Diverse types of bricks from monuments in the city of
Padua (northeastern Italy) were studied using a multi-analytical approach
based on spectrophotometry, X-ray fluorescence (XRF), X-ray powder
diffraction (XRPD), polarized-light optical microscopy (POM) and/or
high-resolution scanning electron microscopy with coupled energy-dispersive X-ray spectroscopy (HRSEM-EDS). The most
representative bricks were yellow or beige and in well-preserved condition.
The results showed that they were made of Mg- and Ca-rich illitic clays,
were fired at high temperatures (from 900 to over 950
The firing of raw clays entails a dynamic and disequilibrium system in which
two main stages take place: the decomposition of the original minerals of
the raw clays and the formation of new phases (Shoval, 1988; Riccardi et
al., 1999). The dehydroxylation of clay minerals contributes to the
development of an amorphous silicate phase that diminishes with increasing
temperature, as the newly formed phases nucleate and grow (Rathossi and
Pontikes, 2010; Heimann and Maggetti, 2019). The firing of carbonate-rich
clays gives rise to newly formed silicates from around 800
Clays with abundant carbonates tolerate a wide range of firing temperatures and result in ceramics with exceptional porosity, mechanical behavior and weather resistance (Kingery and Aronson, 1990; Tite, 1991). The reaction of carbonates and silicates at high temperatures yields newly formed phases such as melilites and pyroxenes (Peters and Iberg, 1978; Trindade et al., 2009; Cultrone and Carrillo, 2020). Despite the fact that Mg-rich clays may yield highly resistant ceramics at low firing temperatures (Lagzdina et al., 1998; Darweesh, 2001), much more research has been carried out on the decomposition and reaction products of calcite than on those of dolomite (Gliozzo, 2020).
In research on historic ceramics, in addition to the residual minerals from the raw clays and the new phases formed during firing, researchers have identified various secondary phases that crystallized after firing. These were related both to the transformation of such new phases and to the precipitation in the pores of chemical elements dissolved in aqueous solutions (Maritan, 2020). Zeolites and calcite – resulting from recarbonation, precipitation and/or the alteration of gehlenite – occur frequently as post-firing (i.e., secondary) mineral phases in Ca-rich ceramic bodies (Buxeda i Garrigos and Cau Ontiveros, 1995; Buxeda i Garrigos et al., 2002; Fabbri et al., 2014).
Multi-analytic studies of ancient bricks offer interesting insights into the transformations that took place within the ceramic bodies during or after firing and shed light on the manufacturing processes. In Padua, fired bricks have been widely used as a building material throughout the city's long history, and the most representative are typically yellow or beige in color. In previous research on this most representative type of brick, Pérez-Monserrat et al. (2021, 2022) found that (i) they were produced with highly calcareous raw clays that were quite rich in magnesium; (ii) they show abundant Ca- and Mg-rich high-temperature phases as well as secondary calcite and zeolite hydration products; (iii) they display good mechanical behavior and resistance to frost and salt crystallization action; and (iv) they are in a good state of conservation, especially those laid in areas subject to high levels of humidity. On the basis of these studies, a range of different bricks from historic constructions in the city are analyzed in this paper. The main aims of this research are (i) to carry out a detailed compositional and microstructural analysis of this representative type of brick in order to understand the formation mechanism of both the high-temperature phases and the secondary phases and (ii) to identify the production technologies that may have brought about the transformations that took place within these yellow- and beige-colored bricks.
Padua's central location in the Veneto region (northeastern Italy) and its numerous waterways fostered the development of the area, which has been a strategic region for trade since Roman times. The province of Padua is located on the eastern side of the Po Plain, which is mainly covered by Pleistocene and Holocene fluvio-glacial deposits. The fact that there are virtually no rocky outcrops in the vicinity and the abundant availability of raw clays have historically favored the use of fired bricks as the main building material in the city. The only nearby rock outcrops are in the Euganean Hills, from which trachyte and rhyolite have traditionally been extracted for construction purposes (Germinario et al., 2017).
The city of Padua was built on top of Quaternary alluvial deposits from the
Brenta and Bacchiglione rivers. The sand fraction from these deposits
contains, above all, silicates and carbonates (calcite and dolomite) as well
as metamorphic and volcanic rock fragments, while the clay fraction consists
mainly of montmorillonite, illite–montmorillonite, chlorite, illite and
kaolinite (Jobstraibitzer and Malesani, 1973). Carbonate leaching and
reprecipitation within the clayey sediments comprise a frequent pedogenic process
in the formation of the soils near the city (Cucato et al., 2008), yielding
impure carbonate fragments aggregated with clay materials and/or clayey
grains cemented by carbonates. This leaching and precipitation of
carbonates, together with alternating oxidizing and reducing conditions
during the Pleistocene–Holocene, resulted in the formation of calcic
horizons, characterized by the accumulation of CaCO
In the province of Padua, the climate is warm and temperate. The summers are
hot and humid, while the winters are very cold, and it is partly cloudy all
year round. There is significant rainfall throughout the year, with an
annual average of circa 1000 mm. The average temperature is 15
Forty fired bricks were sampled (taking fragments or entire bricks) from four emblematic constructions in the city of Padua: (i) the Basilica of Saint Justine (fifth/sixth–sixteenth centuries), from the early Christian (fifth–sixth centuries) and Romanesque (twelfth–thirteenth centuries) periods; (ii) the Elderly Tower (thirteenth century); (iii) the tower of the old castle of the Carraresi family, nowadays known as La Specola (fourteenth century); and (iv) the Renaissance walls (sixteenth century, samples were taken from various different sections of the perimeter). Overall, the bricks were in a good state of conservation. One-half were yellow or beige, and the other half were reddish, orange or brown (Table 1). On a freshly cut surface, some of the yellow and beige bricks displayed fine- and medium-grained inclusions within a compact, uniform matrix. Other samples, including the pale-red-colored and orange-colored bricks, showed large numbers of particles with very heterogeneous sizes. These were mainly lumps of clay and carbonate inclusions, contained within a matrix with a flowing texture, corresponding in turn to Fe-rich and carbonate clay materials. A few rounded or irregularly shaped pores were noted, some of which were partially filled by crystals.
The brick surface color was measured by spectrophotometry, using a portable
3nh NS800 spectrophotometer. On a circular measurement area of 8 mm
diameter, a D65 standard light source and 10
The chemistry of the ceramic bodies was analyzed by X-ray fluorescence (XRF)
with an S2 Ranger EDXRF Bruker AXS spectrometer with a Pd X-ray generator.
The working conditions were 50 kV, 2 mA and 50 W. Major and minor oxides
were determined on pressed pellets. Prior to the analysis, 7 g per sample
was ground to powder in an agate mortar. Loss on ignition was determined
gravimetrically as the weight loss recorded between 110 and
1000
The mineral phases in the samples were identified by X-ray powder diffraction (XRPD), using a PANalytical X'Pert PRO diffractometer in Bragg–Brentano geometry equipped with a Co X-ray tube and an X'Celerator detector. The results were interpreted with the X'Pert HighScore Plus software. The bricks were examined in thin sections by polarized-light optical microscopy (POM), using a Nikon Eclipse E660 microscope equipped with a Canon 650 digital camera and the Canon EOS digital microphotography system.
The microstructural features of the samples were described in detail by high-resolution scanning electron microscopy (HRSEM), using a Carl Zeiss STM (AURIGA series) and microanalysis by means of energy-dispersive X-ray spectroscopy (EDS). Fragments and polished thin sections coated with graphite were observed in back-scattered electron (BSE) and secondary electron (SE) mode.
List of the samples taken, their main textural features and
colorimetric parameters measured on a freshly cut surface. Abbreviations:
CL, clay lumps; CI, carbonate inclusions; FT, flowing texture;
On the basis of the chemical composition, all the samples are rich in CaO
(
Plotting of the chemical composition of bricks in ternary
diagrams. The samples are plotted within the carbonatic body (CB) and
carbonatic-high carbonatic body (CHCB) groups (except RW_11). See Tables 2 and 3 for entire sample numbering. Panels
The mineral phases detected by XRPD – relict components of the base clays,
new (during firing) and secondary (post-firing) phases – are shown in Table 2. Two main mineral assemblages were detected in the samples belonging to
the CB and CHCB groups: (i) high content in relict components (overall
quartz, illite and primary calcite) and very few high-temperature phases
(sub-groups CB
Mineralogical associations detected by XRPD analysis of the
brick samples. Mineral abbreviations after Warr (2021): quartz (Qz);
K-feldspars (Kfs); albite (Ab); illite (Ilt); calcite (Cal); gehlenite (Gh);
diopside (Di); anorthite (An); forsterite (Fo); hematite (Hem); aragonite
(Arg); analcime (Anl); gypsum (Gp). Secondary calcite (Cal (2nd)) was
estimated on the basis of microscopic analysis.
In chemical terms, the ceramic bodies are moderately rich in SiO
The high CaO
The chemical data obtained indicated that the brickmakers used silica-rich clays with considerable amounts of Ca carbonate, Mg carbonate and clay minerals. They also suggest the use of at least two compositionally different raw clays and/or two different recipes for preparing the ceramic pastes (mixtures of various different raw clays), one rich in silica and iron and the other with a high carbonate content.
Chemical composition of the major and minor oxides (in
wt %) of CB and CHCB bricks determined by XRF. Maximum, minimum, average
and standard deviation values of the samples belonging to both groups are
also shown. Abbreviations:
Even though the yellow (positive
A clearer distinction between the CB and CHCB groups can be observed when
their chromatic values are compared with their CaO, CaO
Chromatics of the CB and CHCB samples compared with their
CaO, CaO
The mineral phases identified by XRPD (Table 2) provide information on the
firing dynamics. This shows for example that the chemical reaction of
illite/muscovite dehydroxylation (Reaction R1) occurs between 450 and 780
The gehlenite detected corresponds to an intermediate compound of the solid-solution end-members. It falls somewhere between Mg-rich (åkermanite,
Ca
As the reactivity of lime is higher than that of periclase, the former is
consumed faster. From the excess of periclase, Mg silicates like forsterite
can nucleate as firing temperature rises, both at the expense of Ca–Mg
silicates (Reaction R10) (Trindade et al., 2009) and by the reaction of periclase with
silica (Reaction R11). These transformations occur via the following reactions:
Well-crystallized hematite is a significant component only after heating at
900
For the reactions reported above, the standard enthalpy of reaction (
The mineral assemblages detected point to the use of Ca- and Mg-rich
(calcareous) clays and Fe content in illitic clays, in concordance with the
general composition of the base clays obtained from the chemical data. On
the basis of the presence – and quantity – of phyllosilicates and newly formed
phases, it is possible to estimate the approximate temperatures at which the
samples were fired. The higher the illite concentrations – in line with the
lower amounts of newly formed phases – the lower the firing temperatures. The
samples with the largest amounts of illite were fired at less than 850
Analcime (a zeolite group mineral) and calcite secondary phases were also
detected in significant amounts. In previous research, zeolite was detected
by XRPD as a secondary hydration product in other ancient bricks used in
Padua, probably due to the city's high humidity and the use of calcite-rich
clays (Pérez-Monserrat et al., 2021, 2022). The hydration of the
abundant amorphous phase yielded by highly fired calcareous clays may foster
the formation of zeolites (Buxeda et al., 2002; Schwedt et al., 2006;
Maritan, 2020), as may the presence of alkaline fluids (Pacheco-Torgal et
al., 2008). The glassy phase is unstable under certain weathering conditions,
and, over time, its alteration yields to the leaching of potassium, which in
turn can produce leucite (KAlSi
As pristine carbonates and newly formed silicates cannot theoretically
appear together (Fabbri et al., 2014), the calcite detected in CB
Considering that more than half of the bricks studied belonged to the
CHCB
The particles are represented mostly by pristine Ca- and/or Mg-rich microcrystalline clayey lumps (Fig. 3a), Ca-plagioclase crystals, phyllosilicate pseudomorphs, and Mg-rich dark particles with bright reaction rims (Fig. 3b), formed by the reaction between the partially decomposed grain and the surrounding silicate groundmass (Heimann and Maggetti, 1981). Although Reactions (R6), (R7) and (R8) express the formation of anorthite as firing temperature increases, these Ca-plagioclase crystals mainly correspond to pristine plagioclase that acquired an anorthite-like composition during firing (Cultrone et al., 2014). In fact, plagioclases, with compositions from albite to andesite, are very abundant mineral components of the sands from the Brenta and Bacchiglione rivers, and the calcium terms are rare (Jobstraibitzer and Malesani, 1973). Quartz and K-feldspar relict grains with phase transformations at the edges can also be observed. It is suggested the formation of intermediate members of the åkermanite–gehlenite series from illite (Cultrone and Carrillo, 2020), and the open oval-shaped mica pseudomorphs with vesicles indicate that high temperatures were reached (Fig. 3c and EDS results).
As regards the mineralogical and microstructural changes that took place
during the firing of the CHCB
POM and HRSEM BSE images and associated EDS spectra
showing some features of the CHCB
High-magnification imaging analysis by HRSEM shows that, in many areas,
inclusions are bound by an amorphous phase (Fig. 3d) observed under POM as
compact and optically inactive portions (Fig. 3g). This phase largely
consists of the intermediate compound of the solid solution between the
åkermanite and gehlenite end-members detected by XRPD, and it has been
described as a melilitic composition matrix in other yellow bricks from
monuments in the city of Padua studied in previous research
(Pérez-Monserrat et al., 2022). Within this phase, which has a
Ca-aluminosilicate (and K-rich) composition, very abundant pyroxene-type
crystals with highly sharp edges crystallized, chiefly prism-shaped and
hollow skeletal crystals of diopside (Fig. 3e and f). The larger crystals
seem to have grown suddenly following self-organization and spiral growth
with slight misorientation at the interface. The crystals share a common
crystallographic orientation, in line with the “imperfect oriented
attachment” described by Penn and Banfield (1998). As a result, larger
crystals appear to have grown out of smaller crystallites, thus depleting the
calcium and magnesium from the matrix. Fe-rich bright crystals were also
detected and may correspond to a fassaite-type pyroxene (Fig. 4j), i.e., a
calcium–magnesium silicate with abundant aluminum and ferric iron, which
could also be referred to as a “ceramic pyroxene” (Dondi et al., 1998).
This generic clinopyroxene is formed between 800 and 1000
Wollastonite crystals were occasionally formed around rounded quartz grains,
which suggests that firing temperatures of over 950
The Ca- and/or Mg-rich microcrystalline clayey grains/lumps are probably related to the reprecipitation of carbonates within the pristine clayey sediments (Cucato et al., 2008) that were differentially transformed during firing, mainly depending on the original magnesium and calcium contents, the particle size, and the firing temperatures reached.
Different transformations of the Mg clayey grains can be observed: (i) early melilite rims and rings of Mg-rich crystals at the grain edges, with the pristine microcrystalline texture being quite well preserved at lower temperatures (Fig. 4d); (ii) varying degrees of nucleation achieved by the Mg-silicate crystals as the firing temperature increased (Fig. 4e); and (iii) inclusions transformed into Mg-silicate mineral phases with fully developed melilite reaction rims (Fig. 4f) at the highest temperatures. The higher the firing temperatures, the more transformed the pristine grains are. The formation of Mg-silicate crystals represents the highest degree of transformation as firing temperatures increase. As stated by Pérez-Monserrat et al. (2021), this differential transformation could be used as marker of both the firing temperatures and the base clay composition and provenance.
The microchemical analysis of these crystals indicates that the early
Mg-rich silicate crystals have a composition similar to that of a
monticellite (Ca(Mg,Fe)SiO
The Ca-rich clayey grains/lumps show a different decomposition path in which
porous areas appear, mainly due to CO
HRSEM BSE/SE images and associated EDS spectra of specific
features observed in the ceramic bodies.
Coarse and subrounded inclusions of calcite with a dark corona-like
microstructure (developed during firing) were observed in many samples
(Fig. 5a and b). These correspond to primary calcite from the base clays,
as the local sands are rich in limestones (Jobstraibitzer and Malesani,
1973). These inclusions were largely preserved during firing, only reacting
along the borders with the surrounding groundmass. Therefore, the initial
decomposition of the calcite grains yielded dark rims at temperatures of
700–750
Within the most altered areas of these coarse carbonatic grains, cubic and
fan-shaped crystals of CaK zeolite were formed (Fig. 5c). Even though the
presence of wairakite and/or chabazite-Ca as secondary hydration products
has been previously suggested, the substantial potassium and calcium
contents point more towards the formation of phillipsite-K
((K,Na,Ca)
Secondary calcite was significantly developed. Some recarbonated through the
micromass, but most precipitated within the pores. The recarbonated calcite
appears above all as clusters of sparitic and/or micro-sparitic crystals
scattered through the groundmass (Fig. 3g and h) and is probably the
result of carbonate and calcium transported by rainwater from the lime
binder mortars commonly used in the city's historic buildings (Secco et al.,
2018; Addis et al., 2019). The precipitated calcite partially fills the
pores (Fig. 5g and h), the needle-shaped crystals indicating that they
likely correspond to aragonite, and occasionally causes internal fractures
in the carbonatic microcrystalline grains. The sealing of the shrinkage rims
and the growth of calcite crystals on transformed carbonate grains (Fig. 5i
and j) can also be observed. Another interesting finding was the
precipitation in many samples of a calcite with a highly even texture (Fig. 3i). Besides being spread widely through the matrix, this smooth calcite
completely sealed the shrinkage rims (Figs. 4g and 5k) and even filled the
spaces left between the exfoliated basal planes of the phyllosilicates due
to dihydroxylation. Abundant scalenohedral crystals were formed, mainly
inside the pores (Fig. 5l and m). These morphologies, known as
It is possible that the precipitated calcite was formed both when the bricks were soaked in water and/or due to the circulation of Ca-rich solutions when the bricks were laid. During the firing, the CaO released by the decomposition of the carbonatic particles reacted with the silicate matrix to form the Ca-rich silicates. Although the free lime was almost eliminated due to the high temperatures reached, given the very high calcareous content of the pastes, it seems likely that the bricks were soaked in water just after firing. This practice considerably reduced the hydration of free lime (portlandite) and the further carbonation that takes place if bricks are exposed to the air after firing. In this way it helps prevent the formation of cracks due to the increase in volume caused by the formation of portlandite (Elert et al., 2003), defect known as “lime blowing”. Soaking the bricks in water removes the CaO, which is very reactive and has poor consistency. The CaO precipitates as calcite on the surface of the water and/or within the pores of the bricks, thus minimizing the risk of lime blowing (Saenz et al., 2019). In addition, the uniformity of the even-textured calcite that was widely extended through some of the samples suggests that it formed quickly, at almost the same time as the soluble lime was removed (or redistributed within the pores of the brick, where it crystallizes as a secondary phase). Given that the waters that flow through the city of Padua (in rivers, canals, underground waters, etc.) are Ca-rich, it seems likely that the calcite also precipitates from aqueous solutions often in direct contact with the buildings of which the bricks form a part.
POM and HRSEM BSE/SE images and some EDS spectra on
secondary zeolite and calcite.
The quite uniform chemical and mineralogical composition of the CHCB
The profuse nucleation of the pyroxene-type crystals indicates that a firing technique involving rapid heating and/or soaking was used. It seems that the heating rate was faster than the nucleation and growth of the phases (Grapes, 2006) and/or that some reactions remained incomplete due to the bricks being removed from the kiln too early. It must have been a highly reactive, supersaturated system (Ca-rich and also rich in Mg and Al), which was enhanced in turn by the fine grain of carbonate inclusions, where small pyroxene-type crystals grew fast and early by direct nucleation from the melt (the Ca-aluminosilicate amorphous phase). Likewise, the reactivity of the system was increased by the fact that the crystals were very small, in turn giving rise to very intensive nucleation. Although within a meta-stable system like raw clays under firing, reactions can be delayed. However, in this highly reactive, supersaturated system, some reactions may have taken place earlier. The occurrence of prism-shaped crystals, which had enough time to grow properly, and skeletal-shaped crystals, which did not, indicates that heat inputs of irregular intensity might have taken place inside the kiln. Uneven conditions like these were also reflected in the occurrence (and the quantity) of different mineral phases (Table 2). Therefore, low quantities of newly formed silicates may be due to a fast heating rate and/or excessively short heating time, whereas higher quantities indicate more accurate, better-quality firing, in which the reactions between lime and periclase and the silicate matrix were completed (Fabbri et al., 2014).
Despite this variability in the production process, it is clear that the bricks are in a good state of conservation, a situation that can largely be attributed to the base clays. The firing of the calcareous-rich clays caused the development of an extensive Ca-aluminosilicate amorphous phase during firing, which enhanced the binding action inside the bricks. In fact, the loss of this phase was observed in other similar bricks in the city, prompting the granular disaggregation of the bodies (Pérez-Monserrat et al., 2022). The presence of magnesium can increase the strength of ceramic products (Lagzdina et al., 1998), which is also improved by the copious nucleation of the diopside-type crystals.
The decarbonization of the calcareous-rich clays and the incipient vitrification achieved produced quite porous bricks. This highly porous texture made them ideal for use in areas with high levels of humidity (such as those in the city of Padua). It could also be beneficial for brick conservation as it facilitates the uptake and movement of water inside them. In addition, due to the surrounding humidity, it is possible that the moisture gradient between the surface of the bricks and the nearby environment was quite low, thus enabling the humidity conditions within the bricks to remain fairly constant. Furthermore, this high porosity yielded plenty of spaces that were suitable for precipitation, almost all of which were filled by secondary calcite, which significantly increased the cementation of the bricks. The Ca-rich waters of the city and the lime mortars used to bind the bricks together played an essential role in the precipitation of this secondary calcite.
This research centered around a multi-analytical study of fired bricks from
four emblematic heritage constructions in the city of Padua (northeastern
Italy). More than half the bricks studied are yellow or beige in color and
are well preserved. These bricks, corresponding to CHCB
The results suggest that the CHCB
The information obtained in this multi-analytical study has increased the
knowledge of (i) the mineralogical and microstructural transformations that
take place when very calcareous – especially rich in magnesium – and highly
unevenly textured clays are fired at over 900
No data sets were used in this article.
EMPM, LM and GC conceptualized the research and methodology and interpreted the results. EMPM carried out the formal analysis, supervised the investigations and wrote the original draft; LM facilitated spectrophotometry, POM and XRPD resources and revised the original draft; GC enabled the use of HRSEM-EDS resources and revised the original draft.
The contact author has declared that neither they nor their co-authors have any competing interests.
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This article is part of the special issue “Mineralogy of the built environment”. It is not associated with a conference.
Special thanks are due to Edi Pezzetta and Monica Pregnolato from the Archeology, Fine Arts and Landscape Surveillance Department of Venice's metropolitan area and Belluno, Padua and Treviso provinces; Domenico Lo Bosco from the City Hall of Padua; and Valeria Zanini and Nicola Di Cicco from the National Institute of Astrophysics Astronomical Observatory of Padua for the sampling authorizations. The help and courtesy of Giulio Pagnoni, abbot of Saint Justine Benedictine Monastery; Marie-Ange Causarano from the Cultural Heritage Department of the University of Padua; Elisa Pagan from the City Hall of Padua; and Stefano Tuzzato during the sampling operations are gratefully acknowledged. The authors would also like to thank Francisco Coruña and Sol López de Andrés from the Research Assistance Center of the Faculty of Geological Science of the Complutense University of Madrid (Spain) for the XRF chemical analysis; Leonardo Tauro, Chiara Dalconi, Federico Zorzi and Marco Favero from the Department of Geosciences, University of Padua, for the assistance provided during the thin-section preparation and XRPD analysis; and Jesús Montes from the Faculty of Sciences of the University of Granada and José Damián Montes from the Scientific Instrumentation Centre of the University of Granada for their help preparing samples. The support provided by Alicia González, from the Centre for Scientific Instrumentation of the University of Granada, during the HRSEM-EDS analysis is also deeply appreciated. The authors would finally like to thank Luca Valentini, from the Department of Geosciences of the University of Padua, for the references provided in order to perform the thermodynamic calculations; Nigel Walkington for improving the English language; and the two anonymous reviewers for their thorough reviews and suggestions.
This research has been supported by the Horizon 2020 Marie Skłodowska-Curie Individual Fellowship Action (grant no. 836122); the Consejería de Conocimiento, Investigación y Universidad, Junta de Andalucía, Spain (Research Group RNM-179); and the Ministerio de Economía y Competitividad, Spain (grant no. MAT2016-75889-R).
This paper was edited by Gilberto Artioli and reviewed by two anonymous referees.