In Situ, Non-Destructive Testing for Evaluating the Role of Pointing Mortar in Preventive Conservation Strategies. A Case-Study on Reigate Stone at the Wardrobe Tower, Tower of London

Abstract

The correct choice of pointing mortar is considered crucial to the conservation of historic masonry. A proliferation of cement and eminently hydraulic lime mortars since the late 19th century has accelerated the deterioration of built cultural heritage in many parts of the world. Whilst the use of softer, lime-based mortars in stone conservation is now common practice, their role in the overall conservation strategy of highly vulnerable building stones such as Reigate Stone requires assessment. In this paper non-destructive testing (NDT) is used across a two-year period to investigate the impact of different pointing mortar types in situ. NDT data on surface hardness and moisture are interpreted at different scales to assess moisture regulation of Reigate Stone masonry at the Wardrobe Tower, a ruined structure at the Tower of London, following repointing carried out in Spring 2017. Joints repointed using a hydraulic lime mortar (NHL3.5) are shown to regulate moisture in adjacent Reigate Stone blocks less well than those repointed using a lime putty mortar. However, despite an initially inappropriate recipe, older hydraulic lime mortars are in some instances shown to perform similarly to the lime putty mortar, suggesting that NHL can weather sympathetically. The results also indicate that, whilst pointing mortar type does play a role in the moisture regulation of individual stones, its effect is outweighed by both properties of the stone itself, such as strength and past decay, and by wider micro-contextual factors, such as exposure or adjacent topography. Findings from the Wardrobe Tower indicate that pointing mortar only plays a part in overall moisture regulation; to enable its effective functioning and minimise the need for repeated interventions, it may be necessary to take additional protective measures to mitigate moisture ingress, such as water run-off and channelling following heavy rainfall. The overall implication is that in vulnerable historic masonry such as Reigate Stone, sustainable conservation strategies must incorporate a broad appraisal of, and tailored response to, specific decay mechanisms. With careful calibration across repeated survey campaigns, data collected in situ using NDT can inform the role of pointing mortar within such strategies.