The many faces of Reigate Stone: an assessment of variability in historic masonry based on Medieval London’s principal freestone

Abstract

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.

Full text

Michetteetal. Herit Sci (2020) 8:80
https://doi.org/10.1186/s40494-020-00424-w
RESEARCH ARTICLE
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 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 identifica-
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 classification according to commonly used stone types difficult. Experimental analysis corre-
lates 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.
Keywords: Cultural heritage, Historic architecture, Stone decay, Construction economics, Lithological variation
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Introduction
Reigate Stone was used as a freestone across south-
east England from the eleventh until the sixteenth cen-
tury, contributing significantly to the re-emergence of
masonry architecture in Britain during this period [1].
Freestones are stones used for ashlar and ornamental
masonry. Suitable lithologies can be cut freely in any
direction and worked easily with a chisel; they tend to be
fine-grained, soft and homogeneous. Whilst this makes
them easy to sculpt, it can also make them prone to
rapid decay. Many architecturally important regions have
inherited a legacy of vulnerable historic masonry due to
the nature of their principal freestone; such as Tuffeau
in central France, Lede Stone in Belgium and Opuka in
Prague [2–4]. Other examples exist in England, such as
Clunch and Headington Stone [5, 6]. Given the histori-
cal information stored in regional building stones and the
aesthetic contribution of ornamental masonry, the decay
of these valuable freestones impacts significantly on the
deterioration of architectural heritage.
Reigate Stone masonry displays a wide range of condi-
tions, varying in pattern, rate and state of decay. ese
are frequently visible within single masonry units (Fig.1).
Historically, it was widely replaced due to rapid decay, ini-
tially with fresh Reigate Stone and later with alternative
Open Access
*Correspondence: martin.michette@ouce.ox.ac.uk
1 School of Geography and the Environment, University of Oxford, Oxford,
UK
Full list of author information is available at the end of the article
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Michetteetal. Herit Sci (2020) 8:80
lithologies [7]. However, there are examples of primary
Reigate masonry which has survived in relatively exposed
locations, such as the base of the White Tower, for almost
1000years. Since replacement gave way to conservation
in the mid-twentieth century, there has been persistent
uncertainty regarding the relative role of stone petrology
and environmental mechanisms in decay processes [8].
ere have been attempts to associate exposure with pat-
terns and rates of decay. ese have failed to account for
the full variability evident in historic masonry. ere has
been little consideration of the role historic contingency
can play in long-term decay processes; the cumulative
effect of past context, be it factors pertaining to the initial
construction, environmental exposure, or remedial treat-
ment of masonry. Several attempts at conservation have
accelerated decay. In order to more fully understand the
deterioration of Reigate Stone, and design appropriate
conservation strategies, it is necessary to understand the
variable decay found within historic masonry.
Several factors are likely to contribute to differential
decay in masonry construction.
• Differences in petrology result in different resistance
to decay. ese arise from mineralogical variability
and physical characteristics such as porosity. Petro-
graphic variability is not restricted to different lithol-
ogies; there can be considerable variation across the
facies of a single geological formation [9].
• Differences in workmanship, seasoning or laying can
amplify petrographic variation.
• 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 scientific 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].
• e 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
of heterogeneity in a stone. It is present in many
sedimentary rocks due to bedding. e compres-
sive strength tends to be highest in the direction
of bedding [15]. It also affects capillarity. Incorrect
bedding is not uncommon, particularly in stones
where bedding planes are difficult to determine.
• Once within the building, material variations can
be compounded by micro-climatic variations at the
stone-environment interface, and once decay pat-
terns emerge within a masonry system, non-linear
dynamics can amplify any initial differences [16].
Frequent changes to the micro- or macro-environ-
ment are likely over the long life-span of a building,
for example due to the removal of nearby shelter or
a reduction in atmospheric pollution. Past environ-
ments can have ongoing effect on stone decay [17].
• Finally, repair of the masonry, including selective
replacement or treatment of deteriorated stones and
mortar, will alter existing properties and introduce
new variability, whilst frequently obscuring evidence
of past variability [6].
e cumulative effect of these factors upon historic
masonry can be complex compositions of individual
stones bearing unique mineralogical, chemical and envi-
ronmental signatures.
Studies of Reigate Stone have focussed either on its
conservation [e.g. 7, 8, 18] or an assessment of its quar-
rying and use in architecture [e.g. 1, 13, 19, 20]. ese
valuable contributions have highlighted mineralogical
features and decay processes, or provided detailed his-
torical analysis, which can explain specific differences in
Reigate Stone, but have not formed a combined approach
which can adequately describe the cumulative effect of
processes contributing to variability in historic masonry.
As part of a wider project on understanding Reigate
Stone decay at the Tower of London, this paper aims to
build a hypothetical, general model of these processes.
Fig. 1 Reigate Stone masonry at the Bell Tower, Tower of London,
showing high level of variability. Majority of masonry is in Reigate
Stone, with range of block colours and sizes, and decay patterns and
rates. Second and third course are replaced with other lithologies
(except central stones); approximately 15 stones on right of picture
also replacement lithologies. Boundary between Reigate and other
stone marked by black border
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Building a model of the processes which have contrib-
uted to variability in historic masonry can improve the
understanding of stone decay in general and identify
controls for ongoing conservation strategies. e pur-
pose of this paper is to investigate the role of Reigate
Stone variability in predetermining the emergence of
Fig. 2 a Twelth century Wardrobe Tower, Tower of London, showing decayed, south facing Reigate Stone ashlar in upper part of buttress and to
left of buttress. b Eleventh century White Tower, Tower of London with east facing Reigate Stone ashlar visible in two courses directly above batter,
and predominantly Portland Stone replacements. c Reigate Stone tracery from eleventh century Merton Priory on display in Museum of London. d
Reigate Stone tracery in sixteenth century north cloister of Hampton Court Palace. e Large blocks of stone left in quarry near Chaldon (scale card is
8 cm across). f Quarry face near Merstham from which samples were extracted in 1998
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Michetteetal. Herit Sci (2020) 8:80
differential decay. e investigation will synthesise his-
torical analysis of Reigate Stone economics with experi-
mental analysis of its material properties. is combined
approach is vital for establishing a complete picture of
historic building materials. Synthesising detailed his-
torical analysis can also help to overcome some com-
mon problems in the field of historic building material
analysis, such as limited access to samples. e objectives
are to establish a timeline of Reigate Stone exploitation
which can identify patterns of use and factors pertaining
to inherent variability within masonry systems, define
variability in mineralogical terms, and link mineralogical
composition to physical characteristics known to influ-
ence decay. is will facilitate ongoing identification of
the specific mechanisms which drive the decay of vul-
nerable masonry. It is also intended to contribute to per-
sistent debate on the correct lithological definition for
Reigate Stone [8]. e overall aim is to inform a method-
ological framework for evaluating architectural heritage
under severe threat.
Historical analysis
Geological context
Reigate Stone was quarried from a thin band of Upper
Greensand in northern Surrey. e Upper Greensand is
a Cretaceous deposit of lenticular masses formed in shal-
low sea conditions, which extends across southern Eng-
land [12]. Upper Greensand lithologies are characterised
by fine-grained silicate minerals and green, clay mineral
bearing glauconite; however, an unstable paleogeogra-
phy with changes in sea level, climate and faunal depo-
sition has resulted in a complex geology with inherent
variability at macro- and micro-scales [21, 22]. e facies
exploited for Reigate Stone consist of intercalated lenses
of varying hardness and homogeneity [23p. 84–91]. Early
characterisations of the building stone noted roughly
equal proportions of silica and calcium carbonate [24,
25 p. 9]. Whilst it has been referred to as both a siliceous
limestone and more commonly a calcareous sandstone,
Sanderson and Garner [8] state it cannot truly be clas-
sified as either. Its dominant mineral phase is opal-CT,
precipitated crystalline silica which forms both a highly
micro-porous matrix and functions as the main cement
of very fine-grained quartz and bioclastic components.
e Reigate Stone industry saw the development of a
vast network of mines, running approximately 15 km
from Godstone in the East to Brockham in the West
(Fig. 3) and exploited for several purposes [19]. is
has led to terminological and lithological ambiguities.
In medieval times Reigate Stone and spelling variations
thereof were already being used as superordinate terms
for stones from nearby areas, such as Merstham. Despite
this, specific references to Merstham Stone and Chal-
don Stone are not uncommon. Whilst these may refer
to individual quarries or workings, there is no indica-
tion that they implied a difference in quality or function.
Fig. 3 Map of area around Reigate and London, showing (1) sampling locations in Tower of London, (2) sampled quarries and location of Upper
Greensand in North Surrey, and (main map) other sampling locations along with coding system used for samples
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References to Greensand or green Sandstone may indi-
cate Reigate Stone usage; however, a range of Upper
and Lower Greensand lithologies were used as building
stones [26]. e terms Firestone and Hearthstone relate
to eighteenth and nineteenth century industrial uses;
Firestone was used for lining ovens, Hearthstone for
cleaning (i.e. whitening) doorsteps and other stone sur-
faces. ere is some ongoing confusion as to whether
they are distinct lithological varieties and if so, what
their defining characteristics are. Jukes-Brown [12 p. 97]
distinguishes Firestone and Hearthstone beds within
the geological stratum, however their order is reversed
within some mines [20]. Several sources identify Fire-
stone as being more calcareous, comparable to stone
used in buildings, and Hearthstone as being far softer and
more friable [8, 23]. Other sources claim that Firestone is
more siliceous, and Hearthstone contained more calcite,
lending it its whitening properties [12 p. 93, 26]. Records
of Greensand and Firestone in archaeological or architec-
tural texts relating to London are in most cases likely to
indicate Reigate Stone, particularly when prefixed by Sur-
rey [27]. Reigate Stone should be considered as a descrip-
tive term rather than a fixed material classification; other
names may persist in contemporary literature and refer-
ences thereto are included in this study, yet Reigate Stone
is by far the most common name for building stone from
the North Surrey Upper Greensand.
Past studies of Reigate Stone used within buildings
have noted high lithological variability, even within single
buildings or sites. During excavations at the fourteenth
century abbey St Mary Graces, built beside the Tower
of London in East Smithfield, three different types of
Reigate Stone were identified [28 p. 87]. ese include a
dense variety associated with the earliest phase of con-
struction in the 1350s and a softer variety used in the
main phase of construction. Excavations at St. Mary Spi-
tal, built in the late twelfth and early thirteenth century,
found two distinct typologies of Reigate Stone [29 p. 186–
195]. Variations in mineralogy, including glauconite col-
our and morphology and the nature of microfossils were
identified [30]. e study concluded that several sources
must have been used during construction. Attempts to
determine precise provenance or link to building phases
proved inconclusive. Sanderson and Garner [8] analysed
Reigate Stone from several different quarries and build-
ings ranging from the Medieval to the Victorian periods
and found significant mineralogical variation across sam-
ples. ey suggested that calcite content was a determin-
ing factor in building stone quality.
Reigate Stone building economics
Roman use
ere is clear evidence that Reigate Stone was used in
Roman times. Archaeological excavations in Southwark
have yielded the earliest confirmed use of the stone in
building, with potential first century use documented at
two sites [31, 32]. Second or third century use at a funer-
ary site in Southwark has been linked to a temple [33 p.
9–10]. ere are indications that Reigate Stone was used
in bastions and barracks along the city wall, with some
references to ‘green sandstone’ made during excavation
of the Roman fabric [e.g. 10 p. 31, 33 vol. 3 p. 103]. Sev-
eral Roman sites near Reigate are documented as having
walls built of ‘local Greensand’ [34], including a villa in
Titsey ‘partly built of sandstone (…) probably quarried
on Limpsfield Common’, 15 km to the east of Reigate
[35] (Fig.3). Several sculptures found in Southwark and
Ashtead and dating back to the late-first century have
been identified as Reigate Stone [36]. A tile kiln possi-
bly dating to the late-first century and discovered near
Reigate represents the earliest known use of the stone in
large, squared blocks [37]. e tilery produced ceramics
found at multiple sites across London and Kent [e.g. 33 p.
60, 38 p. 107]. Even if Reigate Stone was not widely used
in pre-medieval masonry architecture, there is enough
evidence of quarrying, artistic and industrial use to indi-
cate it played an important role in the Romano-British
economy of South-East England.
Medieval London’s expansion
Ongoing, large-scale use of the stone coincides with the
re-emergence of masonry architecture in the eleventh
century. ere is some evidence of earlier Anglo-Saxon
quarrying and usage. ‘Local firestone’ was used in parish
churches at Stoke D’Abernon and Fetcham [39 p. 14–15],
with the large, walled-in lintel at St Mary’s in Stoke
D’Abernon suggestive of local quarrying in the late-sev-
enth or early-eighth century [40]. Reigate Stone was used
in large quantities in the mid-eleventh century construc-
tion of churches in Westminster and Waltham [41, 42].
Following the conquest, Norman masons came to regard
Reigate as a local alternative to Caen Stone, although the
latter was still favoured for some time. Primary Reigate
ashlar can be found in the lowest courses of the White
Tower (Fig. 2), indicating use during the first building
phase (1066–1078) [43 p. 54–56]. is suggests existing
stockpiles or supply-chains were exploited immediately
after the conquest. e Domesday Book (1086) refers
to two stone quarries in Surrey, which are likely to have
included the Limpsfield Common site that may have
been used in Roman times, but do not mention locations
closer to Reigate itself [13 p. 11, 44]. Robbing of existing
building stock should also be considered, especially in the
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Michetteetal. Herit Sci (2020) 8:80
form of the Roman city wall adjacent to the White Tower.
Reigate Stone was used in detailed work for the interior
of the White Tower, where its location corresponds with
the building break in the 1080s. Whilst precise reasons
for the break remain unclear, it is notable that courses of
Reigate interspace the use of Caen Stone [43 p. 104–105].
is may reflect complications in supply chains from
northern France. Use in this period, with masonry con-
struction limited to a few key buildings, suggests that
knowledge of the stone had persisted and some degree of
industry in the Reigate area was able to respond to fluctu-
ating demand.
From the mid-twelfth century onwards the use of Rei-
gate Stone became more prevalent [45 vol. 4 p. xxiv]. It
was chosen for detailed masonry in the construction
of the priories and abbeys emerging across the city, but
use is also documented in secular buildings [46 p. 220,
47, 48 p. 58–59]. Starting in the 1170 s, it was used in
the construction of two bridges crossing the ames, at
Kingston and further downstream at the first stone-built
London Bridge [25 p. 67, 27, 48 p. 135]. e loss of the
French possessions in 1206 is likely to have limited the
ongoing use of Caen Stone. e first half of the thirteenth
century sees the establishment of several new quarries
around Reigate and stockpiling sites along the ames [1,
13 p. 17]. is expanding market is reflected in accounts
for building works, with four different sources providing
Reigate Stone for the construction of Westminster Abbey
in the 1250s [45]. Reigate Stone from various sources
continued to arrive at Westminster for over 300years;
it appears that no single quarry consistently provided
stone for more than 4-5years and accounts suggest mate-
rial was frequently sourced from stockpiles rather than
directly from quarries [49]. In other cases, individual
quarries can be more closely associated with specific
sites or buildings. A 1218 quarrying grant for land near
Reigate to the canons of Waltham Abbey, the site of the
eleventh century church, suggests long standing relation-
ships between the mining parishes and their clients [1]. It
also reveals an ongoing need for fresh stone at large sites,
possibly in relation to early repair work. Another quarry
was opened in 1241 solely to provide stone for the Tower
of London [50 vol. 4 p. 271]. is coincides with a major
expansion of the site’s defensive capabilities, including
the construction of several towers along the inner wall
[45 vol. 5 p. 75]. During this period of intense build-
ing, demand may have outstripped the supply of well-
sourced, well-seasoned Reigate Stone.
ere are signs that the industry had largely centralised
by the fourteenth century. Templates for mouldings at
Westminster Palace were being sent to the Reigate quar-
ries so that the stone could be roughly worked on the
spot [51 p. 21–22]. Individual families took ownership
of several quarries; the Prophete family managed quar-
ries supplying Royal Works for over 100years [51 p. 130].
Increased output from Merstham and Chaldon quarries
in this period has been linked to an exhaustion of beds
closer to Reigate [13 p. 44].
Repair, recycling andreplacement
ere is evidence that by the fifteenth century cer-
tain limitations to Reigate Stone were well understood
and large-scale repair programs were underway. Royal
accounts for the late fourteenth century make specific
reference to Reigate Stone intended for repair work [52
vol. 2 p. 1008]. In some fourteenth and fifteenth century
buildings, architectural details evolve to protect exposed
Reigate masonry [47 p. 138, 53]; in others, the exposure
of Reigate Stone to damp is avoided entirely [51 p. 527,
54 p. 20, 55 p. 19]. Significant restoration activity on Rei-
gate masonry at Westminster Abbey took place in the
fourteenth and fifteenth centuries [56 p. 219–227]. is
included extensive repairs to the lower south transept
in 1457–1461, 200years after its construction in Reigate
Stone [49 p. 27]. In the sixteenth century, use of Reigate
Stone as ashlar is predominantly restricted to sheltered
areas; demand is likely to have drastically fallen. In an
indication of declining quarrying activity, the earliest
working faces in the re-opened mines around Reigate can
be dated to the sixteenth century [13].
A decline in use and any apparent decline in quarrying
activity should be considered within the socio-economic
context of late Medieval and Tudor London. e Black
Death changed the dynamic of London’s urban expansion
in the mid-fourteenth century [57] and impacted labour
markets for two centuries. Economic recovery coincided
with the dissolution of the monasteries in the 1530 s,
which provided salvageable material to fuel a renaissance
in masonry architecture. Whilst it had long been used as
flooring material in the form of large slabs, Tudor floors
also used crushed Reigate, which may represent demoli-
tion material [48 p. 80, 118, 136]. e majority of Reigate
Stone for Nonsuch Place (1538) came from the demoli-
tion of Merton Priory; 3643 loads of stone were sourced
from Merton and only 96 loads directly from Reigate
quarries [52 vol. 3 p. 184]. e re-use of building mate-
rial was a common medieval practice; materials used in
royal works were frequently recycled elsewhere within
the royal estate. After the royal residence at Sheen (now
Richmond) was demolished in the 1390s, accounts show
the reuse of materials at other buildings, including the
Tower of London and a new manor built in Sutton, Sur-
rey. Large quantities of Reigate used at Sutton then found
their way back to Sheen when the former was demol-
ished after only 20years and the latter was rebuilt [52 vol.
2 p. 998–1004]. Widespread recycling of building stone
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Michetteetal. Herit Sci (2020) 8:80
is likely to have occurred following economic downturn
and recovery, resulting in a significant increase in the
inherent variability of individual masonry units.
Later centuries saw a further refinement of the use
of Reigate Stone and replacement with other stones. A
major building phase in London began in the late-sev-
enteenth century in response to the Great Fire and saw
the introduction of new freestone, most notably Portland
Stone, and further developments in brick manufactur-
ing [54]. e medieval Reigate quarrying infrastructure
would have been insufficient for the scale of rebuilding
and expansion London underwent. Quarries in places
such as Portland and Oxfordshire could be exploited on
a much larger scale and stone could be transported via
water along the coast or newly built canals [6]. Ongoing
replacement of decaying Reigate masonry was now possi-
ble using these more robust stone types. Freshly quarried
Reigate Stone was still used internally, and recycled stone
was used as rubble infill. Despite reservations, Christo-
pher Wren used large quantities of Reigate Stone in his
rebuilding of the city’s churches [20, 58]. At St. Pauls,
where Wren made deliberate, targeted use of several dif-
ferent types of stone, Reigate Stone was used as infill and
for ashlar and mouldings [58, 59, p. 69–70]. Wren was
particularly attentive to the sourcing, transport and shel-
tering of Reigate Stone; having identified a susceptibility
to frost, he had sheds built to protect the stone during the
winter [20]. As with the large medieval construction pro-
jects, supply came from multiple sources over the years
[58], so despite this quality control, inherent variability is
likely to have been built into the masonry.
Significant mining activity recommenced in the mid-
nineteenth century [19]. is fed a growing demand for
stone used in industrial processes, but also provided
building stone for the rapid development of suburban
Reigate and Redhill. Several mines were newly opened
or vastly expanded in this period [13]. e mines at
Merstham were extensively developed to provide stone
used as infill in the construction of London and Water-
loo bridges. ere is evidence that some material used
in buildings in this period was of poor quality, with the
decay of several Victorian churches near Reigate pro-
gressing particularly rapidly [7]. Increasing pollution
levels and the use of cementitious repair materials accel-
erated masonry decay [17, 60]. It is likely that the impact
was particularly severe on Reigate Stone. Simultaneously,
Victorian stylistic restoration programs drove widescale
replacement of deteriorated fabric at historic sites across
the city, including the Tower of London [61]. Although
the decline in overall stock will have drastically sharp-
ened, it should be considered that fresh Reigate Stone
may have entered the historic fabric as repair material in
this period.
Replacement of decaying Reigate Stone continued
until the mid-twentieth century, when conservation pol-
icy shifted to preservation. Initially little attention was
paid to replacement typologies. Later work recognised
not only the importance of aesthetic compatibility, but
also geochemical compatibility with remaining Reigate
masonry. Chilmark and Ketton limestone were widely
used, however neither is entirely compatible. A wide
range of different consolidants were trialled or exten-
sively used on Reigate Stone in the late-twentieth century,
often with sparse documentation and mixed success [18].
ere are records of isolated replacement programmes
using small quantities of available, fresh Reigate Stone
in recent years. Since the closure of the last Hearthstone
mines in the 1960s ongoing replacement with fresh Rei-
gate Stone has become practically impossible; however,
fresh stone was procured for a new stairwell at Westmin-
ster Abbey completed in 2018.
Experimental analysis
Objective
e documentary analysis has shown that building stone
economies are likely to have resulted in complex masonry
systems, with selective extension, replacement, recycling
and treatment of Reigate Stone masonry occurring across
a period of several centuries. is will have compounded
the effects of environmentally induced changes. Any
inherent differences in individual stones would further
augment the resulting variability; these will be investi-
gated in the following section.
Experiments were conducted on Reigate Stone samples
collected from different quarries and buildings in order
to enable petrographic characterisation and investigate
physical and hygric properties. Samples were selected
to be representative of a broad geographical and histori-
cal range. Non-destructive techniques were favoured to
allow a reuse of samples and enable calibration with field
tests in ongoing work. Measured variables were subject
to statistical analysis in order to determine correlations
(Pearsons method and Principal Component Analysis)
and identify patterns between freshly quarried stone and
stones used in medieval buildings. e objective was to
establish inherent variability in Reigate Stone and link
this to mechanisms likely to drive decay processes.
Materials andmethods
ree samples each from five quarries were selected.
ese make up part of a larger archive of samples gath-
ered to facilitate research by Sanderson and Garner [8].
Quarries in Gatton, Merstham, Chaldon and Godstone
were sampled (Fig.3). e quarries make up networks
of mines that have been gradually re-explored by local
interest groups over the last 50years [13]. Samples were
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Page 8 of 24
Michetteetal. Herit Sci (2020) 8:80
extracted from a single gallery face near an entrance to
each mine as 45mm diameter cylinders using a diamond
tipped core drill (Figs.2f, 4). No water was used during
the drilling. 50mm pieces were cut from these cylinders
for most characterisation tests, with some tests per-
formed on smaller pieces taken from the same cylinder.
Samples from 10 different buildings were investigated
(Figs.2, 3). Four separate building phases at the Tower of
London are represented. Five further sites were selected
to approximate one building per century in Medieval
South East England, although samples may be repre-
sentative of repair material or later work. Samples from
seven buildings were collected for Sanderson and Gar-
ner in the late 1990s [8]: two samples were taken by His-
toric Royal Palaces from the Tower of London; a sample
from St. Mary Spital was supplied by the Museum of
London archives; the other four samples were supplied
by curators or archaeologists working at the individual
sites. Detailed records on sampling locations were not
available. Samples from three further buildings were
made available by HRP during this study. Hampton
Court Palace (HC) samples 1-6 were taken from mate-
rial which had been buried and was rediscovered during
landscaping work in 2016. Wardrobe Tower and Mar-
tin Tower samples, and HC7, were taken from material
which detached from the buildings during conserva-
tion or surveying work. Samples were cut into prisms of
50 × 50 × 50mm from suitably large parent material for
most characterisation tests, with some tests performed
on smaller pieces taken from the same parent. Some
tests were also performed on additional, smaller samples
from Hampton Court Palace and the Wardrobe Tower to
examine variability within single buildings (Fig.4). Sam-
pling sites are listed in Table1 along with approximate
build dates and information on the samples.
Fig. 4 Selection of Reigate Stone specimens showing variation in colour and texture across different samples used in this study and explaining
different sample dimensions
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Michetteetal. Herit Sci (2020) 8:80
Bulk density/open porosity
Bulk density ρb (g/cm3) and open porosity PO were calcu-
lated in accordance with the standard methodology [62].
Ultrasonic wave propagation
A PUNDIT Lab (Proteq) was used to measure ultrasonic
wave propagation through the samples. e pulse veloc-
ity Vp (m/s) is related to strength characteristics [63]. e
standard methodology [64] was adapted, with velocity
measured along each axis of a sample, in order to enable
the determination of the anisotropy coefficient k accord-
ing to the formula
k
=

Vpmax−Vpmin
Vp
mean 100
Table 1 List of quarries and buildings that provided samples forthis study, with approximate date ofconstruction/
quarrying, coding system, andinformation onshape, size andlocation ofsamples
Quarry dates from Burgess (2008)
Building/Quarry Date Sample code Sample size Sample location info
Rockshaw Lodge Quarry,
Chaldon Medieval to nineteenth century RS1 Cylinders 45 mm ø 50 mm h All quarry samples extracted
from gallery faces near quarry
entrances in 1998 (e.g. Figure 2f).
Approx. 14 samples extracted
per quarry at different heights.
Numbered here from quarry ceil-
ing (i.e. 3 closest to floor). Precise
locations known
RS2
RS3
Quarry Dean, Merstham Medieval to nineteenth century QD1 Cylinders 45 mm ø 50 mm h
QD2
QD3
Gatton Quarry Medieval? to nineteenth century GA1 Cylinders 45 mm ø 50 mm h
GA2
GA3
Godstone Quarry seventeenth to twentieth
century GO1 Cylinders 45 mm ø 50 mm h
GO2
GO3
Quarry Field, Merstham Nineteenth century QF1 Cylinders 45 mm ø 50 mm h
QF2
QF3
White Tower, Tower of London 1070 TOL Cube 50 mm Sample removed in 2000. Location
unknown
Wardrobe Tower, ToL 1190 WT1 Cube 50 mm Detached from south facing but-
tress during conservation work
(04.2017)
WT2 Fragment
WT3 Fragment
Balium Wall, ToL Twelfth century BAL Cube 50 mm Date unknown. Location unknown
Martin Tower, ToL Mid- thirteenth century MRT Cube 50 mm Detached from internal door
reveal (2015)
Merton Priory Early Twelfth century MER Cube 50 mm Location unknown
St Mary Spital Early thirteenth century SMS Cube 50 mm Location unknown. Obtained from
Museum of London archive in
c.2000
St Mary Graces Mid-fourteenth century SMG Cube 50 mm Location unknown
Throwley Church Fifteenth century THR Cube 50 mm Location unknown. Marked as
fifteenth century material
Hampton Court Palace Early sixteenth century HC1 Cube 50 mm Excavated from garden in 2016.
Likely to have come from win-
dow jambs and been buried fol-
lowing eighteenth or nineteenth
century replacements with other
stone types
HC2 Cube 50 mm
HC3 Fragment
HC4 Fragment
HC5 Fragment
HC6 Fragment
HC7 Fragment Detached from external east facing
window jamb (2017)
Whitgift Almshouses Late sixteenth century WGA Cube 50 mm Location unknown
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Michetteetal. Herit Sci (2020) 8:80
where Vpmax is the mean maximum velocity obtained
along any one axis, Vpmin is the mean minimum velocity
and Vpmean is the average of all measurements [15, 65].
Capillary absorption
e capillary absorption coefficient wk (g/m2s0.5) was cal-
culated using the standard methodology [66]. Results of
the ultrasonic wave propagation tests were observed in
order to ensure capillary absorption was measured along
comparable planes of anisotropy.
Sorptivity
To measure moisture adsorption due to atmospheric
pressure, small pieces of each sample were taken from the
parent material (between 2 and 10g per sample depend-
ing on availability). ese were oven dried, weighed and
placed in a climate chamber at a constant temperature of
20 C. Relative humidity (RH) was increased in steps until
mass equilibrium (± 5mg). Sorptivity is expressed as the
amount of moisture adsorbed at 95% RH as a percent-
age of total mass of the sample. e hysteresis between
adsorption and desorption at 80% RH was calculated to
provide a measure of moisture retention.
X‑ray diraction (XRD)
XRD was performed to identify minerals present in pow-
dered samples. Powder samples were prepared from
additional pieces of each sample taken from the parent
material, mixed with acetone and deposited on a zero-
background single crystal silicon substrate. Analysis was
performed using a Panalytical Empyrean Series 2 dif-
fractometer operating at 40 kV and 40mA with a Co
Kα source. Measurements were taken in the 5° to 85° 2θ
range using a step size of 0.026° in reflection-transmis-
sion mode. e HighScore Plus software suite was used
to reduce data, and mineral identifications were based on
the correspondence of d-spacings, intensities and profiles
to the International Centre for Diffraction Data Powder
Diffraction File 4+ database and quantified through the
reference intensity ratio method [67]. In addition to bulk
mineral phase analysis, clay mineral phase analysis was
performed on a selection of powder samples following
decarbonation.
Optical microscopy
in sections of selected samples impregnated in blue
resin were analysed using an Olympus BX43. in sec-
tions of WT, HC and MRT were prepared during this
study from additional pieces of each sample taken from
the parent material, all other thin sections were prepared
for Sanderson and Garner [8]. Optical microscopy was
performed qualitatively in order to support and corrobo-
rate findings made using other techniques.
Results anddiscussion
Petrographic variability
XRD reveals quartz, opal-CT, calcite and clay minerals
present in all samples (Table2). ere is notable varia-
tion in the relative proportion of these minerals (Fig.5).
Feldspar was identified in most samples. Gypsum was
identified in several building stone samples. Clay analy-
sis of decarbonated samples yielded fairly amorphous
and low profile peaks which suggest the presence of dis-
crete smectite (trioctahedral smectite, glauconite and/
or montmorillonite) represented by peaks in the region
of 1.513–1.519Å, and 1.507Å respectively, in addition
to illite/muscovite (1.50–1.502Å). e presence of mus-
covite can account for flakes of mica seen both macro-
scopically and in thin section (RS1 in Fig.6). Within the
discrete smectite regions, some samples yielded peaks
at or near 1.511Å which could indicate the presence of
glauconite. Glauconite can refer both to a morphologi-
cal form and a mineral, typically glauconitic smectite,
with considerable variation from one mineral to the
next [68, 69]. Glauconite pellets generally form in semi-
confinement to replace carboniferous parent material
with expandable smectite minerals. e diversity of clay
minerals identified therefore reflects different stages of
glauconization. is is visible in different shades of green,
when comparing thin sections (e.g. comparing GA1 and
WT2 in Fig.6). e distribution of clay minerals is likely
to be inhomogeneous, with mica flakes and glauconite
pellets representing concentrations, but dispersed pore-
filling cement also possible.
Optical microscopy and physical characterisation
tests indicate a physical diversity beyond the fundamen-
tal mineralogical variability. in sections also high-
light the presence of amorphous bioclastic components
such as foraminifera and sponge spicules which will
not have been identified by XRD (RS1 in Fig.6). Physi-
cal differences include the definition of bedding planes,
and degree to which diagenesis and bioturbation have
affected this; the homogeneity of fabric; the size and sort-
ing of grains; and the distribution and shape of porosity.
e overall open porosity varies widely across samples
(Fig.7). e abundance and density of the matrix-filling,
microcrystalline opal-CT appears to affect the shape and
distribution of porosity, with a microporous network
extending throughout all but the densest areas of this
matrix and expanding in sparser areas to larger, highly
connected pores in the 102µm region.
Petrographic variability is likely to relate to the chem-
istry of sedimentary deposition, the frequency and
amplitude of changes inducing diagenesis and the over-
all maturity of the facies. Quartz grain size and content,
and glauconite abundance and saturation are all likely
to increase as a result of diagenesis from native siliceous
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Michetteetal. Herit Sci (2020) 8:80
and calcareous bioclasts, respectively, with opal-CT rep-
resenting a transition stage of the siliceous component
[70, 71]. Demonstrative of two extremes are TOL and
GA2 (Table2; Fig.6):
• TOL is high in opal-CT and calcite and low in quartz
and clay. It has an abundance of bioclastic compo-
nents, sparse, olive-green glauconite pellets and very
fine quartz grains in a very dense, matrix supported
fabric that shows faint signs of discontinuous bed-
ding. e relatively low open porosity is not visible in
thin section other than in some near surface fissures
and is likely to be almost entirely in the microporous
region.
• GA2 is dominated by slightly larger quartz grains
with a high clay content reflected in abundant glau-
conite pellets, ranging from light- to dark-green, in
a grain supported fabric with sparse opal-CT and
highly disturbed bedding. Calcite content is low. e
open porosity is higher, and large pores are visible
between individual grains.
GA2 represents a mature sample which has undergone
much diagenesis, with the majority of siliceous micro-
fauna having transformed fully to quartz via opal-CT,
whilst TOL is likely to have undergone less change, with
most of the silica present in opal-CT form and traces of
initial faunal deposition still present. Significant varia-
tions are also present in samples extracted from the same
facies, probably as a result of initial sedimentation. GA1
displays a denser, more matrix supported fabric, and
is also more calcareous and less clay-bearing than GA2
(Table2; Fig.6). Its open porosity is comparable to TOL;
however, the structure is heterogeneous and includes
pore sizes similar to those in GA2. ese findings serve
to illustrate the relative variability in Upper Greensand
beds ranges across several mineral and physical proper-
ties, and the challenges in extracting building stone of
uniform composition.
Selective quarrying
Several correlations in mineralogical composition indi-
cate selective quarrying of building stone. In quarry
samples there is a good correlation between decreas-
ing opal-CT content and increasing quartz content
(R = − 0.67) (Fig. 5). is reflects the diagenetic transi-
tion from the former to the latter. is correlation is not
present in building stones (R = − 0.15), where quartz
content is generally lower and opal-CT content is more
variable than in quarry samples. ere is a good corre-
lation between decreasing quartz content and increas-
ing clay content in all samples (R = − 0.65) (Table 4).
e average proportion of clay minerals in building
samples (29.4%) tends to be higher than in quarry sam-
ples (19.7%). Viewed together with the lack of correlation
between quartz and opal-CT, this may reflect a process
of selecting suitable building stones which tends towards
reduced quartz content. is could have favoured finer
grained, homogeneous, matrix-supported stone which
proved easier to finely carve.
Further analysis suggests these selection criteria
may have resulted in shifts in mineralogical composi-
tion across different periods of quarrying and use. Cal-
cite content varies but the mean value is comparable in
quarry samples (16.5%) and building samples (17.7%).
Pre-thirteenth century building stones contain rela-
tively higher proportions (M = 24%, or 18.8% without
TOL), particularly when considering the formation of
gypsum in WT1-3 has removed soluble calcium. ir-
teenth to fifteenth century samples contain lower pro-
portions (M = 13.2%). ere is a correlation between
increasing calcite content and increasing opal-CT con-
tent (R = 0.58) ( Table 3), with calcium perhaps able to
precipitate and carbonise more readily in stone with a
more abundant micro-porous matrix. irteenth to fif-
teenth century samples contain a high proportion of clay
minerals (M = 36.4%), at the expense of opal-CT/calcite
(Fig. 8). Clay content in sixteenth century samples is
lower (M = 27.9%), but variability is high. is may reflect
further shifts in selection and use. HC2-4 have a lower
than average clay content (Table2), which could indicate
refined quality control in response to the decay of earlier
building stone. However, the consequent trend in this
sample set is higher quartz content; calcite content is var-
iable. Others, such as WGA, have notably high clay con-
tent and may represent a reuse of earlier material. ese
shifts in selection criteria can be related to the fluctuating
supply and demand discussed in the previous section.
ese findings should be treated with caution given
the small sample set of building stones; however, they
do support claims made elsewhere that building stones
were selectively quarried [8]. is is reflected in a more
uniform bulk density in building stone samples (Fig.7).
e mean value for p-wave propagation is also higher
in building stones (2151 m/s) than in quarry samples
(1973m/s). Extant building stones are characteristically
denser and stronger. However, rather than a selection
based on or even reflected in calcite content, observa-
tions here suggest that ease of working may have been
the principal criterion. is is likely to have favoured
textural homogeneity, present in stones with a more
abundant matrix and/or softer, finer grains. As demand
outstripped the supply of calcareous, matrix-supported
stones, an increase in the proportion of clay minerals is
therefore likely to have become an unintended conse-
quence of stones supplied to buildings.
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Michetteetal. Herit Sci (2020) 8:80
Table 2 Results ofmineralogical andhygro-physical analysis
Mineral components determined using XRD. Bulk density and open porosity determined according to BS EN 772-4:1998. Pulse velocity determined in accordance with BS EN 14579:2004. Anisotropy coecient
determined according to [65]. Capillary absorption determined according to BS EN 1925:1999. Sorptivity dened as mass-percentage gained following adsorption; hysteresis as dierence between mass-percentage
following adsorption and desorption
Sample Quartz (M%) Opal CT
(M%) Calcite (M%) Feldspar
(M%) Clays (M%) Bulk density
(g/cm3)Open
porosity
(V%)
Pulse
velocity
(m/s)
Anisotropy
coecient
(–)
Capillary
absorption
(g/m2s0.5)
Sorptivity
RH95 (%) Hysteresis
RH80 (M%)
GA1 40 12 28 14 6 1.72 27.6 2573 9.5 53.9 7.3 0.8
GA2 49 7 9 10 25 1.52 35.0 1175 22.4 174.9 5.7 0.7
GA3 55 8 8 11 18 1.49 35.7 1591 6.8 131.7 7.3 1.0
RS1 43 15 12 20 10 1.46 33.9 1818 6.5 200.8 7.3 1.7
RS2 49 14 18 14 6 1.51 33.9 2064 2.6 95.6 8.6 1.7
RS3 33 17 16 11 22 1.41 35.2 2034 9.0 125.3 8.7 1.9
QD1 45 16 21 0 17 1.34 35.1 1977 1.2 106.9 9.4 1.7
QD2 23 19 17 0 41 1.38 35.6 2067 6.9 141.5 9.7 1.8
QD3 18 18 17 22 26 1.56 33.7 2203 4.4 105.2 10.0 1.9
GO1 38 12 15 18 17 1.44 36.1 1673 2.3 192.3 8.4 1.1
GO2 48 11 17 10 14 1.45 35.4 1924 6.0 125.8 9.4 1.3
GO3 37 13 16 15 18 1.40 38.6 1687 4.1 139.1 9.3 1.4
QF1 26 17 14 0 42 1.38 33.7 2120 3.1 137.9 8.3 1.4
QF2 57 16 19 0 8 1.38 34.5 2228 9.6 81.7 9.3 2.0
QF3 20 17 20 17 26 1.56 30.6 2465 3.6 60.5 9.2 2.0
TOL 14 25 45 15 1 1.85 26.8 2968 6.5 26.8 7.5 0.8
WT1 28 18 20 0 34 1.68 37.7 1911 1.7 112.2 8.3 1.7
WT2 19 29 18 0 34 – – – – – – –
WT3 26 16 14 17 28 – – – – – – –
BAL 18 18 16 18 30 1.57 36.0 2080 4.4 87.2 9.6 2.0
MRT 11 10 16 17 46 1.57 33.0 2084 5.7 101.2 9.7 1.2
MER 23 27 23 0 28 1.51 35.7 1946 8.1 136.1 8.6 1.6
SMS 17 11 12 22 38 1.49 36.5 2005 7.4 123.7 9.7 1.7
SMG 32 9 10 18 31 1.55 33.2 2062 10.3 85.2 7.7 1.3
THR 14 6 12 31 37 1.57 35.7 2151 3.6 92.9 10.2 1.8
HC1 35 17 24 0 24 1.61 34.8 2184 5.7 69.9 9.4 1.3
HC2 38 14 13 19 15 1.57 37.0 2073 2.2 102.6 6.9 1.6
HC3 36 12 20 14 19 – – – – – – –
HC4 44 11 13 18 14 – – – – – – –
HC5 19 20 19 18 24 – – – – – – –
HC6 19 12 17 10 42 – – – – – – –
HC7 22 8 9 15 45 – – – – – – –
WGA 24 19 17 0 40 1.58 35.0 2200 12.2 60.7 9.8 1.9
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Michetteetal. Herit Sci (2020) 8:80
Variable resilience
In order to investigate the potential impact of variable
mineralogical composition on hygro-physical charac-
teristics, Principal component analysis (PCA) was per-
formed. PCA is a statistical procedure to convert separate
variables into linear, ‘best fit’ composites (called principal
components (PC)), which account for the largest possible
variance within a dataset [72]. e first PC accounts for
as much variability in the data as possible; each succeed-
ing PC has the highest variance possible whilst remaining
orthogonal (uncorrelated) to its precedent. is proce-
dure can be used to plot individual variables and samples
as vectors and points in a low-dimensional data space, in
order to visualise their contribution to correlated groups.
It therefore provides a useful method for identifying cor-
relations and clusters in a dataset. e facto-extra pack-
age was used in RStudio; this automatically scales and
normalises data from separate variables. e analysis
was performed on all samples which were tested both
for mineral and hygro-physical properties, in order to
explore correlations between key mineral components,
strength characteristics, and hygric mechanisms. After
initial runs, feldspar content, anisotropy and hysteresis
were not included in order to simplify the final analysis.
Two dimensions account for almost 70% of variation
(Fig. 9). ese associate two mineral components with
distinct hygro-physical mechanisms. PC1 groups ultra-
sonic wave propagation, surface hardness, water uptake
and calcite content. Bulk density and effective porosity
also contribute. PC1 therefore links calcite content with
strength characteristics such as compressive strength,
elasticity and fabric density. is indicates calcite func-
tions as a pore clogging cement in harder, denser sam-
ples. is results in a decrease in capillarity. PC2groups
clay mineral content with moisture adsorption. e lack
of any significant correlation between hygro-physical
properties and opal-CT content is noteworthy, given
the apparent importance of opal-CT in determining the
shape of the porous matrix. However; PC3 and PC4,
which each account for approximately 10% of variance,
do group opal-CT content with bulk density, moisture
adsorption and water uptake (Table 4). is suggests
more complex interrelations, with the distribution of
components throughout the matrix likely to be crucial
to individual properties. ese results should be treated
with caution given the small number of samples and high
variation within the dataset. PC1 is dominated by TOL,
which accounts for 45% of the contribution. However,
removing TOL from the dataset and running the PCA
again results in a similar dimensionality, with PC1 and
2 accounting for over 60% of variability and respectively
linking increased calcite content with strength and slow
water uptake, and increased clay content with higher
sorptivity (as well as reduced open porosity and quartz
content). e correlations identified here should be
viewed as patterns, which can explain Reigate Stone char-
acteristics in terms of the relative abundance of different
cementing components, and account for different rates
and patterns of decay in relation to hygric mechanisms.
Reigate Stone has several characteristics which are
linked to poor resilience in building stone. e mean
open porosity of 34.5% is high for stones used in building
[73]. Clay minerals have a low weathering resistance and
can induce swelling [73]. Moisture retention of between
1 and 2% at an RH of 80% indicates residual moisture will
be present at high atmospheric humidity and dynamic
equilibrium will increase the risk of salt crystallisation
cycles [74]. Whilst it is uncommon in building stones,
opal-CT forms a weak, highly soluble cement [4]. How-
ever, the picture painted by the relative proportion of
cementing components in relation to hygro-physical
properties likely to control decay, such as strength and
moisture retention, is not as clear as the PCA might sug-
gest (Fig. 8). Beyond baseline mineralogy, lower values
for p-wave propagation in older buildings indicate more
advanced decay in some samples, whilst others such as
TOL appear to remain resilient.
Building stone decay pathways
Different decay patterns visible in the building stone thin
sections shown in Fig. 6 can be indicatively linked to
the dominance of cementing components. In calcareous
TOL, near surface fissures run parallel to faintly visible
bedding orientation and through a pronounced crust,
resulting in a progressive delamination of the dense fab-
ric. In clay-bearing WT2, dissolution of fabric at a greater
depth has resulted in more heterogeneity, with areas of
very high porosity reflected in the high measured open
porosity and low p-wave propagation of its partner sam-
ple WT1 (Table2). ese samples come from buildings
dating to a similar era, and although it is unclear pre-
cisely what part of the White Tower was sampled, TOL
and WT1-3 all come from currently external masonry.
However, whilst large areas of the White Tower base
were enclosed by later structures for long periods, WT
samples were highly exposed in the south facing Ward-
robe Tower for several centuries and there is evidence of
repair using inappropriate materials. e aspect and con-
tingency of exposure, and past repair programs can be a
further factor in the varying emergence and rate of decay.
Several results provide further evidence that varia-
tion is amplified insitu as a result historic contingency,
micro-environmental and/or baseline mineralogical vari-
ation. Gypsum was identified in WT1-3, MRT and HC7.
It is likely to have formed from soluble calcite reacting
with atmospheric sulphur dioxide. Gypsum is absent in
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Page 14 of 24
Michetteetal. Herit Sci (2020) 8:80
samples HC1–6, which are likely to have been buried
before industrial scale coal burning. e different levels
present in WT1-3 indicate that variable contamination
is possible in highly proximate stones, possibly due to
slight petrographic or microclimatic differences. Initial
hygric characterisation tests on MRT indicate the effect
contamination can have on physical properties: capillary
absorption was very low (28.2 g/m2s0.5), presumably as
a result of the pore clogging effect of near surface gyp-
sum; sorptivity was very high (46.2%), possibly due to
the hygroscopicity of other trace salts. After the sample
was thoroughly rinsed and retested, results were closer to
those of uncontaminated samples (Table2). e mineral-
ogical variability present in samples from the same build-
ing implies that in some cases stone came from a variety
of sources. Variability in HC1–7 is as high as any of the
measured variability within individual quarries (Fig.3).
Macroscopic inspection of the samples suggests that
they are representative of at least two distinct typologies,
evident in different colour. Provenance from multiple
quarries and/or material recycled from several buildings
appears likely.
0.00
0.25
0.50
0.75
1.00
GA1GA2GA3QF1QF2QF3TOLWT1WT2WT3HC1HC2HC3HC4HC5HC6HC7
Composition
Mineral
Calcite
Clays
Feldspar
Opal.CT
Quartz
a
R = − 0.67 , p = 0.0068
R = − 0.15 , p = 0.54
0
10
20
30
10 20 30 40 50
Quartz (%)
Opal CT (%)
Class
Building
Quarry
Group
●
●
●
●
●
●
●
C11−12
C13−15
C16
Chaldon
Gatton
Godstone
Merstham
b
Fig. 5 a Selection of results from XRD analysis showing mineralogical variation across and within separate quarries (Gatton and Quarry Field,
Merstham), and across and within individual buildings and sites (White Tower and Wardrobe Tower, Tower of London, and Hampton Court Palace). b
Chart showing differing correlation between opal CT and quartz within freshly quarried stone (R = − 0.67) and stone used in buildings (R = − 0.15),
lower quartz content in building stone, and higher opal CT content in phase 1 building stone
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Page 15 of 24
Michetteetal. Herit Sci (2020) 8:80
Other results indicate that some degree of homogeni-
sation can result from decay. Whilst there is no signifi-
cant difference in the average or range of open porosity
between quarry samples and building stone samples, it is
notable that building stones have a slightly higher open
porosity (Fig.7). is does not reflect the overall correla-
tion between higher bulk density and lower open poros-
ity (R = − 0.57) (Table3). is could partly be due to the
dissolution of matrix opening previously closed areas of
porous network. e pore-size distribution, which has
not been measured here, may be a crucial factor in deter-
mining variable durability. Although the primary, opal-
CT matrix will form a regular porosity in the 102µm
region, the clay mineral fraction may result in highly
bimodal distributions. is will increase the risk of salt
decay mechanisms [75]. e role of calcite as a pore fill-
ing cement will play an important role in pore size dis-
tribution and open porosity. Weathering of calcite could
increase connected, open porosity in near surface areas,
especially in case hardened stones.
ere is no clear pattern to anisotropy (Table2). e
highest value is in GA2, an example of mature Reigate
Stone, with more advanced diagenesis and bio-perturbed
bedding; its anisotropy is therefore unlikely to be a con-
sequence of bedding structure. With variation within
and across individual quarries high, neither maturity
nor paleoenvironment are good indicators of anisotropy.
Samples in which bedding direction is visible in thin sec-
tion, such as QF3 and TOL (Fig.6), have low anisotropy.
e low anisotropy in WT1 is notable given the advanced
state of decay of its partner WT2 (Fig. 6). Rather than
compounding pre-existing bedding structure, this sug-
gests that decay has had a levelling effect on the internal
fabric. e implication of these findings is that Reigate
Stone is at most weakly anisotropic. Post-sedimentary
processes prior to quarrying and decay processes follow-
ing construction can override bedding structure.
Synthesis
Discussion
e largely nineteenth century differentiation between
Firestone and Hearthstone, used to distinguish stones for
different purposes as much as of different properties, has
led to contemporary discourse describing two distinct
typologies of Reigate Stone [8, 19]. is typically associ-
ates Firestone with calcite and Hearthstone with clay.
e reality is that there are likely to be as many different
mineralogical compositions as there are intercalated beds
in the geological stratum. When defining the macro-
geology of the Upper Greensand, Jukes-Brown [12 p. 38
onwards] described zones of deposition defined by space,
time and life (i.e. organic sediment). Particularly due to
the characteristic species of a thus defined zone, there
can be considerable overlap in resulting mineralogy.
is is reflected in the findings presented here; compo-
sitional variability is present in stones sampled at relative
proximity within single quarry faces. Furthermore, the
stones used in medieval construction do not conform to
a clear typology or necessarily contain increased calcite.
Firestone and Hearthstone may be representative of two
selectively exploited sub-types; however, when discussing
the full history of Reigate Stone used in buildings, it may
be more useful to consider a broad mineralogical range.
is will not alleviate persistent difficulties in classify-
ing Reigate Stone according to common lithologies such
as sandstone or limestone [e.g. 8, 12]. Instead it demands
that each use in masonry is assessed individually and
conservation strategies are tailored accordingly.
ere are nevertheless patterns evident in Reigate
Stone selection and use over time. Identifying these has
benefited from synthesising documentary research with
material analysis, which was only possible on a small
sample set due to the restrictions of the historic built
environment. e samples investigated here do not
reflect the full variability of quarry samples. Whilst they
vary in mineralogical composition, density and strength
characteristics are more uniform. e earliest building
stone investigated here (TOL), sampled from the elev-
enth century White Tower, supports anecdotal evidence
that early use Reigate Stone conformed to a particu-
larly durable variety, which became exhausted during
London’s rapid growth. Only one quarry sample (GA1)
matches the hygro-physical properties of this building
stone. As with many of the quarry samples investigated
here, its coarse, less homogeneous texture suggests it may
not have been a suitable freestone. No quarry face reli-
ably provides stone that is comparable to those sampled
from buildings. Large quantities of waste material found
in later medieval quarries suggest quarrymen became
adept at purposefully selecting stone [19]. It is possible
that this was a process of trial and error, and that particu-
larly the Reigate Stone industry boom of the thirteenth
(See figure on next page.)
Fig. 6 Photomicrographs of selected thin sections, showing variability of mineralogy, texture and grain size within and across different quarries
and buildings, different decay patterns within building stones, and highlighting some common mineral components. GA1 and GA2 taken from
same quarry face show high level of petrographic and physical variation, neither is similar in texture to building stone samples. QF3 is more similar
to building stone, but shows high porosity with some suggestion of bedding direction. TOL and WT2 show evidence of varying decay phenomena.
RS1 shows some of the minerals described in this study
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Michetteetal. Herit Sci (2020) 8:80
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Page 17 of 24
Michetteetal. Herit Sci (2020) 8:80
century saw lesser quality material extracted and used in
buildings. is is supported by findings presented here,
which associate a second phase of Reigate Stone exploi-
tation with increased clay content. is could be a rea-
son for some of the early failings of the stone, which had
tarnished its reputation by the fifteenth century. is
does not contradict findings made elsewhere, which sug-
gest extant Reigate Stone is more calcareous [8]. Any
attempted characterisation of building stone according
to extant masonry is however to ignore potentially vast
amounts of stone which were being replaced by the thir-
teenth century. Following centuries of limited, targeted
use, which developed a quarrying industry around reli-
able sources, selection criteria for easily workable stone
during the rapid expansion of quarrying activity in the
twelfth century are likely to have introduced inferior
quality.
Beyond the selection of suitable stone, there is fur-
ther evidence that workmanship may have influenced
the performance of stone in situ. e historical analy-
sis presented here builds on previous assessments of
changing use throughout history, to suggest that Reigate
Stone economies were well established before the Nor-
man conquest and the stone was a valued resource [1,
36, 37]. However, significant growth in demand in the
High Medieval and Early Modern periods is likely to
have affected workmanship and logistics. e presence
of calcite and its correlation with harder, denser stone
suggest that seasoning of freshly cut stone will have been
important. Roughly cutting stone to shape before this
case hardening will also have greatly improved resilience.
Knowledge and mastery of these processes are likely to
have refined over time, with templates being sent to quar-
ries and adequate shelter for seasoning being provided
perhaps only after initial difficulties became apparent.
When Christopher Wren did make use of the stone in
later centuries, he demanded great care be taken during
storage [20]. Anisotropy does not appear to affect Reigate
Stone, however there is some evidence of flaking occur-
ring along bedding planes in samples where these are
visible (e.g. TOL). is suggests in some environments
incorrect laying of the stone may have accelerated decay.
Especially given the poor visibility of bedding planes,
errors during construction following lengthy transport
and stockpiling are likely to have occurred. In the case of
TOL, the stone appears to have been face bedded rather
than surface bedded. ese factors will have contributed
to divergent decay pathways.
is study identifies correlations between different
cementing components and hygric mechanisms affecting
●
●
●
●●
●
●
●
●
●
●
●
●
●
●●
●
●
●
●
●
●
●
●
●
●
1.4
1.5
1.6
1.7
1.8
Building Quarry
Bulk density (g/cm^3)
a
●
●
●
●
●
●
●
●
●
●
●
●
●
●●
●
●
●
●
●
●
●
●
●●
●
0.27
0.30
0.33
0.36
0.39
BuildingQuarry
Open porosity
b
●
●
●
●
●
●
●
●
●
●
●●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
1500
2000
2500
3000
BuildingQuarry
Pulse velocity (m/s)
c
Group
●
●
●
●
●
●
●
C11−12
C13−15
C16
Chaldon
Gatton
Godstone
Merstham
Fig. 7 Box and whisker plots, showing a higher and more uniform bulk density in building stone, b slightly higher open porosity in building stone,
and c more uniform response to pulse velocity measurement in building stone. Central line of box depicts mean value; lower and upper edges of
box correspond to 25th and 75th percentile (IQR); whiskers extend to the largest/smallest value within 1.5 * IQR
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

Page 18 of 24
Michetteetal. Herit Sci (2020) 8:80
Table 3 Correlation matrix using Pearson’s method onresults shown inTable3
Mineralogical correlations made using all 33 samples. Mineralogical-physical and physical correlations made using 26 samples for which this data was available. R greater than 0.4 (or less than − 0.4) marked in italic. R
greater than 0.6 (or less than − 0.6) marked in bold-italic
Quartz Opal CT Feldspar Calcite Clays Bulk density Open
porosity Pulse velocity Capillary
absorption Sorptivity Hysteresis
Quartz 1
Opal CT − 0.34 1
Feldspar − 0.21 − 0.49 1
Calcite − 0.21 0.58 − 0.23 1
Clays − 0.65 − 0.06 − 0.12 − 0.42 1
Bulk density − 0.40 0.18 0.24 0.59 − 0.14 1
Open porosity 0.17 − 0.19 − 0.12 − 0.65 0.35 − 0.57 1
Pulse velocity − 0.46 0.48 0.05 0.75 − 0.16 0.55 − 0.72 1
Capillary
absorption 0.31 − 0.26 0.03 − 0.60 0.09 − 0.62 0.55 − 0.82 1
Sorptivity − 0.45 0.17 − 0.02 0.02 0.43 − 0.24 0.24 0.26 − 0.24 1
Hysteresis − 0.23 0.31 − 0.04 − 0.21 0.28 − 0.35 0.37 0.14 − 0.08 0.64 1
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

Page 19 of 24
Michetteetal. Herit Sci (2020) 8:80
resilience. ese underline complex decay processes in
relation to the environment, which will be affected by
spatial, micro-climatic variations and temporal, macro-
climatic changes. Increased calcite content impacts
positively on strength characteristics; its pore-clogging
properties are shown to reduce capillarity, but water
uptake is still rapid in most stones. Coupled with high
moisture absorption and retention, particularly in stones
with abundant clay minerals, Reigate Stone will be prone
to moisture related decay processes and is likely to be
highly sensitive to alterations caused by salt contamina-
tion. Associated decay phenomena such as gypsum crust
formation and spalling have been well documented in
other vulnerable historic freestones [2, 3]. Varying calcite
and clay contents are likely to preclude standard stone
conservation treatments being suitable for all types of
Reigate Stone, with targeted selection necessary depend-
ing on mineralogical composition. Assessment of past
treatments indicate that effective long-term consolida-
tion is only possible in more calcareous, and therefore
naturally less vulnerable stones [18].
e cumulative effect of decay and changing
approaches to conservation will have increased variabil-
ity over time. e dynamics of building stone decay are
known to be complex and non-linear [16]. Minor physi-
cal differences may diverge thresholds at which decay
becomes evident. Besides the impact of environmental
change upon rates and patterns of decay, the impact of
cultural change upon the perception of deterioration
has been a significant factor in shaping variation in the
20
40
60
80
100
20
40
60
80
100
20
40
60
80
100
Opal.CT
Clays
Calcite
Opal.CT
Clays Calcite
class
C11−12
C13−15
C16
quarry
Hyst80
0.0100
0.0125
0.0150
0.0175
0.0200
2000
2250
2500
2750
pulse(m/s)
Fig. 8 Ternary plot showing relationship between proportion of opal CT, calcite and clay (cementing components of stone), sorption hysteresis
at RH80, and pulse velocity. Hygro-physical properties selected to represent performance of stone. One sample from each quarry deemed most
suitable for building stone selected. Limited number of high calcite stones shown to perform well. Thirteenth to fifteenth century stones are
relatively higher in clay content and perform poorly. Other stones with variable performance
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Page 20 of 24
Michetteetal. Herit Sci (2020) 8:80
●
●
●●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●●
●
●
GA1
GA2
GA3
RS1
RS2
RS3
QD1
QD2
QD3
GO1
GO2
GO3
QF1
QF2
QF3
TOL
WT1
BAL
MRT
MER
SMS
SMG
THR HC1
HC2
WGA
Quartz
Opal.CT
Calcite
Clays
Bulk.density
Open.porosity
Pulse.velocity
Capillary.absorption
Sorptivity
−2
−1
0
1
2
−2.5 0.0 2.55.0
Dim1 (46.1%)
Dim2 (21.1%)
8
10
12
14
contrib
Fig. 9 Biplot of Principal Component Analysis showing variation of samples according to dimensionality of two principal components and
contribution of variables to the components. PC1 groups calcite with density and strength characteristics; PC2 groups clay content with sorptivity.
The distribution of individuals shows that there is no clear typological clustering, although there is some grouping according to quarry provenance
(e.g. GO1–3) and TOL is a notable outlier. Removing TOL does not significantly alter the dimensionality
Table 4 Results of PCA, showing contribution ofPC1–4 to overall variability, andcontribution of individual variables
toeach PC
Variable PC1 (46%) PC2 (21%) PC3 (11%) PC4 (10%)
Quartz 9.9 23.4 4.7 3.1
Opal CT 5.8 1 33.6 28.
Calcite 14.5 6.5 14.1 4.9
Clays 2.2 33 7.2 1.7
Bulk density 7.1 2.7 25.6 23
Open porosity 7.2 16 5.8 0.9
Pulse velocity 24.5 1.4 0 5.6
Capillary absorption 19.4 2.4 2.3 7.3
Sorptivity 9.4 13.7 6.7 25.1
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Page 21 of 24
Michetteetal. Herit Sci (2020) 8:80
historic built environment. Within an overall pattern that
has shifted from replacement and renewal to preservation
and consolidation over the past two centuries, there has
been nuance and disparity in response to socio-economic
context. Overlaying these two complex trajectories, non-
linear physical decay and idiosyncratic perception of
deterioration, imparts a lack of contingency on conser-
vation strategy; one area of Reigate masonry may cross
a decay threshold and be replaced one decade, only for
the rest to cross the same threshold yet be treated with
a consolidant the following decade. Introducing the new
materials of replacement or consolidation, and the mem-
ory effect of past environmental stresses, into this already
intricate system dynamic serves only to augment the var-
iability of initially minor, physical differences.
Synopsis
e objectives of this investigation were to establish pat-
terns of use in Reigate Stone exploitation pertaining to
variability, define variability in mineralogical terms, and
link mineralogical composition to physical character-
istics known to influence decay. is was intended to
contribute to the overall aim of building a model of the
processes which have contributed to variability in his-
toric masonry. is model can be visualised as a timeline,
tracking Reigate Stone use in building through several
phases (Fig.10).
1. Early use, prior to and in the first century following
the Norman conquest, was limited and targeted. e
focus was on detailing and supplementation of other
more widely used lithologies such as Kentish Rag-
stone and Caen Stone. Quarrying was restricted to a
Phase 2: Ra
p
id ex
p
ansion
o
f
quarr
y
ing activit
y
to
supp
ly
h
uge growt
h
in
m
asonr
y
construct
i
on.
P
hase 3: Initial re
p
air,
r
ep
l
acement an
d
rec
y
c
l
ing
of deca
y
ed primar
y
Reigate
Stone;
d
ec
l
ine in quarr
y
ing
.
P
hase 4: Accelerated deca
y
,
r
ep
l
acement wit
h
non-
R
ei
g
ate. New use interna
l
o
nl
y
. Brie
f
C19
r
eviva
l
.
Ph
a
se
5:
Rep
l
acement
gives wa
y
to
conservat
i
on.
Phase 1: Localised quarr
y
ing
f
or targeted use in ke
y
b
ui
ld
in
g
s. Normans exp
l
oit
e
sta
bl
is
h
e
d
in
d
ustr
y
.
G
oo
d
qua
l
it
y
st
o
ne fr
o
m
se
l
ecte
d
q
uarr
i
es.
Va
ri
abl
e
qua
l
it
y
stone
f
rom across
r
eg
i
on.
Repair
m
a
teri
al
wit
h
qua
l
ity
contro
l
.
V
ict
o
ri
a
n
r
epair an
d
n
ew
b
ui
ld
m
ateria
l.
Historic context
n
Reigate Stone
d
istri
b
ution (in
d
icative sca
l
e
)
Selected Reigate
e
g
Stone buildings
d
f
Ph
a
ses
of
u
s
e
Tower of London
Tower of London
n
p
Other sampled
l
Other mentioned
i
1066: Norman Conquest
c.1050: St Peter’s Abbey
c.1070: White Tower
c.1520: Hampton Court Palace
1114: Merton Abbey
1538: Nonsuch Place
c.1140: Wardrobe Tower
c.1240: Martin Tower
1245: Westminster Abbey 1832: New
London bridge
1176: Old London bridge 1390: St Thomas Chapel rebuilt
c.40: Londinium founded
late C1: Brick firing kilns
C1: Roman Southwark
1206: Loss of French possesions
1340s: Black Death
1530s: Dissolution of the
monasteries
1666: Great Fire of London
1661: Fumifugum
published 1956: Clean Air
Act passed
C19: Stylistic restoration
c.1600: Whitgift Almshouse
1197: St Mary Spital
1350: St Mary Graces
mid C19:
Suburban Reigate
1675-1710
St Paul’s Cathedral
c.1460: Major repairs
1
1
00
0
A
.
D
.
1
300
1500
17
00
1900
Fig. 10 Hypothetical model of Reigate Stone distribution, distinguishing broadly classified stone types, aligned with timeline of Reigate Stone
quarrying and construction, showing different phases of use, and key events and buildings mentioned in this study
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

Page 22 of 24
Michetteetal. Herit Sci (2020) 8:80
small number of locations and it was possible to sup-
ply durable stone of uniform quality.
2. e second phase of use, from the twelfth to the fif-
teenth centuries, witnessed huge growth in masonry
building and accordingly a significant expansion of
quarrying activity. Whether due to inherent variabil-
ity found in new quarrying locations, or due to the
stresses of increased demand leading to a decline in
workmanship and quality control, this phase saw the
introduction of less resistant stones into the built fab-
ric. Growing logistical complexity may have resulted
in masonry with stones of variable provenance or
maturity, as stockpiling locations became a necessary
juncture between quarry and building site and cen-
tralisation streamlined supply.
3. e third phase of use sees a first wave of replace-
ment, repair and refinement in response to the decay
of less durable stones, beginning in the fifteenth cen-
tury following several centuries of weathering activ-
ity. Understanding of the limitations is likely to have
improved and ongoing use may have been subject to
more stringent quality control. However, it is possi-
ble that supplies of the best quality Reigate Stone had
been exhausted. Compounded by socio-economic
factors, which led to reduced demand for freestone,
quarrying activity gradually declined. Good quality
Reigate Stone was still a valued resource; robbing of
disused buildings and reuse in new buildings is likely
to have occurred in many cases. is will have fur-
ther increased the inherent variability found within
individual masonry units.
4. e fourth phase brings gradual changes to London’s
environment, beginning in the seventeenth century
and climaxing in the intense pollution of the late-
nineteenth century, which accelerated the decay of
even good quality Reigate Stone. e value of the
stone decreases and as the construction industry
grows anew, replacement with newly available alter-
natives becomes more common. Whilst a brief Victo-
rian revival introduced a small amount of fresh Rei-
gate Stone into the historic fabric, the overall stock
drastically falls.
5. A final phase beginning in the twentieth century
focussed on attempts at conserving and consolidat-
ing remaining Reigate masonry. Selective treatment
is likely to have further amplified inherent variability.
Conclusion
is paper has investigated the causes of inherent vari-
ability in historic Reigate Stone masonry. e methodo-
logical approach has been to synthesise historical analysis
of its use with scientific examination of its properties.
is approach has revealed patterns which can be used
to build a hypothetical model of processes resulting in
variability. Reigate Stone was Medieval London’s prin-
cipal freestone. It was used in vast quantities during a
key growth period in London’s history. Huge demand
may have outstripped the supply of good quality build-
ing stone as quarrying adapted from localised supply for
specific projects prior to the twelfth century, to industrial
exploitation in step with rapid, regional economic growth
in the following centuries. is introduced mineralogical
variability into the built fabric.
Different mineralogical components are shown here to
influence key material properties known to control the
onset of decay in building stones. Calcite content influ-
ences strength and capillarity. e abundance of clay
mineral phases affects adsorption. As the main matrix
forming cement, the highly soluble opal-CT content
will play an additional role in long-term moisture trans-
port and durability. Whilst the precise interplay of these
cementing components and their net effect on the resil-
ience of individual building stones is likely to be complex,
small initial differences can result in variable response
to environmental processes and the divergence of decay
pathways.
In historic buildings, any resulting variation in the
onset of decay would have been further augmented in
response to changing economic and environmental con-
text. Successive material interventions and recycling of
built fabric occurred as a reflection of changing attitudes
to architectural heritage and introduced further com-
plexity. Understanding not only the material and envi-
ronmental factors, but also the historical contingency
underlying inherent variability in Reigate Stone masonry
is key to the design of ongoing conservation strategies. In
terms of practical guidelines, this demands a thorough
documentary analysis of past changes to a masonry sys-
tem, and careful documentation and archiving of any
new changes for the benefit of future work.
Abbreviations
XRD: X-ray diffraction; RH: Relative humidity; PCA: Principal component
analysis; PC: Principal component; Opal-CT: Opal-cristobalite-tridymite; RS:
Rockshaw Lodge; QD: Quarry Dean; GA: Gatton; GO: Godstone; QF: Quarry
Field; TOL: (White Tower) Tower of London; WT: Wardrobe Tower; BAL: Balium
Wall; MRT: Martin Tower; MER: Merton Priory; SMS: St Mary Spital; SMG: St
Mary Graces; THR: Throwley Church; HC: Hampton Court Palace; WGA : Whitgift
Almshouses.
Acknowledgements
The authors would like to thank Hong Zhang, Owen Green and Katherine
Clayton from the University of Oxford for technical and analytical support
with the petrographic experiments; Peter Burgess and his colleagues from the
Wealden Cave and Mine Society for facilitating a tour of the Reigate mines and
providing useful historical information; Robin Sanderson and Keith Garner for
making available an extensive sample collection and unpublished data from
their previous research; Xiaoke Liu from University College London for help
with the statistical analysis; colleagues from HRP for support and guidance, in
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Page 23 of 24
Michetteetal. Herit Sci (2020) 8:80
particular Jo Thwaites and William Page for locating and providing additional
samples; and three anonymous reviewers for valuable feedback.
Authors’ contributions
MM contributed to all stages of work. HV, CV and IA contributed to the con-
ception of the work. HV additionally contributed to the design of the work. All
authors read and approved the final manuscript.
Funding
This work was supported by funding from the Engineering and Physical Sci-
ences Research Council (EPSRC) and Historic Royal Palaces (HRP) as a part of
the Centre for Doctoral Training in Science and Engineering in Arts, Heritage
and Archaeology (SEAHA) (Grant Number: EP/L016036/1).
Availability of data and materials
The datasets used and/or analysed during the current study are available from
the corresponding author on reasonable request.
Competing interests
The authors declare that they have no competing interests.
Author details
1 School of Geography and the Environment, University of Oxford, Oxford, UK.
2 Historic Royal Palaces, London, UK. 3 Carden & Godfrey Architects, London,
UK.
Received: 27 March 2020 Accepted: 25 July 2020
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