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Insight into the conservation problems of the stone building “Bab Agnaou”, a XII cent. monumental gate in Marrakech (Morocco)
Journal of Cultural Heritage 8 (2007) 315e322 http://france.elsevier.com/direct/CULHER/
Case study
Insight into the conservation problems of the stone building ‘‘Bab Agnaou’’, a XII cent. monumental gate in Marrakech (Morocco) Lorenzo Lazzarini a,*, Ernesto Borrelli b, Mohamed Bouabdelli c, Fabrizio Antonelli a a Laboratorio di Analisi dei Materiali Antichi, DSA, Universita` IUAV di Venezia, S. Polo 2468, 30125 Venice, Italy ICCROM, International Centre for the Study of the Preservation and Restoration of Cultural Property, Via di S. Michele, 13, 00153 Rome, Italy c Universite´ Cadi Ayyad, Faculte` de Sciences, Laboratoire de Ge´ologie Structurale, Bd. Amir Mly Abdellah, B.P. S 15, 4000 Marrakech, Morocco b
Received 25 October 2006; accepted 26 February 2007
Abstract Bab Agnaou is one of the most beautiful gates of the 12th century town walls of Marrakech (Morocco). It is faced with poorly preserved stone, which has recently been subjected to preliminary laboratory studies for the purpose of collecting data useful for its restoration. The results of such studies indicate the presence of two macroscopically similar grey stones, classified as slates, which were probably sourced from quarries situated not far from the city of Marrakech. The schistosity of such stones, namely their laminated fabric, has much influenced the morphology of deterioration. The causes and mechanisms of decay have been investigated and connected to the presence of soluble salts, mainly chlorides and sulphates deriving from the mortar used to fix the stone blocks to the wall, and to local air pollution. Their concentration in the bottom 3 m of the gate due to rising damp has produced strong exfoliation and flaking phenomena through rapidly repeating crystallization/dissolution cycles. A minor contribution to the overall deterioration is due to hydrolytic phenomena partly responsible for the formation of natural brown patinas on the stone surface. Another brown patina formed of Ca-oxalates and with residual proteinaceous matter was also detected and is probably due to partly mineralized protection-treatments made with natural products such as animal glue. General indications for the restoration of the gate, based on these results, are proposed. Ó 2007 Elsevier Masson SAS. All rights reserved. Keywords: Marrakech; Bab Agnaou; Stone; Origin; Deterioration; Conservation
1. Introduction The Bab Agnaou gate (Fig. 1) is one of the oldest and most beautiful gates in the famous Medieval wall of the city of Marrakech. It is located in front of the minaret of the great Mosque of the Casbah from which it is separated by a heavy-traffic road. Its architectural style and importance pertain to the Almohad period (12th century). The origin of its name is uncertain; Agnaou has often been translated as ‘‘Guinean’’, and in the Berber language the name concerns a person whose * Corresponding author. E-mail addresses: [email protected], [email protected] (L. Lazzarini), fabria@ iuav.it (F. Antonelli). 1296-2074/$ - see front matter Ó 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2007.02.002
language is not understandable, thus, by extension, a person coming from black (sub-Sarahan) Africa. Therefore, this gate was probably initially the entrance to the Casbah reserved to common people. Nevertheless, the gate had an essentially decorative role; it was, above all, an elegant guardroom with a sumptuous design in stone facing a wall made of brick under which an archivolt, richly embroidered with three interlacing festoons, rests on upright supports that have a sinuous layout. The corner-pieces are decorated with wide and solid floral decorations extending around a shell and converging at the keystone of the arch in a quadrilobate finial. Originally it was certainly more imposing than it is today and it must have reminded people of the huge gate of the Oudaya Casbah in Rabat built in the same period during the Almohad
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Fig. 1. General view of Bab Agnaou.
sovereignty [1]. In fact, the gate has been partially rebuilt and restored many times since its creation. The width and length of the open arch were, for example, reduced, probably during the reign of Sultan Sidi Mohammed Ben Abdellah (1757e 1790) who was responsible for the reconstruction of the Casbah of Marrakech. As regards more recent interventions, two significant restorations undertaken in 1930 and in the Sixties are roughly documented. In the first, the Fine Arts and Historical Monuments Service in Rabat directed the restoration of the lower right section and the decoration at the top of the arch. The stone used for the substitutions made during this work was brought from the Oued N’Fiss valley, close to Marrakech, in the High Atlas Mountains and consisted of small carved, brick-sized blocks of a greenish-grey pelitic greywacke, which is slightly lighter than the older dark grey stone. The second restoration started at the beginning of the 1960s: apparently there exist no documents on the nature of the intervention or the origin of the material used. However, analysis and comparison with historical photographs clearly indicate that a new entrance with two arches was created besides the monumental gate. Nowadays the Bab Agnaou gate is in a poor state of conservation with some portions of the structure severely deteriorated and needing urgent treatment. In preparation for a new restoration of the monument, a preliminary scientific study was undertaken by a team of Italian and Moroccan experts in the framework of a INCO-MED project sponsored by the European Community. In the framework of this study, a representative number of samples was taken for laboratory analysis, in order to provide the necessary information about the nature, origin and causes of deterioration of the original stone, information that may be of primary importance for the forthcoming restoration project.
2. Experimental methods In line with the conditions attached to the permission granted by the local authorities, sampling for preliminary laboratory investigations had to be kept to a minimum. So it was decided to concentrate the sampling only on the stone: no investigation was made of the brick wall underneath. Fifteen samples of stone and its deterioration products were taken from various areas of the lower portion of the gate (at a height of between 1 and 3 m from the ground; Table 1), within and just above the section restored in the past. This is the most damaged part of the monument and corresponds to the upper limit of the rising damp. Six geological samples were taken from different stone outcrops identified as potential source areas of the Bab Agnaou building stone. X-ray powder diffraction analysis (XRD; Philips PW 1830: Cu Ka/Ni, 40 Kv, 20 mA) and scanning electron microscopy (SEM; Philips XL 30) were used to investigate the mineralogical components of whole samples and alteration products. Optical microscopy on thin and polished sections as well as SEM þ EDS analyses were used to determine the nature and the texture of the building stone. The results were compared with those obtained from the same analyses made on the geological samples. Porosimetric analyses of the sound and deteriorated stones were performed with a mercury porosimeter (Carlo Erba Porosimeter 2000) according to the Normal Recommendation [2]. Conductimetric (with a Hanna HI 8820N conductimeter) and quantitative chemical analyses of the anions composing the soluble salts were made with a ion chromatographer (Dionex DX-120) according to the Normal Recommendation [3]. Finally, a search for proteic and lipidic substances was made on a sample of the brown patina using GC-MS analysis was made with an
L. Lazzarini et al. / Journal of Cultural Heritage 8 (2007) 315e322 Table 1 Description and location of the samples taken from the door Sample
Description
Location
PA1
Left side, lower part, h. 1.5 m
PA3
Detaching flake of substitution stone Stone flake with brown patina Flaking and powdering stone
PA4
Powdered stone
PA5
Flaking Stone
PA6 PA7
Detaching flake of substitution stone Flaking Stone
PA8
Flaking and powdering stone
PA9
Detaching flake of substitution stone Stone flake with brown patina
PA2
PA10
PA1/2 PA2/2
PA3/2 PA4/2 PA5/2
Q1eQ6
Reddish patina on the stone surface Microflaking and powdering stone White powder at the surface of stone Stone flake with reddish-brown patina Powdery mortar between stone blocks
Right side, as above, close to the base of the internal arch Right side, as above, base of the external arch Left side, lower part of the internal arch, h. 1.9 m Left side, lower part, external arch, h. 2.5 m Left side, lower part, h. 1.5 m Right side, lower part, external arch, h. 2.8 m Left side, lower part, internal arch, h. 1.7 m Right side, lower part, h. 1.5 m Left side, lower part, close to the base of the internal arch, h. 1.6 m Right side, 30 cm above the base of the internal arch Right side, 40 cm above the base of the right side of the internal arch Right side, 45 cm above the base of the external arch Right side, 60 cm above the base of the external arch Right side, 30 cm above the base of the right side of the internal arch
Field samples: grey shale sampled from the slopes of the Jbilet massif, to the north of Marrakech
HP-5890 series II gas chromatograph coupled to an HP-5971A mass selective detector (Hewlett-Packard).
317
quartz alternating with others of phillosilicates (sericite-muscovite and chlorite) and iron oxides (Fig. 2). Calcite, and sometimes also dolomite is present in small amounts and forms single isodiametric individuals. Opaque minerals, mostly carbonaceous matter/graphite, and iron ore, of irregular shapes or dispersed/concentrated in fine particles, are quite abundant. Iron ores are sometimes associated with small limonitic masses. As confirmed also by the X-ray diffraction analysis (Table 2), the main mineral components are quartz, muscovite and clinochlore. This kind of stone represents the main lithotype (type-1) used to build the Bab Agnaou gate; nevertheless it was noticed that a second stone variety, macroscopically very similar to the first one, with which it alternates randomly, was employed in the core of the monument (particularly in the lower arch of the gate). Such a stone may be classified as a shale, and is macroscopically very similar to the metamorphosed siltstone previously described. Microscopically it shows an arenitic to microconglomeratic fabric (Fig. 3) characterised by predominantly well-rounded clasts with average sphericity composed of: e a volcanite formed of plagioclasic phenocrysts in a redbrown groundmass, sometimes containing plagioclasic microliths and abundant opaques; e a volcanite with a very low porphyritic index, formed by few (plagioclasic?) microliths in a red glassy groundmass coloured by dispersed hematite; e a quartz-arenite with abundant quartz, minor k-feldspar and muscovite in a brownish cement formed by iron oxides. Individuals of various sizes of sparitic calcite (abundant), quartz, chalcedony, twinned plagioclase, pertitic feldspar and chlorite complete the composition. The cement shows a micritic appearance and a high porosity. Generally, the external surfaces of the samples are intensely weathered (Fig. 7) and many systems of iso-parallel micro-fissures due to in situ deterioration can be seen close to it (Fig. 8). An enrichment of
3. Results and discussion The results obtained from the laboratory studies are subdivided into two sections, one regarding the provenance of the stone of the monument, and one to its decay-phenomena. 3.1. The provenance of the stone The initial study concentrated on the petrographic characterisation of the building stone in order to identify its intrinsic properties and locate its source area (and thus an ideal supply of the restoration material). Most of the rock samples taken from the monument showed homogeneous petrographic features; they always proved to be composed of a grey-greenish graphitic slate (metapelite) formed from the low-grade metamorphism of a siltstone in the green-schist facies [4], showing a lineated structure formed by very thin levels of fine-grained
Fig. 2. Sample # PA5: photomicrograph of the thin section showing the lineated fabric of the main stone of the gate (underlined by the presence of small sericite needles) that has encouraged its isoparallel micro-cracking. Nþ, 16.
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Table 2 Results of the diffractometric analyses of deteriorated stone samples and of samples from the quarry Sample
Qtz
Cal
PA1 PA2 PA3 PA4 PA6 PA9 PA10 PA1/2 PA2/2 PA3/2 PA4/2 PA5/2 Q1 Q2 Q4 Q6
þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ
þþ þ
Dol
Pl
þþ þ
þ þþ þþþ
þ þ
þþ
Chl
Ms
þþþ þþ þþ þþþ þþ þþ þþþ þþ þþþ þþþ þþ
þþ þ þ þþ þþ þþ þ þþ þþ þþ þ
þ þ þ þ
þþ þ þ þþ
Gp
Hl
þ þ
þ
Hem
W
Wd
Tlc
Atg
CM
þ
þþ
þ
þ þ
þ þ
Qtz ¼ Quartz; Cal ¼ Calcite; Dol ¼ Dolomite; Pl ¼ Plagioclase; Chl ¼ Chlorite; Ms ¼ Muscovite; Gp ¼ Gypsum; Hl ¼ Halite; Hem ¼ Hematite; W ¼ Whewellite; Wd ¼ Weddellite; Tlc ¼ Talc; Atg ¼ Antigorite; CM ¼ Clay Minerals. þþþ ¼ very abundant; þþ ¼ average presence; þ ¼ present; ¼ trace.
limonite and calcite often characterises the very surface of most of the building stones. The former component is most probably due to hydrolytic leaching of iron-bearing minerals from the interior of the rock. The latter is likely to be connected to re-precipitation of the calcite present close to the surface. To search for the origin of these stones, a geological map of the region surrounding Marrakech was studied [5], local geologists were interviewed and some field-work was done in selected areas north of the city, in the Jbilet mountains, Ourika and Oued N’Fiss valleys. A grey-dark grey siltstone similar to that of the gate was found outcropping in the neighbourhood to the north of Marrakech. Schists from Jbilet mountains and Ourika valley were also considered, but they were quite different from the stone of the gate. The rocks from the Oued N’Fiss valley, which were used for the restoration works of the 1930s, were excluded because they are not shale but greenish-grey greywackes and meta-greywackes. Thin-section and XRD
(Table 2) analyses suggest a strong compatibility of the stones from the gate with the grey shale from the slopes of the Jbilet massif, to the north of Marrakech, even if the samples collected also contain small amounts of talc and antigorite (Table 2) not present in the original stone of the gate. It is very likely, however, that the formation including such a stone exists somewhere in the massif considered. Unfortunately, the Carboniferous schist formation occupies about 80% of the surface of this massif and it was not possible to find the exact quarrying area (it should be, however, along the Safi-Casablanca or Fe`s roads) due in part to the small size of the outcrops when compared with the scale of the geological map (1:500,000) and in part to the lack of historical information. Two samples of the original stones, one macroscopically showing slight (PA3) and one average (PA4) deterioration, and one sample of the substitution-stone (PA1) were subjected to Hg-intrusion porosimetry. The results collected in Table 3 and shown in Fig. 4, indicate a generally low total open porosity for the original stone, with prevailing amounts of small pores (0.01e1 mm), both parameters being slightly higher in the most deteriorated stone sample (PA4): these data point to an original stone of decent quality with a low total porosity, but also with a relatively large number of small pores, a feature that negatively influenced its resistance to salt crystallisation. Conversely, the stone recently used to replace the seriously damaged old stone blocks, feature a much higher total porosity and a more evenly distributed pore-size distribution, thus accounting for a slightly better behaviour as regards salt crystallisation. 3.2. The main causes of deterioration
Fig. 3. Sample # PA1: as for Fig. 2, but showing the micro-conglomeratic fabric of the other stone of the gate, with clasts of slates and volcanites in a micritic matrix. Nþ, 32.
Most of the present research was dedicated to the study of the macro-and micro-morphologies of deterioration, and to the causes and mechanisms of stone decay. The deterioration macro-morphologies observed on the original and the restored
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319
Table 3 Results of the porosimetric analyses Sample
P (%)
rr (g/cm3)
APR (mm)
Pore radius distribution (% relative volume) 7.5e2 (mm)
2e1 (mm)
1e0.1 (mm)
0.1e0.01 (mm)
0.01e0.0037 (mm)
PA3 PA4 PA1
1.27 2.21 6.41
2.68 2.77 2.53
0.06 0.07 0.41
05.77 05.95 11.31
11.55 07.14 18.72
36.52 39.27 37.83
46.09 47.60 20.28
e e 12.09
P: total open porosity; rr: bulk density; APR: average pore radius.
stones of the gate were studied and mapped on a provisional drawing (no graphic relief was available) according to standard definitions listed in the Italian Normal recommendation [6]. The most evident of the deterioration phenomenawere a very evident brown patina covering most of the stone surface (Fig. 5); flaking (Fig. 6), and subordinately, exfoliation,
Fig. 4. Porosity diagram of the original (PA3 and PA4) and substitution (PA1) stones.
powdering, cracking and losses which were variously distributed over the fac¸ade, but mostly concentrated in its lower part where a strong water capillary-rise reaches a height of about 3e4 m. Localised white salt efflorescences were also observed (Fig. 7). The brown patina has an average thickness of 50 mm and covers all the best preserved parts of the gate. In plane polarised light it appears to be composed of very small, slightly opaque particles like micrite (calcite <4 mm) including larger particles/aggregates of yellow-brown colour quite similar to limonite. In cross polarised light the matrix shows, further to small angular quartz crystals and the limonite particles, a mass of very small particles with the very high birefringence typical of calcium oxalates. This identification was confirmed by the XRD-analyses performed on powder samples revealing a significant content of whewellite and minor effects of weddellite (Table 2). Observation of several polished cross sections of the patina under UV light often showed its penetration in microcracks of the stone (Fig. 8), and an intense yellow-greenish fluorescence typical of animal glue. The use of an animal glue was confirmed by the positive results of a GC-MS analysis on powder of the patina (Fig. 9), which also showed the absence of lipids. The presence of the oxalate-layer may relate to the action of micro-organisms such as lichens (these are not present now, but they may have disappeared due to air pollution), or to a conservation treatment of the stone with organic matter that may have undergone mineralization processes through the action of micro-organisms [7]. This second hypothesis is the most likely to have occurred because of the homogeneity of the distribution and aspect of the patina, and for the residual presence in it of animal glue. It was probably after a treatment with such a medium applied for conservation purposes that the patina formed and became coloured from the embedding of desert dust mainly containing quartz and iron oxides (mostly limonites, but also hematite, detected in sample PA5/2: Table 2). The hypothesis of a treatment is also supported by the penetration of the patina inside microcracks of the stone formed as a consequence of its deterioration (Fig. 8). The presence should be noted in some samples of exclusively natural, non-fluorescent patinas, formed by the deposition of desert dust as above, probably fixed by small amounts of re-crystallised calcite. In such patinas one cannot however exclude the contribution of the fixing action of limonite coming from the leached-iron produced by hydrolysis and migrated from the interior of the stone towards the surface. In deteriorated stone samples in fact a strong limonitisation of opaque iron ores was frequently observed.
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Fig. 5. Detail of the right hand side of the gate showing heavy deterioration due to flaking mainly caused by salt crystallisation in an area reached by rising damp.
Fig. 7. White efflorescence of halite on the intonaco covering the wall of the tower to the right of the gate.
As regards the composition of the deterioration-products, X-ray diffraction analysis showed the presence of small, but not negligible, amounts of gypsum and halite (Table 2). These two soluble salts are the main cause of the surface exfoliation, flaking and powdering (Fig. 10) of the stone [8,9]. Halite is a common salt in inhabited desert areas where the strong evaporation and human use of NaCl tend to concentrate this salt in the soil. As is often the case in a sulphur-polluted atmosphere, the presence of gypsum is probably due to the progressive sulphation of the calcite contained in the rock. However, as suggested by local workers, the origin of gypsum, as well as that of halite, may also be related to the composition of the mortar used to cement the stone blocks to the wall, presumably manufactured with salt-rich sand. This hypothesis seems confirmed by XRD analysis of three mortar samples that showed evidence of a moderate presence of halite and gypsum (cf. Table 2 and Fig. 7). In effect, most of the Triassic and Cretaceous clay and sandy soil outcrops inspected in the outskirts of Marrakech feature a strong superficial presence of salt layers due to evaporation. The city of Marrakech is in fact located in the Haouz plain whose alluvial deposits derive from the
Atlas mountain chain. Several watercourses such as the R’dat, Zat, Ourika, Rheraira and N’fiss originate from the High Atlas and all flow into the Oued Tensift, the main river draining the Haouz plain. Alluvial deposits of these rivers all contain varying amounts of salts originating from the geological formations of the High Atlas [5]. Chemical analysis of a few samples of deteriorated stone products (Table 4) has also shown the presence of small amounts of nitrites, nitrates and of phosphates, most probably of biological origin. All these salts are mobilised by rising capillary-water (reaching a visible maximum height of about 3 m), which corresponded not only to rainfall, but also to the water currently used for washing the road passing before and under the gate. The source of the salts may be multiple, from the brick-core of the wall and from the outside (e.g. bird’s droppings): all being very soluble, they easily migrate towards the surface. The sometimes strong exfoliation and flaking produced by the salts may be explained by their concentration (and magnified effectiveness) along the foliation of the rock and the following dissolution-crystallisation cycles that are very frequent in
Fig. 6. Detail of Fig. 5: note the brown patina and the serious exfoliation and flaking.
Fig. 8. UV photomicrograph showing the penetration of the brown patina and its yellow-greenish fluorescence. 16.
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321
Fig. 9. Mass-spectrogram of the patina of sample # PA 2 showing the residual presence of a few proteins.
the particular semi-arid climate of Marrakech, with considerable day-night variation of relative humidity. This mechanism starts with a micro-cracking parallel to foliation favoured by the fabric of the rock (well observed in deteriorated samples of the first stone-type, Fig. 2), which soon evolves into macro-cracking and then loss of foils and flakes. The exposure of the gate to the west, and the dark colour of the stones, may also favour some deterioration due to thermal stress, which may add to its overall decay. 3.3. Considerations for conservation treatments To decrease the speed and intensity of the deterioration phenomena occurring in the stone of the gate it is obviously first of all necessary to stop rising damp, the main vehicle transporting soluble salts within the stone and the other building materials. This may be achieved safely only by cutting the walls at the base of the gate and introducing a water barrier extending to the two flanking towers. This operation is not simple, owing to the considerable thickness of the walls, but it is possible by cutting them at the same level on both sides
Fig. 10. SEM micrograph of sample # PA7 showing decohesioned small flakes of the type-1 stone.
as is a current practice, for example, in Venice. A possible, less intrusive and alternative solution may be a drainage system at the base of the walls around the gate and the introduction of ventilation channels [10]. The following action should be the desalination of all deteriorated areas. This treatment is long and time-consuming, but absolutely necessary. It may be performed by repeated poulticing with a mud obtained by mixing deionised water with special clays, such as bentonite or sepiolite, or paper pulp [11,12]. A control of the progress of such treatment will be possible by checking the conductivity of the removed poultice [13]. This operation should lead to the removal of most of the salts present in the stone; it should be done by an experienced restorer, since very deteriorated areas may need the careful prior application of sheets of paper between the stone and the poultice, and in some areas even a pre-consolidation treatment with ethyl-silicate which does not completely fill the pores and is thus compatible with the subsequent removal of salts. A light cleaning of the surface from soot and other deposits (such as the removal of desert dust, bird droppings, etc.) is necessary for all the surface and may be done by washing with the minimum amount of water before the consolidation of the damaged areas. Such a treatment needs preliminary laboratory research to find out the best materials for the types of stones used in the gate, and for the preservation of the oxalate patinas. The deteriorated stone should in principle be structurally consolidated with ethyl silicate, if necessary by repeated applications: the silica thus deposited can glue together the loose particles and thin sheets of the powdering and flaking stone. The choice of ethyl silicate is based on the fact that this consolidant is very fluid, chemically compatible with the silicatic composition of the stones and, as shown by preliminary tests made on a small scale in the lab, it does not alter their colour. In the most difficult situations, e.g. in heavily flaking areas, the only possibility of success is to rely on the use of small amounts of strong adhesives, such as an epoxy resin, used to bridge together in a few points the two parts of a flake, and of a putty that may be made with stone powder mixed with ethyl silicate, with fibres added to strengthen the paste if necessary, and pigments to match the colour. Only in very limited
L. Lazzarini et al. / Journal of Cultural Heritage 8 (2007) 315e322
322 Table 4 Results of the chemical quantitative analyses PA1
þ
Li Naþ NHþ 4 Kþ Mgþþ Caþþ F Cl NO 2 NO 3 PO 4 SO4 C2O 4
PA2-2
PA3-2
PA5-2
Tot. cond. ¼ 25 mS
Tot. cond. ¼ 69 mS
Tot. cond. ¼ 75 mS
Tot. cond. ¼ 145 mS
%w/w
meq/100 mg
%w/w
meq/100 mg
%w/w
meq/100 mg
%w/w
meq/100 mg
e e e 0.02 0.01 0.82 e 0.02 e e e 0.14 e
e e e 0.05 102 0.08 102 4.09 102 e 0.06 102 e e e 0.29 102 e
e 0.50 e 0.03 0.06 1.02 e 0.59 e 0.59 0.14 0.27 e
e 2.2 102 e 0.08 102 0.49 102 5.09 102 e 1.66 102 e 0.95 102 0.44 102 0.56 102 e
e 0.63 e 0.05 0.14 0.66 0.02 0.41 0.01 1.31 e 0.69 e
e 2.7 102 e 0.12 102 1.15 102 3.29 102 0.11 102 1.16 102 0.02 102 2.11 102 e 1.44 102 e
e 0.74 e 0.30 0.06 1.81 0.02 1.12 e 2.96 e 0.42 e
e 3.2 102 e 0.77 102 0.49 102 9.03 102 0.11 102 3.16 102 e 4.77 102 e 0.87 102 e
areas with deep stone loss, should stone be replaced with the similar rock identified above. The reduced amount of rainfall in the area of Marrakech (less than 300 mm/year), the high temperatures reached by the stone surface in summer, and the overall strong UV radiation for many months of the year, make it inadvisable to apply any surface-protective treatment. Acknowledgements This research was conducted in the framework of the European Project INCO-MED (Concerted Action Contract ERB-IC18-CT98-0384, Prospective Study 3). The authors are grateful to Prof. Antonella Casoli of the University of Parma for the GC-MS analysis. References [1] G. Deverdun, Marrakech, des origins a` 1912, Rabat, 1966. [2] NORMAL 4/80, Distribuzione del volume dei pori in funzione del loro diametro, CNR-ICR Roma (1980). [3] NORMAL 13/83/UNI-Beni Culturali 11087 (2003), Dosaggio dei Sali solubili, CNR-ICR, Roma, 1983, Milano, 2003.
[4] B.W.D. Yardley, An Introduction to Metamorphic Petrology, Longman, Harlow, 1989. [5] R. Ambroggi, G. Thuille, Geological map of the Marrakech region, 1/ 500.000, Notes Me´m. Serv. Ge´ol., Maroc. (1952). [6] NORMAL 1/88, Alterazioni macroscopiche dei materiali lapidei: lessico, CNR-ICR, Roma, 1988. [7] L. Lazzarini, O. Salvadori, A reassessment of the formation of the patina called scialbatura, Studies in Conservation 34 (1) (1989) 20e26. [8] E. Borrelli, Salts, ARC Laboratory Handbook, ICCROM, Rome, 1999. [9] L. Lazzarini, General issues on the deterioration of stone, in: Proceedings of the Meeting ‘‘The Building Stone in Monuments’’, Athens-Mytilene 9e11/11-9-2001, Athens, 2002, pp. 149e160. [10] N. Spinosa, A.C. Alabiso, D. Pagano, D. Ferragni, P. Mora, G. Torraca, I. Massari, Il restauro del chiostro maiolicato di S. Chiara in Napoli. In: Le superfici dell’architettura: il cotto, caratterizzazioni e trattamenti, Libreria Progetto Ed., Padova, 1992, pp. 517e526. [11] L. Lazzarini, M. Laurenzi Tabasso, Il restauro della pietra, CEDAM, Padova, 1986. [12] S. Simon, Desalination by poulticing: laboratory study on controlling parameters, in: D. Kwiatkowski (Ed.), Proceedings of the 10th International Congress on Deterioration and Conservation of Stone, Stockholm, June 27eJuly 2, 2004, pp. 323-330. [13] E. Borrelli, Standardizzazione di un metodo di estrazione di sali da supporti porosi e suo utilizzo a scopo diagnostico, in: V. Fassina, H. Ott, F. Zezza (Eds.), The Conservation of Monuments in the Mediterranean Basin. Proceedings of the 3rd International Symposium, Padova, Venice, 22e25 June 1994, pp. 163e167.
Case study
Insight into the conservation problems of the stone building ‘‘Bab Agnaou’’, a XII cent. monumental gate in Marrakech (Morocco) Lorenzo Lazzarini a,*, Ernesto Borrelli b, Mohamed Bouabdelli c, Fabrizio Antonelli a a Laboratorio di Analisi dei Materiali Antichi, DSA, Universita` IUAV di Venezia, S. Polo 2468, 30125 Venice, Italy ICCROM, International Centre for the Study of the Preservation and Restoration of Cultural Property, Via di S. Michele, 13, 00153 Rome, Italy c Universite´ Cadi Ayyad, Faculte` de Sciences, Laboratoire de Ge´ologie Structurale, Bd. Amir Mly Abdellah, B.P. S 15, 4000 Marrakech, Morocco b
Received 25 October 2006; accepted 26 February 2007
Abstract Bab Agnaou is one of the most beautiful gates of the 12th century town walls of Marrakech (Morocco). It is faced with poorly preserved stone, which has recently been subjected to preliminary laboratory studies for the purpose of collecting data useful for its restoration. The results of such studies indicate the presence of two macroscopically similar grey stones, classified as slates, which were probably sourced from quarries situated not far from the city of Marrakech. The schistosity of such stones, namely their laminated fabric, has much influenced the morphology of deterioration. The causes and mechanisms of decay have been investigated and connected to the presence of soluble salts, mainly chlorides and sulphates deriving from the mortar used to fix the stone blocks to the wall, and to local air pollution. Their concentration in the bottom 3 m of the gate due to rising damp has produced strong exfoliation and flaking phenomena through rapidly repeating crystallization/dissolution cycles. A minor contribution to the overall deterioration is due to hydrolytic phenomena partly responsible for the formation of natural brown patinas on the stone surface. Another brown patina formed of Ca-oxalates and with residual proteinaceous matter was also detected and is probably due to partly mineralized protection-treatments made with natural products such as animal glue. General indications for the restoration of the gate, based on these results, are proposed. Ó 2007 Elsevier Masson SAS. All rights reserved. Keywords: Marrakech; Bab Agnaou; Stone; Origin; Deterioration; Conservation
1. Introduction The Bab Agnaou gate (Fig. 1) is one of the oldest and most beautiful gates in the famous Medieval wall of the city of Marrakech. It is located in front of the minaret of the great Mosque of the Casbah from which it is separated by a heavy-traffic road. Its architectural style and importance pertain to the Almohad period (12th century). The origin of its name is uncertain; Agnaou has often been translated as ‘‘Guinean’’, and in the Berber language the name concerns a person whose * Corresponding author. E-mail addresses: [email protected], [email protected] (L. Lazzarini), fabria@ iuav.it (F. Antonelli). 1296-2074/$ - see front matter Ó 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2007.02.002
language is not understandable, thus, by extension, a person coming from black (sub-Sarahan) Africa. Therefore, this gate was probably initially the entrance to the Casbah reserved to common people. Nevertheless, the gate had an essentially decorative role; it was, above all, an elegant guardroom with a sumptuous design in stone facing a wall made of brick under which an archivolt, richly embroidered with three interlacing festoons, rests on upright supports that have a sinuous layout. The corner-pieces are decorated with wide and solid floral decorations extending around a shell and converging at the keystone of the arch in a quadrilobate finial. Originally it was certainly more imposing than it is today and it must have reminded people of the huge gate of the Oudaya Casbah in Rabat built in the same period during the Almohad
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Fig. 1. General view of Bab Agnaou.
sovereignty [1]. In fact, the gate has been partially rebuilt and restored many times since its creation. The width and length of the open arch were, for example, reduced, probably during the reign of Sultan Sidi Mohammed Ben Abdellah (1757e 1790) who was responsible for the reconstruction of the Casbah of Marrakech. As regards more recent interventions, two significant restorations undertaken in 1930 and in the Sixties are roughly documented. In the first, the Fine Arts and Historical Monuments Service in Rabat directed the restoration of the lower right section and the decoration at the top of the arch. The stone used for the substitutions made during this work was brought from the Oued N’Fiss valley, close to Marrakech, in the High Atlas Mountains and consisted of small carved, brick-sized blocks of a greenish-grey pelitic greywacke, which is slightly lighter than the older dark grey stone. The second restoration started at the beginning of the 1960s: apparently there exist no documents on the nature of the intervention or the origin of the material used. However, analysis and comparison with historical photographs clearly indicate that a new entrance with two arches was created besides the monumental gate. Nowadays the Bab Agnaou gate is in a poor state of conservation with some portions of the structure severely deteriorated and needing urgent treatment. In preparation for a new restoration of the monument, a preliminary scientific study was undertaken by a team of Italian and Moroccan experts in the framework of a INCO-MED project sponsored by the European Community. In the framework of this study, a representative number of samples was taken for laboratory analysis, in order to provide the necessary information about the nature, origin and causes of deterioration of the original stone, information that may be of primary importance for the forthcoming restoration project.
2. Experimental methods In line with the conditions attached to the permission granted by the local authorities, sampling for preliminary laboratory investigations had to be kept to a minimum. So it was decided to concentrate the sampling only on the stone: no investigation was made of the brick wall underneath. Fifteen samples of stone and its deterioration products were taken from various areas of the lower portion of the gate (at a height of between 1 and 3 m from the ground; Table 1), within and just above the section restored in the past. This is the most damaged part of the monument and corresponds to the upper limit of the rising damp. Six geological samples were taken from different stone outcrops identified as potential source areas of the Bab Agnaou building stone. X-ray powder diffraction analysis (XRD; Philips PW 1830: Cu Ka/Ni, 40 Kv, 20 mA) and scanning electron microscopy (SEM; Philips XL 30) were used to investigate the mineralogical components of whole samples and alteration products. Optical microscopy on thin and polished sections as well as SEM þ EDS analyses were used to determine the nature and the texture of the building stone. The results were compared with those obtained from the same analyses made on the geological samples. Porosimetric analyses of the sound and deteriorated stones were performed with a mercury porosimeter (Carlo Erba Porosimeter 2000) according to the Normal Recommendation [2]. Conductimetric (with a Hanna HI 8820N conductimeter) and quantitative chemical analyses of the anions composing the soluble salts were made with a ion chromatographer (Dionex DX-120) according to the Normal Recommendation [3]. Finally, a search for proteic and lipidic substances was made on a sample of the brown patina using GC-MS analysis was made with an
L. Lazzarini et al. / Journal of Cultural Heritage 8 (2007) 315e322 Table 1 Description and location of the samples taken from the door Sample
Description
Location
PA1
Left side, lower part, h. 1.5 m
PA3
Detaching flake of substitution stone Stone flake with brown patina Flaking and powdering stone
PA4
Powdered stone
PA5
Flaking Stone
PA6 PA7
Detaching flake of substitution stone Flaking Stone
PA8
Flaking and powdering stone
PA9
Detaching flake of substitution stone Stone flake with brown patina
PA2
PA10
PA1/2 PA2/2
PA3/2 PA4/2 PA5/2
Q1eQ6
Reddish patina on the stone surface Microflaking and powdering stone White powder at the surface of stone Stone flake with reddish-brown patina Powdery mortar between stone blocks
Right side, as above, close to the base of the internal arch Right side, as above, base of the external arch Left side, lower part of the internal arch, h. 1.9 m Left side, lower part, external arch, h. 2.5 m Left side, lower part, h. 1.5 m Right side, lower part, external arch, h. 2.8 m Left side, lower part, internal arch, h. 1.7 m Right side, lower part, h. 1.5 m Left side, lower part, close to the base of the internal arch, h. 1.6 m Right side, 30 cm above the base of the internal arch Right side, 40 cm above the base of the right side of the internal arch Right side, 45 cm above the base of the external arch Right side, 60 cm above the base of the external arch Right side, 30 cm above the base of the right side of the internal arch
Field samples: grey shale sampled from the slopes of the Jbilet massif, to the north of Marrakech
HP-5890 series II gas chromatograph coupled to an HP-5971A mass selective detector (Hewlett-Packard).
317
quartz alternating with others of phillosilicates (sericite-muscovite and chlorite) and iron oxides (Fig. 2). Calcite, and sometimes also dolomite is present in small amounts and forms single isodiametric individuals. Opaque minerals, mostly carbonaceous matter/graphite, and iron ore, of irregular shapes or dispersed/concentrated in fine particles, are quite abundant. Iron ores are sometimes associated with small limonitic masses. As confirmed also by the X-ray diffraction analysis (Table 2), the main mineral components are quartz, muscovite and clinochlore. This kind of stone represents the main lithotype (type-1) used to build the Bab Agnaou gate; nevertheless it was noticed that a second stone variety, macroscopically very similar to the first one, with which it alternates randomly, was employed in the core of the monument (particularly in the lower arch of the gate). Such a stone may be classified as a shale, and is macroscopically very similar to the metamorphosed siltstone previously described. Microscopically it shows an arenitic to microconglomeratic fabric (Fig. 3) characterised by predominantly well-rounded clasts with average sphericity composed of: e a volcanite formed of plagioclasic phenocrysts in a redbrown groundmass, sometimes containing plagioclasic microliths and abundant opaques; e a volcanite with a very low porphyritic index, formed by few (plagioclasic?) microliths in a red glassy groundmass coloured by dispersed hematite; e a quartz-arenite with abundant quartz, minor k-feldspar and muscovite in a brownish cement formed by iron oxides. Individuals of various sizes of sparitic calcite (abundant), quartz, chalcedony, twinned plagioclase, pertitic feldspar and chlorite complete the composition. The cement shows a micritic appearance and a high porosity. Generally, the external surfaces of the samples are intensely weathered (Fig. 7) and many systems of iso-parallel micro-fissures due to in situ deterioration can be seen close to it (Fig. 8). An enrichment of
3. Results and discussion The results obtained from the laboratory studies are subdivided into two sections, one regarding the provenance of the stone of the monument, and one to its decay-phenomena. 3.1. The provenance of the stone The initial study concentrated on the petrographic characterisation of the building stone in order to identify its intrinsic properties and locate its source area (and thus an ideal supply of the restoration material). Most of the rock samples taken from the monument showed homogeneous petrographic features; they always proved to be composed of a grey-greenish graphitic slate (metapelite) formed from the low-grade metamorphism of a siltstone in the green-schist facies [4], showing a lineated structure formed by very thin levels of fine-grained
Fig. 2. Sample # PA5: photomicrograph of the thin section showing the lineated fabric of the main stone of the gate (underlined by the presence of small sericite needles) that has encouraged its isoparallel micro-cracking. Nþ, 16.
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318
Table 2 Results of the diffractometric analyses of deteriorated stone samples and of samples from the quarry Sample
Qtz
Cal
PA1 PA2 PA3 PA4 PA6 PA9 PA10 PA1/2 PA2/2 PA3/2 PA4/2 PA5/2 Q1 Q2 Q4 Q6
þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ þþþ
þþ þ
Dol
Pl
þþ þ
þ þþ þþþ
þ þ
þþ
Chl
Ms
þþþ þþ þþ þþþ þþ þþ þþþ þþ þþþ þþþ þþ
þþ þ þ þþ þþ þþ þ þþ þþ þþ þ
þ þ þ þ
þþ þ þ þþ
Gp
Hl
þ þ
þ
Hem
W
Wd
Tlc
Atg
CM
þ
þþ
þ
þ þ
þ þ
Qtz ¼ Quartz; Cal ¼ Calcite; Dol ¼ Dolomite; Pl ¼ Plagioclase; Chl ¼ Chlorite; Ms ¼ Muscovite; Gp ¼ Gypsum; Hl ¼ Halite; Hem ¼ Hematite; W ¼ Whewellite; Wd ¼ Weddellite; Tlc ¼ Talc; Atg ¼ Antigorite; CM ¼ Clay Minerals. þþþ ¼ very abundant; þþ ¼ average presence; þ ¼ present; ¼ trace.
limonite and calcite often characterises the very surface of most of the building stones. The former component is most probably due to hydrolytic leaching of iron-bearing minerals from the interior of the rock. The latter is likely to be connected to re-precipitation of the calcite present close to the surface. To search for the origin of these stones, a geological map of the region surrounding Marrakech was studied [5], local geologists were interviewed and some field-work was done in selected areas north of the city, in the Jbilet mountains, Ourika and Oued N’Fiss valleys. A grey-dark grey siltstone similar to that of the gate was found outcropping in the neighbourhood to the north of Marrakech. Schists from Jbilet mountains and Ourika valley were also considered, but they were quite different from the stone of the gate. The rocks from the Oued N’Fiss valley, which were used for the restoration works of the 1930s, were excluded because they are not shale but greenish-grey greywackes and meta-greywackes. Thin-section and XRD
(Table 2) analyses suggest a strong compatibility of the stones from the gate with the grey shale from the slopes of the Jbilet massif, to the north of Marrakech, even if the samples collected also contain small amounts of talc and antigorite (Table 2) not present in the original stone of the gate. It is very likely, however, that the formation including such a stone exists somewhere in the massif considered. Unfortunately, the Carboniferous schist formation occupies about 80% of the surface of this massif and it was not possible to find the exact quarrying area (it should be, however, along the Safi-Casablanca or Fe`s roads) due in part to the small size of the outcrops when compared with the scale of the geological map (1:500,000) and in part to the lack of historical information. Two samples of the original stones, one macroscopically showing slight (PA3) and one average (PA4) deterioration, and one sample of the substitution-stone (PA1) were subjected to Hg-intrusion porosimetry. The results collected in Table 3 and shown in Fig. 4, indicate a generally low total open porosity for the original stone, with prevailing amounts of small pores (0.01e1 mm), both parameters being slightly higher in the most deteriorated stone sample (PA4): these data point to an original stone of decent quality with a low total porosity, but also with a relatively large number of small pores, a feature that negatively influenced its resistance to salt crystallisation. Conversely, the stone recently used to replace the seriously damaged old stone blocks, feature a much higher total porosity and a more evenly distributed pore-size distribution, thus accounting for a slightly better behaviour as regards salt crystallisation. 3.2. The main causes of deterioration
Fig. 3. Sample # PA1: as for Fig. 2, but showing the micro-conglomeratic fabric of the other stone of the gate, with clasts of slates and volcanites in a micritic matrix. Nþ, 32.
Most of the present research was dedicated to the study of the macro-and micro-morphologies of deterioration, and to the causes and mechanisms of stone decay. The deterioration macro-morphologies observed on the original and the restored
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319
Table 3 Results of the porosimetric analyses Sample
P (%)
rr (g/cm3)
APR (mm)
Pore radius distribution (% relative volume) 7.5e2 (mm)
2e1 (mm)
1e0.1 (mm)
0.1e0.01 (mm)
0.01e0.0037 (mm)
PA3 PA4 PA1
1.27 2.21 6.41
2.68 2.77 2.53
0.06 0.07 0.41
05.77 05.95 11.31
11.55 07.14 18.72
36.52 39.27 37.83
46.09 47.60 20.28
e e 12.09
P: total open porosity; rr: bulk density; APR: average pore radius.
stones of the gate were studied and mapped on a provisional drawing (no graphic relief was available) according to standard definitions listed in the Italian Normal recommendation [6]. The most evident of the deterioration phenomenawere a very evident brown patina covering most of the stone surface (Fig. 5); flaking (Fig. 6), and subordinately, exfoliation,
Fig. 4. Porosity diagram of the original (PA3 and PA4) and substitution (PA1) stones.
powdering, cracking and losses which were variously distributed over the fac¸ade, but mostly concentrated in its lower part where a strong water capillary-rise reaches a height of about 3e4 m. Localised white salt efflorescences were also observed (Fig. 7). The brown patina has an average thickness of 50 mm and covers all the best preserved parts of the gate. In plane polarised light it appears to be composed of very small, slightly opaque particles like micrite (calcite <4 mm) including larger particles/aggregates of yellow-brown colour quite similar to limonite. In cross polarised light the matrix shows, further to small angular quartz crystals and the limonite particles, a mass of very small particles with the very high birefringence typical of calcium oxalates. This identification was confirmed by the XRD-analyses performed on powder samples revealing a significant content of whewellite and minor effects of weddellite (Table 2). Observation of several polished cross sections of the patina under UV light often showed its penetration in microcracks of the stone (Fig. 8), and an intense yellow-greenish fluorescence typical of animal glue. The use of an animal glue was confirmed by the positive results of a GC-MS analysis on powder of the patina (Fig. 9), which also showed the absence of lipids. The presence of the oxalate-layer may relate to the action of micro-organisms such as lichens (these are not present now, but they may have disappeared due to air pollution), or to a conservation treatment of the stone with organic matter that may have undergone mineralization processes through the action of micro-organisms [7]. This second hypothesis is the most likely to have occurred because of the homogeneity of the distribution and aspect of the patina, and for the residual presence in it of animal glue. It was probably after a treatment with such a medium applied for conservation purposes that the patina formed and became coloured from the embedding of desert dust mainly containing quartz and iron oxides (mostly limonites, but also hematite, detected in sample PA5/2: Table 2). The hypothesis of a treatment is also supported by the penetration of the patina inside microcracks of the stone formed as a consequence of its deterioration (Fig. 8). The presence should be noted in some samples of exclusively natural, non-fluorescent patinas, formed by the deposition of desert dust as above, probably fixed by small amounts of re-crystallised calcite. In such patinas one cannot however exclude the contribution of the fixing action of limonite coming from the leached-iron produced by hydrolysis and migrated from the interior of the stone towards the surface. In deteriorated stone samples in fact a strong limonitisation of opaque iron ores was frequently observed.
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Fig. 5. Detail of the right hand side of the gate showing heavy deterioration due to flaking mainly caused by salt crystallisation in an area reached by rising damp.
Fig. 7. White efflorescence of halite on the intonaco covering the wall of the tower to the right of the gate.
As regards the composition of the deterioration-products, X-ray diffraction analysis showed the presence of small, but not negligible, amounts of gypsum and halite (Table 2). These two soluble salts are the main cause of the surface exfoliation, flaking and powdering (Fig. 10) of the stone [8,9]. Halite is a common salt in inhabited desert areas where the strong evaporation and human use of NaCl tend to concentrate this salt in the soil. As is often the case in a sulphur-polluted atmosphere, the presence of gypsum is probably due to the progressive sulphation of the calcite contained in the rock. However, as suggested by local workers, the origin of gypsum, as well as that of halite, may also be related to the composition of the mortar used to cement the stone blocks to the wall, presumably manufactured with salt-rich sand. This hypothesis seems confirmed by XRD analysis of three mortar samples that showed evidence of a moderate presence of halite and gypsum (cf. Table 2 and Fig. 7). In effect, most of the Triassic and Cretaceous clay and sandy soil outcrops inspected in the outskirts of Marrakech feature a strong superficial presence of salt layers due to evaporation. The city of Marrakech is in fact located in the Haouz plain whose alluvial deposits derive from the
Atlas mountain chain. Several watercourses such as the R’dat, Zat, Ourika, Rheraira and N’fiss originate from the High Atlas and all flow into the Oued Tensift, the main river draining the Haouz plain. Alluvial deposits of these rivers all contain varying amounts of salts originating from the geological formations of the High Atlas [5]. Chemical analysis of a few samples of deteriorated stone products (Table 4) has also shown the presence of small amounts of nitrites, nitrates and of phosphates, most probably of biological origin. All these salts are mobilised by rising capillary-water (reaching a visible maximum height of about 3 m), which corresponded not only to rainfall, but also to the water currently used for washing the road passing before and under the gate. The source of the salts may be multiple, from the brick-core of the wall and from the outside (e.g. bird’s droppings): all being very soluble, they easily migrate towards the surface. The sometimes strong exfoliation and flaking produced by the salts may be explained by their concentration (and magnified effectiveness) along the foliation of the rock and the following dissolution-crystallisation cycles that are very frequent in
Fig. 6. Detail of Fig. 5: note the brown patina and the serious exfoliation and flaking.
Fig. 8. UV photomicrograph showing the penetration of the brown patina and its yellow-greenish fluorescence. 16.
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Fig. 9. Mass-spectrogram of the patina of sample # PA 2 showing the residual presence of a few proteins.
the particular semi-arid climate of Marrakech, with considerable day-night variation of relative humidity. This mechanism starts with a micro-cracking parallel to foliation favoured by the fabric of the rock (well observed in deteriorated samples of the first stone-type, Fig. 2), which soon evolves into macro-cracking and then loss of foils and flakes. The exposure of the gate to the west, and the dark colour of the stones, may also favour some deterioration due to thermal stress, which may add to its overall decay. 3.3. Considerations for conservation treatments To decrease the speed and intensity of the deterioration phenomena occurring in the stone of the gate it is obviously first of all necessary to stop rising damp, the main vehicle transporting soluble salts within the stone and the other building materials. This may be achieved safely only by cutting the walls at the base of the gate and introducing a water barrier extending to the two flanking towers. This operation is not simple, owing to the considerable thickness of the walls, but it is possible by cutting them at the same level on both sides
Fig. 10. SEM micrograph of sample # PA7 showing decohesioned small flakes of the type-1 stone.
as is a current practice, for example, in Venice. A possible, less intrusive and alternative solution may be a drainage system at the base of the walls around the gate and the introduction of ventilation channels [10]. The following action should be the desalination of all deteriorated areas. This treatment is long and time-consuming, but absolutely necessary. It may be performed by repeated poulticing with a mud obtained by mixing deionised water with special clays, such as bentonite or sepiolite, or paper pulp [11,12]. A control of the progress of such treatment will be possible by checking the conductivity of the removed poultice [13]. This operation should lead to the removal of most of the salts present in the stone; it should be done by an experienced restorer, since very deteriorated areas may need the careful prior application of sheets of paper between the stone and the poultice, and in some areas even a pre-consolidation treatment with ethyl-silicate which does not completely fill the pores and is thus compatible with the subsequent removal of salts. A light cleaning of the surface from soot and other deposits (such as the removal of desert dust, bird droppings, etc.) is necessary for all the surface and may be done by washing with the minimum amount of water before the consolidation of the damaged areas. Such a treatment needs preliminary laboratory research to find out the best materials for the types of stones used in the gate, and for the preservation of the oxalate patinas. The deteriorated stone should in principle be structurally consolidated with ethyl silicate, if necessary by repeated applications: the silica thus deposited can glue together the loose particles and thin sheets of the powdering and flaking stone. The choice of ethyl silicate is based on the fact that this consolidant is very fluid, chemically compatible with the silicatic composition of the stones and, as shown by preliminary tests made on a small scale in the lab, it does not alter their colour. In the most difficult situations, e.g. in heavily flaking areas, the only possibility of success is to rely on the use of small amounts of strong adhesives, such as an epoxy resin, used to bridge together in a few points the two parts of a flake, and of a putty that may be made with stone powder mixed with ethyl silicate, with fibres added to strengthen the paste if necessary, and pigments to match the colour. Only in very limited
L. Lazzarini et al. / Journal of Cultural Heritage 8 (2007) 315e322
322 Table 4 Results of the chemical quantitative analyses PA1
þ
Li Naþ NHþ 4 Kþ Mgþþ Caþþ F Cl NO 2 NO 3 PO 4 SO4 C2O 4
PA2-2
PA3-2
PA5-2
Tot. cond. ¼ 25 mS
Tot. cond. ¼ 69 mS
Tot. cond. ¼ 75 mS
Tot. cond. ¼ 145 mS
%w/w
meq/100 mg
%w/w
meq/100 mg
%w/w
meq/100 mg
%w/w
meq/100 mg
e e e 0.02 0.01 0.82 e 0.02 e e e 0.14 e
e e e 0.05 102 0.08 102 4.09 102 e 0.06 102 e e e 0.29 102 e
e 0.50 e 0.03 0.06 1.02 e 0.59 e 0.59 0.14 0.27 e
e 2.2 102 e 0.08 102 0.49 102 5.09 102 e 1.66 102 e 0.95 102 0.44 102 0.56 102 e
e 0.63 e 0.05 0.14 0.66 0.02 0.41 0.01 1.31 e 0.69 e
e 2.7 102 e 0.12 102 1.15 102 3.29 102 0.11 102 1.16 102 0.02 102 2.11 102 e 1.44 102 e
e 0.74 e 0.30 0.06 1.81 0.02 1.12 e 2.96 e 0.42 e
e 3.2 102 e 0.77 102 0.49 102 9.03 102 0.11 102 3.16 102 e 4.77 102 e 0.87 102 e
areas with deep stone loss, should stone be replaced with the similar rock identified above. The reduced amount of rainfall in the area of Marrakech (less than 300 mm/year), the high temperatures reached by the stone surface in summer, and the overall strong UV radiation for many months of the year, make it inadvisable to apply any surface-protective treatment. Acknowledgements This research was conducted in the framework of the European Project INCO-MED (Concerted Action Contract ERB-IC18-CT98-0384, Prospective Study 3). The authors are grateful to Prof. Antonella Casoli of the University of Parma for the GC-MS analysis. References [1] G. Deverdun, Marrakech, des origins a` 1912, Rabat, 1966. [2] NORMAL 4/80, Distribuzione del volume dei pori in funzione del loro diametro, CNR-ICR Roma (1980). [3] NORMAL 13/83/UNI-Beni Culturali 11087 (2003), Dosaggio dei Sali solubili, CNR-ICR, Roma, 1983, Milano, 2003.
[4] B.W.D. Yardley, An Introduction to Metamorphic Petrology, Longman, Harlow, 1989. [5] R. Ambroggi, G. Thuille, Geological map of the Marrakech region, 1/ 500.000, Notes Me´m. Serv. Ge´ol., Maroc. (1952). [6] NORMAL 1/88, Alterazioni macroscopiche dei materiali lapidei: lessico, CNR-ICR, Roma, 1988. [7] L. Lazzarini, O. Salvadori, A reassessment of the formation of the patina called scialbatura, Studies in Conservation 34 (1) (1989) 20e26. [8] E. Borrelli, Salts, ARC Laboratory Handbook, ICCROM, Rome, 1999. [9] L. Lazzarini, General issues on the deterioration of stone, in: Proceedings of the Meeting ‘‘The Building Stone in Monuments’’, Athens-Mytilene 9e11/11-9-2001, Athens, 2002, pp. 149e160. [10] N. Spinosa, A.C. Alabiso, D. Pagano, D. Ferragni, P. Mora, G. Torraca, I. Massari, Il restauro del chiostro maiolicato di S. Chiara in Napoli. In: Le superfici dell’architettura: il cotto, caratterizzazioni e trattamenti, Libreria Progetto Ed., Padova, 1992, pp. 517e526. [11] L. Lazzarini, M. Laurenzi Tabasso, Il restauro della pietra, CEDAM, Padova, 1986. [12] S. Simon, Desalination by poulticing: laboratory study on controlling parameters, in: D. Kwiatkowski (Ed.), Proceedings of the 10th International Congress on Deterioration and Conservation of Stone, Stockholm, June 27eJuly 2, 2004, pp. 323-330. [13] E. Borrelli, Standardizzazione di un metodo di estrazione di sali da supporti porosi e suo utilizzo a scopo diagnostico, in: V. Fassina, H. Ott, F. Zezza (Eds.), The Conservation of Monuments in the Mediterranean Basin. Proceedings of the 3rd International Symposium, Padova, Venice, 22e25 June 1994, pp. 163e167.