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Preliminary investigations into Caen Stone in the UK; its use, weathering and comparison with repair stone
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Building and Environment 38 (2003) 1143 – 1149 www.elsevier.com/locate/buildenv
Preliminary investigations into Caen Stone in the UK; its use, weathering and comparison with repair stone J.F. Blows∗ , P.J. Carey, A.B. Poole Department of Earth and Environmental Sciences, The University of Greenwich at Medway, Chatham Maritime, Kent ME4 4TB, UK
Abstract Many late 11th and early 12th century buildings in the UK were originally constructed of French Caen Stone. However, alternative sources of limestone had to be used periodically for subsequent repairs. With time, the di2ering weathering characteristics of the external dressed stones in these buildings have exacerbated the di2erences in the stonework, even in similar weathering environments. Initial petrographical studies highlight the role of petrography as a key indicator of the likely weathering characteristics of dressed limestone and therefore its long-term appearance in the buildings. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Building Stone; Caen Stone; Weathering
1. Introduction The use of Caen Stone as a construction material in prestigious buildings in England is well documented, for example by Howe [1] and Hull [2]. Caen Stone buildings include the Cathedrals of Canterbury, Rochester and Norwich, the Tower of London, and Westminster Abbey. The Caen Stone is a cream coloured =ne grained limestone from the Bathonian stage of the Middle Jurassic, quarried from the vicinity of Caen in Normandy, France. There is little detailed petrographical information regarding the nature of the material or its behaviour in service. Research by Samson-Gombert [3] has provided some background, but her work focussed on the in?uence of urban and marine environments on the behaviour of the stone. Lack of availability of Caen Stone has, at times, necessitated the use of English limestones for restoration works and in the1960s an alternative French limestone, the Lepine, was introduced as a repair stone in the UK. The weathering mechanisms of limestones are brie?y reviewed in this paper and illustrated with Caen Stone examples from Canterbury Cathedral. The weathering characteristics and potential durability of Caen and Lepine stones are discussed in relation to initial petrographical =ndings and test results for these two stones and compared ∗
Corresponding author. Tel.: +44-20-8311-8000. E-mail address: [email protected] (J.F. Blows).
0360-1323/03/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0360-1323(03)00075-1
with the behaviour of a third French limestone, the SavonniEeres, currently used in the repair of another Caen Stone historic building, Chichester Cathedral in Sussex. 2. The use of Caen Stone in historic buildings Caen Stone, from Normandy in France, was probably =rst used in the South of England in Roman buildings [4]. However, the large-scale use of Caen Stone in England occurred after the accession of William, Duke of Normandy, following the battle of Hastings in 1066 [5]. Prior to his arrival in England, William had commissioned the building of two cathedrals in the city of Caen, St. Etienne and the Holy Trinity, for which the local stone was used [6]. Prior to the Norman conquest, building stone in London and the south east of England had been essentially limited to ?int, limestone from the Lower Greensand in Kent, (‘Kentish Ragstone’), glauconitic =ne sandstone from the Upper Greensand in Surrey, (‘Reigate Stone’), and laminated =ne sandstone/siltstone from the Thanet Beds in Kent. In 1070 the =rst of the Norman Archbishops, Lanfranc, from the cathedral of St. Etienne in Caen, was consecrated Archbishop of Canterbury and at his right hand was a monk named Gundulf who originated from Caen. Gundulf was very skilled in masonry and supervised the building of both Caen Cathedral and Canterbury Cathedral. He became Bishop of Rochester in Kent in 1077 [7], where rebuilding
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J.F. Blows et al. / Building and Environment 38 (2003) 1143 – 1149
Table 1 The principal weathering agencies acting on building stones (after Sha2er [9]) Chemical agents
Physical agents
Biological agents
Acid gases and solutions Salts in solution Oxidisers and reducers Oxidiser and reducers
Temperature variations Freeze–thaw Moisture variations Particulate matter
Plants Algae, fungi and lichens Bacteria Birds & man
of the cathedral took place. The previously used sources of English stone were insuOcient for the increased demands for stone in the Norman building programme and the requirements of the Norman builders were readily met by the use of Caen Stone imported across the Channel [8]. In addition to the two major cathedrals of Kent, Rochester and Canterbury, Caen Stone was widely used in London and elsewhere in the south east of England; it was also used in Chichester Cathedral in Sussex and its use in the UK has been reported as far distant as Durham and South Wales [5]. The material was imported intermittently in large quantities until the Reformation, when the destruction of the monastic stone buildings made much Caen Stone available for re-use. At the end of the 18th century and in particular in the 19th century, Caen Stone was imported once more for major restoration works. However, by this time the stone was generally of poorer quality [7]. 3. Weathering and deterioration The weathering processes of building stone and the in?uence of the local environment have been, and remain, the subject of intensive scienti=c research. An extensive review of the weathering agencies acting on building stone is given by Scha2er [9] and this can be summarised for limestone as shown in Table 1. Scha2er divided the phenomena associated with weathering into two classes, chemical and physical, but he recognised that no weathering agent acts alone. Loughnan [10], quoted in Eden [11], recognised three principal processes of chemical weathering, namely: (i) The breakdown of the parent material structure and the resultant mobilisation of the elements as ions or molecules. (ii) The removal in solution of some of these constituents. (iii) The formation of new minerals from reactions with the atmosphere and hydrosphere and with the mobilised ions or molecules. An extensive quantitative study of the mechanisms of weathering has been produced by Lal Gauri and Bandyopadhyay [12]. They set out the chemical reaction which occurs when carbonate minerals react with carbon dioxide. This reaction takes place slowly and only in the presence of water. In
ENVIRONMENT
MATERIAL
USE
Weathering Processes
Properties
Architectural Form & Finishes
Rates of Weathering
Observed Deterioration
Fig. 1. Interrelationship between environment, material and use (after Smith [14]).
contrast, the pollutant gases SO2 and NOx are stated to be more potent in their e2ect on limestone. They react with surfaces protected from rain by absorbing moisture from the air, producing a crust which eventually exfoliates. The two pollutants will also produce an acid rain which dissolves the carbonate minerals on rain washed surfaces The mechanism of physical weathering is the same in both unpolluted and polluted environments. Reiche [13] de=ned physical weathering as weathering which causes in situ fragmentation without contributory chemical change. The presence of and interconnection between pores and fractures govern the susceptibility of a rock to physical weathering by allowing the passage of water through the material. Depending on the geometry of the spaces between the mineral grains, water can cause disruption to varying degrees by wetting, expansion on freezing, crystallisation of salts on drying and the expansion of certain clay minerals in the presence of water. There is, of course, interaction and overlap between these di2erent agencies, for example biological agents can produce both chemical and physical e2ects. It is also clear that deterioration of stone is typically the result of several of these factors acting together in a complex way. Smith [14] has drawn attention to the need to consider the complex interaction between the material properties, the weathering processes and the environmental conditions over time in order to understand the resultant deterioration. The interrelationship noted by Smith [14] between all the factors forming the decay system has been modi=ed and developed to be appropriate to weathering deterioration in buildings, (Fig. 1). The e2ect of the weathering processes on a particular piece of masonry will be dependent on the properties of the material and the speci=c surface form and exposure conditions. Further consideration also needs to be made of the short-term changes or cycles in the severity of attack, due for example to variation in temperature or solution concentration, and these may lead to the introduction of an episodic element into the weathering pattern. The diOculties in predicting stone decay owing to the possibility of numerous decay routes acting on the stone at various stages in the lifetime of a structure have been discussed by Smith [14]. The local environmental conditions around a historic stone building will have varied over long time scales and such changes may a2ect both rate and type of weathering of the stone over time. It is only necessary to consider the contrast
J.F. Blows et al. / Building and Environment 38 (2003) 1143 – 1149
between the UK coal burning urban economy of Victorian times with the present situation, following from the Clean Air Act of 1956, to realise that signi=cant changes to the environment have occurred. A notable example of a study of the weathering of limestones is the House of Commons report of 1861 [16]. More recently, deterioration processes and their e2ects have been reviewed by the Building E2ects Review Group (BERG) [17] and the Building Research Establishment [18,19]. The alteration of newly cut limestone in a building begins with at least some discolouration of the fresh surfaces driven by the action of atmospheric gases, and moisture, particularly the oxygen and ionic solutions present. Chemical reactions, notably oxidation of coloured minerals such as ferrous compounds within the limestone, are the primary cause of discolouration, although biodeteriogens, such as algae, also form coloured crusts. The airborne particulates within polluted environments may enhance the surface discolouration. The process then generally proceeds to ?aking, due to expansive reactions taking place in the surface zone as a result of thermal stresses, freezing of pore water and/or salt crystallisation. Textural changes become apparent with time as the less resistant minerals, generally the cement matrix, are eroded leaving a surface form speci=c to the nature and structure of the limestone. This typical pattern is set out in Fig. 2, and examples from Canterbury Cathedral can be seen in Fig. 3. The main di2erence between the original stone of the building and a replacement stone used in repair is that
i) Fresh, cut face
1145
Colour Changes
Flaking and Delaminating (and occasional pitting)
Surface Textural Changes
Rounded Blocks Fig. 2. Typical weathering sequence of Caen Stone.
the new material would have been freshly prepared in the mason’s yard. Most of the other factors arising from contemporary local environmental conditions will be identical for both the new material and for the adjacent existing stones. However, the repair stone will begin to weather in the current environmental climate, whereas the original materials will have had a weathering history dating back to when environmental conditions were di2erent from those of today and which have already altered the existing stone. This response variation between ‘old’ and ‘new’, similar stone to current decay systems has been addressed by researchers such as Smith [14] and Warke [20], who have adopted the terms ‘memory’ or ‘inheritance’ e2ects. Geomorphological approaches, such as that of Smith et al. [15], have
iii) Flaking of stone
iv) Surface Pitting, uncommon but dramatic
v) Rounded stone blocks
ii) Colour Changes
Fig. 3. Examples of weathering changes in Caen Stone at Canterbury Cathedral.
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J.F. Blows et al. / Building and Environment 38 (2003) 1143 – 1149
LEPINE
CAEN
SAVONNIERES Fig. 4. Comparison of surface textures on fresh, cut faces.
been introduced into the study of building stone decay, with the production of visual assessment forms for stone decay features. 4. Selection of repair stone for historic Caen Stone buildings The Caen Stone quarries, which produced the limestone originally used in the construction of many UK buildings, have long been worked out of the better quality stone and periodically Caen Stone has been unavailable for use in England. English limestones, such as Kentish Ragstone [21], and in the early 20th, the Jurassic Bath, Doulting and Portland limestones [7] were then used in repair works. The variation in weathering characteristics can be seen on the south wall of Canterbury Cathedral where the Bath Stone is an orange-brown colour with a coarse oolitic texture and the Ragstone, greenish-grey. It is clear that, ideally, a replacement material to be used in restoration works must not only be a good match as regards initial colour and texture to the original, but should also weather in a similar way and be suOciently durable to provide resistance to disintegration and decay over an acceptable time span. Thus it is important to consider and compare all the weathering characteristics of the original and replacement materials. In most cases, a programme of cleaning and the replacement of damaged masonry are undertaken together. Both old and replacement stone will begin to weather again in the prevailing environmental conditions, but owing to the
‘memory e2ect’ on the old stone their future responses are unlikely to be the same [14]. If the mineralogy, grain size distributions and the fabrics of two building stones were identical in all respects and the stones were subjected to identical environmental conditions, they would weather in an identical manner, as long as they were placed in the same orientation. In recent times, the need for repair of some buildings has necessitated the continuing use of alternatives to the unavailable Caen Stone. The most recently used alternative, the Lepine limestone, is being compared with the Caen Stone in the present study. A careful comparison of the Caen Stone with the Lepine should give insights into those factors in the limestones’ petrography which in?uence the types and scales of the weathering deterioration which can be expected, and focus on the di2erences between the original and repair materials. 5. Petrography of Caen and repair stones Caen Stone was described by North [22] as an oolitic limestone, not unlike Bath Stone, although earlier, Howe [1] and Hull [2] had recorded that the former is a =ne grained pale cream and yellowish, homogeneous limestone, with rarely any obvious indications of oolitic structure. The Middle Jurassic Bath and Doulting Stones contrast with the Caen Stone in that they are darker and essentially coarser grained, with the Bath Stone in particular being characteristically oolitic. The oolitic Portland Stone from the Upper Jurassic weathers to a distinctive light greyish-white colour
J.F. Blows et al. / Building and Environment 38 (2003) 1143 – 1149
1147
Table 2 Summary of constituent proportions Approx % Caen Micrite Sparite Opaques
(a)
(b)
(c)
50 50 ¡5
Lepine 70 25 5
SavonniEeres 30 70 ¡5
Stone [23] and this characteristic has been con=rmed by petrographical examinations carried out as part of the current research work. In contrast, the Upper Jurassic, Lepine Stone from the Poitiers area in France is described as an oolitic limestone [24]. The Caen and the Lepine limestones are not only being compared with each other but also with a third French limestone, SavonniEeres Stone, currently also being used in repair works in Chichester Cathedral, not as a direct replacement for Caen stone but for Quarr limestone from the Isle of Wight. The SavonniEeres is also described as an oolitic limestone [24] and is a substantially coarser grained limestone than the other two stones. The macro texture of the fresh Lepine Stone is super=cially similar to fresh Caen Stone, as indicated in Fig. 4, however, the Lepine is slightly whiter and close examination reveals distinct calcite crystals approximately 1 mm in size. However, in thin section notable di2erences are observed in each of the three stones (Fig. 5). The Caen Stone has been found to comprise a bioclastic limestone, with unstructured discrete particles, approximately 0:08 mm in size, of micrite (microcrystalline calcite) (Fig. 5a). The Lepine can be described as a micritic, peloidal limestone. Most of the discrete particles in the latter are typically 0:2 mm in size, with generally only some of the larger features showing any oolitic structure (Fig. 5b). The SavonniEeres limestone is an oolitic limestone (ooliths typically 0:4 mm in size) in a sparry calcite cement (Fig. 5c). The constituent proportions from the thin section examinations are presented as Table 2 and reveal, in particular, di2erences in the ratio of sparite (coarse crystalline calcite) to micrite (microcrystalline calcite). In addition, the micropores are found to be typically associated with the micrite. The insoluble residue contents (which would include quartz, clay minerals and some iron compounds) are similar, less than 5%, in all the samples.
Fig. 5. Thin section photographs—widths of photographs 15 mm: (a) Caen Stone—plane polarised light, (b) Lepine Stone—plane polarised light and (c) SavonniEeres Stone—plane polarised light.
6. Sodium sulphate soundness tests
whilst the Kentish Ragstone stone retains its greenish-grey colour. The commentary accompanying the 1/50,000 geological map of Caen refers to the bioclastic nature of the Caen
In the present study comparative sodium sulphate soundness tests have been carried out on samples of each of the three French limestones, as soundness tests have been recognised as an indicator of stone durability [24,25]. The tests were carried out in general accordance with the method of the Building Research Establishment [25]. In this test, three
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J.F. Blows et al. / Building and Environment 38 (2003) 1143 – 1149
Cathedral and also Mr. R. Tyreman, Clerk of Works, Chichester Cathedral, for their co-operation.
Table 3 Initial sodium sulphate soundness results
Mean weight loss (%) after 5 cycles
Caen
Lepine
SavonniEeres
2
11
0
specimens from each variety of limestone were subjected to cycles of soaking in a solution of sodium sulphate, followed by heating, drying and weighing, with weight loss recorded at the end of each cycle. Table 3 shows the results after =ve cycles. It is clear from this preliminary study that the coarser, SavonniEeres limestone is more resistant at this stage of the test than the Caen and Lepine limestones.
7. Conclusions Comparison of the petrographical information with the initial =ndings from the sodium sulphate soundness tests suggests that Jurassic limestones with a high micritic content, particularly as a cementing medium, may be less durable than those with a high proportion of sparite and ooliths, possibly because of the apparent association of micropores with the micritic material. Microporosity is known to be an important controlling factor in the weathering characteristics of limestones and it is intended to carry out further work on this aspect in the near future [25]. Petrographical examinations are an essential feature of any investigation into the characteristics and likely behaviour of a building stone. They provide not only information on the composition of stone but also essential information on the texture and structure of the material. It is acknowledged that developing any link between petrographical characteristics and weathering characteristics requires additional work and this forms part of the current research project. XRD and SEM techniques are being used to identify those constituents and features which cannot be resolved by optical microscope techniques, in particular opaques and clay minerals. In addition, physical tests will be carried out to help quantify the =ndings in relation to previously acceptable stones and additional, possible new sources of repair material which have been added into the research programme.
Acknowledgements Thanks to Mr. P. Burton, Surveyor to the Works, Canterbury Cathedral; Mrs. H. Newton, Conservator, Canterbury
References [1] Howe JA. The geology of building stones. Edward Arnold 1910— reprinted edition. London: Donhead Publishing, 2001. [2] Hull E. A treatise on the building and ornamental stones of Great Britain and foreign countries. London: Macmillan and Co., 1872. [3] Samson-Gombert C. In?uences d’un environment urbain et maritime sur les alterations d’un calcaire en oeuvre; la Pierre de Caen. Ph.D. Thesis, University of Caen, Centre de Geomorphologie du CNRS, Caen, 1993. [4] Cunli2e BW. Excavations at Fishbourne 1961–1969. Res. Reports of the Society of Antiquaries 1971. p. 27. [5] Dujardin L. L’aire de disposition de la pierre de Caen. In: 2e Coll. Carriere et Construction 117e Congress Nat Soc. Sav., Clermont– Ferrand, 1992. p. 431– 44. [6] Marin JY Fichet-Clairefontaine F. Archeologie urbaine: les carrieres de Caen. Archeologia 1984; (195). [7] Tatton-Brown TWT. La pierre de Caen en Angleterre. L’architecture Normande au moyen age. University of Caen, 1997. p. 305 –14. [8] Tatton-Brown TWT. Building stone in Canterbury c1070 –1525. In: Parsons D, editor. Stone: quarrying and building in England AD 43-1525. London: Phillimore and Co. Ltd., 1990. p. 70–82. [9] Scha2er RJ. The weathering of natural building stones. Special Report No. 18, Department of Scienti=c and Industrial Research. London: HMSO, 1932. [10] Loughnan FC. Chemical weathering of silicate minerals. New York: Elsevier, 1969. [11] Eden MA. The in-service performance of limestone building stones and their predominant mechanisms of decay. Unpublished M.Sc. dissertation, Queen Mary and West=eld College, University of London, 1990. [12] Lal Gauri K, Bandyopadhyay JK. Carbonate Stone: chemical behavior, durability and conservation. New York: Wiley, 1999. [13] Reiche PA. Survey of Weathering processes and products. New Mexico University Publication in Geology 3. New Mexico: University of New Mexico Press, 1950. [14] Smith BJ. Scale problems in the interpretation of urban stone decay. In: Smith BJ, Warke PA, editors. Processes of urban stone decay. London: Donhead, 1996. p. 3–18. [15] Smith BJ, Whalley B, Magee R. Assessment of building stone decay: a geomorphological approach. In: Webster Robin GM, editor. Stone cleaning and the nature, soiling and decay mechanisms of stone. London: Donhead, 1992. p. 249–57. [16] Commissioners for the Stone for the New Houses of Parliament. Report of the commissioners appointed to visit the quarries and to inquire into the qualities of the stone to be used in the building the new Houses of Parliament, London, 1839. [17] Building E2ects Review Group (BERG). The e2ects of acid rain deposition on buildings and building materials. London: HMSO, 1989. [18] Building Research Establishment (BRE). The decay and conservation of stone masonry. BRE Digest 177, Part 2. London: HMSO, 1975. [19] Building Research Establishment (BRE). Selecting natural building stones. BRE Digest 420. London: HMSO, 1997. [20] Warke PA. Inheritance e2ects in building stone decay. In: Smith BJ, Warke PA, editors. Processes of urban stone decay. London: Donhead, 1996. p. 32–44. [21] Worssam BC, Tatton-Brown T. Kentish Rag and other Kent building stones. Archaeologia Cantiana 1993;CXII:93–125. [22] North FJ. Limestones. Atlanta, GA: Thomas Murby and Co., 1930.
J.F. Blows et al. / Building and Environment 38 (2003) 1143 – 1149 [23] Rioult et al. Caen. Carte geologique de la France a 1/50 000. Ministere de l’industrie et de l’ amenagement du territoire, Bureau de recherches Geologiques et Minieres, Service Geologique National, 1971. [24] Honeybourne DB. The building limestones of France. London: HMSO, 1982.
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[25] Miglio BF, Richardson DM, Yates TS, West D. Assessment of the durability of porous limestones: speci=cation and interpretation of test data. In: Dimension stone cladding; design, construction, evaluation and repair, ASTM STP 1394. West Conshohocken, PA: American Society for Testing and Materials, 2002.
Building and Environment 38 (2003) 1143 – 1149 www.elsevier.com/locate/buildenv
Preliminary investigations into Caen Stone in the UK; its use, weathering and comparison with repair stone J.F. Blows∗ , P.J. Carey, A.B. Poole Department of Earth and Environmental Sciences, The University of Greenwich at Medway, Chatham Maritime, Kent ME4 4TB, UK
Abstract Many late 11th and early 12th century buildings in the UK were originally constructed of French Caen Stone. However, alternative sources of limestone had to be used periodically for subsequent repairs. With time, the di2ering weathering characteristics of the external dressed stones in these buildings have exacerbated the di2erences in the stonework, even in similar weathering environments. Initial petrographical studies highlight the role of petrography as a key indicator of the likely weathering characteristics of dressed limestone and therefore its long-term appearance in the buildings. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Building Stone; Caen Stone; Weathering
1. Introduction The use of Caen Stone as a construction material in prestigious buildings in England is well documented, for example by Howe [1] and Hull [2]. Caen Stone buildings include the Cathedrals of Canterbury, Rochester and Norwich, the Tower of London, and Westminster Abbey. The Caen Stone is a cream coloured =ne grained limestone from the Bathonian stage of the Middle Jurassic, quarried from the vicinity of Caen in Normandy, France. There is little detailed petrographical information regarding the nature of the material or its behaviour in service. Research by Samson-Gombert [3] has provided some background, but her work focussed on the in?uence of urban and marine environments on the behaviour of the stone. Lack of availability of Caen Stone has, at times, necessitated the use of English limestones for restoration works and in the1960s an alternative French limestone, the Lepine, was introduced as a repair stone in the UK. The weathering mechanisms of limestones are brie?y reviewed in this paper and illustrated with Caen Stone examples from Canterbury Cathedral. The weathering characteristics and potential durability of Caen and Lepine stones are discussed in relation to initial petrographical =ndings and test results for these two stones and compared ∗
Corresponding author. Tel.: +44-20-8311-8000. E-mail address: [email protected] (J.F. Blows).
0360-1323/03/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0360-1323(03)00075-1
with the behaviour of a third French limestone, the SavonniEeres, currently used in the repair of another Caen Stone historic building, Chichester Cathedral in Sussex. 2. The use of Caen Stone in historic buildings Caen Stone, from Normandy in France, was probably =rst used in the South of England in Roman buildings [4]. However, the large-scale use of Caen Stone in England occurred after the accession of William, Duke of Normandy, following the battle of Hastings in 1066 [5]. Prior to his arrival in England, William had commissioned the building of two cathedrals in the city of Caen, St. Etienne and the Holy Trinity, for which the local stone was used [6]. Prior to the Norman conquest, building stone in London and the south east of England had been essentially limited to ?int, limestone from the Lower Greensand in Kent, (‘Kentish Ragstone’), glauconitic =ne sandstone from the Upper Greensand in Surrey, (‘Reigate Stone’), and laminated =ne sandstone/siltstone from the Thanet Beds in Kent. In 1070 the =rst of the Norman Archbishops, Lanfranc, from the cathedral of St. Etienne in Caen, was consecrated Archbishop of Canterbury and at his right hand was a monk named Gundulf who originated from Caen. Gundulf was very skilled in masonry and supervised the building of both Caen Cathedral and Canterbury Cathedral. He became Bishop of Rochester in Kent in 1077 [7], where rebuilding
1144
J.F. Blows et al. / Building and Environment 38 (2003) 1143 – 1149
Table 1 The principal weathering agencies acting on building stones (after Sha2er [9]) Chemical agents
Physical agents
Biological agents
Acid gases and solutions Salts in solution Oxidisers and reducers Oxidiser and reducers
Temperature variations Freeze–thaw Moisture variations Particulate matter
Plants Algae, fungi and lichens Bacteria Birds & man
of the cathedral took place. The previously used sources of English stone were insuOcient for the increased demands for stone in the Norman building programme and the requirements of the Norman builders were readily met by the use of Caen Stone imported across the Channel [8]. In addition to the two major cathedrals of Kent, Rochester and Canterbury, Caen Stone was widely used in London and elsewhere in the south east of England; it was also used in Chichester Cathedral in Sussex and its use in the UK has been reported as far distant as Durham and South Wales [5]. The material was imported intermittently in large quantities until the Reformation, when the destruction of the monastic stone buildings made much Caen Stone available for re-use. At the end of the 18th century and in particular in the 19th century, Caen Stone was imported once more for major restoration works. However, by this time the stone was generally of poorer quality [7]. 3. Weathering and deterioration The weathering processes of building stone and the in?uence of the local environment have been, and remain, the subject of intensive scienti=c research. An extensive review of the weathering agencies acting on building stone is given by Scha2er [9] and this can be summarised for limestone as shown in Table 1. Scha2er divided the phenomena associated with weathering into two classes, chemical and physical, but he recognised that no weathering agent acts alone. Loughnan [10], quoted in Eden [11], recognised three principal processes of chemical weathering, namely: (i) The breakdown of the parent material structure and the resultant mobilisation of the elements as ions or molecules. (ii) The removal in solution of some of these constituents. (iii) The formation of new minerals from reactions with the atmosphere and hydrosphere and with the mobilised ions or molecules. An extensive quantitative study of the mechanisms of weathering has been produced by Lal Gauri and Bandyopadhyay [12]. They set out the chemical reaction which occurs when carbonate minerals react with carbon dioxide. This reaction takes place slowly and only in the presence of water. In
ENVIRONMENT
MATERIAL
USE
Weathering Processes
Properties
Architectural Form & Finishes
Rates of Weathering
Observed Deterioration
Fig. 1. Interrelationship between environment, material and use (after Smith [14]).
contrast, the pollutant gases SO2 and NOx are stated to be more potent in their e2ect on limestone. They react with surfaces protected from rain by absorbing moisture from the air, producing a crust which eventually exfoliates. The two pollutants will also produce an acid rain which dissolves the carbonate minerals on rain washed surfaces The mechanism of physical weathering is the same in both unpolluted and polluted environments. Reiche [13] de=ned physical weathering as weathering which causes in situ fragmentation without contributory chemical change. The presence of and interconnection between pores and fractures govern the susceptibility of a rock to physical weathering by allowing the passage of water through the material. Depending on the geometry of the spaces between the mineral grains, water can cause disruption to varying degrees by wetting, expansion on freezing, crystallisation of salts on drying and the expansion of certain clay minerals in the presence of water. There is, of course, interaction and overlap between these di2erent agencies, for example biological agents can produce both chemical and physical e2ects. It is also clear that deterioration of stone is typically the result of several of these factors acting together in a complex way. Smith [14] has drawn attention to the need to consider the complex interaction between the material properties, the weathering processes and the environmental conditions over time in order to understand the resultant deterioration. The interrelationship noted by Smith [14] between all the factors forming the decay system has been modi=ed and developed to be appropriate to weathering deterioration in buildings, (Fig. 1). The e2ect of the weathering processes on a particular piece of masonry will be dependent on the properties of the material and the speci=c surface form and exposure conditions. Further consideration also needs to be made of the short-term changes or cycles in the severity of attack, due for example to variation in temperature or solution concentration, and these may lead to the introduction of an episodic element into the weathering pattern. The diOculties in predicting stone decay owing to the possibility of numerous decay routes acting on the stone at various stages in the lifetime of a structure have been discussed by Smith [14]. The local environmental conditions around a historic stone building will have varied over long time scales and such changes may a2ect both rate and type of weathering of the stone over time. It is only necessary to consider the contrast
J.F. Blows et al. / Building and Environment 38 (2003) 1143 – 1149
between the UK coal burning urban economy of Victorian times with the present situation, following from the Clean Air Act of 1956, to realise that signi=cant changes to the environment have occurred. A notable example of a study of the weathering of limestones is the House of Commons report of 1861 [16]. More recently, deterioration processes and their e2ects have been reviewed by the Building E2ects Review Group (BERG) [17] and the Building Research Establishment [18,19]. The alteration of newly cut limestone in a building begins with at least some discolouration of the fresh surfaces driven by the action of atmospheric gases, and moisture, particularly the oxygen and ionic solutions present. Chemical reactions, notably oxidation of coloured minerals such as ferrous compounds within the limestone, are the primary cause of discolouration, although biodeteriogens, such as algae, also form coloured crusts. The airborne particulates within polluted environments may enhance the surface discolouration. The process then generally proceeds to ?aking, due to expansive reactions taking place in the surface zone as a result of thermal stresses, freezing of pore water and/or salt crystallisation. Textural changes become apparent with time as the less resistant minerals, generally the cement matrix, are eroded leaving a surface form speci=c to the nature and structure of the limestone. This typical pattern is set out in Fig. 2, and examples from Canterbury Cathedral can be seen in Fig. 3. The main di2erence between the original stone of the building and a replacement stone used in repair is that
i) Fresh, cut face
1145
Colour Changes
Flaking and Delaminating (and occasional pitting)
Surface Textural Changes
Rounded Blocks Fig. 2. Typical weathering sequence of Caen Stone.
the new material would have been freshly prepared in the mason’s yard. Most of the other factors arising from contemporary local environmental conditions will be identical for both the new material and for the adjacent existing stones. However, the repair stone will begin to weather in the current environmental climate, whereas the original materials will have had a weathering history dating back to when environmental conditions were di2erent from those of today and which have already altered the existing stone. This response variation between ‘old’ and ‘new’, similar stone to current decay systems has been addressed by researchers such as Smith [14] and Warke [20], who have adopted the terms ‘memory’ or ‘inheritance’ e2ects. Geomorphological approaches, such as that of Smith et al. [15], have
iii) Flaking of stone
iv) Surface Pitting, uncommon but dramatic
v) Rounded stone blocks
ii) Colour Changes
Fig. 3. Examples of weathering changes in Caen Stone at Canterbury Cathedral.
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J.F. Blows et al. / Building and Environment 38 (2003) 1143 – 1149
LEPINE
CAEN
SAVONNIERES Fig. 4. Comparison of surface textures on fresh, cut faces.
been introduced into the study of building stone decay, with the production of visual assessment forms for stone decay features. 4. Selection of repair stone for historic Caen Stone buildings The Caen Stone quarries, which produced the limestone originally used in the construction of many UK buildings, have long been worked out of the better quality stone and periodically Caen Stone has been unavailable for use in England. English limestones, such as Kentish Ragstone [21], and in the early 20th, the Jurassic Bath, Doulting and Portland limestones [7] were then used in repair works. The variation in weathering characteristics can be seen on the south wall of Canterbury Cathedral where the Bath Stone is an orange-brown colour with a coarse oolitic texture and the Ragstone, greenish-grey. It is clear that, ideally, a replacement material to be used in restoration works must not only be a good match as regards initial colour and texture to the original, but should also weather in a similar way and be suOciently durable to provide resistance to disintegration and decay over an acceptable time span. Thus it is important to consider and compare all the weathering characteristics of the original and replacement materials. In most cases, a programme of cleaning and the replacement of damaged masonry are undertaken together. Both old and replacement stone will begin to weather again in the prevailing environmental conditions, but owing to the
‘memory e2ect’ on the old stone their future responses are unlikely to be the same [14]. If the mineralogy, grain size distributions and the fabrics of two building stones were identical in all respects and the stones were subjected to identical environmental conditions, they would weather in an identical manner, as long as they were placed in the same orientation. In recent times, the need for repair of some buildings has necessitated the continuing use of alternatives to the unavailable Caen Stone. The most recently used alternative, the Lepine limestone, is being compared with the Caen Stone in the present study. A careful comparison of the Caen Stone with the Lepine should give insights into those factors in the limestones’ petrography which in?uence the types and scales of the weathering deterioration which can be expected, and focus on the di2erences between the original and repair materials. 5. Petrography of Caen and repair stones Caen Stone was described by North [22] as an oolitic limestone, not unlike Bath Stone, although earlier, Howe [1] and Hull [2] had recorded that the former is a =ne grained pale cream and yellowish, homogeneous limestone, with rarely any obvious indications of oolitic structure. The Middle Jurassic Bath and Doulting Stones contrast with the Caen Stone in that they are darker and essentially coarser grained, with the Bath Stone in particular being characteristically oolitic. The oolitic Portland Stone from the Upper Jurassic weathers to a distinctive light greyish-white colour
J.F. Blows et al. / Building and Environment 38 (2003) 1143 – 1149
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Table 2 Summary of constituent proportions Approx % Caen Micrite Sparite Opaques
(a)
(b)
(c)
50 50 ¡5
Lepine 70 25 5
SavonniEeres 30 70 ¡5
Stone [23] and this characteristic has been con=rmed by petrographical examinations carried out as part of the current research work. In contrast, the Upper Jurassic, Lepine Stone from the Poitiers area in France is described as an oolitic limestone [24]. The Caen and the Lepine limestones are not only being compared with each other but also with a third French limestone, SavonniEeres Stone, currently also being used in repair works in Chichester Cathedral, not as a direct replacement for Caen stone but for Quarr limestone from the Isle of Wight. The SavonniEeres is also described as an oolitic limestone [24] and is a substantially coarser grained limestone than the other two stones. The macro texture of the fresh Lepine Stone is super=cially similar to fresh Caen Stone, as indicated in Fig. 4, however, the Lepine is slightly whiter and close examination reveals distinct calcite crystals approximately 1 mm in size. However, in thin section notable di2erences are observed in each of the three stones (Fig. 5). The Caen Stone has been found to comprise a bioclastic limestone, with unstructured discrete particles, approximately 0:08 mm in size, of micrite (microcrystalline calcite) (Fig. 5a). The Lepine can be described as a micritic, peloidal limestone. Most of the discrete particles in the latter are typically 0:2 mm in size, with generally only some of the larger features showing any oolitic structure (Fig. 5b). The SavonniEeres limestone is an oolitic limestone (ooliths typically 0:4 mm in size) in a sparry calcite cement (Fig. 5c). The constituent proportions from the thin section examinations are presented as Table 2 and reveal, in particular, di2erences in the ratio of sparite (coarse crystalline calcite) to micrite (microcrystalline calcite). In addition, the micropores are found to be typically associated with the micrite. The insoluble residue contents (which would include quartz, clay minerals and some iron compounds) are similar, less than 5%, in all the samples.
Fig. 5. Thin section photographs—widths of photographs 15 mm: (a) Caen Stone—plane polarised light, (b) Lepine Stone—plane polarised light and (c) SavonniEeres Stone—plane polarised light.
6. Sodium sulphate soundness tests
whilst the Kentish Ragstone stone retains its greenish-grey colour. The commentary accompanying the 1/50,000 geological map of Caen refers to the bioclastic nature of the Caen
In the present study comparative sodium sulphate soundness tests have been carried out on samples of each of the three French limestones, as soundness tests have been recognised as an indicator of stone durability [24,25]. The tests were carried out in general accordance with the method of the Building Research Establishment [25]. In this test, three
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J.F. Blows et al. / Building and Environment 38 (2003) 1143 – 1149
Cathedral and also Mr. R. Tyreman, Clerk of Works, Chichester Cathedral, for their co-operation.
Table 3 Initial sodium sulphate soundness results
Mean weight loss (%) after 5 cycles
Caen
Lepine
SavonniEeres
2
11
0
specimens from each variety of limestone were subjected to cycles of soaking in a solution of sodium sulphate, followed by heating, drying and weighing, with weight loss recorded at the end of each cycle. Table 3 shows the results after =ve cycles. It is clear from this preliminary study that the coarser, SavonniEeres limestone is more resistant at this stage of the test than the Caen and Lepine limestones.
7. Conclusions Comparison of the petrographical information with the initial =ndings from the sodium sulphate soundness tests suggests that Jurassic limestones with a high micritic content, particularly as a cementing medium, may be less durable than those with a high proportion of sparite and ooliths, possibly because of the apparent association of micropores with the micritic material. Microporosity is known to be an important controlling factor in the weathering characteristics of limestones and it is intended to carry out further work on this aspect in the near future [25]. Petrographical examinations are an essential feature of any investigation into the characteristics and likely behaviour of a building stone. They provide not only information on the composition of stone but also essential information on the texture and structure of the material. It is acknowledged that developing any link between petrographical characteristics and weathering characteristics requires additional work and this forms part of the current research project. XRD and SEM techniques are being used to identify those constituents and features which cannot be resolved by optical microscope techniques, in particular opaques and clay minerals. In addition, physical tests will be carried out to help quantify the =ndings in relation to previously acceptable stones and additional, possible new sources of repair material which have been added into the research programme.
Acknowledgements Thanks to Mr. P. Burton, Surveyor to the Works, Canterbury Cathedral; Mrs. H. Newton, Conservator, Canterbury
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[25] Miglio BF, Richardson DM, Yates TS, West D. Assessment of the durability of porous limestones: speci=cation and interpretation of test data. In: Dimension stone cladding; design, construction, evaluation and repair, ASTM STP 1394. West Conshohocken, PA: American Society for Testing and Materials, 2002.