Journal of Cultural Heritage 10 (2009) 248–257
Case study
Vaterite in the mortars of a mosaic in the Saint Peter basilica,
Vatican (Rome)
Cesare Fiori a,1 , Mariangela Vandini a,∗,1 , Silvia Prati b,2 , Giuseppe Chiavari c,3
a
Dipartimento di Storie e Metodi per la Conservazione dei Beni Culturali, Alma Mater Studiorum, Università di Bologna,
via degli Ariani 1, 48110 Ravenna, Italy
b Microchemistry and Microscopy Art Diagnostic Laboratory (M2ADL), Alma Mater Studiorum, Università di Bologna,
via Guaccimanni 42, 48100 Ravenna, Italy
c CIRSA, Alma Mater Studiorum, Università di Bologna, via S. Alberto 163, Ravenna, Italy
Received 16 January 2008; accepted 30 July 2008
Abstract
The vaults of the Saint Peter basilica in Vatican (Rome) are decorated with mosaics whose realisation is dated to the end of the 16th century. The
mortar layers beneath the mosaics are realised with the so-called “Roman stucco”, a kind of mastic specifically employed as a binder in the mosaic’s
supporting layers. Its empirical recipe was known and reported by 18th century authors, accounting for the use of lime, travertine powder added to a
mixture of herbs and linseed oil. A recent restoration of the mosaics has allowed to characterise the mortars from a compositional point of view by
individuating the inorganic mineralogical fraction and by chemically characterising the organic components. The study of stucco samples has been
performed through polarising microscope observations, X-ray diffraction (XRD) analyses, thermal analyses (TA) (thermodifferential-DTA and
termogravimetric-TGA analysis), Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy, pyrolysis-gascromatography
(PY-GC)–mass spectrometry. The analyses have allowed to distinguish between original stucco, produced and utilized at the same time of the
realisation of the mosaics, and other mortars, presumably employed in later times during restoration interventions. The outcomes of the mineralogical
investigation and TA indicate the presence of four different types of stuccos, here considered as four characteristic groups. The mineralogical analyses
indicate that all the samples are constituted of two main phases: calcite and vaterite and the TA, beyond the quantification of the calcium carbonate
content, have shown the presence of organic components in the stucco. The organic fraction was characterised by PY-GC–mass spectrometry,
confirming the presence of the linseed oil cited in the ancient recipes. The very interesting outcome of this study is the occurrence of the rare
calcium carbonate polymorph vaterite. The ATR-FTIR spectroscopy on the stucco gives further contribution to a better understanding of the FTIR
spectrum of the rare mineral and an explanation of its formation is tentatively given.
© 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Saint Peter basilica; Vatican; Mosaic mortars; Roman stucco; Vaterite; Linseed oil; X-ray diffraction (XRD) analyses; Thermal analyses (TA); Pyrolysisgaschromatography (PY-GC)–mass spectrometry; ATR-FTIR spectroscopy
1. Research aims
A recent restoration of the mosaics that decorates one of
the web beneath the dome of the Saint Peter basilica in Rome
(Fig. 1) has allowed to study the mortars of the layers support-
∗
Corresponding author.
E-mail addresses: [email protected] (C. Fiori),
[email protected] (M. Vandini), [email protected] (S. Prati),
[email protected] (G. Chiavari).
1 Tel.: +390544936711; Fax: +390544936717.
2 Tel.: +390544937152; Fax: +390544937159.
3 Tel.: +390544937311; Fax: +390544937411.
1296-2074/$ – see front matter © 2009 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.culher.2008.07.011
ing the mosaic made of glass tesserae and realised at the end
of the 16th century (1598–1599). Although the recipes of the
so-called “Roman stucco” are written and known, the bedding
mortars of the mosaics had never been characterised. The aim
of this work is the characterisation of the mortars by individuating the inorganic mineralogical fraction and by chemically
characterising the organic components: this allows to distinguish
between original stucco, produced and utilized at the same time
of the realisation of the mosaics, and other mortars, presumably
employed in later times during restoration interventions. The
very interesting outcome of this study is the occurrence of the
rare calcium carbonate polymorph vaterite. An explanation of
its formation is tentatively given.
C. Fiori et al. / Journal of Cultural Heritage 10 (2009) 248–257
2. Introduction
The sample collection has been realised during the restoration intervention in 2005. All the collected samples were termed
as “Roman stucco”, referring to a kind of mastic specifically
employed as a binder in the mosaic’s backgrounds. The recipe
of this kind of stucco is known. The use of the Roman stucco
has an ancient origin that goes back to Gerolamo Muziano in
the 16th century [1]. In a document of the Archivio della Reverenda Fabbrica di San Pietro (archive of the reverend edifice
of Saint Peter), an anonymous writer in 1764 describes this
stucco or “mastic” without a precise recipe [2]: “Questa composizione fu trovata nel milleseicento e tanti quali viene composto
di calce, polvere di travertino e olio di lino” (this composition
was found in the 17th century and is made of lime, travertine
powder and linseed oil in equal parts). Another document of
the same archive, dating back to the 16th century [3], explains
the details to prepare the mastic: “Non adoperano stucchi ma
colla fatta come segue: pigliano calcina viva di sasso fatta di
recente perchè riesce migliore e la bagnano e l’adoperano subito
perchè fa miglior opera ma prima la depurano con setazza e la
mischiano per metà con la seguente materia. Pigliano erba detta
malmischio, orzo con scorza e semenze di lino, di tutto ugual
porzione e fanno bollire insieme con acqua fino che si consumino
i due terzi della medesima, questa mista colla suddetta calcina
fa colla buonissima per il lavoro e questi si servono. . .” (they
don’t use stuccos but glue made as follows: they take a recently
made caustic lime and, after purification by sieving, they wet and
use it immediately because it comes out better, then they mix it
with an equal part of the following material. They take a kind
of grass called “malmischio”, barley with bark and linseeds, in
equal parts, and boil them together in water until the two thirds
of it are eliminated; mixing this material with the above mentioned lime makes a very good glue for the work and they use
it. . .). This compound had to season for several months and,
249
on application, it was grounded to dust and mixed with linseed
oil.
3. Experimental
3.1. Materials and methods
The study of a dozen of stucco samples collected from
different parts of the mosaics, has been performed through
polarising microscope observations, X-ray diffraction (XRD)
analyses, thermal analyses (TA) (thermodifferential-DTA and
termogravimetric-TGA analysis), ATR-FTIR spectroscopy,
pyrolysis-gascromatography (PY-GC)–mass spectrometry.
Polarising microscopy has been carried out on thin sections
by a Wild optical microscope with 40, 100 and 400 magnifications.
X-ray diffractometry was carried out on powdered samples
manually pressed on an Ag sample holder in a Rigaku Miniflex
diffractometer employing CuK␣1 radiation, in the range 2:
4–64 ◦ , scan speed: 1◦ /minute.
TA were conducted in air flow on powdered sample (about
10 mg) by a TA instruments Q600, ramp from room temperature
to 1100 ◦ C at 20 ◦ C/minute.
ATR-FTIR spectra were acquired in the range of
4000–400 cm–1 on a Bruker Tensor 27 spectrometer with a
Harrick MVP2 single reflection 45 ◦ angle of incidence ATR
accessory with diamond crystal. A total of 32 scans were
recorded and the resulting interferogram averaged, with a resolution of 4 cm–1 .
For PY-GC–mass spectrometry, samples (about 10 mg) were
inserted into a pre-pyrolysed quartz capillary tube and analysed
both in normal pyrolysis and after the addition of 5 ␮L of tetramethylammonium hydroxide 25% in water (Aldrich® analytical
reagent).
Pyrolysis was carried out at 600 ◦ C for 10 seconds at the
maximum heating rate using a CDS 1000 pyroprobe heated filament pyrolyser (Chemical Data System, Oxford, USA) directly
connected to the injection port of a Varian 3400 gas chromatograph coupled to a Saturn II ion trap mass spectrometer (Varian
Analytical Instruments, Walnut Creek, USA). A Supelco SPB5
capillary column (30 m, 0.25 mm I.D., 0.25 ␮m film thickness)
was used with a temperature programme starting from 50 ◦ C
(held for 10 minutes) to 300 ◦ C (held for 10 minutes) with a
temperature ramp of 5 ◦ C/minute in with helium as carrier gas.
Temperature of split/splitless injector (split mode) was kept at
250 ◦ C. Py-GC interface was kept at 250 ◦ C. Mass spectra were
recorded at one scan per second under electron impact at 70 eV,
scan range 45 to 650 m/z. Structural assignment of the products
was based on match with the NIST 1998 mass spectra library.
3.2. Results and discussion
Fig. 1. Mosaic decoration in one of the webs beneath the San Pietro dome; *:part
subject to the restoration.
The mineralogical analysis with XRD indicates that all
the samples are constituted of two main phases: calcite and
vaterite. They are two different crystalline forms (polymorphs)
of calcium carbonate (hexagonal system for calcite, hexagonal
space group P 63 /m mc for vaterite [4]). On the basis of some
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Fig. 2. Sample of group SP0; a: XRD analysis; b: thermodifferential (DTA) and termogravimetric (TGA) analysis.
noticeable compositional differences, the set of 12 samples can
be divided into four main groups of stucco: SP0, SP1, SP2
and SP3. One sample has been chosen to be representative of
each group, whose characteristics are given below. The diffractograms and the TA graphs of the four samples representative
of original fragments and of restoration materials are reported
(Figs. 2–5).
The crystalline phase identification through XRD investigation has revealed that all the samples are made of the two main
phases calcite and vaterite and of small quantities (or traces)
of quartz; the sample representing group SP2 contains also an
appreciable amount of gypsum.
The thermogravimetric analysis (Table 1) has allowed to calculate the calcium carbonate content (sum of the quantities of
calcite and vaterite) through the weight loss due to the decomposition that occurs in the range 600–800 ◦ C and the weight loss
attributable to the organic component of the stucco (i.e. residues
of linseed oil, of seeds and of herbs utilised). The organic frac-
tion weight loss, that occurs in the range 200–550 ◦ C, indicates
the amount of volatile substances released during heating and
burning of the above mentioned organic materials and do not
represent the whole organic fraction content, since it has to be
taken into account that the ash residue after combustion is not
detectable by TA.
Table 1
TGA analysis: percentage weight losses.
Sample 20–200 ◦ C 200–500 ◦ C 500–600 ◦ C 600–850 ◦ C Notes
SP0
1.35
1.45
0.42
41.61
SP1
SP2
1.71
3.76
2.04
3.42
0.46
0.59
39.88
36.16
SP3
4.22
11.43
0.72
35.00
CaCO3 ≈ 94%
magnesite tr.
CaCO3 ≈ 91%
CaCO3 ≈ 82%
gypsum ≈ 8%
CaCO3 ≈ 79%
C. Fiori et al. / Journal of Cultural Heritage 10 (2009) 248–257
251
Fig. 3. Sample of group SP1; a: XRD analysis; b: thermodifferential (DTA) and termogravimetric (TGA) analysis.
A quantitative estimation of the contents of calcite and
vaterite through XRD analyses, has been obtained by the addition method by known pure calcite additions for each sample,
taking also into consideration the sum of the two calcium carbonates, as determined by TA.
On the basis of the diffractometric and TA outcomes, the
compositional characteristics of the four groups, chosen to be
representative of the compositional variability of the whole set
of the samples under investigation can be described as follows:
• SP0: from in situ observations and restorers’ evaluations, this
material corresponds to the stucco originally employed for
the realisation of the mosaics; it contains 94% of calcium
carbonate and presents a weight loss due to organic material
combustion around 1.5%. The probable occurrence of traces
of magnesite is indicated by a small endothermic effect on
the DTA curve, around 430 ◦ C (the content of the compound
is below the detection limit of our XRD apparatus). The estimated content of vaterite is between 11.7 and 14.3%;
• SP1: it represents a kind of stucco presumably employed in
ancient restoration interventions; it can be macroscopically
distinguished from the previous group since it contains coarse
stone fragments (size of a few centimetres), indeed its compositional character – excluding the stone fragments – is very
similar to that of the preceding group, i.e. 91% of CaCO3 and
weight loss due to the organic component around 2%. The
estimated content of vaterite is fairly low, around 6.5–8.5%;
• SP2: the material of this group is definitely attributable to
restoration interventions carried out in later times since it
comes from a bedding mortar overlapped to the original layer;
it is a very degraded material powdery and de-coherent; it
contains 82% of calcium carbonate and shows an organic
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Fig. 4. Sample of group SP2; a: XRD analysis; b: thermodifferential (DTA) and termogravimetric (TGA) analysis.
weight loss of about 3.5%. It also contains 8% of gypsum.
The content of vaterite is the lowest found, corresponding to
a range of 5–7%;
• SP3: it is probably the most recent restoration material; it
contains the highest organic fraction (relative weight loss
higher than 11%) and the lowest content of calcium carbonate
(CaCO3 around 79%). In this case, the estimated content of
vaterite is almost as high as in the sample SP0, being included
in the range 10.8–13.4%.
The polarizing microscope thin section observations put into
evidence some noticeable differences between sample SP0 and
SP1. The main difference regards the inert material powder:
in SP0 grounded travertine (Fig. 6a) was used whereas in SP1
marble powder can be clearly recognised by the well-grown
crystals of calcite (Fig. 7a). Other differences concern the higher
homogeneity of SP0 with respect to SP1 (Figs. 6 and 7b,c).
In the sample SP2, gypsum is recognised as an additional
component added to the traditional mix of the Roman stucco.
Since its content is relatively high, whereas it is not detectable
in the other nearby groups of samples although exposed to the
same environmental conditions, this component is considered
an additive to the stucco and it is not an alteration product of the
calcium carbonate.
ATR-FTIR spectroscopy was performed on powdered samples representing the four groups. In the collected spectra, the
absorption peaks of the carbonates occupy substantially the same
positions for the four typologies of materials (Fig. 8). The presence of gypsum is confirmed in the spectrum of the sample SP2
(1140–1080 cm–1 , SO4 2− stretching band and ∼605 cm–1 bend-
C. Fiori et al. / Journal of Cultural Heritage 10 (2009) 248–257
253
Fig. 5. Sample of group SP3; a: XRD analysis; b: thermodifferential (DTA) and termogravimetric (TGA) analysis.
ing band). The presence of linseed oil is shown in the spectrum
of the sample SP3 (and secondly in that of SP2) by the presence of 3600–3200 cm–1 (OH, R-OOH) and 3000–2800 cm–1
(C–H) stretching bands, the former being characteristic of the
linseed oil due to the presence of oxi-polymerised triglycerides
[5]. The SP3 and, although less evidently, the SP2 samples show
two peaks in the region 2360–2320 cm–1 range, attributable
to CO2 [5]; however, the intensity of these peaks is proportional to the content of organic material in the samples and
perfectly overlapping to the same feature shown by the spectrum
of a modern commercial linseed oil (refined extract of linseed
Maimeri – Italy, product code 650). The main difficult is to distinguish between calcite and vaterite since they are both present
in all the samples of the four groups and the two minerals have
very similar spectra. FTIR spectroscopy data about vaterite have
been reported in literature [6–11]. By comparing the spectra with
the data found in the literature and basing our discussion on the
review reported by White [12], three peaks in our spectra are
definitely to be considered of interest in the definition of the
non-calcite calcium carbonates: a broad peak between 1087 and
1070 cm–1 , corresponding to the ␯1 symmetric stretching [5–12]
and within which amourphous calcium carbonate has also been
associated [13], and two bands at 848 cm–1 [7] and 745 cm–1
[9–12], the former close to the 864 cm–1 assigned to amorphous
calcium carbonate [13] and the latter being well assigned to
the presence of vaterite by Andersen and Kralj [11]. This result
grants a confirmation of the presence of vaterite in the stuccos
and gives a further contribution to a better understanding of the
FTIR spectrum of the rare mineral.
PY-GC–mass spectrometry has been performed in methylating conditions in order to characterise the organic substances.
In SP0 and SP1, the low amount of organic materials (1.5 and
2% respectively as determined by thermogravimetric analysis)
did not allow its identification. In SP2 and SP3, the detection of
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C. Fiori et al. / Journal of Cultural Heritage 10 (2009) 248–257
Fig. 6. Polarising microscope micrographs of a sample from group SP0; a: 40×, nicols//; b: 40×, nicols//; c: 100×, nicols//.
azelaic, palmitic, oleic and stearic acids confirm the presence of
linseed oil (Fig. 9) [14–16].
3.3. Vaterite formation
Calcium carbonate appears in nature in various crystalline
solid forms both anhydrous (calcite, aragonite, vaterite) and
hydrated polymorphs (calcium carbonate monohydrate and
ikaite); besides, an amorphous form is also reported [6,13].
Among the three anhydrous polymorphs, calcite exhibits an
hexagonal structure and it is the stable phase at room temperature and atmospheric pressure; aragonite, with orthorhombic
structure, is the high pressure phase; vaterite, with distinctive
hexagonal structure [4], is the less stable polymorph [17]. The
three polymorphs are different also with respect to their solubility, vaterite being the most and calcite the least soluble phase
over a range between 0 ◦ C and 90 ◦ C [7]. Even if calcite is the
thermodynamically stable modification under standard conditions into which all the metastable forms transform, a mixture
of polymorphs can be found, depending on the chemical environment in which precipitation takes place. In nature, aragonite
and vaterite preferentially form in the biomineralisation process in marine organisms [9,10]. The physicochemical processes
involved in the precipitation of CaCO3 have been extensively
studied. Nonetheless, some of the mechanisms leading to the
formation of one or another polymorph are not fully understood.
Many studies have dealt with crystallization and transformation
mechanisms of calcium carbonate polymorphs from aqueous
solutions [6–8,17–24], with particular interest to the biomineralization process, through which CaCO3 is naturally formed
in vivo. CaCO3 polymorphism is a very important problem in
biomineralization since many organisms are able to exert control over the mineral form and all the three anhydrous crystalline
forms have been observed in mollusc shells and fish otoliths
macromolecules [9,10]. This means that vaterite, although characterised by the lowest stability, can form and stabilize under
favourable conditions.
From highly supersaturated solutions the precipitation of
CaCO3 occurs through nucleation and growth of metastable
forms followed by transformation into more stable phases
according to the Ostwald’s step rule [6]. Vaterite precipitates in
supersaturated solutions that are far from equilibrium [10]. The
previous mentioned studies have generally identified vaterite as
the initial crystalline form precipitating in high superstauration
conditions [18]. After an amorphous phase is formed vaterite
and calcite nucleate simultaneously; the former, being unstable,
should transform in the thermodynamically stable calcite, but
several studies have demonstrated that suitable conditions can
stabilize the vaterite phase. Although the mechanisms are not
clearly understood, a key role is played by factors such as temperature, pH and degree of supersaturation [19]. Tai and Chen
[18] found that, at room temperature, vaterite is obtained at a
C. Fiori et al. / Journal of Cultural Heritage 10 (2009) 248–257
255
Fig. 7. Polarising microscope micrographs of a sample from group SP1; a: 40×, nicols//; b: 40×, nicols//; c: 100×, nicols //.
pH lower than 10, being the dominant polymorph in moderate
supersaturation and pH between 9.0 and 9.5.
Furthermore, stabilisation of vaterite is influenced by various additives (surfactants, polymers, proteins). It has been
shown that vaterite is formed on polymeric substrates in
the presence of carboxylate groups [8]. Jada and Jradi have
studied the influence of anionic polyelectrolites on size and
shape of calcite and vaterite crystallisation from supersaturated solutions [20] and various studies have demonstrated
that the formation of vaterite is favoured by the presence of
amino acids and alcohols. In the presence of the amino acids
alanine, glycine, lysine, polyglycine, polymethionine, polylysine [21] and leucine [22] in supersaturated solutions vaterite
becomes stable. Similarly, ethanol, isopropanol and diethylene
glycol stabilize the vaterite phase, preventing its transformation into calcite [7] and Xie et al. [23] have demonstrated
that high concentrations of polyethylene glycol promote the
formation of vaterite. In addition, in his studies on crystallisation and transformation mechanisms of calcium carbonate,
Kitamura [24] has found that also magnesium ions suppresses
the transformation of vaterite by inhibiting the growth of calcite.
To the authors’ knowledge, the only case of occurrence of
vaterite in cultural heritage materials is that reported by Prinsloo
in her study about African San rock art [25]. Vaterite is recognised in the precipitates from the urine of rock hyraces in the
salts deposed on the rocks. Vaterite and monohydrocalcite were
formed where the urine of a species of rock hyrax was in contact
with the faeces. In the urine/faeces mix, the presence of various
substances (such as abundant organic molecules, phosphates,
magnesium, urea, potassium chloride) are thought to be respon-
Fig. 8. FTIR spectra of the samples from the four investigated groups.
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C. Fiori et al. / Journal of Cultural Heritage 10 (2009) 248–257
degradation products. The stabilisation of vaterite is definitely
promoted by the presence of the organic compounds and it
depends on the seasoning conditions of the stucco. Although
the main contribution to the presence of calcite in the stucco is
derived from the travertine powder added as a inert component,
a part of this polymorph calcite found in the binder could also be
formed as an initial precipitate or as a product of the metastable
vaterite transformation.
Acknowledgements
Fig. 9. Chromatograms arising by pyrolisis methylation of (a) sample from SPS2
group; (b) sample from SPS3 group A: azelaic acid; P: palmitic acid; O: oleic
acid; S: stearic acid.
sible of calcite growth inhibition, thus promoting the formation
of the two rare calcium carbonates.
In our case, the occurrence of this rare polymorph can be
associated with the presence of the organic compounds from the
additives employed in the preparation of the stucco. Vaterite
was probably formed during the very first step of precipitation of the calcium carbonate from the binder lime, prior to
hardening reactions and it did not completely transform into
calcite. The organic fraction of siccative oil and cellulosic
residues and their degradation products acted as stabilizer for
the vaterite polymorph, partially suppressing its transformation
into calcite. The alteration of organic components could also be
the result of microorganisms metabolic activity. In our study,
the amount of vaterite does not appear to be directly related
to the organic matter content: the two samples with higher
vaterite are the stucco with highest organic fraction and the
most ancient stucco with the lowest detectable organic component. Our hypothesis is that the stabilisation of vaterite is
definitely promoted by the presence of the organic compounds
and strongly dependant on the seasoning conditions of the
stucco.
4. Conclusions
This work reports a characterisation of the so-called “Roman
stucco”, a kind of mortar employed in the bedding layers of
the 16th century mosaics of the San Pietro basilica in Vatican (Rome). Apart from the importance and relevance of the
site, the outcomes of the analyses show the occurrence of the
very rare calcium carbonate polymorph vaterite. To the authors’
knowledge, it is the first time that this is reported for mosaic
mortars.
Vaterite was probably formed during the very first step of
precipitation of the calcium carbonate from the binder lime and
can definitely be associated with the presence of the organic
compounds from the additives employed in the preparation of
the stucco (siccative oil and cellulosic residues) and from their
This project was financially supported by the University of
Bologna Strategic Project: “Archaeometrical study of ancient
glass”. The authors are indebted to Claudia Tedeschi who
restored the mosaics and provided the samples and to Carlo
Stefano Salerno for fruitful discussions on stucco’s recipes.
They also would like to thank His Excellency Most Reverend
A.Comastri – president of Fabbrica di San Pietro (F.S.P.) – Archpriest of the Saint Peter basilica, His Excellency Most Reverend
V.Lanzani – delegate of the F.S.P. – president of the Study of
the mosaic, Dr. Carlo-Stella – office supervisor of the F.S.P.,
Dr.Gabrielli – advisor for the restoration interventions of the
F.S.P. and Dr. Di Buono – Responsible of the Study of the mosaic
of the F.S.P.
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