Scientific publications

These are lists of articles published in academic journals and conference proceedings. The top list includes publications from the Reigate Stone doctoral research carried out at the Tower of London between 2017 and 2020. The bottom list includes other publications relating principally or significantly to Reigate Stone research. The lists are sorted by publication date. Links open in a new tab.

Reigate Stone at the Tower of London research project (2017-2020)

Michette, M., Viles, H., Vlachou, C., & Angus, I. (2022): Engineering Geology, 303, 106641.

Abstract: In order to evaluate the potential for using environmental controls as a preventive conservation strategy for Reigate Stone masonry, this paper tests the hypothesis that powdering and flaking decay patterns are a direct consequence of climatic variables. Samples of masonry affected by each decay pattern are analysed for soluble salt content using ion chromatography. The results are used to build thermodynamic models in ECOS Runsalt. These identify NaCl and NaNO3 crystallisation in both environments, with NaCl dominating in powdering masonry and NaNO3 dominating in flaking masonry. Freshly quarried samples of Reigate Stone with a range of physical characteristics are contaminated with a mixture of these salts. Replicates of each sample are subjected to two distinct accelerated ageing regimes. These regimes are based on environmental monitoring and designed to reproduce the observed decay pathways. One set is subject to partial immersion and relatively stable, high RH, to stimulate a zone of NaCl crystallisation near the surface. The replicate set is subject to occasional wetting and frequent drops in RH, to stimulate NaNO3 crystallisation within the fabric of the stone. The results provide some indication that the intended decay mechanisms were achieved, supporting the hypothesis; however, physical variation in the stone was a far greater control on the rate of decay and the emergence of distinctive decay phenomena. This extended to both the mineralogical composition of the stone and to pre-existing decay features, such as crusts and cracks. The implication of these results is that the emergence of decay patterns in Reigate Stone cannot be explained solely by environmental mechanisms; baseline mineralogy and historical contingency can play a crucial role. Whilst environmental controls may provide a conservation strategy in some cases, it is likely that detailed assessment of the case specifics will be necessary.

Michette, M. (2021): Journal of Building Survey, Appraisal & Valuation, 10(3), 242-256.

Abstract: This paper summarises a scientific methodology for assessing masonry decay, developed over the course of a four-year research project on Reigate Stone at the Tower of London and Hampton Court Palace. Reigate Stone is a vulnerable building stone with a complex history of use, high cultural value and poorly understood decay processes. The Tower of London and Hampton Court Palace, managed by Historic Royal Palaces, contain a large amount of Reigate Stone masonry. As such, these important historic sites provided a range of different case studies for investigating Reigate Stone decay. Masonry decay is increasingly being understood in terms of complex system dynamics, in which the interactions between primary building stone, replacement stones, mortar and invasive agents of anthropogenic or environmental origin are as important as the nature of the building stones themselves in controlling non-linear response to environmental mechanisms. Integrating this diverse set of variables significantly increases the complexity of scientifically robust stone conservation. Non-destructive techniques (NDT) play an important role in addressing this complexity, but in order to make sense of the data they capture, it is vital to appreciate different scales of investigation and distinguish between rapid and in-depth protocols.

Michette, M., Viles, H., Vlachou, C., & Angus, I. (2021): Minerals, 11(4), 345.

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.

Michette, M., Viles, H., Vlachou, C., & Angus, I. (2020): Heritage Science, 8(1), 1-24.

Abstract: 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.

Michette, M., Viles, H., Vlachou-Mogire, C., & Angus, I. (2020): Studies in Conservation, 65(sup1), P225-P232.

Abstract: 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 on-going 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 on-going decay than treatment histories.

Michette, M., Viles, H., Vlachou, C., & Angus, I. (2019): Monum. Monum, 2019, 53-64.

Abstract: At the Bell Tower, Tower of London, Reigate Stone decay patterns can be associated with different internal locations. The decay phenomena are surveyed using non-destructive techniques, and the micro-climates of these locations are monitored over an 18-month period in order to associate specific environments with patterns of decay. Results are compatible with the hypothesis that distinct environmental mechanisms govern the emergence of powdering and flaking decay phenomena. Powdering is driven by deep wetting of the stone and a steady ambient climate; evaporation and salt crystallisation can take place at or near the stone surface. Flaking is driven by a dynamic moisture profile and ambient temperature cycles; salt crystallisation takes place beneath the stone surface. Identifying these controls with appropriate, low-impact methodologies is crucial to developing preventive conservation strategies.

Other Reigate Stone research

Price, C. A. (2007) Environmental geology, 52(2), 369-374.

Abstract: The paper compares the results of two studies of salt damage at the Wakefield Tower, Tower of London. The first, in 1993, was based on semi-quantitative salt analysis and hygroscopicity measurements. The second was based on quantitative analysis and a thermodynamic model, ECOS. Both studies aimed to predict environmental conditions that would minimise salt damage to the interior stonework. Their predictions are markedly different, and this paper examines the reasons for the differences. It is shown that hygroscopicity alone is not a reliable indicator of crystallisation from mixed salt solutions.

Lott, G., & Cameron, D. (2005): 10th Euroseminar on Microscopy Applied to Building Materials, Paisley, Scotland, 21-25 June 2005.

Abstract: The South East of England (London, Kent, Sussex, Surrey and Hampshire), is characterized by a range of distinctive, traditional vernacular buildings. Stone from the local strata, which ranges from Late Jurassic to Palaeogene in age, has been widely used throughout the area for building purposes (footings, walling, roofing) and was also frequently imported into London, which itself has no building stone resource of any significance cropping out within its boundaries. Determining the provenance of some of these stones, away from their local context, however, can be difficult, but is greatly facilitated by studying their mineralogy, textures and fabrics under the microscope.

Sanderson, R., & Garner, K. (2001): Journal of architectural conservation, 7(3), 7-23.

Abstract: This paper describes a research project on Reigate stone, undertaken by Historic Royal Palaces (HRP) over the last four years. Reigate Stone was used extensively at the royal palaces from the medieval period, and is infamously nondurable. Reigate Stone was replaced with more durable stones, such as Bath Stone, in the nineteenth century, but more recently there have been efforts at conservation. To date, the HRP project has concentrated on gaming a more complete understanding of the nature of the stone, which is unique in the British Isles. The project has involved re-entering the abandoned underground quarries in East Surrey to take core samples, which have been analysed by various techniques. Results were incomplete at the time of writing (June 2001), but much useful new information about the stone has been gained. Comparative treatment trials have gone ahead in recent months, in collaboration with US-based researchers. Further discussions with academics and practitioners are planned.

Tatton-Brown, T. (2001): Medieval Archaeology, 45(1), 189-201.

Abstract: Reigate stone was one of the most important building stones used in London and along the Thames littoral in the Middle Ages but, despite this, its use has been largely ignored by archaeologists. This introductory essay attempts to look at both the geology and documented history of the stone quarries, and at the Battersea and other Thames-side sites from which the stone was distributed.

Lockwood, S. (1994): Structural Survey

Abstract: Describes the geological distribution of Reigate Stone, a calcareous sandstone, and comments on its extraction in medieval times. Examines its characteristics to explain its popularity for carved work on many important historic buildings and advises on its identification. Explains the distribution of Reigate stone in buildings by reference to transport links and summarizes the results of archival and field investigations in a distribution map. Discusses the significance of chemical decay using examples from Surrey churches.

Robinson, E., & Worssam, B. (1989): Proceedings of the Geologists' Association, 100(4), 595-603.

Abstract: Most of the older churches of Middlesex (11th to 14th century) contain a building material which has not received serious attention until recent years, variously called ‘gravel-stone’, ‘rubblestone’ or ‘ironstone’. On analysis, and plotted in relationship to superficial deposits and solid geology across the area, this material can be demonstrated as reflecting the underlying subsoil and outcrop, locally consolidated by mineral cementation from groundwaters. Otherwise, Kentish Rag (from the Lower Greensand near Maidstone) and Reigate Stone (Upper Greensand) were widely used in Middlesex churches in such a way as to show that their differing resistance to weathering was clearly appreciated.

Sowan, P. W. (1975): Proceedings of the Geologists’ Association, 86(4), 571-591.

Abstract: The history of development of mines for the extraction of stone from the Upper Greensand of east Surrey is reviewed. Twenty or thirty disused mines occur between Brockham and Godstone. Archaeological and documentary evidence indicates that the stone mines were worked for a thousand years, from the seventh or eighth centuries until 1961. Stone from the mines was used in some of the earliest Saxon churches and the mines were of sufficient importance in the Middle Ages to be taken into Crown ownership. With the improvement of transport in the eighteenth and nineteenth centuries and the exploitation of other sources of building materials the stone mines went into decline, to revive briefly as a source of material (hearthstone) for whitening doorsteps, stone floors and hearths. Now entirely abandoned, the mines have presented problems to the contractors constructing the M23 and M25 motorways at Merstham, and are currently the subject of considerable interest and pressure for conservation from specialist groups with such diverse interests as economic geology, industrial archaeology, bat hibernacula, and spelaeological training.