Reigate Stone at the Tower of London: Improving resilience in vulnerable historic masonry
Keywords: stone conservation
historic built environment
cultural heritage
architectural conservation
non-destructive testing
heritage science

This thesis constructs a scientific framework for examining resilience in vulnerable historic masonry, focusing on Reigate Stone at the Tower of London. Reigate Stone was medieval London’s principal freestone; it was used in high profile projects across South East England during a key economic growth period and represents an important chapter of architectural heritage. The UNESCO world heritage Tower of London is one of the UK’s most recognisable and culturally valuable sites. This combination affords opportunity for evaluating the deterioration of the historic built environment and aligning scientific methodologies with emerging paradigms in the interpretation and management of cultural heritage.
An integrated methodology was developed, linking non-destructive field surveys at several scales of investigation with archival research and experiments conducted in controlled environments. This enabled scientific investigation of the complex system dynamics affecting decay. Rather than system balance along a static material/ environment interface, decay is controlled by three axes of variability. (a) An unusual geological context coupled with vast exploitation of limited resources resulted in a wide physical variation in building stone. Mineralogical variability can be expressed according to the relative content of three different cementing components: opal-CT forms a weakly cemented porous matrix; clay minerals include hygroscopic swelling clays; calcite improves strength. (b) The multifaceted character of the historic built environment results in a high degree of present-day environmental variability and associated decay mechanisms. This can be expressed in degrees of shelter at a micro-contextual scale, with complex topographies providing distinct microclimates within single masonry units. Variation is also expressed at larger scales, e.g. Reigate Stone in urban or rural environments. (c) Variability in the past treatment and exposure of masonry has formed a high level of contingency, with the memory effect of past decay pathways contributing to ongoing decay. This axis is also manifest in changing attitudes to deterioration across time, which establish a complex reciprocity between decay process and remediation.
The implication of these findings on practical stone conservation is that no single strategy can be successful for all Reigate Stone masonry. Calcareous stones respond well to Hydroxylating Conversion Treatment. However, this consolidation technique is at best benign when used on more vulnerable argillaceous stones; water uptake during application can accelerate decay. Limewashing is safer, although in damp, salty environments coats have detached quickly. Salts are a common source of decay. A variety of mechanisms contribute to different decay patterns. The precise relationship between salt, microclimate and decay pattern is highly intricate; observable decay patterns are not a clear indicator of underlying mechanisms. The identification of any appropriate environmental controls must occur on a case-by-case basis. Constructing detailed case histories of vulnerable masonry will facilitate a more holistic approach to treatment. This can move from active conservation of individual stones to a more preventive approach based on revealing underlying system dynamics. In highly exposed Reigate Stone for example, improving water run-off must not only mitigate moisture ingress in the stone, but also protect the correct functioning of pointing mortar to minimise conservation cycles. This systems approach can improve overall resilience by aligning the appreciation of historic built environment with cultural landscape. This paradigm recognises the importance of symbiosis and diversity to system sustainability and prioritises the safe-guarding of intrinsic value over the preservation of materiality.