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Available online at www.sciencedirect.com
Journal of the European Ceramic Society xxx (2013) xxx–xxx
The steeple spire of the Parma Cathedral: An analysis of the
glazed bricks and mortars
Elisa Adorni ∗
University of Parma, Department of Civil-Environmental Engineering and Architecture, via G. Usberti 181/A, 43124 Parma, Italy
Received 18 February 2013; received in revised form 9 April 2013; accepted 18 April 2013
Abstract
In October 2009, a terrible lightning struck the steeple spire of the Parma Cathedral, causing a fire. The fire-fighting operation made possible the
discovery of the original spire ceiling made up by dichromatic glazed bricks, white and black, dating from the 14th century. Original materials
presented a relevant decay, both for the high temperatures reached during the fire and for lack of maintenance. The research presents the first study
of glazed bricks of the 14th century in Po Valley (Italy) with the purpose of collect chemical, mineralogical and petrographic data on the dichromatic
glazed bricks. Brick samples with different kind of glazes and mortars exposed at different condition of fire were analyzed. The following techniques
were used in the study: X-ray powder diffraction, optical microscopy, scanning electron microscopy analysis, inductively coupled plasma atomic
emission spectroscopy and Raman spectroscopy. Glazes, applied on to Ca-rich paste, have a high lead content (41–57 wt%), with an high amount
of tin (19–24 wt%) for the white opacified glazes and manganese (about 4.0 wt%) for the black ones. Typological and historical analysis allowed
us to define the production technique of bricks and glazes. Mortars are mainly composed of lime binder and carbonate aggregate.
© 2013 Elsevier Ltd. All rights reserved.
Keywords: Glaze; Brick; Mortar; SEM-EDS; ICP-AES
1. Introduction
In October 2009, a lightning struck the spire of the Parma
Cathedral (Fig. 1). Following the fire-fighting operation was possible to discover the original roof of the conical spire, built in the
14th century with rounded profile brick elements with white and
black glazes (Fig. 2). The precious dichromate roof was covered
at the end of 16th century.
In literature, the glazing technique of the Asian, Syrian and
Egyptian areas was studied extensively: early Islamic,1,2 preIslamic3 and both.4 Several studies have been performed on
glazed ceramics from Spain and Portugal.5–8
A detailed description of the history and production techniques of glazing can be found in literature.2,10–12 In particular,
some studies analyze high lead glazes and alkali-lead glazes,2
tin-opacifier glazes,4,9 coloring,8,12,13 methods of glaze application, directly over the air-dried ceramic body or after a first
∗
Tel.: +39 0521 905961.
E-mail address: elisa.adorni@unipr.it
firing (biscuit-fired bodies)2 and the interaction between glazes
and the ceramic body.2,5
In Italy most of studies have been performed on glazed pottery
of the central regions,14 e.g. Gubbio and Orvieto,15 Siena,16 no
studies have been done on glazed bricks in the Po valley.
The literature also reports a series of studies to define the
firing temperature of ceramics, especially in archeology: mineralogical, textural and chemical transformation of clay-rich
materials during firing.17–19
The study presents the first original study of 14th century glazed bricks in Po Valley, North Italy. From the original
masonry a number of samples were collected, glazed bricks and
mortars, to characterize the original materials of the cover of
the ancient spire of the Parma Cathedral by determining the
production technique through a mineralogical, petrographic and
chemical analysis of bricks and glazes. In particular, the firing
process was defined by means of the study of the shape of the
bricks and the discovery of some defects of glazing.
14th century and 16th century mortars of the spire were also
characterized to define the provenance of the materials and the
original recipes.
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Fig. 3. (A) Dimensions and shape of the bricks. (B) and (C) Overlapping model
of the bricks.
Fig. 1. Panoramic view of the Cathedral of Parma before the fire. The spire still
had the copper covering, now removed.
2. Historical note
The Cathedral of Parma was edified starting from 1050 on
the site of an early Christian cathedral, which dates back to the
fifth century.
The edifice was built following a unique compositional
scheme with three naves completed by apses, a protruding
transept and a crypt, in agreement with the tradition of the
Romanesque churches in the Po area.
Fig. 2. Particular of the conic spire, realized with dichromatic glazed bricks:
white and black. The masonry shows deterioration, cracks and erosion of the
bricks.
The building of the bell tower began in 1284, when the tower
of the ancient cathedral was demolished. In 1291 the edifice was
progressed to the height of the castle of the bells.
Documents attest that the whole mural-architectonic parts
of the tower were already ended in 1294.20 According to the
Chronicle by Salimbene De Adam,20 the conical spire was built
after 1336 and before the 15th century.
The model of the conic spire, realized with curved radial
bricks (Fig. 2), was common in the Po Valley since the 13th
century: see the bell tower of the Cathedral of Piacenza and
the San Donnino Basilica in Fidenza, Parma. Other more recent
examples are the majolica spires in Sicily, documented since
1579 for the construction of the spire of S. Giacomo in Collesano
(Palermo), now collapsed.21
In Parma the primitive glazed brick masonry of the spire
endures to the end of the 16th century when, because of the
materials deterioration, spire was covered with a copper roofing.
The project, by Giorgio Edoari da Erba, was executed in 1596.
Since the end of the 16th century to the recent fire the original
covering was not visible.
The dichromatic brick decoration remained occulted for
centuries.22
The conic spire, 15 m high, was realized in masonry with
trapeze-shaped quoins, which have a curved external profile disposed in alternated courses (Fig. 3). The external part of bricks
is covered by a dichromatic glaze, white and black (Fig. 2). The
bicolor courses consist of 4 rows of white bricks and 4 of blacks.
Some cuneiform bricks are more tapered to close the circumference of the spire without an excessive amount of mortar and
to correct the radial position of the bricks.
The structure leans on a polygonal basement with 16 sides,
composed by regular courses of bricks which have smooth surface and reduced mortar joints. The masonry is about 33–35 cm
thick, i.e. the depth of the bricks which constitute the structure.
At the top of the spire a cone of stones (arenaceous stone and
Vicenza stone) supports a golden anemograph angel.
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2.1. Conservation state
After the fire, the whole copper cover was removed; except a
wooden beam of the basement, nothing of the ancient frame was
left. Iron nails remained linked to masonry; they were inserted
in the 16th century as anchorage for the cover to the masonry.
The ancient masonry structure was in poor state of preservation and, thus, the fire worsened an already advanced
deterioration (Fig. 2).
The masonry showed deep fractures and some areas presented
deep erosions and lacunas. Glazes showed superficial cracks and
alterations which were probably due to the high temperature
reached during the fire and by the following thermal shock produced by the water used to extinguish it. Glazes have portions
of copper (from the cover) recast on the surface, pink shades are
present on the surface, probably due to a reducing environment
during the fire (the fire burn up in the space between the copper
covering and the masonry), the glazes show small surface cracks
with detached parts and swellings.
Mortar was affected by decohesion (powder consistency) and
blackening on the surface caused by the thermal shock and the
smoke.
3. Analytical methods
Nine samples of glazed bricks of different colors, white and
black, six samples of mortars and two sample of plaster were
collected at different level from the steeple spire (Fig. 4).
3
Sampling was planned in order to represent the two construction phases of the spire: the building (14th century) and the later
intervention of covering (16th century).
Glazed bricks and mortars were investigated by petrographic,
mineralogical and geochemical methods with the aim of defining
glaze, brick and mortar characteristics. The methods of investigation were:
- In situ macroscopic observations; in particular, color, structure, glaze-brick interaction, surface aspect, manufacturing
techniques, and pathology.
- Optical mineralogical microscopy, transmitted and reflected
light (Nikon Eclipse LV100 POL), three thin sections for each
sample.
- Scanning electron microscopy (JEOL 6400, Jeol Ltd., Tokyo,
Japan) and energy-dispersive system microanalysis (Oxford
Isis 300, Oxford Instruments, Abingdon, UK) on polished thin
section, gold or carbon coated.
- X-ray powder diffraction (Philips PW-1710 diffractometer, Cu
K␣ radiation, [Philips Analytical Inc., Natick, MA]; Step Size:
0.05◦ .; Scan Rate: 0.750000; Scan Mode: Step; λ: 1.540562)
with XPowder Ver. 2004.04.47 Pro program (PDF2 data
base).23
- Raman spectroscopy (Jobin-Yvon Horiba LabRam Raman
micro-spectrometer).
- Chemical analysis of major and trace elements by Inductively
Coupled Plasma Atomic Emission Spectroscopy (ICP-AES –
FUS-ICP analysis method) of the bricks and mortar powder
(8 g each sample, binder and aggregate together).
Fig. 4. Survey of the spire with sampling location.
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Fig. 5. (A) Black drops of black glaze over some white glazed bricks. (B) Reconstruction of the stacked bricks in the kiln during firing.
Mortars characteristics, porosity, grain morphology and size,
have been determined using optical analysis, according to
the Italian Standard Normal 12/83.24 To estimate the aggregate/binder ratio, the Volume % estimation diagrams was
used25,26 ; image analysis was used to evaluate the porosity.27
4. Experimental results and discussion
4.1. Production technique
The bricks and masonry analyses allowed defining the
ancient production techniques. Through the survey of the bricks
(Fig. 3A) and removing part of the masonry during the restoration works, the assembly system of the masonry has been defined
(Figs. 3B and C).
The analysis of the characteristic shape of the bricks and the
discovery of horizontal glaze casting over some bricks (Fig. 5A),
helped to define the production and firing processes: the bricks,
still uncooked, were immersed manually in the glaze, stacked as
shown in Fig. 6B and fired in the kiln.
Is presumable that, to optimize the available space in the kiln,
the bricks were stacked close to each other. These suppositions
have been confirmed by mineralogical analysis of the bricks that
show an uneven single-firing.
Furthermore, the presence of horizontal sagging (Fig. 5A)
indicate that the white and black bricks were fired together,
stacked on each other.
4.2. Bricks
The ceramic paste is a highly calcareous clay, 19–21 wt%
CaO and 47–52 wt% SiO2 (Table 1), with an iron oxide contents
lower than 5 wt%. The chemical results show a substantially
homogeneous composition. In the calcareous clay bodies the
iron oxides are incorporated into calcium iron silicates, for this
reason the brick paste has a pale pink color, allowing a minor
quantity of opacifier into the glaze to cover the brick.2,5,14
Together with the chemical analysis, a mineralogical characterization of the paste was carried out. Powder diffractions reveal
significant amounts of quartz, residual calcite, neo-formed diopside and gehlenite, anorthite and sanidine in trace. Trace of
neo-formed diopside, gehlenite and anorthite were detected in
all the samples. The decomposition of calcite in oxidizing atmosphere occurs at a low temperature, above 800 ◦ C the free CaO
can react with clay and SiO2 , forming new phases: gehlenite, diopside and plagioclase (anorthite). Experimental studies
showed that gehlenite crystals begins to form at 800 ◦ C through
a reaction that occurs at the grain boundaries of CaO, Al2 O3 and
SiO2 and from 900 ◦ C it begins to disappear quickly.13,17,28,29
The presence of diopside, anorthite and gehlenite of neoformation is indicative of a firing temperature of about
900–950 ◦ C. The high content of calcite in the samples CD 1,
CD 2 and CD 14, together with high temperature phases, may
be indicative of an uneven firing, caused by a low circulation of
the air, a stacking of bricks (Fig. 5B) or a different position in
the kiln with respect to the fire.
The clayey matrix of the samples has a mainly homogeneous texture characterized by open and closed pores (pores
size: 0.02–4 mm), in a few cases they are of primary origin due
to the preparation of the dough, while for the most part are of
secondary origin, generated after the release of fluids during
decomposition of the same phase which produces them. In the
brick body are present quartz crystals, lithic fragments, altered
mica minerals, apatite minerals, Fe-oxides and-Ti-oxides. Minerals show zoned compositions, as a result of the firing process,
and coronal reaction, indices of a slow kinetic.30
4.3. Glazes
Brick samples consist of a portion of ceramic body with a
layer of high lead glaze (PbO 41–57 wt%) of two different colors: white and black. Glazes have been investigated by scanning
electron microscopy with EDS microanalysis to have information about their chemical (Table 2) and mineralogical features.
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Fig. 6. Sample CD 2. Throughout the black glaze are present thin skeletal crystals formed during the firing; the blacks grains are constituted by Si, Ca and Mn (ED
spectrum). Thus, the black pigment is attributable to a MnO–FeO compound.
Table 1
Major (oxide wt%) and trace elements (wt ppm) in the investigated bricks.
Samples
Brick
CD 2
CD 3
CD 4
CD 8
Mortar
CD 13
CD 18
CD 9
CD 12
CD 10
CD 16
SiO2
Al2 O3
Fe2 O3 (T)
MnO
MgO
CaO
Na2 O
K2 O
TiO2
P2 O5
LOI
Total
Ba
52.59
47.01
51.36
48.36
12.33
10.55
11.65
11.33
4.86
4.31
4.79
4.49
0.118
0.122
0.148
0.113
2.36
2.16
2.43
2.23
19.32
21.69
21.38
19.76
0.91
0.91
1.01
0.77
1.66
1.68
2.38
1.82
0.608
0.516
0.581
0.555
0.19
0.28
0.41
0.22
5.59
8.98
2.25
10.65
100.54
98.21
98.39
100.30
368
352
435
391
26.11
20.67
27.01
24.00
5.30
5.97
3.61
3.05
3.64
3.08
1.29
1.45
2.37
1.76
2.23
1.91
0.57
0.61
0.151
0.123
0.168
0.144
0.021
0.021
1.28
1.00
1.12
1.11
0.98
1.02
36.54
38.61
35.08
36.04
34.01
35.71
0.32
0.36
0.22
0.28
0.23
0.18
0.70
0.58
0.42
0.56
0.36
0.46
0.150
0.133
0.153
0.130
0.060
0.067
0.16
0.18
0.14
0.19
0.26
0.05
29.03
32.5
29.54
30.87
11.49
9.42
100.42
98.97
99.72
98.31
54.57
54.96
198
263
131
205
58
65
Sr
Y
Sc
Zr
V
588
672
750
597
25
23
24
24
12
10
11
11
151
139
144
136
82
79
76
88
1043
1054
655
780
1034
1076
12
12
13
12
2
3
5
4
5
4
2
2
49
36
45
41
22
19
44
42
53
58
17
17
wt% = per cent weight; ppm wt = part per million weight; LOI includes H2 O and CO2 ; T = total Fe as Fe2 O3 .
The analyses reveal a similar composition of white and black
glazes: samples have the same proportion of SiO2 –PbO and differ only for the presence of FeO–MnO in black glazes and tin
as opacifier in the whites.
The total alkaline content is less than about 1 wt%
(Na2 O + K2 O) and Al2 O3 about 1% (except CD 2), suggesting
the use in all the sample of a quite pure quartz sand for glazes
production5 .
White glazes belong to the class of ‘tin-opacified high lead
glazes’: in addition to lead, they have a high SnO2 content. Cassiterite crystals were added to improve glaze flux during firing
and to confer an opaque shade to the glaze layer.31
Table 2
Energy-dispersive X-ray spectroscopy microanalysis results of glazes (wt%).a
Samples
CD
CD
CD
CD
CD
CD
CD
1
2
3
6
8
11
13
B
B
W
W
W
W
W
Na2 O
MgO
AlO3
SiO2
K2 O
CaO
MnO
SnO2
FeO
PbO
0
0.34
0.61
0.71
0.62
0.71
0.95
0.40
0.90
0.34
0.36
0
0.38
0
1.00
5.17
0.81
1.06
1.27
1.05
2.07
34.60
33.01
29.40
27.64
28.16
30.11
33.75
0
1.41
0
0
0
0
0
2.52
10.02
2.40
2.68
1.99
2.55
2.57
3.80
4.20
0
0
0
0
0
0
0
24.30
21.08
21.94
21.35
19.38
0
2.80
0
0
0
0
0
57.48
42.19
42.14
44.34
46.02
43.85
41.28
a Normalized to 100%.
B = black color glaze; W = white color glaze.
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Fig. 7. Sample CD 6. (A) Interface: illite grains from the clay body show the growth of lead–potassium feldspar crystallites around. Within the glaze may be noted
bright inclusions of tin oxide. (B) Heterogeneous distribution of tin oxide crystals in the glaze.
The analysis of the samples showed that the composition and
microstructure are similar for all the white glazed, which could
suggest a common production technique. The tin content are in
the range 19–24 wt% SnO2 , with SnO2 /PbO ratios in the range
0.5–0.6 wt%.14 The content of SnO2 is very high compared
to the contemporary productions from Eastern Spain (13–14th
centuries), with SnO2 in the range between 6 and 7 wt%,5 to
the previous productions from Egypt, with tin oxide rates of
5–9 wt%4 and to the Italian Renaissance glazed pottery form
Gubbio and Deruta (about 5.8 wt% SnO2 ).15
SEM-EDS analysis evidence the uniform distribution of cassiterite crystals (SnO2 ), confirmed by Raman Spectroscopy.32
Tin oxide was first dissolved in silica and lead vitreous matrix;
during the firing process, tin oxide recrystallized to well develop
cassiterite crystals. Recrystallization of cassiterite was dependent on time and temperature. In some samples, CD 3 and CD 13,
the punctual SEM microanalysis revealed heterogeneity in the
distribution of SnO2 .
The study of geological features and the identification of the
quarries already existing at the time of the tower construction,
allowed to hypothesize that tin used could be sourced from the
mines of Campiglia Marittima (Tuscany), not far from Parma.33
Black glazes have an high lead content. From the analyses, the
samples have different composition (Table 2): CD 2 has an high
content of iron (FeO 2.8 wt%) and alkaline components, instead
absent in CD 1. Manganese content (MnO about 4.0 wt%) is
similar in the two samples and gives the black color, tinged
with brown, to the glaze.34,35 SEM-EDS analysis evidenced the
presence, especially near the external edge, of very thin skeletal crystals containing lead, Mn, Fe, and Si, formed during the
firing and black grains constituted by Si, Ca and Mn (Fig. 6).
Microanalysis and microscopic examination indicated that an
opacifier was not present in black glazes.
4.4. Body-glaze interface
During firing, interaction between glaze and clay body
produces an interface layer, that is strongly related to the clay
body typology, the temperature and time of firing.1,2,11 In particular, in dry unfired bodies, elements such as aluminum, iron,
potassium, calcium and magnesium diffuse from the body into
the glaze and, as the concentration of these elements in the
glaze increases, crystals of potassium–lead–aluminum-silicate
are formed at the body–glaze interface, this has been detected
Fig. 8. The thickness of glaze and interface layer. (A) Sample CD 6, unreacted quartz crystals and elongated potassium–lead–aluminum-silicate crystals are present
at the interface. (B) Sample CD 2, extended interface with coal particles, manganese dioxide and trace of Fe-oxide.
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Fig. 9. Glaze damages: (A) fracture parallel to the surface in the sample CD 6; (B) sample CD 4, glaze damages during the fire, with large irregular bubbles and
cracks.
in all analyzed samples (Figs. 6 and 7A). The development at
the interface is significantly greater when the glaze suspension
is applied to the unfired body.2 In the samples, tin-oxide crystals are heterogeneously distributed, suggesting a non-fritting
process for the glaze production or a not accurate preparation
technique5 (Fig. 7B).
Morphological analysis of these crystals and their “dragging”
into the glaze, allowed to hypothesize that, in the production
process, the cooling of the ceramics has been slow, probably
inside the kiln.36
Many sub-rectangular particles corresponding to quartz in
form of ‘unreacted quartz’ are detected at the interface glazebrick, these depend on the glaze composition, in this case the
SiO2 content is 12.9–16.9 wt%. In all samples, calcium incorporated into the glaze formed Ca-rich pyroxene.
The noticeable amount of bubbles in the glaze, produced by
release of carbon dioxide generated by decomposition of carbonate of the clay, and the large amount of crystals developed
at the interface indicate a single firing process (Fig. 8).
In the samples, the thickness of glaze and interface layer is
variable and irregular (Fig. 8): (1) glaze thickness: 30–700 m
and (2) interface layer: 20–150 m. White glazes have a higher
thickness than the black ones, with a more developed interface
layer.
A particular case concerns the sample CD 4, profoundly
altered during the blaze. The sample shows signs of re-firing,
with swellings and large irregular bubbles in the glaze and an
accentuated vitrification of the brick (Fig. 9B).
Notwithstanding the high lead glazes have a low coefficient
of thermal expansion than the alkali lead glazes, the few cracks
present in the glaze are probably due to the shrinkage of the
ceramic body during firing or to thermal shock suffered during
the recent fire (Fig. 9A).
4.5. Mortars
Joint mortars and repair mortars have been investigated by
optical microscopy, electron microscopy (SEM-EDS), ICP-AES
analysis and X-ray diffraction (Table 3) to have information
on their chemical (Table 1), mineralogical and petrographic
features.37,38
Plaster samples were took in correspondence of one of the
ancient scaffolding hole; mortar samples were taken at different
heights of the spire, sampling points are reported in Fig. 5.
Samples, despite having different compositions and mixtures,
are mostly composed of carbonates both in aggregate and binder.
On the basis of the sampling point, two main typologies of
mortars could be identified: mortars from the original masonry,
14th century, collected inside the masonry; samples taken in
correspondence of the covering structure installed in the 16th
century.
The ancientest mortars (CD 13, CD 15, CD 18; 14th century) are composed of a lime-based binder with carbonate and
silicate aggregate, in a 1:1 ratio. The interaction between binder
and aggregate is poor and reaction rims are not developed around
grains boundaries. Small amounts of phyllosilicates, talc and
Table 3
Principal mineralogical composition of the mortars (XRD).
Sample
CD
CD
CD
CD
CD
CD
13
15
9
12
10
16
Phases
Anhydrite
Calcite
Chlorite
Gypsum
Phyllosilicates
Quartz
–
t
–
t
+++
+
+++
+++
+++
+++
+
++
–
–
t
t
–
–
t
+
t
–
+++
+++
++
t
+
+
–
–
+++
+++
++
++
–
–
+++ = major; ++ = small; + = minor; t = trace.
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Fig. 10. (A) Sample CD 13. Microfossils (Pliocene gastropoda) (optical microscope, 10×, transmitted light, parallel polarizers). (B) Sample CD 9. Ophiolite grain
found in the mortar of the end of the XVI century (optical microscope, 10×, transmitted light, crossed polarizers).
microfossils are found in all three samples (Fig. 10A). By optical analysis and visual estimation,25,26,38 the size of the grains
(aggregates size: 0.2–3 mm) and the pores (macropores size:
0.1–2 mm) are homogeneous.
Mortars, CD 9 and CD 12, employed during the installation of copper roofing in the 16th century, are very similar
in composition and grain size distribution (aggregates size:
0.08–6 mm), with a good interaction between binder and aggregate, as observed at the carbonate–phyllosilicates interface; high
porosity (macropores size: 0.1–0.8 mm) and fracture are present.
The ratio between binder and aggregate is visually estimated to
1:2. Ophiolite grains are present in both the samples, reveling the
use of aggregate maybe from Parma Apennine area (Fig. 10B).
As shown by chemical analysis, for samples CD 9, CD 12,
CD 13, CD 18, CaO is mostly related to carbonates suggested
by the high value of LOI.
Plaster samples CD 10 and CD 16 have predominant content
of gypsum and anhydrite (Table 3), as observed by XRD and
SEM-EDS analysis and also showed in the chemical analysis
by the high content of CaO as well as low LOI and high Sr
values (Table 1). In the samples, the dimension of the aggregates
varies from 0.1 to 7 mm, with a macroporosity (size > 1/16 mm)
of 10–15%. The binder and aggregate ratio is 1:2.
The presence of anhydrite in CD 15, CD 12, CD 16 and
especially in CD 10 (Table 3) could be related to the gypsum
dehydration as effect of the recent fire.
5. Conclusions
The results of this study allow characterizing the dichromatic
glazed masonry of the conical spire of the steeple of the Parma
Cathedral and defining the production techniques of the 14th
century glazed bricks.
Samples consist of a portion of calcareous clay body with a
layer of high lead glaze of two different colors: white and black.
White glazes belong to the class of ‘tin-opacified high lead
glazes’, analysis reveal an high tin content (19–24 wt% SnO2 ),
probably extracted from Campiglia Marittima tin mines in Tuscany (Italy), not far from Parma. The high content of SnO2
distinguishes this production from the contemporary from Eastern Spain, from the Italian Renaissance glazed pottery and from
the previous Egyptian production, which have lower values
(SnO2 about 5–9 wt%).
For black glazes, the color tinged with brown is conferred
by the addition of manganese, without trace of opacifier. The
samples have different composition: compared to CD 1, CD 2
contains iron (2.8 wt%) and alkali.
The calcareous paste has a low content of iron oxides and a
pale pink color.
Traces of neo-formed gehlenite, diopside and anorthite,
detected by XRD, together with calcite suggest a uneven firing temperature, about 900–950 ◦ C. Little difference of firing
temperature could be probably due to poor air circulation in the
kiln and closeness between the bricks.
The interface between brick and glaze has a variable thickness
of 20–150 m, in general higher in the white glazes.
The thickness of the interface and the presence of crystals
of potassium–lead–aluminum silicate are indicative of a single cooking, with glaze applied on an un-fired body. Unreacted
quartz are also present at the interface and into the glaze, together
with bubbles generated by carbonate decomposition of the clay.
Mortars of the 14th and 16th centuries have been analyzed.
All the samples are composed of a lime-based binder with different aggregate, in particular: the mortar of 14th century (CD 13,
CD 15, CD 18) has a low content of silicate, small amounts
of phyllosilicates, talc and microfossils and poor interaction
between binder and aggregate; the mortars of the 16th century
(CD 9 and CD 12) are very similar in composition and grain size
distribution, with a good interaction between binder and aggregate, aggregates contain ophiolite grains, probably deriving from
Parma Apennines.
Plaster samples, CD 10 and CD 16, have predominant content of gypsum and anhydrite, the latter could be related to the
gypsum dehydration as effect of the recent fire.
Acknowledgments
Thanks to Silvia Simeti and Stefano Volta of Archè Restauri
society for the essential and kind assistance in the building site.
A special thanks to professor Judit Molera of the University of
Vic for her essential advices and corrections and to professor
Please cite this article in press as: Adorni E. The steeple spire of the Parma Cathedral: An analysis of the glazed bricks and mortars. J Eur
Ceram Soc (2013), http://dx.doi.org/10.1016/j.jeurceramsoc.2013.04.019
+Model
JECS-9176; No. of Pages 9
ARTICLE IN PRESS
E. Adorni / Journal of the European Ceramic Society xxx (2013) xxx–xxx
Giampiero Venturelli of the University of Parma for his
suggestions during the research.
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Please cite this article in press as: Adorni E. The steeple spire of the Parma Cathedral: An analysis of the glazed bricks and mortars. J Eur
Ceram Soc (2013), http://dx.doi.org/10.1016/j.jeurceramsoc.2013.04.019