Reigate stone was extensively used in medieval London and is prone to rapid decay. A variety of different conservation treatments has been applied in the past; in many cases, these have not mitigated ongoing decay. This paper presents an overview of wax, limewash, silane and ammonium tartrate treatment at the Tower of London and Hampton Court Palace. Documentary analysis and visual inspection indicate that whilst these methods have provided protection to some stones, no single method has resulted in the protection of all stones. Non-destructive and minimally-destructive testing is used to more closely assess the effects of ammonium tartrate treatment. The results imply that inherent stone mineralogy, past decay pathways and/or present environmental factors are a greater influence on ongoing decay than treatment histories.

Background: Conservators and architects working with built heritage at risk from ground moisture intrusion face a difficult choice. Leaving the building unprotected can lead to moisture related deterioration; installing vertical and/ or horizontal barriers using standard materials requires chemical or mechanical intervention with the historic fabric. The use of compacted clay to form a barrier is commonplace in some areas of environmental engineering, and can be applied to built heritage conservation. Naturally occurring or slightly modified soils were traditionally used for protecting buildings in a number of vernacular techniques. Knowledge of what types of soils are suitable for use could provide certain regions with a low-cost, low-impact alternative for protecting historic buildings. Three commercially available products previously shown to have low hydraulic conductivity were analysed in order to identify waterproofing mechanisms; these included two specialised bentonite-sand mixtures and a Saxonian glacial till. Results:The till relies on a high proportion of densely agglomerated fine grains to achieve a low permeability when consolidated, possibly assisted by the presence of calcite. The specialised mixtures rely on an engineered grain size distribution, with practically no silt sized grains, and a fraction of sodium bentonites capable of forming colloidal suspension when compacted. All materials showed a low risk of shrinkage when compacted at optimum moisture content. Conclusion:Different mechanisms appear to account for the low hydraulic conductivities found in clay barrier material. The implication is that a range of soil material may be suitable for protecting built heritage from ground moisture intrusion and its associated decay mechanisms. The suitability of glacial till may present large areas of Northern Europe with locally available barrier material.

Reigate Stone was used in high profile projects across London during a key growth period and represents an important chapter of architectural heritage. Historic Reigate masonry is subject to inherent variability. It is prone to rapid decay; however, highly decayed and well-preserved stones are frequently adjacent. This inherent variability in masonry can present a challenge to the design of conservation strategies by obscuring or complicating the identification of decay processes. This paper presents a model for assessing the combined impact of construction economies and mineralogical variability (Graphical abstract), by synthesising archival research on the history of Reigate Stone with experimental analysis of its properties. The limitations of the local geography coupled with the demands of the medieval building industry are shown to have introduced inherent variability into the built fabric at an early stage. Later socio-economic factors are shown to have compounded these by contributing to selective recycling, replacement and contamination of Reigate Stone. These historic factors augmented pre-existing mineralogical variability. This variability makes classification according to commonly used stone types difficult. Experimental analysis correlates variable cementing components with hygro-physical properties related to resilience. Calcite content influences strength properties and capillarity; clay content influences moisture adsorption and retention; opal-CT forms a weakly cemented, porous matrix. These presented different decay pathways to a range of environmental mechanisms and agents of decay. The findings suggest that inherent mineralogical variability, environmental changes, and historic contingency must all be considered in the design of ongoing Reigate Stone conservation strategies.

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