Context 154 - May 2018

26 C O N T E X T 1 5 4 : M A Y 2 0 1 8 JESSICA HUNNISETT Mortar analysis for specifiers Used correctly, mortar analysis can provide a useful tool in the development of a specification. At worst, it may lead to inappropriate specifications being accepted. Mortar analysis is well established as a method of determining the composition of original mortar mixes found on traditional and historic buildings, and is often seen as the starting point for a repair specification. But how often is scientific analysis really required, and what value can it add for clients and specifiers? Minimal intervention and like-for-like repair is widely accepted and generally considered best practice for conservationwork.When repairing or replacing mortar or plaster finishes a like-for-like approach ensures a good match from an aesthetic and technical perspective, while keeping repairs to the minimum can ensure that earlier finishes are not needlessly removed and replaced for the sake of uniformity.Technically, being able to match the characteristics of earlier mortars and plasters reduces the risk of cracking due to differential movement or weathering due to incompatible materials. Retaining breathability by matching repair mortars can help maintain the equilibrium between mortar and masonry which helps keep buildings dry. Used correctly, analysis can form part of a holistic approach to understanding the construction of a building and for developing an appropriate repair strategy. Mortar analysis can range from a basic visual and physical assessment to highly sophisticated scientific testing to determine various properties, as outlined below, so it is useful to know from the outset what kind of information is required and how it might inform future decisions.The more information is required, the more expensive it is (a visual analysis and acid digestion testalone costs around £300), so the level of analysis needs to be justifiable from a client’s perspective.¹ Visual analysis Inspection in normal conditions and under a microscope of the fresh broken face of the sample, allows an assessment of colour, matched against standardised colour charts, identification of types and sizes of aggregate, the presence of unburned lime, shell or coal inclusions, and additives such as hair. A visual inspection will normally include a basic description of physical qualities, including whether it is soft/hard or friable, and can allow some assumptions to be made of the strength of the mortar. A test for carbonation The Haa of Sand, an 18th century Laird’s House in an exposed location in Shetland. A new harling specification was required to replace defective cement-based harling on a gable-end wall which was suffering from water damp penetration. Mortar analysis of a range of harling mortars from similar buildings in the area was commissioned to help find a suitable specification for the re-harling works. (Photo: Jessica Hunnisett, Historic Environment Scotland) Reference ¹ For more information on analysis methods, see Historic England (2012) Practical Building Conservation: mortars, renders and plasters and Peter Ellis (2002) The Analysis of Mortar: the past 20 years , Historic Churches.