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Material composition, provenance and production technology using analytical techniques from the physical sciences

Technical and scientific data on existing historic materials and construction technologies are needed in order to repair and conserve historic and traditional fabrics.  Ancient mortars, stone and ceramics often weather over time and need to be looked after. New repairs should be quality, durable materials compatible with the original fabric.

CNV00016Petrographic micrograph of a mortar from Skellig St. Michael in Co. Kerry including coarse rounded sandstone aggregate. Finer aggregate includes sandstone, quartz, microsilica, clay minerals (probably ilite), chlorite fragments and occasional shell (red). The lime binder, consisting of carbonated lime with occasional amorphous hydraulic cements and crystalline phases, is locally missing. Parallel polars, 2X. Field of view 7.1 mm.

This work applies established analytical techniques of the physical sciences to historic mortar, stone and ceramics in order to gather evidence of production technologies, provenance, composition and sources of raw materials. Petrographic microscopy, XRD, XRF, SEM/EDX, firing and physical laboratory tests were used to study the materials. Based on the results, stone quarries were located for replacement, and quality, durable replicas of the mortars, renders and ceramics were produced.

This research has assisted the repair and conservation of National Monuments to preserve them, in good health, for future generations. Historic buildings reflect culture, architecture, history, tradition and identity and are of strategic importance to both the society and the economy. Hence, tourism in Ireland is one of the biggest contributors to the economy, accounting for about 4% of GNP and employing over 200,000 people.

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Microphotograph of Roman masonry mortar (I to III c. AD) including angular ceramic aggregate in a homogeneous, cohesive binder of carbonated lime which remains unaltered. A perfect binder-aggregate bond is also evident. Plane polars 2X.

General microfabric of Roman plaster (finishing coat) including abundant angular ceramics in a carbonated lime binder which remains unweathered. Plane polars 2X.

Facts on lime technology relating calcination and slaking were obtained through petrographic analysis. The analyses also revealed the composition and origin of raw materials, pozzolanic additions and mortar hydraulicity. Based on these outcomes, new repair mortars for historic structures were designed including Clonmacnoise Monastic site, Clonfert Cathedral, Ardfert Cathedral, Askeaton Castle, Skellig Michael, Adare Castle and others. Often, the bricks studied were hand-moulded with a silica-based, predominantly non-calcareous clay gathered locally, including fluxes and a high percentage of temper. The mineralogy and petrography of the brick together with the presence of pebbles and a coarse-matrix suggest that the clay was gathered from glacial deposits. The transformation of limestone temper involving breakdown of calcite and generation of calcium silicates, new-formation of plagioclase, high-temperature quartz, hematite and spinel indicate that they were fired in clamps at temperatures ranging from 750 to above 900 ºC. The presence of spinel in ‘hot spots’ suggests that fuel was added to the raw clay to assist firing.

Project coordinator: Associate Prof. Sara Pavia, Funded by: Office of Public Works (OPW)