Rates of Stone Recession on Mediaeval Monuments: Some Thoughts and Methodological Perspectives

Cadernos Lab. Xeolóxico de Laxe Coruña. 2010. Vol. 35, pp. 13 - 40 ISSN: 0213-4497

Rates of stone recession on Mediaeval monuments: some thoughts and methodological perspectives

ANDRÉ, M. F.1 and PHALIP, B.2

(1) Laboratory of Physical and Environmental Geography / GEOLAB – UMR 6042, CNRS. - Blaise Pascal University, Clermont-Ferrand (France) (2) Centre of History ‘Spaces and Cultures’ – Department of Art History and Archaeology - Blaise Pascal University, Clermont. Ferrand (France) Contact: [email protected]

Recibido: 9/8/2007 Revisado: 10/3/2008 Aceptado: 15/6/2008

Abstract

Studies of stone decay contribute signifi cantly to an understanding of weathering processes and landform development. However, compared to tombstones, coastal defensive structures and Roman temples, Mediaeval monuments appear to be underrepresented in quantitative assessments of historical stone decay. As a result of the close cooperation between geomorphologists and archaeologists of the Mediaeval period, it has proved possible to improve the chronological control of stone recession measurements, to reconstruct the rates of stone decay, and to investigate some of its causes.

Key words: Rock weathering, stone decay, weathering rates, Mediaeval monuments, Romanesque churches, Angkor temples 14 ��Andr�é�� and Phalip � CAD. LAB. XEOL. LAXE 35 (2010)

INTRODUCTION increasing attention during the last fifteen years, with special reference to Jordan tem- Over the last several years studies of ples and theatres (PARADISE, 1998, 2005) weathering acquired considerable status and Brittany megaliths (SELLIER, 1991, within physical geography. Though most 1997). The same is true of the 500 yr-old progress has resulted from field investiga- coastal defensive structures of the British tions in natural environments and laborato- Isles, which have been thoroughly investi- ry simulations, stone decay studies have also gated (MOTTERSHEAD, 1997, 2000a). made valuable contributions. More than one The main reason for this database het- century after Geikie’s pioneer investigations erogeneity is that tombstones, coastal defen- of stone decay in Edinburgh churchyards sive structures and ancient monuments usu- (GEIKIE, 1880), a considerable internation- ally offer a much more uniform corpus for al and multidisciplinary research effort has stone recession measurements than Medi- been made in cultural stone weathering re- aeval (i.e. 5th-15th centuries CE) monuments. search (see reviews in POPE et al., 2002 and Many of these, especially in Europe, have TURKINGTON and PARADISE, 2005). been subject to several building and/or res- Various descriptive schemes have been de- toration phases, and look like giant jig-saw veloped for classifying and mapping weath- puzzles (figure 1). In parts of some so-called ering forms (e.g. FITZNER, 1990; WARKE ‘Romanesque’ churches, constructed of rock et al., 2003). types susceptible to frost action (e.g. the Based on less integrated though still ef- Saint-Etienne church of Nevers), up to 50% fective methodologies, monuments have of the stones date back to post-Romanesque been used by geomorphologists as ‘natural periods. Such a complexity renders difficult laboratories’ to assess rates of long-term any quantification of weathering rates. Even surface back-wearing in various lithologies in the same wall bricks of essentially simi- and environments. As noted by POPE et al. lar composition and texture display strongly (2002, p. 216), “through measurement of contrasted rates of weathering (IKEDA, surface recession in various contexts, a very 2004). However, apart from stone by stone large database is available now for weath- assessments of percentage areas of decayed ering rates”. However, this database is not surfaces (ROBINSON and WILLIAMS, homogeneous. Some monument types are 1996), some attempts have been made to overrepresented (e.g. ≤200 yr marble tomb- measure surficial stone recession. Most of stones, see MEIERDING, 1981; BAER and this exploratory work has been conducted in BERMAN, 1983; DRAGOVICH, 1986; France on Romanesque and Gothic church- NEIL, 1989; INKPEN and JACKSON, es of Brittany and the Massif Central, us- 2000), whereas others have not been given ing basic tools such as steel tapes, callipers enough attention (e.g. Mediaeval sandstone and profile gauges. The resulting weathering monuments, see ROBINSON and WIL- rates, which are summarized on Table 1, are LIAMS, 1996). Between these extremes, an- at this stage provisional. cient monuments (>2000 yr) have received CAD. LAB. XEOL. LAXE 35 (2010) Rates of stone recession on Mediaeval monuments 15

A B C D

E F G

Fig. 1. Polygenetic origin of ‘Mediaeval’ European churches. © M. F. André A. Mid-12th century Gothic arch inserted in late 11th century Romanesque arch (Saint-Front Cathedral, Périgueux, Dordogne, 21-11-08). B. Decayed and replaced sandstone elements in early 13th century portal (Saint-Magnus Cathedral, Kirkwall, Orkney Islands, 27-7-1987). C. Intriguing weathering pattern on a Me- diaeval column (Sainte-Foy abbey church, Conques, Aveyron, 18-6-00). D. Contemporary restoration site (Sainte-Valérie abbey church, Chambon-sur-Voueize, Creuse, 19-10-08). E. Late 12th century whitewashed portal columns (Notre-Dame church, Fleuriel, Allier, 15-10-08). F. Polychrome apsidiole, restored in mid-19th c. and cleaned in the early 2000s. (Notre-Dame du Port church, Clermont-Ferrand, Puy-de-Dôme, 31-1-06). G. Original limestones and darker arkosic sandstones incorporated during the 1995-2000 restoration (Sainte- Martine church, Pont-du-Château, Puy-de-Dôme, 4-10-08). 16 ��André and Phalip � CAD. LAB. XEOL. LAXE 35 (2010)

Stone recession Monument Building Monument location Lithology rate in mm per Source corpus century century FRENCH CHURCHES West and South Brittany 6 15-16th Granite 3.1 – 6.5 Sellier 1998 South Brittany 29 11-17th Granite 2.6 – 4.5 Paris 1998 South Brittany 35 11-17th Granite 1.6 – 3.9 Magré et al. 2007 French Massif Central 133 11-15th Various including: Bonneau 2001 • Leucocratic granite <1 Robert 2002 • Basalt, Trachyandesite <1 André et al. 2007 • Oolitic & biocon- structed limestones <1 • Marble <1 • Biotite/chlorite-rich granite 3 – 5 • Soft limestones 6 – 7 • Sandstones 0.3 - 16

KHMER TEMPLES Angkor temples 11 10-13th Sandstone 0.2 – 5.0 André 2006 Ta Keo temple (Angkor) 1 11th Sandstone 0.0 – 5.5 André et al. 2008

Table 1. Tentative rates of stone recession on Mediaeval French churches and Khmer temples.

Based on this reconnaissance work in- (ANDRÉ et al., 2007a, 2007b; ANDRÉ et volving more than 200 churches, it is clear al., 2008; PHALIP and ANDRÉ, 2008; AN- that further use of Mediaeval monuments DRÉ and PHALIP, in press). with the assessment of historical weather- The main objective of this paper, which ing rates in mind requires collaboration. Of is based on these current interdisciplinary special interest is the current tightening of exchanges, is to suggest research that ad- links between French geomorphologists, dresses the following questions: archaeologists concerned with Mediaeval 1. How to improve the chronological buildings and art historians, architects, re- control of stone recession measure- storers and stonemasons. Such a collabora- ments in Mediaeval monuments? tive approach is being developed to ensure 2. How to reconstruct the tempo of stone that stone recession measurements are rep- decay in such monuments? resentative and reliable. This will improve 3. What specific causes of stone decay the current knowledge and understanding deserve further investigations? of the spatial and temporal variability of The examination of the crucial question stone decay rates since the Middle Ages. The of the mapping methods and sampling strate- two main study areas presently investigated gies is beyond the scope of this paper. Insights by this interdisciplinary network are the of special interest are provided by previous Mediaeval churches of the Massif Central reviews and discussions (e.g. MOTTERS- and the Khmer temples of the Angkor site HEAD, 2000b; WILLIAMS and SWANTES- CAD. LAB. XEOL. LAXE 35 (2010) Rates of stone recession on Mediaeval monuments 17

SON, 2000; INKPEN et al., 2004) and recent 2005; McCABE et al., 2007a; ANDRÉ et al. case studies (INKPEN et al., 2001; TURK- 2008; HEINRICHS, 2008). INGTON and SMITH, 2004; PARADISE,

CHRONOLOGICAL CONTROL TIS, 2005). The resulting maps, such as the OF STONE RECESSION one recently obtained of the southern wall MEASUREMENTS of Notre-Dame du Port church in Cler- mont-Ferrand by Phalip and Morel (figure Deciphering the monument history 2), are a first-class data source for the geo- morphologist. Not only do they help in the In spite of the development of the tech- selection of chronologically homogeneous nology involved in methods such as laser surfaces for quantifying purposes, but the scanning, the stone by stone survey is still accompanying descriptions of stone fac- considered by the archaeologists as the ings (e.g. stone dressing marks, lime and ce- most efficient and informative method by ment mortars, etc.) provide valuable indica- which to decipher the intricate history of tors by which to identify zero datum levels Mediaeval monuments (PARRON-KON- (REVEYRON, 2005).

Fig. 2. Stone by stone map of the restored parts of the southern façade of the nave of Notre-Dame du Port church (Clermont-Ferrand, Puy-de-Dôme, 2006). © B. Phalip and D. Morel 18 ��André and Phalip � CAD. LAB. XEOL. LAXE 35 (2010)

Searching for secure zero-datum levels WINKLER, 1986; INKPEN, 1998; MOT- TERSHEAD, 2000b). Of special interest to Reliability of reference surfaces (i.e. the inventory of reference surfaces provided dated ‘zero-datum levels’) is a pre-requisite by Mediaeval monuments, are four explora- to ensure the validity of any weathering/ tory studies conducted in Brittany and Au- decay rates, that may be obtained either in vergne, France (PARIS, 1998; MAGRÉ, natural or cultural contexts. The question 1999; BONNEAU, 2001; ROBERT, 2002; of the reference surfaces has long been ad- see also SELLIER, 1998, and MAGRÉ et dressed by cold-climate geomorphologists al., 2007). Based on these studies, and on who have made extensive use of protrud- complementary fieldwork by the authors in ing ice-polished quartz veins to assess Post- the Massif Central (PHALIP, 2001; AN- glacial weathering rates (e.g. DAHL, 1967; DRÉ et al., 2007b) and in Cambodia (AN- ANDRÉ, 1995; NICHOLSON, 2008), and, DRÉ, 2006; ANDRÉ et al., 2008), a variety indeed in previous studies on cultural stone of zero datum levels can be listed (Table 2, (e.g. LIVINGSTON and BAER, 1986; figures 3 and 4).

Mediaeval churches Khmer temples Zero-datum levels (Central and West France) (Angkor, Cambodia)

Stone-dressing marks ++++ ++

Mason’s marks ++

Inscriptions + ++

Moulded surfaces ++ +++

Columns and pillars ++ +++

Ornamentation (carved details) + +++++

Mortar joints (lime and cement) +++

Slate wedges +

More resistant stones ++ (polychrome monuments) CAD. LAB. XEOL. LAXE 35 (2010) Rates of stone recession on Mediaeval monuments 19

A B C

D E F

Fig. 3. Zero-datum levels provided by Mediaeval churches of the Massif Central. © M.-F. André A. Late 12th century ‘fern-leaved’ or ‘fishbone’ finish and mason’s marks on arkosic sandstone (Saint-Aus- tremoine abbey church of Issoire, Puy-de-Dôme, 18-10-08). B. 14th century brettelé finish and mason mark on trachyandesite (Clermont-Ferrand Cathedral, Puy-de-Dôme, 31-1-06). C. Bush-hammered trachy-andesite stone, 19th century (Michel de l’Hospital’s Place, Clermont-Ferrand, Puy-de-Dôme, 31-1-06); in a Mediaeval monument, a bush-hammered stone surface indicates a post-late 18th century, often 19th century, restoration. D. Late 12th century sandstone door jambs affected by scaling (Saint-Martin church, Besson, Allier, 29-09- 08). E. Mid-12th century ornamented capitals in arkosic sandstones (Saint-Pierre church, Gipcy, Allier, 29-9- 08). F. Cement mortar joints in arkosic sandstones (Saint-Marc church, Souvigny, Allier, 29-9-08).

A B C

D E F

Fig. 4. Zero-datum levels provided by sandstone Khmer temples to evaluate recession depth from scaling and flaking (Angkor, Cambodia). © M.-F. André A. Stone dressing marks (Ta Keo temple, South face, second tier, early 11th century, 7-2-08). B. Stele with inscriptions ( Lolei temple, 9th century, 20-3-08). C. Moulded surface ( Ta Keo temple, South face, first tier, early 11th century, 6-12-08). D. Ornamented pillar (west gallery, Angkor Wat, 12th century, 6-12-08). E. Scal- ing decorated toric rib (eastern face, Ta Keo temple, early 11th century, 6-2-08). F. Flaking sandstone on the polished surface of a garuda statue (terrace of the elephants, Late 12th century, 28-12-04). 20 ��André and Phalip � CAD. LAB. XEOL. LAXE 35 (2010)

Stone-dressing marks Greek alphabets, sometimes used in reverse. Stone-dressing marks provide authen- As to the Khmer temples of Cambodia, they tic reference surfaces on the wall facings of provide plenty of steles bearing inscriptions churches and temples dating back as far as (figure 4b), of which thousands have been the Antiquity (e.g. BESSAC, 1986, 1993; translated from Sanskrit and ancient Khmer PARADISE, 1998, 2005). Good inscription by COEDÈS (1937-1966). They frequently and dressing marks are more likely to survive provide information on the building or con- on sandstones, limestones, marbles, and hard secration date of the monument, but some volcanic rocks, than on for example conglom- may postdate the temple itself, a factor that erates and coarse-grained granites. In Medi- must be taken into account when using them aeval churches, stone-dressing marks can be as datum surfaces to assess weathering rates. classified in three major categories: th(1) 12 century stones with their mason marks and Mouldings and columns fern-leaved or ‘fish-bone’ finish, with irregu- In Romanesque churches, moulded sur- lar and obliterated furrows every 5 mm (see faces are present in cornices, drips and por- figure 3a); (2) 13-15th century brettelé finish tals, with polychrome archivolt of special in- (see figure 3b); (3) 19th century bush-ham- terest when quantifying selective weathering mered finish (see figure 3c). This chronologi- (as long as stones have not been replaced). In cal succession provides a general framework portals of Mediaeval churches, door jambs whereby reference surfaces can be identified, can be systematically used to measure stone but the reliability varies according to regions recession, for they display standardized sec- and monuments. Close cooperation with tions, either cylindrical (in Romanesque archaeologists of the Mediaeval period is style, cf. figure 3d) and/or polygonal section required to ensure the chronology of stone- (in Gothic style). The polished surface of dressing marks, as it is to detect Mediaeval door jambs and columns can be preserved, re-cuts of Gallo-Roman stones and 19th- particularly in marble, but this material of- 21st century imitations of Mediaeval stone ten comes from Gallo-Roman columns, re- dressing techniques, with very sharp 1-2 used during the Middle Ages. Therefore, the mm-spaced tooling marks. In Cambodian chronological control of the datum is not sandstones, stone-dressing marks are mostly easy to establish precisely. Edges of corner- represented in the least ornamented or unfin- stones can be used to quantify stone reces- ished parts of temples (figure 4a). sion, though because of the sensitivity to weathering of their sharp edge, their use Mason marks and inscriptions leads to over-estimations of the amount of Mason marks are widespread on build- weathering (as do, the measurements taken ing stones of Mediaeval churches of the at the base of monuments, where accelerat- Massif Central, where they have been thor- ed weathering operates within the capillary oughly studied and classified (e.g. MOREL, fringe). In Cambodia, the numerous pillars 2004). They are present both on Roman and of the Khmer temple galleries can be used Gothic churches (figures 3a and 3b), and to quantify historical weathering, together have various forms such as swirls, triangles, with horizontal mouldings such as toric ribs stars, keys, and letters from the Latin and which are affected by scaling (figure 4c). CAD. LAB. XEOL. LAXE 35 (2010) Rates of stone recession on Mediaeval monuments 21

Decorative carved elements It should be done with caution, for joints Romanesque churches do not display belong to several generations and are not abundant ornamentation, except for deli- always incorporated into the walls at the cately chiselled capitals (figure 3e), modil- same level as building stones. Lime mortars lions and tympanums, which provide good have been used from the 3-4th century BC to zero datum levels, provided they were not the middle of the 19th century. Those which repeatedly restored or hammered during the date back to the 12th century are usually very French Revolution. Gothic cathedrals and lumpy, lime-rich, with non sieved coarse later monuments partly inspired by the Late sands. From the 13th century onwards, mor- Gothic style (e.g. during the 16th century in tar joints are less lime-rich, they contain Brittany) are richer in carved elements such more and more sieved sands, and their wa- as gargoyles and pinnacles, but they do not ter content is higher. This frequently makes provide good datum surfaces in all sites. For it necessary to use wedges to keep constant instance, in coarse-grained granite located in the spacing between the stones. Most Port- coastal regions (e.g. Brittany), they have of- land cement joints indicate repointing op- ten been too severely affected by salt weath- erations having taking place from the 1860s ering for their initial shape to be reconstruct- to 1950. Cement joints contain fine-grained ed (SELLIER, 1998). Nevertheless, based on sieved sands, and often protrude some 2 mm a scale of ornamentation legibility inspired out from the stone surface. At places, these from the scales of inscription legibility pro- joints follow the shape of pre-existing irreg- posed by RAHN (1971) and MEIERDING ularities and can be used, jointly with older (1993), they provide valuable information for datum surfaces such as dressing marks, to purposes of comparison (PARIS, 1998). assess both pre- and post-repointing reces- In contrast to Romanesque churches, the sion rates. Khmer temples of the Angkor site are richly ornamented, offering almost everywhere, ze- Slate wedges ro-datum levels by which to assess sandstone Up to c. 3 cm thick slate wedges in- recession rates: at the base of pillars in the corporated in mortars during building are Angkor Wat galleries, on the sculpted facings widespread in churches of western France, of the Ta Keo temple-mountain, and on the and that used in Brittany tends to be more bas-reliefs of the Terrace of Elephants (fig- resistant than the granite building material. ures 4d–4f). Carved details are particularly Therefore these protruding slate wedges can secure datum surfaces in monuments or parts be used to assess rates of granite stone re- of monuments that have never been restored. cession since the 15th century, though they Thus, the ornamented patterns in Khmer ar- only provide minimum rates for they are chitecture provide large, statistically signifi- commonly affected by flaking (see Fig. 58 cant, datasets, on stone recession. in PARIS, 1998, and Fig. 2a in SELLIER, 1998). In the Massif Central, wedges are Lime and cement mortar joints of 11-12th century broken tile except for the Mortar joints can be used, frequently in Saint-Pierre church of Souvigny, where slate association with other datum surfaces, to wedges were used. assess rates of stone recession (figure 3f). 22 ��André and Phalip � CAD. LAB. XEOL. LAXE 35 (2010)

More resistant stones later churches are made of granite, which The polychrome Romanesque churches displays protruding quartz and feldspar of the Massif Central display many exam- veins or phenocrysts, that can be used to re- ples of resistant building materials, pro- construct the initial stone surface; of special viding reference surfaces to assess rates of interest are the remnants of vein walls many stone recession in less resistant lithologies of which have remained intact since granite such as sandstones. Among these resistant extraction (PARIS, 1998; SELLIER, 1998; stones are such magmatic rocks as leuco- MAGRÉ, 1999; MAGRÉ et al., 2007). cratic granite, basalt and trachyandesite, Although resistant rock types do not pro- and some sedimentary materials as siliceous vide as precise reference surfaces as do dress- nodules or recrystallized layers in lime- ing marks, they are frequently considered as stone, laterite and brick. This last material particularly reliable when reconstructing the is more resistant than chlorite-rich granite position of the original stone surface (e.g. in the Massif Central (e.g. in the Meroving- WINKLER, 1986). However, as they have ian wall at Neris-les-Bains) and the volcanic been subjected to scaling, the measurements tuffs in Roman sites like Herculaneum and of stone recession made should be regarded Pompeii. In Brittany, building stone materi- as providing minimum values. als are less diversified. Most Mediaeval and

RECONSTRUCTING THE TEMPO OF 1984; NEIL, 1989; WELLS et al., 2008). In STONE DECAY: THE DIACHRONOUS fact, it seems that nonlinearity prevails in AND HETEROCHRONOUS stone decay/rock weathering systems, which APPROACH operate in an episodic rather than gradual mode, in a ‘stepwise’ fashion; brief decay ep- Average rates of stone recession expressed isodes follow the crossing of thresholds as- in millimetres per century or millennium are sociated with intrinsic or extrinsic variables, relevant for comparison purposes between separated by often prolonged intervening different lithologies or environments, but periods of relative or apparent quiescence the mean values derived pertain to a suite of (SMITH et al., 1992, 1994; cf. figure 5). weathering processes and frequently mask Extrinsic triggering factors of accelerated the non-linear nature of weathering rates decay can be climatic (e.g. severe frost) or over time (POPE et al., 2002). At the centen- anthropogenic (e.g. stone cleaning). In some nial to millennial scales, previous weathering instances, building stone materials suffer studies have suggested constant rates over catastrophic decay, and conceptual models time (e.g. PETUSKEY et al., 1995; MOT- of such decay have been recently proposed TERSHEAD, 1997), decreasing rates (e.g. for sandstone and limestone (SMITH AND WINKLER, 1973; MATSUKURA and VILES, 2006). It has even been suggested MATSUOKA, 1991; ROBINSON AND that stone decay is an erratic, random or WILLIAMS, 1996, see Fig. 6A; MAGRÉ, chaotic process (e.g. ROBINSON and WIL- 1999), and increasing rates (e.g. KLEIN, LIAMS, 1996; VILES, 2005). CAD. LAB. XEOL. LAXE 35 (2010) Rates of stone recession on Mediaeval monuments 23

Fig. 5. Stone decay: a pulsatory process (SMITH et al. 1994).

Addressing the episodicity of stone decay quences of old photographs have been used calls for research strategies extending back by INKPEN et al. (2001) to plot changes in in time. Anamnesis (FITZNER et al., 1992), the extent of weathering forms on an Oxford inheritance effects (WARKE, 1994), the his- building over the 1892-1932 period. The tory of monuments, building materials, and abundance and relevance of documentation their environment are considered essential in for Mediaeval monuments of the Massif the assessment of present-day stone decay. Central varies. A long search can be neces- In Mediaeval monuments, the episodicity sary, both at the national and local levels, of stone recession can be investigated using to discover key illustrations. On the other diachronic photogrammetry, stepped datum hand, there is an abundant iconographic surfaces, and comparative studies of stone documentation covering the last century for recession in younger and older monuments Khmer temples, due to the conservation and in similar lithologies as those found in Me- restoration work carried out by the Ecole diaeval constructions. Française d’Extrême-Orient and the Conser- vation d’Angkor. For example, just for the Diachronous photogrammetry 1930s, some 5300 photographs document the deterioration of the Angkor monuments The value of historical photographs as a and the conservation and restoration opera- means of documenting the extent and tim- tions (POTTIER, 2008). However, the cru- ing of stone decay have been illustrated in cial archival documents are held by various many studies (e.g. VILES, 1993). Time se- research centres, museums and private col- 24 ��André and Phalip � CAD. LAB. XEOL. LAXE 35 (2010)

lections, and a methodical and determined tion of orthorectified photographs with the photographic hunt is needed. For example, MapInfo GIS (ANDRÉ et al., 2008). This figure 6 documents the formation and devel- diachronic approach is useful both in the re- opment of an ‘erosion’ scar during at least six construction of the calendar of decay and in scaling episodes over the 1905-2006 period, the tentative prediction of future damage, or on a toric rib of Ta Keo temple. More exten- at least the delineation of risk zones (based sive photogrammetry being undertaken on on scenarios of degradation, which differ the sculpted facings of the central pyramid according to the sculpted levels). of this temple-mountain, is based on integra-

1905-1930 1930-1967 1967-1994

1994-2006

Fig. 6. Scenario of formation and development of an ‘erosion’ scar on an ornamented sandstone toric rib, reconstructed after a collection of old photographs (first tier of the eastern face of the central pyramid of Ta Keo temple, early 11th century, 19-3-08). © M.-F. André

Stepped datum surfaces of Portland cement joints (see Fig 55 in PARIS, 1998, p. 96). These heterochronous In Romanesque and Gothic churches, datum surfaces can be used to discriminate two generations of zero-datum levels are of- the ‘modern’ (i.e. post-repointing) stone ten present on the stone facings: one dating recession depth from the previous one (i.e. back to the Middle Ages, the other to the pre-repointing). Preliminary observations 19th or 20th century. The old datum consists by PARIS (1998) in Brittany and by the either of Mediaeval stone-dressing marks authors in the Massif Central suggest ac- or masons’ marks, or of resistant protrud- celerated weathering rates in the last 150 ing stones or nodules, or of carved elements, years, that are possibly a consequence of whereas the recent one generally consists the Portland cement repointing, although CAD. LAB. XEOL. LAXE 35 (2010) Rates of stone recession on Mediaeval monuments 25

atmospheric pollution may also have played stone is. Interestingly, SELLIER (2007) has a role in the accelerated stone decay seen in also described substitutions of weathering Rodez’s Cathedral. forms over time in the granite of Brittany. In England, ROBINSON and WIL- Comparison of stone recession depths in LIAMS (1996) have investigated hetero- heterochronous monuments chronous Wealden sandstone churches and question the significance of the resulting In Brittany, SELLIER (1998) and MA- curve (figure 7). This suggests either a grad- GRÉ (1999) have collected data on rates ual decrease of decay due to the formation of granite weathering on heterochronous of protective crusts or patinas over time, or a monuments such as 500 yr-old churches and contemporary acceleration of decay, possibly 5000 yr-old megaliths. As a result, MAGRÉ due to pollution and acid deposition. Such an (1999, p. 109) suggests that weathering was acceleration is also suggested by BONNEAU more rapid in the first centuries, and then (2001) in a preliminary study on the Ro- slowed down, possibly because the deeper manesque and Neo-romanesque sandstone the weathering proceeds the more sound the churches of the French Massif Central.

Fig. 7. Graph showing mean weathering scores for churches of different ages in Hastings Beds sandstones of central Weald, SE England (ROBINSON and WILLIAMS, 1996, Fig. 12.7 p. 144).

INVESTIGATING SOME CAUSES OF the second half of the 20th century is due ACCELERATED STONE DECAY to the disastrous impact of air pollution on the calcium-rich building sandstone, and the Accelerated stone decay of cultural recent improvement of the situation is due stone is due to a suite of weathering proc- both to the reduction of air pollution and esses resulting from the interplay of various the replacement of calcium-rich sandstones extrinsic and intrinsic factors. For instance, (‘grès à meules’) by calcium-free sandstones in Strasburg’s cathedral (France) the con- (‘grès vosgien’). temporary accelerated decay observed in 26 ��André and Phalip � CAD. LAB. XEOL. LAXE 35 (2010)

Intrinsic causes: low durability of stone rank ordering of weathering rates values are There is a persistent contemporary trend provided, which suggest controls on high to emphasize the role of external factors such versus low durability (e.g. high quartz and as environmental stress in accelerated stone muscovite content versus high feldspar and decay. However, a recent assessment of 112 chlorite content). Irish monuments under the auspices of the The polychrome Romanesque churches Heritage Council points to stone properties of the Massif Central are particularly suit- as the main control on stone decay: ‘Only in able for comparative measurements of stone aggressive polluted atmospheres is the envi- recession (figure 8a–8d). These monuments ronment more determinant on the type and are particularly abundant in the sandstone- rate of decay than the nature of the stone rich Brive, Espalion and Limagne basins, itself’ (PAVIA and BOLTON, 2001). which are surrounded by basement rocks The ‘durability’ of a stone, which is de- and/or limestones or volcanics. Studies are fined as ‘a measure of its ability to resist in progress, but as a working hypothesis, the weathering’ (BELL, 1993), is a complex phe- following scale of increasing durability can nomenon. The significance and relevance of be suggested (Table 1; see also ANDRÉ et this term have been extensively discussed in al., 2007b): (1) weak limestones, (2) chlorite/ the literature (see review in INKPEN et al., biotite-rich granites, (3) oolitic and biocon- 2004). CARR et al. (1996) have suggested structed limestones, and magmatic rocks that proven use over time is the best test of such as basalt, trachyandesite and leuco- durability. It is therefore of interest to try cratic granite. The behaviour of sandstones and evaluate the behaviour over time of var- is highly variable, with some arkosic sand- ious building stones by carrying out on-site stones as the most durable building stones, comparative studies of stone recession in a that display well preserved Mediaeval given environment. Such field studies, which builder’s and dressing marks (Issoire abbey should take into account the history of the church, Puy-de-Dôme, cf. MOREL, 2003), monument, are complemented by labora- and some psammitic sandstones as the least tory simulations. resistant (Estivals church, Corrèze, cf. figure Of special interest is the ‘500 year stone 8b). This last example of heavily weathered durability trial’ carried out by MOTTER- sandstone is comparable with ‘the most se- SHEAD (2000a) on sixteen coastal defen- vere cases of stone decay’ and ‘loss of carved sive structures embracing a wide range of detail’ reported from the recent assessment lithologies. Based on measurements of stone of sandstone Irish monuments (PAVIA and recession at selected representative sites, BOLTON, 2001). CAD. LAB. XEOL. LAXE 35 (2010) Rates of stone recession on Mediaeval monuments 27

A B C D

E F

Fig. 8. Selective weathering in Mediaeval monuments: examples from Massif Central churches and Angkor temples. © M.-F. André A. Well preserved laterite blocks and decayed light-coloured granites on the left (Saint-Didier church, Saint- Dier-d’Auvergne, Puy-de-Dôme, 22-4-01). B. Well preserved limestone capital and severely decayed psammitic sandstone door jamb (Saint-Barthélémy church, Estivals, Corrèze, 16-6-00). C. Well preserved iron-rich sandstones and decayed white sandstones (Saint-Prejet church, Malicorne, Allier, 10-1-01). D. Enigmatic alveolized reddish sandstone block (Saint-Jacques-le-Majeur church, Villefranche-d’Allier, Allier, 10-1-01). E. Well preserved decorative pattern in pink sandstone (Banteay Srei temple, 10th century, 7-12-08). F. Decayed decorative pattern in grey sandstone (East Mebon temple, 10th century, 7-12-08).

In the French Massif Central, the highly otite, glauconite) is also involved in the de- variable durability of sandstones stimulated cay. Grain size is not as important as other an ongoing study carried out in the Limagne textural characteristics and mineralogy, but Basin north and south of Clermont-Ferrand it is not uncommon to observe fine-grained to investigate the main controls of sand- sandstones that are more decayed than stone decay in similar environmental con- coarse equivalents. Similar observations ditions. At first glance, the nature of the have been made on granite in west Wicklow cement seems to be crucial for high or low (Ireland), where the finely textured granite durability (e.g. siliceous/ferruginous versus of the Mediaeval Baltinglas Abbey is much argillaceous/calcitic cements). The presence more severely affected by stone decay than or abundance of certain minerals (e.g. bi- the coarser grained granite at the Early 28 ��André and Phalip � CAD. LAB. XEOL. LAXE 35 (2010)

Bronze Age Athgreany stone circle to the BLECOMBE, 2003), but other controls call north of Baltinglas (PAVIA and BOLTON, for further investigation, and with regard to 2001). Similarly, in nordic environments, the cultural heritage of rural and ‘natural’ granite weathering rates are known to be areas, for instance the impact of forest clear- primarily controlled by the biotite content, ing and tourism pressure in Cambodia, the with pegmatite veins lacking biotite being maintenance and restoration procedures in more resistant than fine-grained granite (e.g. France, and the stone processing in both ANDRÉ, 1995). The only example of grain countries. size influencing stone decay in the Massif Central is found in conglomeratic sand- Impact of forest clearing on stone decay in stones, with accelerated decay due to the de- Khmer temples tachment of gravels (e.g. Charroux church, Following an exploratory survey of 17 Allier). Documentation on the monument Angkor temples (ANDRÉ, 2006), the main history can be crucial to interpret properly objective of the ongoing Ta Keo research contrasting patterns and rates of weathering project is to assess the pre- and post-clearing between different sandstones. For instance, rates of stone decay on the central pyramid in figure 8d, the selective weathering between of this temple-mountain (ANDRÉ et al., white and darker sandstones corresponds to 2007a). Ta Keo temple has remained prac- concomitant decay, whereas in figure 8d, the tically untouched since building (c. 1000 alveolized stone might have been incorpo- A.D.). It was surrounded by forest for some rated just as it is during restoration, and the five centuries before the forest clearing of honeycombing being an inherited feature. the 1920s. The history of the contemporary In Cambodia, the opportunities to stone decay at the surface of its sandstone quantify differential weathering on a single facings is documented by photographs monument are rare, for most Khmer temples covering the period 1905-2008 (figure 6). are made of a unique sandstone type. Therefore, it provides a rare opportunity to However, it is of interest to compare the evaluate the impact of recent forest clear- states of preservation of similar decorative ance on stone decay. Various sampling strat- patterns in monuments of the same date but egies were developed to assess weathering of differing lithotypes (figures 8e and 8f). rates, based on a combination of methods such as photogrammetry, laser scanning Extrinsic causes: environmental and and manual measurements of stone reces- anthropogenic stresses sion. Comparisons were made with Ta Nei, a neighbouring temple still located in a for- The variety of environmental stresses ested environment (figure 9), regarding both conducive to decay has been examined in the state of deterioration of similar deco- previous studies (e.g. WARKE, 1996). There rative patterns and the climatic conditions is no doubt that enhanced temperature/hu- (meteorological monitoring in progress). midity fluctuations (both in magnitude and The first results point to a contemporary ac- frequency) can trigger or accelerate stone celeration of stone decay, with the area of deterioration. The influence of pollution has deteriorated surfaces having tripled between also been extensively studied (e.g. BRIM- 1963 and 2008 on the investigated test-zone CAD. LAB. XEOL. LAXE 35 (2010) Rates of stone recession on Mediaeval monuments 29

(ANDRÉ et al., 2008). Touristic pressure since the 2000s, but at Ta Keo temple, apart has been shown to be an accelerating factor from some obvious anthropogenic impacts, of stone decay at world heritage sites (e.g. it is at this stage difficult to discriminate the in Petra, Jordan; see PARADISE, 2005). No role of direct human impact from that of the doubt that it has played a role in Angkor consequences of forest clearing.

A B

Fig. 9. Forested environment at Ta Nei temple (A) and non forested environment at Ta Keo temple, cleared from the forest in the 1920s (B). © M.-F. André (left: 8-12-08; right: 4-12-08).

Influence of conservation and restoration me- of Portland cement repointing and cleaning asures in European Mediaeval monuments methods which have taken place between the Restoration campaigns have flourished mid-19th and the mid-20th century. in Europe over the last two centuries, and the negative effects of some restoration Portland cement repointing practices were adversely criticised early on In the Middle Ages, the builders of (e.g. in France in the 1830s Prosper Mérimée the Romanesque churches had found vari- and Charles de Montalembert denounced ous solutions to ensure the ventilation and ‘vandalism’ in restorations as well as in de- drainage of masonry stones, such as the structions). Among the restoration/conser- use of putlog holes across the walls and of vation measures, it seems of particular inter- porous lime mortars for jointing (PHALIP, est to investigate the effects on stone decay 2001; PHALIP and ANDRÉ, 2008). The 30 ��André and Phalip � CAD. LAB. XEOL. LAXE 35 (2010)

introduction of watertight Portland cement ing rates are three times faster than pre-re- and the blocking of putlog holes (figure 10) pointing rates (PARIS, 1998). The accelera- caused drastic changes in the water balance tion of stone decay in rural areas of Ireland and the chemistry of masonry stones (e.g. has been blamed on repointing, based on a HERNÁNDEZ, BALLESTEROS AND national assessment commissioned by the ARNAU, 1993). They caused accelerated Heritage Council. Of the 112 investigated granular disintegration (e.g. GRUNAU, monuments, 32.5% had been repointed with 1971; COLLOMBET, 1989). In rural ar- modern Portland cement which had reacted eas of the Massif Central, the most severe chemically with the original masonry, induc- damage occurred from the end of the 19th ing damage in 46% of these cases (PAVIA century to the middle of the 20th, due to and BOLTON, 2001). In the Massif Cen- the replacement of lime mortars by cement tral, preliminary studies suggest that post- mortars, e.g. on Saint-Nectaire church, Puy- repointing weathering rates are two to three de-Dôme (PHALIP and ANDRÉ, 2008). times faster than pre-repointing rates, but in In Brittany, measurements on nine granite churches located in urban areas, it is difficult churches located in non polluted environ- to distinguish the responsibility of repoint- ments suggest that post-repointing weathering itself from the one of pollution.

Cement mortar

Fig. 10. Repointing with cement mortar and ob- Blocked putlog hole turation of putlog hogs: a widespread practice between 1860 and 1950 in the Mediaeval churches of the French Massif Central. Examples are from two Puy-de-Dôme abbey churches: Saint-Sébastien B of Manglieu (A) and Saint-Austremoine of Issoire (B). © N. Duracka / M.-F. André. CAD. LAB. XEOL. LAXE 35 (2010) Rates of stone recession on Mediaeval monuments 31

Cleaning methods 1994, 1996), and recently investigated by Cleaning and resurfacing can destroy McCABE et al. (2007b), based partly on lab- crusts and patinas (biogenic or not), which oratory simulations. Among the wide range act as a natural protection against rainfall of inheritance effects, stone processing was dissolution, wind abrasion, atmospheric pol- identified long ago as a major control on pat- lution and salt weathering. Such effects have terns and rates of stone decay (e.g. SCHAF- been demonstrated in the British Isles, in the FER, 1932). Quarrying, dressing and carv- late 1980s in Scotland (WEBSTER, 1992) ing of stone can induce or reveal structural and in the late 1990s in Ireland, where the weaknesses in the building stone material. rate of particle removal by lichens is lower For example, the use of explosives for stone than the one by the atmospheric agents, es- extraction and of bush hammers for stone pecially in the exposed environments of the dressing can induce macrofracturing or west coast (PAVIA and BOLTON, 2001). In microcracking, paving the way to further France, the acceleration of stone decay dur- weathering processes. Of course, the sensi- ing and after cleaning operations has been tivity of stone to such operations depends recognised, mostly based on qualitative ob- on its inherent properties. The impact of the servations. For example, on Brittany granite Mediaeval and recent stonemasonry tech- churches affected by scaling, the systematic niques on European monuments deserves removal of scales during ‘conservation’ op- further investigations, as does the mode of erations is evidently common practice (MA- incorporation of stones into the structure. GRÉ et al., 2007). Unfortunately it induces For instance, checked weathering patterns in a much faster rate of decay than previous Romanesque churches are frequently linked biogenic granular disintegration due to li- with the alternating placement of stones chen colonization. Repeated short range la- in parallel and perpendicular to bedding ser scanning of test-zones would be helpful planes (e.g. in the Escurolles church, Al- in the evaluation of the short term and long lier – see photo 52 in BONNEAU, 2001 p. term impact of various cleaning procedures 117). In Cambodia, accelerated decay due on a range of lithologies. But at this stage, to vertical stone placement is widespread at despite recent progress, the implementation the base of pillars in the Angkor Wat galler- of such standardized procedures is some- ies (ANDRÉ, 2006). The unfinished Ta Keo what difficult due to institutional barriers. temple offers exceptional conditions whereby to assess the role of stone processing in Role of inheritance effects related to stone the amount and forms of stone decay, and processing: from quarrying to carving comparative studies of dressed, moulded The importance of inheritance effects in and delicately ornamented sandstones are in patterns and rates of stone decay have been progress (figure 11). emphasized by WARKE (e.g. WARKE 32 ��André and Phalip � CAD. LAB. XEOL. LAXE 35 (2010)

Fig. 11. Three types of stone finish on sandstone facings of the central pyramid of Ta Keo temple, Angkor. © M.- C F. André. A. Rough-hewed stones. B. Moulded stones. C. Chiselled stones. CAD. LAB. XEOL. LAXE 35 (2010) Rates of stone recession on Mediaeval monuments 33

CONCLUSIONS specific methods for understanding the multi- phase history of monuments, and with stone- Stone decay is a particularly exciting top- masons, who have a highly valuable practical ic for geomorphologists because it provides experience and documentation (e.g. CON- opportunities to address a range of funda- NIER, 2000; see figure 12). This is necessary mental questions such as: (1) the rhythm of in order to ensure the reliability and validity weathering and erosion, (2) the process/form of stone decay assessments carried out by linkages, (3) the interplay between intrinsic geomorphologists, who while pursuing their and extrinsic controls, (4) the importance of own objectives, share their views with ar- microenvironmental conditions, (5) the eval- chaeologists, and provide data to contribute uation of the human impact, and (6) the as- to the determination of conservation strate- sessment of the destructive versus the protec- gies. Informal interdisciplinary cooperations tive role played by living organisms. Working and institutional research groups such as the on cultural stone encourages the ‘cross-polli- Oxford Preserving Our Past research cluster nation of ideas’ between disciplines (POPE et (that includes fields ranging from synchro- al., 2002, p. 215). There is a crucial need for tron radiation applications to video art), are further cooperation with archaeologists of crucial for further progress in the assessment the Mediaeval period, who have developed of stone decay rates.

Fig. 12. Stone by stone mapping of the axial ambulatory window of Chamalières church, Puy-de-Dôme, by Yves Connier, stonemason (CONNIER, 2000, Fig. 1 p. 266). Domite = Puy-de-Dôme trachyte; Lave = Vesicular lava; Lave fine = Close-grained lava; Restauration = Restoration. 34 ��André and Phalip � CAD. LAB. XEOL. LAXE 35 (2010)

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approach. Building and Environment 38: Appendix: Glossary of Mediaeval 1113-1123. architecture WEBSTER, R. G. M., Ed. (1992). Stone cleaning and the nature, soiling and decay APSIDIOLE. Small apsidal chapel, espe- mechanisms of stone. Donhead Publish- cially one projecting from an apse. ing Ltd, London, 308 p. ARCHIVOLT. Bands or mouldings sur- WELLS, T., HANCOCK, G. and FRYER, rounding an arched opening. J. (2008). Weathering rates of sandstone CAPITAL. Decorative element that divides in a semi-arid environment (Hunter Val- a column or pier from the masonry ley, Australia). Environmental Geology, which it supports. 54 (5): 1047-1057. LINTEL. Horizontal beam spanning an WILLIAMS, R. B .G. and SWANTESSON, opening, as over a door or portal. J. O. H. (2000). Measuring rates of sur- MODILLION. Ornamental bracket used in face downwearing and mapping micro- series under a cornice. topography: The use of micro-erosion PUTLOG HOLE. Hole left in a masonry meters and laser scanners in rock weath- wall to provide support for a horizontal ering studies. Zeitschrift für Geomor- framing member of scaffolding. phologie, Supplementband, 120: 51-66. STELE. Upright stone or slab with an in- WINKLER, E. M. (1973). Stone: properties, scribed or sculptured surface. durability in man’s environment. Springer TYMPANUM. Area above a door enclosed Verlag, Berlin – Heidelberg – New York, by an arch and a lintel, frequently deco- 230 p. rated with relief sculpture (e.g. Christ in WINKLER, E. M. (1986). The measure- majesty and Judgment day). ment of weathering rates of stone structures. Association for Preservation Tech- nology International, XVIII (4): 65-70 (including R.A. Livingston’ discussion comments).