Insight into the conservation problems o (1)

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Insight into the conservation problems of the
stone building “ Bab Agnaou”, a XII cent.
monumental gate in...
Article in Journal of Cultural Heritage · July 2007
DOI: 10.1016/j.culher.2007.02.002

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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: lorenzo@iuav.it, lama@iuav.it (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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L. Lazzarini et al. / Journal of Cultural Heritage 8 (2007) 315e322

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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L. Lazzarini et al. / Journal of Cultural Heritage 8 (2007) 315e322

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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L. Lazzarini et al. / Journal of Cultural Heritage 8 (2007) 315e322
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