmore.org.uk/event/1089983

NM 86079 43497 A programme of landscape, buildings and materials analysis is being carried out at Lismore Cathedral within the framework of this project. This has included examination of both main parts of the building, including the W area of the nave and W tower (excavated in a community dig over two seasons), and the standing structure widely recognised as the former cathedral choir. Both of these parts of the building have previously been regarded as multiphase medieval structures. The maintenance of a strip of burial ground between these two multiphase parts of the building has precluded direct assessment of any surviving

stratigraphic relationship between them, however; and the architectural style of the surviving fabric is also typologically broad. As a result, the overall phasing and chronology of the former cathedral is poorly understood.

Buildings analysis suggests the structure of the building is generally consistent with 20th-century descriptions. The stratigraphic relationship between the W tower and nave is clearly displayed in the robbed-out basal plinth socket in the external face of the W nave wall, which is overbuilt by the abutting S and N walls of the tower. Further evidence for this event is presented by incidental (probably secondary) sandstone use in the tower foundation. Any abutment between the choir walls and pultpitum in the upstanding part of the building is currently obscured by internal cladding and external mortar coatings, although some fragmentary evidence for rubble tusking at the SW corner (just above ground level) suggests that these side walls did previously continue westward. All these different parts of the building

have been constructed of lime-bonded rubble. On-site analysis of the surviving and exposed constructional mortars in the building, however, suggests these are predominantly wood-fired calcined oyster shell-lime mortars of remarkably consistent character.

The cathedral has been subject to a continuing programme of materials sampling and lab-based analysis. At the time of writing the sample assemblage contains 39 samples, including: 7 mortar fragments, 7 sandstone fragments, 3 marine shell fragments, and 22 relict fuel inclusions. Lab-based analysis has included thick and/or thin section microscopic analysis of the mortar and sandstone assemblage, shell and fuel assemblage, and radiocarbon analysis of selected fuel samples from the W Nave and W Tower.

The evidence from the analysis suggests the fabric of Lismore Cathedral can be separated into four main suites of masonry evidence, of contrasting character. This includes:

A – The surviving masonry of the choir and nave walls. This is characterised by a formally coursed rubble masonry style, dominated by large basalt face blocks, bound and/or coated with a wood-fired shell-lime mortar, and framed with buff coloured sandstone dressings consistent with an Inninmore source. The relict fuel evidence in the constructional mortar of the W Nave includes a mixture of taxonomies, but is dominated by Quercus.

B – The surviving masonry of the W Tower. This is characterised by an informal rubble masonry style, dominated by limestone slabs, bound with a wood-fired shell-lime mortar, but with no surviving sandstone dressings (except incidental probable secondary use). The relict fuel evidence in the constructional mortar of the W Tower includes a mixture of taxonomies but is dominated by Betula.

C – The surviving masonry of the S nave doorway. This is characterised by at least two different sandstone types, consistent with Lochaline and Carsaig sources in Morvern and Mull, and a more fine-textured mortar with some evidence for altered limestone.

D – The visible masonry of the choir Pulpitum. This is characterised by two different blue-green and very light buff/white sandstones, core rubble dominated by limestone blocks, and a probable wood-fired shell-lime mortar.

The Lismore Cathedral evidence for calcined oyster shell-lime mortars (on an island dominated by limestone geology), changing fuel taxonomies and changing multiple

sandstone sources, is striking, and these four suites of masonry evidence appear to be consistent with at least three and probably four constructional phases. Despite some previously recognised contrasts in moulding details and floor heights, however, this study also draws attention to similarities in the character and form of the nave and choir walls. The evidence for contrasting mortar in the S nave doorway is more fragmentary, and requires further work, but does at least suggest that this feature (with its wider range of sandstones and later mouldings) is likely to be secondary.

Archive: NRHE (intended)

Funder: University of Stirling and Historic Environment Scotland

Mark Thacker – University of Stirling

(Source: DES, Volume 19)