Michette et al. Herit Sci (2020) 8:80 https://doi.org/10.1186/s40494-020-00424-w RESEARCH ARTICLE Open Access The many faces of Reigate Stone: an assessment of variability in historic masonry based on Medieval London’s principal freestone Martin Michette1* , Heather Viles1, Constantina Vlachou2 and Ian Angus3 Abstract Reigate Stone was used in high profle 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 identifca- tion 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, replace- ment and contamination of Reigate Stone. These historic factors augmented pre-existing mineralogical variability. This variability makes classifcation according to commonly used stone types difcult. Experimental analysis corre- lates variable cementing components with hygro-physical properties related to resilience. Calcite content infuences strength properties and capillarity; clay content infuences moisture adsorption and retention; opal-CT forms a weakly cemented, porous matrix. These presented diferent decay pathways to a range of environmental mechanisms and agents of decay. The fndings suggest that inherent mineralogical variability, environmental changes, and historic contingency must all be considered in the design of ongoing Reigate Stone conservation strategies. Keywords: Cultural heritage, Historic architecture, Stone decay, Construction economics, Lithological variation Introduction rapid decay. Many architecturally important regions have Reigate Stone was used as a freestone across south- inherited a legacy of vulnerable historic masonry due to east England from the eleventh until the sixteenth cen- the nature of their principal freestone; such as Tufeau tury, contributing signifcantly to the re-emergence of in central France, Lede Stone in Belgium and Opuka in masonry architecture in Britain during this period [1]. Prague [2–4]. Other examples exist in England, such as Freestones are stones used for ashlar and ornamental Clunch and Headington Stone [5, 6]. Given the histori- masonry. Suitable lithologies can be cut freely in any cal information stored in regional building stones and the direction and worked easily with a chisel; they tend to be aesthetic contribution of ornamental masonry, the decay fne-grained, soft and homogeneous. Whilst this makes of these valuable freestones impacts signifcantly on the them easy to sculpt, it can also make them prone to deterioration of architectural heritage. Reigate Stone masonry displays a wide range of condi- *Correspondence: [email protected] tions, varying in pattern, rate and state of decay. Tese 1 School of Geography and the Environment, University of Oxford, Oxford, are frequently visible within single masonry units (Fig. 1). UK Historically, it was widely replaced due to rapid decay, ini- Full list of author information is available at the end of the article tially with fresh Reigate Stone and later with alternative © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat​iveco​ mmons​.org/licen​ses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creat​iveco​mmons​.org/publi​cdoma​in/ zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Michette et al. Herit Sci (2020) 8:80 Page 2 of 24 • Seasoning is an important process in calcare- ous freestones [10 p. 15–17]. It involves the case hardening of the stone following its extraction, as moisture from within the pore matrix migrates to the surface, a process which could take sev- eral years. Its importance has long been noted in stone conservation practice, but it has received little scientifc attention. Case hardening has been noted in Reigate Stone [11, 12 p. 420]; stone was sometimes stored within the mines for seasoning (Fig. 2e) [13]. • Te orientation of stones laid within masonry can greatly impact their resistance to decay due to ani- sotropy [14 p. 52–53]. Anisotropy is an expression Fig. 1 Reigate Stone masonry at the Bell Tower, Tower of London, of heterogeneity in a stone. It is present in many showing high level of variability. Majority of masonry is in Reigate sedimentary rocks due to bedding. Te compres- Stone, with range of block colours and sizes, and decay patterns and sive strength tends to be highest in the direction rates. Second and third course are replaced with other lithologies (except central stones); approximately 15 stones on right of picture of bedding [15]. It also afects capillarity. Incorrect also replacement lithologies. Boundary between Reigate and other bedding is not uncommon, particularly in stones stone marked by black border where bedding planes are difcult to determine. • Once within the building, material variations can be compounded by micro-climatic variations at the lithologies [7]. However, there are examples of primary stone-environment interface, and once decay pat- Reigate masonry which has survived in relatively exposed terns emerge within a masonry system, non-linear locations, such as the base of the White Tower, for almost dynamics can amplify any initial diferences [16]. 1000 years. Since replacement gave way to conservation Frequent changes to the micro- or macro-environ- in the mid-twentieth century, there has been persistent ment are likely over the long life-span of a building, uncertainty regarding the relative role of stone petrology for example due to the removal of nearby shelter or and environmental mechanisms in decay processes [8]. a reduction in atmospheric pollution. Past environ- Tere have been attempts to associate exposure with pat- ments can have ongoing efect on stone decay [17]. terns and rates of decay. Tese have failed to account for • Finally, repair of the masonry, including selective the full variability evident in historic masonry. Tere has replacement or treatment of deteriorated stones and been little consideration of the role historic contingency mortar, will alter existing properties and introduce can play in long-term decay processes; the cumulative new variability, whilst frequently obscuring evidence efect of past context, be it factors pertaining to the initial of past variability [6]. construction, environmental exposure, or remedial treat- ment of masonry. Several attempts at conservation have Te cumulative efect of these factors upon historic accelerated decay. In order to more fully understand the masonry can be complex compositions of individual deterioration of Reigate Stone, and design appropriate stones bearing unique mineralogical, chemical and envi- conservation strategies, it is necessary to understand the ronmental signatures. variable decay found within historic masonry. Studies of Reigate Stone have focussed either on its Several factors are likely to contribute to diferential conservation [e.g. 7, 8, 18] or an assessment of its quar- decay in masonry construction. rying and use in architecture [e.g. 1, 13, 19, 20]. Tese valuable contributions have highlighted mineralogical • Diferences in petrology result in diferent resistance features and decay processes, or provided detailed his- to decay. Tese arise from mineralogical variability torical analysis, which can explain specifc diferences in and physical characteristics such as porosity. Petro- Reigate Stone, but have not formed a combined approach graphic variability is not restricted to diferent lithol- which can adequately describe the cumulative efect of ogies; there can be considerable variation across the processes contributing to variability in historic masonry. facies of a single geological formation [9]. As part of a wider project on understanding Reigate • Diferences in workmanship, seasoning or laying can Stone decay at the Tower of London, this paper aims to amplify petrographic variation. build a hypothetical, general model of these processes. Michette et al. Herit Sci (2020) 8:80 Page 3 of 24 Fig. 2 a Twelth century Wardrobe Tower, Tower of London, showing decayed,