Context 154 - May 2018

C O N T E X T 1 5 4 : M A Y 2 0 1 8 29 LIME AND STONE used in the original mix, although common additions such as hair or shell will generally be picked up by visual inspection. Mortar analysis can give an indication of the method of manufacture (whether it was hot-mixed, putty or dry hydrate) but, again, while the clues may be evident within the basic visual inspection, they are elucidated only by advanced methods of analysis, such as SEM microscopy or pore-structure analysis. With all these assumptions and qualifications, when should specifiers or conservation officers request a mortar analysis, and how should they interpret the results? When is mortar analysis required? On some level, an analytical approach to observing and understanding lime mortar should be a skill that all conservation professionals and particularly specifiers should develop. The ability to recognise features of lime mortars, and describe the colour, texture, presence of lime and other inclusions, shell and aggregate size and appearance is something that can be developed over time simply by routine observation. This is not to undermine the value of specialist analysis, but the question should not be ‘should I get a mortar analysis?’ but ‘what do we want to learn from analysing the mortar and how will we use the information?’ A competent conservation-accredited building surveyor, architect or other professional tasked with specifying mortars should be able to make observations about the mortars found in buildings, and to understand when further information from a specialist analysis is necessary. Mortar analysis is required whenever there is insuf- ficient information available about the existing mortar to enable a repair specification to be developed, or where there is a need to understand the particular characteristics of a mortar which can not be identified from a site inspection (such as reasons for failure). In this respect it would normally form part of any significant mortar-repair proposal. Clients and statutory bodies should request a mortar analysis if there is uncertainty about the appropriate repair specification for a traditional or historic building, and particularly if large-scale works are proposed using lime-based or other materials. Mortar analysis results should always be considered in the light of the condition of the building, its proposed and existing uses, the overall repair approach being implemented and the maintenance plan. If used to help with matching new mortar to existing or original materials, the mortar analysis should be accompanied by a comprehensive explanation of the findings, and should be read with an awareness of the limitations of the data. 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 simply by virtue of having an analysis attached. A mortar sample with coal and lime inclusions, from Doune Castle (Photo: Callum Graham, Historic Environment Scotland) A thin-section image (blue resin) (Photo: Bill Revie) The author acknowledges the contributions of Dr Callum Graham and Dr William Napier in the preparation of this article. Jessica Hunnisett, a conservation-accredited chartered building surveyor, is senior technical officer at Historic Environment Scotland.