RESEARCH ARTICLE

Application of spectrometric analysis to the identificationof pollution sources causing cultural heritage damage

C. M. Belfiore & D. Barca & A. Bonazza & V. Comite &

M. F. La Russa & A. Pezzino & S. A. Ruffolo & C. Sabbioni

Received: 12 February 2013 /Accepted: 6 May 2013 /Published online: 7 June 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Black crusts are recognized to have been, up tonow, one of the major deterioration forms affecting the builtheritage in urban areas. Their formation is demonstrated tooccur mainly on carbonate building materials, whose inter-action with an SO2-loaded atmosphere leads to the transfor-mation of calcium carbonate (calcite) into calcium sulfatedihydrate (gypsum) which, together with embedded carbo-naceous particles, consequently forms the black crusts onthe stone surface. An analytical study was carried out onblack crust samples collected from limestone monumentalbuildings and churches belonging to the European builtHeritage, i.e., the Corner Palace in Venice (Italy), theCathedral of St. Rombouts in Mechelen (Belgium), andthe Church of St. Eustache in Paris (France). For a completecharacterization of the black crusts, an approach integratingdifferent and complementary techniques was used, includ-ing laser ablation inductively coupled plasma mass spec-trometry (LA-ICP-MS), Fourier transform infraredspectroscopy, optical and scanning electron microscopy. In

particular, the application of LA-ICP-MS permitted to ob-tain a complete geochemical characterization in terms oftrace elements of the black crusts from the inner parts tothe external layers contributing to the identification of themajor combustion sources responsible for the deteriorationover time of the monuments under study. In addition, theobtained results revealed a relation between the height ofsampling and the concentration of heavy metals and provedthat the crust composition can be a marker to evaluate thevariation of the fuels used over time.

Keywords European built heritage . Black crusts . LA-ICP-MS analysis . Heavy metals . Environmental pollution

Introduction

The formation of black crusts and damage layers on carbon-ate stones in urban areas is a complex and dynamic phe-nomenon that induces their degradation (Amoroso andFassina 1983; Bonazza et al. 2007a; Brimblecombe 1999,2000; Del Monte et al. 1981; Turkington et al. 1997; Zappiaet al. 1998). In fact, buildings and monuments in urban andindustrial areas are soiled and “blackened” for the accumu-lation of atmospheric pollutants due to human activity. From1950s until today, initially because of the use of differentfuels in industrial plants, domestic heating, and transport,later thanks to the introduction of low-sulfur fuels andmitigation policies, a change in the atmospheric composi-tion occurred and is still ongoing, mainly related to theemissions of prevailing pollutant sources. Today, emissionsfrom mobile combustion sources are among the main agentsresponsible for the pollution encountered in most westernEuropean cities, and the negative effects of traffic areexpected to continue and multiply in the future.Undoubtedly, these changes will have an impact on theblackening and surface deterioration of historic monuments

Responsible editor: Philippe Garrigues

Electronic supplementary material The online version of this article(doi:10.1007/s11356-013-1810-y) contains supplementary material,which is available to authorized users.

C. M. Belfiore :D. Barca :M. F. La Russa : S. A. RuffoloDipartimento di Biologia, Ecologia e Scienze della Terra(DiBEST), Università della Calabria, Via Pietro Bucci,87036 Arcavacata di Rende, CS, Italy

C. M. Belfiore (*) :V. Comite :A. PezzinoDipartimento di Scienze Biologiche, Geologiche eAmbientali–Sezione di Scienze della Terra,Università di Catania, Corso Italia 57,95129 Catania, Italye-mail: cbelfio@unict.it

A. Bonazza : C. SabbioniIstituto di Scienze dell’Atmosfera e del Clima, ISAC-CNR,Via Gobetti 101,40129 Bologna, Italy

Environ Sci Pollut Res (2013) 20:8848–8859DOI 10.1007/s11356-013-1810-y

and buildings located in urban centers, implying the forma-tion of damage layers of different chemical compositionwith respect to the past (Ausset et al. 1998; Bonazza et al.2007a; Brimblecombe and Grossi 2006; Ghedini et al. 2006;Sabbioni and Zappia 1992a; Sabbioni et al. 1996a; Saiz-Jimenez 2004, 1993). The pollutants are transferred tobuilding material surfaces by two mechanisms: (a) dry de-position, which proceeds by aerodynamic transfer of gassesand particulate to material surfaces without the aid of hy-drometeors and (b) wet deposition, consisting in the transferof trace gasses and particles occurring in an aqueous form(i.e., rain, snow, or fog; Brimblecombe 2001; Camuffo et al.1982; Ghedini et al. 2006; Marinoni et al. 2003; Saiz-Jimenez 1993). The dry and wet deposition of particulateand gasses (particularly SO2) on the carbonate surface, inthe presence of water and catalysts such as carbonaceousparticles and heavy metals, allow the chemical reactions thattrigger the formation of black crusts (Maravelaki-Kalaitzakiand Biscontin 1999; Maravelaki et al. 1997). Several au-thors (e.g., Bityukova 2006; Fobe et al. 1995) pointed outthat the analysis of trace elements occurring in the blackcrusts can help to understand the influence of some pollut-ants in their formation. Accordingly, an exhaustive geo-chemical study of black crusts can be crucial to planstrategies for the protection and maintenance of monumentsand historic buildings (Ghedini et al. 2006; Sabbioni andZappia 1992b). In this work, we investigate black crustssampled from monuments located in three differentEuropean urban contexts, i.e., Venice (Italy), Mechelen(Belgium), and Paris (France), which undergo differentkinds of pollution due to diverse environmental conditions.The selected monuments are: (a) the Corner Palace inVenice, (b) the St. Rombouts Cathedral in Mechelen, and(c) the Church of St. Eustache in Paris. The purpose of thiswork is to evaluate the type and degree of deteriorationwhich affect the investigated monuments and determinewhich are the pollutants mainly causing the damage layersformation. Several analytical techniques have been used,including polarized optical microscopy (OM), scanning

electron microscopy coupled with energy dispersive X-rayspectrometry (SEM-EDS), Fourier transform infrared spec-troscopy (FT-IR), and laser ablation inductively coupledplasma mass spectrometry (LA-ICP-MS). The latter, insome recent works (Barca et al. 2010, 2011; Comite et al.2012), had been proved to be a powerful tool in the study ofblack crusts, since it allows to detect and quantify traceelements with high sensitivity in a short span of time andusing small amounts of sample. Moreover, this techniqueallows to obtain information about heavy metals’ concen-trations that can be considered as indicators of the environ-mental pollution affecting the built heritage.

Materials and methods

Black crusts examined here had been sampled from relevantbuildings and churches (Fig. 1) of three European cities(Corner Palace in Venice, St. Eustache Church in Paris,and St. Rombouts Cathedral in Mechelen), as part of theresearch activity carried out within the EC projectCARAMEL “Carbon content and origin of damage layersin European monuments (2001–2003)”, which aimed at thediscrimination and quantification of carbon fractions (organ-ic and elemental) within black crusts from several Europeanmonuments as tool for determining the stationary and mo-bile combustion sources major responsible for the blacken-ing and soiling encountered (Bonazza et al. 2005, 2007b;Saiz-Jimenes et al. 2004). In this work, the interpretation ofresults of a deeper chemical and mineralogical–petrographiccharacterization is presented and discussed. As already in-dicated by Bonazza et al. (2005), the three monuments wereselected for their historical–artistic importance, the locationin urban contexts characterized by different prevailing pol-lution sources and the stone materials they are built (i.e.,limestone). Unfortunately, information on possible previousrestoration works on the considered buildings are not avail-able. The Corner Palace in Venice (Fig. 1a) represents apeculiar site, being located in an area with local pollution

Fig. 1 a Corner Palace in Venice (Italy), b Cathedral of St. Rombouts in Mechelen (Belgium), and c Church of St. Eustache in Paris (France)

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primarily due to boat, industrial, and traffic emissions fromMestre, Porto Marghera, and Murano areas (Masiol et al.2012; Toscano et al. 2011). The edifice, in both Romanicand Venetian styles, was built in the second half of the XVIcentury by the rich and influential Cornaro family. Thepalace, facing on the Canal Grande, opposite theGuggenheim Museum, nowadays hosts the head office ofthe Provincial Prefecture and Administration of Venice. Thebuilding material used for the construction of this edifice isthe Istria stone, a compact and highly resistant ivory lime-stone, quarried in Istria and widely used in the past, partic-ularly in the Venetian Republic (Bonazza et al. 2005).

The Cathedral of St. Rombouts (Fig. 1b) is located inMechelen (Belgium), a city next to the heavily industrial-ized area around Brussels and Antwerp, one of the mostpolluted regions in Europe. The cathedral undergoes also theimpact of the city-center vehicular traffic (Bonazza et al.2005; Fobe et al. 1995; Saiz-Jimenes 2004). The Cathedralof Mechelen is an ancient Gothic building, whose oldest partis represented by the eastern wall, dating to 1217. Theconstruction of the tower started in 1452 and lasted fornearly two centuries. Like many ancient buildings in north-ern Belgium and in the Low Lands, the St. RomboutsCathedral was built with local materials, i.e., Begelem andGobertange stones. In this work, we focused the attention onthe Begelem lithotype, a yellowish gray sandy limestone ofthe Tertiary age. It is a typical building stone of the Gothic,Renaissance, and Baroque monuments of Flanders, butquarries were exhausted meanwhile (Saiz-Jimenez et al.2004). The Church of St. Eustache in Paris (Fig. 1c) islocated in the Halles district, an area closed to vehiculartraffic since 25 years ago. Designed by the Italian architectDomenico da Cortona, it was built between 1532 and 1637and has a ground plan similar to that of the Gothic Cathedralof Notre Dame. The main building material of this church isrepresented by the so-called Parisian Lutetian stone (knownalso as “Pierre de Curville”), a gray fine-grained limestonewhich was also used for the construction of some other

relevant monuments in Paris, such as the Cathedral ofNotre Dame and the Louvre museum (Lefrévre et al.2007). As regards the Corner Palace, among the 12 blackcrusts originally collected, 3 samples referred at differentheights in the main façade, have been selected for this study.Instead, just one sample from the Cathedral of St. Romboutsand one from the Church of St. Eustache have been collected(for in situ operating difficulties) and here examined (Table 1;for sampling details see also Bonazza et al. 2005, 2007a).

For a complete characterization of black crusts, an ap-proach integrating different and complementary techniqueswas used. The microscopic techniques include polarized OMand scanning electron microscopy coupled with SEM-EDS.Observations by OM were carried out on polished thin sec-tions under a Zeiss Axiolab microscope (equipped with adigital camera), in order to characterize both substrate andblack crusts and investigate the substrate/black crust interface.SEM-EDS analyses were performed on polished transversalsections to obtain information on the micromorphology andchemical composition (in term of major elements) of the blackcrusts. Analyses were performed with a Tescan Vega LMUscanning electron microscope equipped with an EDAXNeptune XM4 60 microanalysis working in energy-dispersive spectrometry, with an ultrathin Be window to en-sure lower detection limits (of the order of 0,1 %) for allelements analyzed. Operating conditions were set at 20-kV138 accelerating voltages, 0.2-mA beam current, 100-s acqui-sition time, and 30–35 % dead time. Precision was better than1 % for major elements and better than 3 % for minor ele-ments. Accuracy was of the same order of magnitude asprecision. FT-IR investigations were carried out to identifythe mineralogical phases constituting the examined damagelayers. The spectrophotometer used was a Perkin ElmerSpectrum 100, equipped with an attenuated total reflectance(ATR) accessory. Infrared spectra were recorded in ATRmode, in the range of 500–4,000 cm−1 at a resolution of4 cm−1. Chemical analyses of black crusts in terms of traceelements were performed by LA-ICP-MS. This method can

Table 1 List of monuments selected for sampling, along with a brief description of samples collected, details about the sampling sites and the typeof substrate

City Monument Sample location Samples Site and height of sampling Description of crusts Substrate

Venice Corner Palace Main facade CV5 From a vertical surfaceat 25 m

Compact black crust Istria stone(micritic limestone)

CV8 From the external horizontalsurface on an arch at 15 m

Compact black crust

CV12 From the external horizontalsurface on an arch at 5 m

Compact black crust

Mechelen Cathedral of St.Rombouts

Main facade CM From the main facadeat 1.5–2 m

Dendritic black crust Begelem stone(sandy limestone)

Paris Church of St.Eustache

North Terrace EP From the north terrace,at 40 m

Dendritic black crust Parisian Lutetian stone(biosparitic limestone)

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investigate a great number of elements with spot resolutions ofabout 40–50 μm, also allowing the determination ofmicrometric compositional variations (Gratuze 1999; VanderPutten et al. 1999; Wyndham et al. 2004). Analyses werecarried out using an Elan DRCe instrument (PerkinElmer/SCIEX), connected to a New Wave UP213 solid-stateNd-YAG laser probe (213 nm). Samples were ablated by alaser beam in a cell following the method tested by Guntherand Heinrich (1999). The ablation was performed with spotsof 40–50 μmwith a constant laser repetition rate of 10 Hz andfluence of ∼20 J/cm2 (Barca et al. 2010; 2011). Calibrationwas performed using the NIST 612-50 ppm glass referencematerial as external standard (Pearce et al. 1997). Internalstandardization to correct instrumental instability and driftwas achieved using CaO concentrations from SEM-EDS anal-yses (Fryer et al. 1995). Accuracy was evaluated on BCR 2Gglass reference material and on an in-house pressed-powdercylinder of the standard Argillaceous Limestone SRM1d ofNIST (Barca et al. 2011). The resulting element concentra-tions were compared with reference values from the literature(Gao et al. 2002). Accuracy, as the relative difference fromreference values, was always better than 12 %, and mostelements plotted in the range of ±8 %. With the aim of betterinvestigating the geochemical variability of black crusts fromthe external to internal portions, analyses were performed on100 μm thick cross-sections. A variable number of spot anal-yses were performed on each sample, depending on the thick-ness of black crusts.

Results

Optical microscopy analysis

Petrographic characterization was performed on all samples,except for CV8 and CV12 due to an insufficient amount ofavailable material, including both building substrate andsuperimposed damage layer. The stone from the CornerPalace (Fig. 2a) can be classified as a micritic limestone(Folk 1959). It is composed of microcrystalline calcite(crystals size of <4 μm) with some bioclasts (<10 %) andappears rather compact without any porosity detachableunder microscope. The analyzed black crusts contain micro-crystalline gypsum into an orange film having a thickness ofabout 200 μm. A high amount of carbonaceous particles andreddish iron oxides was also recognized. The layer is firmlyattached to the substrate which appears to be not particularlyaltered, with only sporadic penetrations of the black crustinto the stone. The Begelem stone (Fig. 2b) from theCathedral of St. Rombouts can be classified as a biosparite,according to Folk (1959). This lithotype is composed ofmacrocrystalline calcite (crystals size≈400 μm) and onlysporadic (<7 %) bioclasts (foraminifera). The substrate

appears strongly degraded because of the presence of manyfractures (some of which partially filled with microcrystal-line gypsum) and dissolution phenomena, these latter gen-erating a secondary porosity (around 20–25 %). The crust,containing few carbonaceous particles and reddish iron ox-ides, does not appear continuous along the surface of thesubstrate and its thickness varies between 300 and 100 μm.

Thin-section observations of the stone from the Church ofSt. Eustache (Fig. 2c) allowed to classify it as a biospariticlimestone (Folk 1959). The allochemical component mainlyconsists of bioclasts (foraminifera≈20%) and rare small-sizedquartz crystals. The substrate is strongly fractured, so thatmicrocrystalline gypsum often penetrates inside the rock andgrows with its typical acicular habitus. Because of this evident

Fig. 2 Photomicrographs representative of the investigated substratesand relative black crusts on the surface (plane polars, 5×). CV5 CornerPalace (Venice), CM Cathedral of St. Rombouts (Mechelen), EPChurch of St. Eustache (Paris)

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degradation state, the secondary porosity is about 25 %. Theblack crust shows an irregular external profile, whereas thecontact with the substrate is continuous. Its thickness variesfrom 250 to 950 μm. It is composed of microcrystallinegypsum and a high amount of carbonaceous particles andreddish iron oxides, among which some quartz grains andportions of the substrate are also recognized.

SEM-EDS analysis

SEM analyses provided detailed information on the mor-phological features of examined crusts and allowed to betterinvestigate the interaction with substrates. Observations re-vealed different morphological characteristics of the crustsamples both in relation to the three monuments taken intoaccount and, in the case of Palazzo Corner, the differentheight of sampling (Fig. 3). As regards Corner Palace,sample CV5 (Fig. 3a) displays a crust with thickness rang-ing between 400 and 200 μm, with a homogeneous mor-phology and well adhering to substrate. Different sphericalparticles’ typologies (both smooth and porous) were identi-fied. CV8 sample (Fig. 3b) has a crust with a homogeneousmorphology and thickness of about 200 μm. It does notproperly adhere to the underlying substrate due to the pres-ence of numerous fractures. Gypsum crystals along with fewsmall particles can be identified within the crust. CV12sample (Fig. 3c) shows a crust consisting of four distinctlayers, with variable thickness and morphology. The mostexternal layer (indicated with number “1” in Fig. 3c) has a

small thickness (varying from about 50 to 100 μm) anddisplays the presence of acicular gypsum crystals togetherwith many spherical particles all along the surface. Thesecond layer (“2”) is thicker (180–380 μm) than the previ-ous one and has a darker color. The two layers closest to thesubstrate have a similar thickness (between 130 and250 μm), but different morphology. Specifically, the thirdlayer (“3”) has abundant particles and gypsum crystals,whereas the fourth one (“4”), which is at direct contact withthe substrate, appears to be strongly fractured and displaysfew gypsum crystals, while particles are totally lacking. Thecrust from the Cathedral of St. Rombouts (Fig. 3d) shows amoderate thickness, ranging between 40 and 150 μm, andhave a discontinuous development along the stone surface.The substrate has numerous fractures, containing gypsumcrystals and spherical particles with both smooth and porousmorphology. The crust from the Church of St. Eustache(Fig. 3e) has a thickness variable between 250 and950 μm and is highly heterogeneous due to the presenceof alternating dark and light zones, the latter showing theoccurrence of gypsum crystals and spherical smooth andporous particles. The crust is well adhering to the underlyingsubstrate that appears to be strongly degraded (manymicrofractures can be observed). EDS analyses carried outon the same samples permitted to achieve information on thechemical composition of examined damage layers. Resultspoint to a quite similar composition, in terms of majorelements, for all crusts from the different monuments andfrom the three sampling sites in the case of Corner Palace.

Fig. 3 BSE-SEM microphotographs of the examined samples: CV5(a), CV8 (b), and CV12 (Corner Palace, Venice) (c); CM (Cathedral ofSt. Rombouts, Mechelen) (d); EP (Church of St. Eustache, Paris) (e).

The holes caused by LA-ICP-MS spot analyses (carried out on blackcrusts and unaltered substrates) are also visible

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Within all crusts, CaO and SO3 are the major components,followed by SiO2, Al2O3, Na2O, and lesser amounts ofMgO, K2O, Cl2O, Fe2O3, and P2O5. These results point togypsum as the essential component of black crusts, asexpected. In addition, for EP and CV12 samples, the darkportions observed (Fig. 3) showed a high C content proba-bly consisting in elemental carbon.

FT-IR analysis

Infrared spectra of all black crusts (Fig. 4) exhibited thecharacteristic absorption peaks of gypsum (CaSO4·2H2O)centered at 1,109, 667, and 596 cm−1, as well as the distinctivesignatures of O–H stretching and bending vibrations at 3,525,3,492, 3,401, and at 1,682 and 1,627 cm−1, respectively.

Furthermore, the stretching vibrations of calcite (refer-able to the substrate), with peaks at 1,409, 871, and710 cm−1, were identified in the sample CM taken fromthe Cathedral of Mechelen. Finally, in the crust CV5 fromthe Corner Palace, the presence of calcium oxalate (peakedat 1,324 and 780 cm−1) was also recognized, thusconfirming the data published in Bonazza et al. (2007a).

LA-ICP-MS analysis

Analyses through LA-ICP-MS have been performed alongselected crust–substrate profiles, with the aim of investigatingthe possible migration towards the substrate of those elementsoccurring within the crust and achieving a better understand-ing of the major stationary and mobile combustion sourcescausing the damage of architectural surfaces over time. Thenumber of spot analyses carried out in each sample varieddepending on the thickness and morphology of the crust. Thefull dataset (including the average concentrations for each

black crust and substrate sample and the relative standarddeviations) is available as electronic supplementary material.

Corner Palace

Analyses have been performed on both substrate and crustof the three samples. Results obtained for the three blackcrusts have shown a general enrichment in heavy metals(As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Sn, Ti, V, andZn), compared to the substrate. Among these, the followingelements resulted to be particularly enriched: Fe (maximumand minimum average values (in ppm): 12,600 in CV12-4and 5,264 in CV5, respectively), Cu (maximum and mini-mum average values: 173 in CV12-4 and 46 in CV8, re-spectively), As (maximum and minimum average values:224 in CV8 and 23.3 in CV12-2, respectively), Cr (maxi-mum and minimum average values: 7,124 in CV12-1and17.8 in CV8, respectively), Ni (maximum and minimumaverage values: 97.2 in CV12-1and 14.68 in CV12-4, respec-tively) and V (maximum and minimum average values: 203 inCV12-1 and 39.7 in CV12-4, respectively). The average con-centrations (ppm) of the same elements in the unaltered sub-strate were 1,158 for Fe, 18.5 for Cu, 9.04 for As, 12.6 for Cr,4.80 for Ni, and 11.13 for V.

As regards Pb and Zn, these resulted to be enriched onlyin the inner layers of sample CV-12 (3 and 4) (Pb withmaximum and minimum average values: 12,401 in CV12-4 and 375 in CV12-3, respectively; and Zn with maximumand minimum average values: 2,901 in CV12-4 and 303 inCV12-3, respectively), whereas the outer layers of CV-12 (1and 2), CV5 and CV8 show lower Pb and Zn concentrationsthan the substrate (130 for Pb and 265 for Zn).

Cathedral of St. Rombouts

Even in this case, the black crust displayed higher concentra-tions than the substrate for almost all heavy metals and par-ticularly for the following elements: Fe (38,843 and 535 ppmin crust and substrate, respectively), Zn (3,437 and 44.8 ppmin crust and substrate, respectively), Pb (1,737 and 2.95 ppmin crust and substrate, respectively), V (385 and 3.02 ppm incrust and substrate, respectively), Cu (186 and 2.38 ppm incrust and substrate, respectively), Cr (131 and 30.5 ppm incrust and substrate, respectively), As (50.4 and 3.68 ppm incrust and substrate, respectively), and Ni (40.1 and 4.81 ppmin crust and substrate, respectively).

Church of St. Eustache

Even the analysis of this sample revealed a general enrich-ment in heavy metals (As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni,Pb, Sb, Sn, Ti, V, and Zn) in the black crust with respect tosubstrate. In particular, the following elements displayed the

Fig. 4 FT-IR spectra of samples CV5 (Corner Palace), CM (Cathedralof St. Rombouts), and EP (Church of St. Eustache)

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higher concentrations: Fe (8,170 and 1,085 ppm in crust andsubstrate, respectively), Pb (811 and 66.9 ppm in crust andsubstrate, respectively), Zn (442 and 133 ppm in crust andsubstrate, respectively), Sn (163 and 0.71 ppm in crust andsubstrate, respectively), Cu (91.8 and 18.95 ppm in crustand substrate, respectively), As (75.9 and 9.61 ppm in crustand substrate, respectively), V (69.4 and 5.03 ppm in crustand substrate, respectively), and Ni (50.2 and 8.41 ppm incrust and substrate, respectively).

Discussion

Optical and scanning electron microscopy, along with infra-red spectroscopy, provided information on the mineralogicaland textural features of both substrate and black crusts andthe degree of deterioration of the stone surfaces of the threeanalyzed monuments. In particular, substrates of St.Rombouts Cathedral (Mechelen) and Church of St.Eustache (Paris) appear to be strongly altered by dissolutionphenomena, which cause many fractures where microcrys-talline gypsum grows with its typical acicular habitus.Conversely, the stone of Corner Palace (Venice) is onlyslightly damaged. Black crusts of all examined samples arecomposed of microcrystalline gypsum, together with carbo-naceous particles and iron oxides in variable amountsdepending on the sample (very common in EP and CV12and scarce in CV5, CV8, and CM) and, only in EP sample,quartz grains, and substrate fragments. The particles, mostlyspherical shaped and with variable sizes, display differentmorphologies, varying from smooth to porous. Morphologyand thickness of crusts are quite different in the three exam-ined monuments. Specifically, the damage layer in the CMdoes not appear continuous along the surface of the sub-strate and its thickness ranges between 40 and 150 μm.Conversely, the black crusts of CV5 and CV8 samples fromCorner Palace show a regular profile and a homogeneousthickness of about 200 μm. CV12 sample from the sameedifice has a heterogeneous crust (whose total thicknessranges between 500 and 700 μm), composed of four distinctlayers, having a variable amount of acicular gypsum crystalsand spherical particles. The morphology of the crust fromthe Church of St. Eustache is irregular, even if the contactwith the substrate is continuous, and its thickness variesbetween 250 and 950 μm. It consists of alternating darkand light areas, which are characterized by an abundantorganic component and predominant acicular crystals ofgypsum, respectively. FT-IR analyses allowed to recognizegypsum as the main mineralogical component in all exam-ined crusts, as expected. In addition, only in one samplefrom the Corner Palace, calcium oxalate was also recog-nized. Its presence can be due to either previous restorationworks or activity of microorganisms’ colonies (Barone et al.

2008; Bonazza et al. 2007a; Rampazzi et al. 2005; Sabbioniand Zappia 1991). A complete geochemical characterizationof black crusts and substrates in terms of trace elements wasobtained through LA-ICP-MS investigations. In general, arather high variability characterizes the obtained resultsreferred to single spot analyses performed on each blackcrust sample. However, this is due to the process itselfleading to the formation of black crusts which, as statedabove, is a complex and dynamic phenomenon that is af-fected by a variety of mobile and fixed pollution sources thatcan change over time. On the whole, data obtained suggestthat concentrations of heavy metals in the crusts are higherthan those in substrates, especially for Fe, Pb, Zn, V, Cu, Cr,Ni, and Sb, as observable in the histograms of Fig. 5, whichdisplay the average concentrations of such elements in bothblack crusts and unaltered substrates. Metals have beengrouped on the basis of their abundances. The concentra-tions of four of the most significant trace elements havebeen also plotted into binary diagrams. Here we report, Asvs. Pb and Zn vs. Fe (Fig. 6), giving the most significantresults. The diagram As vs. Pb (Fig. 6a) shows an overallincrease in As from unaltered stone to black crusts for allsamples from the three monuments; as regards Pb, only thetwo inner layers (3 and 4) of sample CV12 (Corner Palace)display concentrations higher than the substrate, whereassamples CV5, CV8, and the outermost layers of CV12 havecontents comparable with those of substrate. Also the binarydiagram Zn vs. Fe (Fig. 6b) displays an enrichment of bothelements in all the crust samples with respect to the sub-strate, with the exception of CV5, CV8, and the outer layers(1 and 2) of CV12 samples, which show a depletion of theZn content. With the aim of estimating the degree of enrich-ment of heavy metals in black crusts with respect to thesubstrate, the average concentrations of each element incrusts have been normalized with respect to those in theunaltered stones. The logarithmic spider diagrams obtainedare reported in Fig. 7, where the enrichment factor (EF;Barca et al. 2010, 2011) for each element can be observed.In detail, samples from the Corner Palace (CV5,CV8,CV12-1-2-3-4) display an enrichment of almost all heavymetals (Fig. 7a–b) from moderate (EF ∼40 in CV5, CV8,and the outer layers of CV12) to high (EF up to 100 in theinnermost layers of CV12). The different concentrations oftrace elements and the EF observed in the three crusts fromthe Corner Palace can be ascribed to several factors, such as:different height of sampling, different morphology of thesampled surfaces (vertical or horizontal), exposure to atmo-spheric agents, wash out, and marine aerosol. The highestconcentrations of several heavy metals were found in CV8and CV12 samples, which have been taken out from hori-zontal surfaces at 15 and 5 m, respectively. Conversely, CV5sample, collected from a vertical surface at 25 m from theground, displays lower concentrations of the same heavy

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metals. In general, all three samples from the Corner Palaceshow high contents of As and Fe (Fig. 6a–b). The occur-rence of As can be explained with the emission of highamounts of this element in atmosphere as a product of theglass manufacturing in the near island of Murano(Rampazzo et al. 2008; Rossini et al. 2010; Zonta et al.2007). Whereas, the high concentrations of Fe could be

due to emissions from the numerous industries (oil refining,storage, shipbuilding, metal extraction and metallurgy, en-ergy production and distribution, and waste incinerator)sited in the near industrial center of Porto Marghera(Bellucci et al. 2002; Pavoni et al. 1992; Zonta et al.2007). The highest concentrations of this element, alongwith Pb and Zn, have been found in the innermost layers

Fig. 5 Histograms showing the average concentrations (in parts per million) of heavy metals in all examined samples. The elements are groupedaccording to their order of abundance. bc black crust, s substrate

Fig. 6 As vs. Pb (a) and Zn vs. Fe (b) binary diagrams for black crusts (bc) and unaltered substrates (s) of all samples analyzed by LA-ICP-MS

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(3 and 4) of CV12 sample (Figs. 5 and 6a–b). On thecontrary, the outer layers (1 and 2) of the same crust resultto be highly enriched in Cu, Ni, Cr, and V. Such a peculiartrend observed for this sample can be firstly explained withthe relatively low height of sampling (5 m). Indeed, thestone substrate at this height is significantly affected by bothfumes from marine transport vehicles crossing the CanalGrande and marine spray, this latter being highly enrichedin metallic contaminants, mainly Zn, Pb, and Cu (Ministerodelle Infrastrutture e dei Trasporti 2010). In addition, thehighest concentrations of Pb, Fe, and Zn found in the innerlayers of the crust (which probably represent the oldestdeposit) are in agreement with the use of lead gasoline(Rodriguez-Navarro and Sebastian 1996), employed untilabout 20 years ago. Similarly, the high amounts of Cu, Ni,Cr, and V in the outermost portion of the crust, together with

very low concentrations of Pb, Fe, and Zn, well match theuse of other combustibles such as: oil combustible, diesel,and gasoline (Contini et al. 2011; Geller et al. 2006;Rodriguez-Navarro and Sebastian 1996), widespread afterthe abolition of leaded gasoline. Therefore, the detailedanalysis of such a multilayered crust which characterizessample CV12 represents almost a fingerprint of the variationof the fuels used by boats over time. The geochemicalcharacterization of the crust sample from St. RomboutsCathedral (sample CM) shows a considerable enrichmentin some heavy metals (Fe, Zn, Pb, V, Cr, Cu, As, and Ni)with respect to the substrate. This behavior can be observedin the histograms and binary diagrams of Figs. 5 and 6a–b,respectively, and in the spider diagram of Fig. 7c, where theEF values range from 10 to 100, with a peak of 600 for Pb.The high concentrations of these elements are probably due

Fig. 7 Spider diagrams(logarithmic scale) of meanvalues of trace elements(determined through LA-ICP-MS) in black crusts normalizedto the unaltered substrate for: asamples CV5 and CV8 fromCorner Palace, b the four layersof CV12 from Corner Palace,and c sample EP from theChurch of St. Eustache andsample CM from the Cathedralof St. Rombouts

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to the height of sampling of this crust in the facade (2 mfrom the ground) and consequently to the direct impact ofvehicular emissions, which result to be particularly high inthis area. Specifically, the emission in the atmosphere ofhigh amounts of the aforementioned heavy metals can beascribed to the combustion of fossil fuels (Geller et al. 2006;Rodriguez-Navarro and Sebastian 1996) or the attrition andfriction of mechanical parts of vehicles (Harmens and Norriset al. 2008), or the consumption of the asphalt (Winther andSlentø 2010). However, a contribution from several indus-trial centers (manufacturing industries, extraction and work-ing industries of iron, steel, and nonferrous metals), thatrelease in the atmosphere high quantities of Fe, Zn, Pb, V,and As (Harmens and Norris 2008; Harmens et al. 2007)and located in the surrounding area between Antwerp andBrussels, cannot be excluded. Finally, the black crust samplefrom the Church of St. Eustache (EP sample) shows anenrichment in Fe, Zn, Pb, Cu, Ni, V, and Sn (Figs. 5 and6a–b) with respect to the unaltered substrate. It is worthnoting that almost all heavy metals’ concentrations are low-er than those in the samples from the other two monuments.In fact, a moderate enrichment with EF values not higherthan 30 can be observed in Fig. 7c, except for Sn whose EFreaches a value of about 230.

This can be due to the location of the monument in an areawhich is closed to the vehicular traffic since about 25 years, aswell as to the height of the sampling (40 m from the ground),so that the monument is not exposed to direct sources ofpollutants from mobile combustion sources. However, thepresence of industries located in the neighborhood of the city(electric power plants, refineries, petrochemical andmanufacturing industries, steelworks, and waste incinerators;Harmens and Norris 2008; Harmens et al. 2007; Motelay-Massei et al. 2005) may have contributed to the enrichmentin the above mentioned heavy metals found in the crust, inparticular, Sn whose markedly high concentrations can beascribable to emissions in the air by steelworks.

Conclusions

In this work, black crusts collected from three Europeanmonuments have been analyzed. The characterization ofthese damage layers by means of several analytical methodsprovided information on their chemical composition, thestate of conservation of the substrate and the interactionsbetween crust and stone. On the whole, the following con-clusions can be drawn:

(a) In a monument, architectural surfaces located at lowerheight from the ground are characterized by highestconcentration of heavy metals. This is linked to the majorimpact of mobile combustion sources (boats/vehicles).

This confirms the driven role of local sources of pollutionin determining the damage.

(b) Areas at highest height (about 40 m) goes to a majorimpact of background pollution from fixed combustionsources (e.g., industries).

(c) The results prove that crust composition can be afingerprint of the variation of the used fuels over time(see case of Venice).

A final consideration regards the high sensitivity of LA-ICP-MS technique in the study of black crusts, since itpermits to identify the different sources of pollution in urbancontexts with diverse environmental conditions, which cer-tainly contribute to the formation of damage layers onmonuments.

Acknowledgments We would express our gratitude to PhilippeGarrigues for his editorial guidance and to the anonymous reviewersfor the constructive comments and suggestions provided that certainlycontributed to increase the quality of the manuscript.

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