mapping and characterizationofstone materials and their...

Per. Minerai. (2002), 71, SPECIAL ISSUE: Archaeometl)' and Cultural Heritage, 1-16 http://go.to/permin

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Mapping and characterization of stone materials and theiralterationldeterioration products in the historical center of Palermo (Italy)

ROSARIO ALAIM0 1, ETTORE AZZAR0 1, RENATO GIARRUSS02, PIETRO MARESCALCHI3 and GIUSEPPE MONTANA I *

l Dipartimento di Chimica e Fisica della Terra ed Applicazioni alle Georisorse e ai Rischi Naturali (C.F.T.A.),Università di Palermo, Via Archirafi, 36, 1-90123, Palermo, Italy

2 CEPA s.r.l., Via Fonderia Oretea, 23 1-90139, Palermo, Italy3 Dipartimento di Rappresentazione, Università di Palermo, Viale delle Scienze, 1-90lO0, Palermo, Italy

ABSTRACT. - In the last years, natural and man-made building materials from several importanthistorical palaces and/or churches of Palermo (Italy)and their alteration/deterioration products started tobe colIected and analyzed.

In this paper are reported the relief and digitaImapping of stone material' s typology andconservation state of the Baroque palaces Lungarini(XVII century), Alliata di Villafranca (XVIIIcentury), Ugo delle Favare (XVIII century) and thechurch of Santa Maria dei Miracoli (XVI century),alI located in the historical center of the city. Theresults of mineralogical-petrographic analysisperformed on the originaI building materials andtheir alteration/deterioration products are alsodiscussed.

Graphic relief and materials mapping of the threeBaroque palaces were realized starting from aclassical graphic procedure which has beensuccessively digitalized by means of standardcommerciaI software. The main façade of the churchof Santa Maria dei Miracoli, on the contrary, wasrelieved by the «total station» photogrammetricmethod, consisting of an electronic theodoliteequipped with a laser-pointing diastimeter. Graphicrestitution has been carried out via commerciaIsoftware.

Mineralogical-petrographic analysis, XRD, thinsection microscopy and SEMIEDS alIowed to obtainthe compositional and textural characterization ofnatural stones (Pleistocene biocalcarenite andMesozoic welI cemented limestone) and air lime

Con'esponding author, E-mail: [email protected]

plasters as well as their alteration/deteriorationproducts. According to previous studies, thealteration/deterioration pathologies affecting bothbiocalcarenites and limestones are represented bybrownish-blackish encrustations and sub-efflorescences of soluble salts (mainly gypsum andhalite). Cyclic crystallization of soluble salts (mainlycomposed of magnesian sulphates such as epsomite,hexahydrite and kieserite) is the major cause ofdeterioration of air lime plasters. The predominanceof magnesium sulphates is to be related to thewidespread use of magnesian lime in themanufacture of Palermo's plasters from the XVII tothe XIX century.

The acquisition of a thematic cartography relativeto the most important natural or artificial buildingmaterials used in the monumental constructions ofthe historical center of Palermo supply a usefulinstrument for programming restorationinterventions. The future realization of a databaseregarding compositions and mechanisms ofdegradation will concur to choose the besttechnologies and products.

RIASSUNTO. - Negli ultimi anni, i materialilapidei naturali ed artificiali utilizzati nei piùimportanti edifici storici, civili e religiosi, diPalermo sono stati oggetto di una sistematicacampagna di campionamento ed analisi.

Nella presente nota sono riportati i rilievi e lemappe digitali che riguardano la tipologia deimateriali lapidei ed il loro stato di conservazione,relativamente ai seguenti edifici: Palazzo Lungarini(XVII sec), AZZiata di Villafranca (XVIII sec), Ugo

2 R. ALAIMO, E. AZZARO, R. GIARRUSSO, P. MARESCALCHI and G. MONTANA

delle Favare (XVIII sec), chiesa di Santa Maria deiMiracoli (XVI sec). Vengono commentati i risultatidelle indagini mineralogico-petrografiche effettuatesui materiali originali, finalizzate alla lorocaratterizzazione ed alla identificazione dei prodottidi alterazione e degrado.

I rilievi e le mappature dei tre palazzi barocchi,sono stati realizzati a partire da una classicaprocedura grafica che è stata successivamentedigitalizzata per mezzo di un software disponibile incommercio. Diversamente, per la rappresentazionedel prospetto della chiesa di Santa Maria deiMiracoli è stato impiegato il metodofotogrammetrico della "Stazione Totale" compostada un teodolite elettronico corredato didistanziometro a puntamento laser. La restituzionegrafica anche in questo caso è stata ottenuta tramitel'utilizzo di un software commerciale.

Le analisi mineralogico-petrografiche, (XRD,microscopia ottica su sezione sottile, SEM/EDS)hanno consentito di caratterizzare, dal punto di vistacomposizionale e tessiturale, gli elementi lapidei, gliintonaci ed i loro prodotti d'alterazione.Conformemente a quanto ricavato da studiprecedenti, le patologie degenerative che interessanosia le biocalcareniti che i calcari compatti simanifestano attraverso la formazione diincrostazioni bruno nerastre e sub-efflorescenze disali solubili (costituite principalmente da gesso ehalite). La cristallizzazione ciclica di sali solubili(maggiormente rappresentati da solfati di magnesioquali epsomite, hexahydrite e kieserite) è laprincipale causa di degrado degli intonaci. Inquest'ultimo caso, la presenza di solfati di magnesioè da porre in relazione al largo utilizzo di una calcemagnesiaca per la realizzazione degli intonacipalermitani, soprattutto durante il periodo compresotra il XVII e il XIX sec.

L'acquisizione di una cartografia tematica relativaai più importanti materiali lapidei naturali edartificiali impiegati negli edifici monumentali delcentro storico di Palermo fornisce un utile strumentoper la programmazione degli interventi di restauro.La futura realizzazione di una banca daticoncernente composizioni e meccanismi di degradoconcorrerà alla valutazione dei prodotti e tecnologiedi restauro più idonei.

KEY WORDS: Sicily; Palermo; building materials;alte ra tion/dete rio ration; res to ration;Archaeometry.

INTRODUCTION

In the restoration of monumental buildings itis necessary to know in a comprehensive waythe composition of the employed stonematerials, their functionality in thearchitectonic structure as well as the state ofconservation.

In the particular case of the recovery of thehistorical center of Palermo, the demand for agood scientific knowledge concerning naturaland man-made building materials, traditionallyused in the architectural practice of the pastcenturies, increased more and more. In fact,lacking of this kind of data in support ofrestoration projects has often induced to chooseinadequate technological procedures, like as,for example, the use of integration materials orconsolidating products scarcely compatiblewith the earliest stone substratum. It is wellknown that every kind of stone material has gota quite typical pattern of alteration/degradation(also depending on micro-environmentalconditions) and it is therefore necessary toknow in detail its nature to be aware of themechanisms and provide for suitablerestoration tests.

On the base of the above generaIconsiderations it has been made active, since afew years, an organic and systematic studyof the building materials used in thehistorical architecture of Palermo, in order toobtain:

- mineralogical, petrographic, chemical andpetro-physical characterization of numerousvarieties of lower Pleistocene biocalcarenitecropping out in the Palermo' s surroundings,whose utilization is documented in severalbuildings of great historical-artistic relief(Alaimo et al., 1998);

- review of lime production techniques in theold city area during past centuries (Montana,1997-a);

- mineralogical, petrographic and chemicalcharacterization of mortars and plasterscollected from monumental buildings of thehistorical center (Alaimo et al., 2000-b);

- mineralogical, petrographic and chemical

Mapping and characterization ofstone materia/s and their alterationldeterioration ... 3

characterization of decorative stucco-worksin several Baroque oratories andchurches (Montana, 1997-b; Montanaand Ronca 2001).

The present contribution concerns with thecompositional characterization of natural andman-made building materials as well as theproducts of alteration/degradation which havebeen identified in the main façades of threeimportant Baroque palaces and a church, alllocated in the historical center of Palermo:

- Ugo delle Favare palace dates back to thefirst years of the XVII century. The main gateas well as the lateral ones with polygonal archare decorated with mythological figuresrealized in manneristic style.

- Lurzgarini Palace was built up towards thehalf of XVII the century The wall surface iscoated by two different layers of plaster. Themost recent can be dated back to the XVIIIcentury; it covers an older layer (XVII century)which is decorated with a painted «diamond»ashlar-work.

- In its current appearance the Alliatadi Vill[~franca palace goes back to 1753,due to the transformations that becamenecessary after the 1751 earthquake, whichproduced notable damages to the originaIstructure (La Duca, 1994; Chirco, 1996).Its wall structure is covered by two layers ofplaster respectively manufactured in the XVIIand XVIII century. Both the two gates aredecorated with monolithic limestone columnsand stucco medallions of valuableworkmanship.

- The church of Santa Maria dei Miracoliwas founded in 1547. Within a Renaissanceoutline the main façade (constituted of porouscalcarenite) introduces several successiveelements in Baroque style.

This study is also expected to acquirepreliminary information on the frequency ofemployment of originaI lithotypes in thehistorical center of Palermo and it comprises afirst attempt of realization of a thematiccartography concerning with the monumentalbuildings of the city.

METHODs, TECNIQUES AND SAMPLING

Mineralogical-petrographic analysis

Mineralogical and petrographiccharacterization of the collected materials andof their alteration/deterioration forms has beencarried out according to Normal ICR/CNR10/82, 12/83, 14/83, 16/84 and 23/86recommendations. AlI the samples wereinitially studied by visible light microscopy.Plaster samples have been preventivelyimpregnated with epoxy resin under vacuum, inorder to realize polished cross sections and thinsections suitable for the study of stratigraphy.Abundance of aggregate components has beenestimated by routine quantitative pointcounting procedures. X-ray powder analyses(XRD) were carried out with a Rigaku D/maxIIlc diffractometer (CuKa 40 KV, 20 mA,graphite monochromator). SEM/EDS analysishas been performed on carbon-coated samplesby a Leica Stereoscan 440 (Link Isis OxfordInstruments, Pentafet Si-Ge detector), usingboth secondary electron (SE) and back-scattered electron (BSE) imaging mode.

Reliefof the façades

For the representation of the façade of theSanta Maria dei Miracoli church it has beenemployed the method of the photogrammetricrelief, by an instrumentation composed of anelectronic theodolite equipped with a laser-pointing diastimeter. The theodolite was alsoprovided with a display that supplies dataaccording to Cartesian coordinates. This «totalstation» methodology makes available theselection of points of the façade as well as themeasurement of the angle between the line ofview and the optical axis of theinstrumentation. The sequence of measurementcan be summarized as folIow: 1)photogrammetric relief and reading of the data;2) photographic survey of the prospect of thechurch by means of Rollei nutrieinstrumentation (semi-metric, with calibratedlens and precision micro-grooving); 3) Roto-translation of the relieved points; 4) image


flattening (correction of the distortion); 5)C.A.D. plotting; 6) mapping of natural andman-made materials and alteration/degradationforms.

Graphic relief and materials mapping of thethree Baroque palaces were realized startingfrom a classical graphic procedure which hasbeen successively digitalized by means ofstandard commerciaI software.

Sal11plìng

Up to 26 samples considered fullyrepresentative of the employed natural andman-made materials and of the most diffusedalteration/degradation products have beencollected from the main façades of thepreviously cited monumental buildings in orderto be characterized by mineralogical-petrographic methods.

RESULTS AND DISCUSSION

Mapping of facades: employed l11aterials andtheir conservation state

The three Baroque palaces resultedcharacterized by the employ of the similarbuilding materials. Figures l a and l b show, asexample, the digitalized relieves referred toLungarini and Alliata di Villafranca palaces.The load-bearing walls are composed ofsquared blocks of the local porous Pleistocenebiocalcarenite. They show only slightmacroscopic differences, from pIace to pIace,which can be easiIy explained considering theuse of the same lithotype although exploited indifferent quarries of Palermo' s territory. Thewall structure is constantly covered by one ortwo layers of plaster, whitish or light pinkish incolor. The biocalcarenite is in full sight only inthe balcony where constitutes brackets,members and various decorative sculptures.Monolithic columns decorating the main gateand their plinths are manufactured, in most ofthe cases, with a local hard gray limestone(breccias) of Mesozoic age, locally namedpietra di Billiemi. More or less recent

integrations made by bricks and cement alsooccur.

The forms of alteration/degradation affectingthe studied monumental buildings have beenmapped and described on the basis of therecommendation ICR/CNR Normal 1/88. Themost important and diffuse are:

- black crusts: are mostly concentrated onthe surfaces of the building repaired from thewashing-action of rain waters (fig. 2a). Theyshow variable thickness and morphologiccharacteristics according to the nature of thestone to which they adhere tenaciously. Whenspontaneously detached the substratum appearsto be strongIy disaggregated.

- brovvnish-blackish fill11s: show smallthickness and preferentially develop oncompact materials (fig. 2b). They can beremoved from the substratum which generallyremains integraI.

- biological patinas: are thin, soft andhomogenous, adherent to the stone surface,with color from dark green to blackish.

- diffe rential degradati0l1: is in closerelationship with the compositional or texturalheterogeneity of the building material, resultingparticularly evident in the calcarenite (fig. 2c).

- decohesion: manifests itself by theseparation of grains under minimal mechanicalsolicitations.

- pulve rization: is represented by thespontaneous fall of powdered material orgrains.

- erosion: mechanical removal of materialfrom the surface due to various processes.

- gaps: are due to loss of parts of the surfaceplaster which brings to light the inner stonesubstratum (fig. 2d).

The main prospect of the chm"ch of SantaMaria dei Miracoli has been realized with awhitish Pleistocene biocalcarenite, veryprobably exploited in the north zone ofPalermo. The whole façade is covered by anocher film. The decorative elements are alsomade of calcarenite, with the exception for acoat of arms realized in marble (fig. 3). It hasto be noticed that the upper part of the buildinghas been realized in a successive age (XVIII

Mapping and characterization ofstone materials and their alterationldeterioration ...

Lungarini Palace

10 ml

5

Plast(?r (XVIII ccnt.)

Plaster (XVII cent.)

BiocaJcarenite

Bricks integrations

Concrete integrations

Recent reconstructions

Plaster (XVIII cenI.)

Plaster (XVII cenI.)

Concrete integration

Biocalcarenitc

Bricks intcgration

Alliata di Villqfi"anca Palace

8 mI.

Fig. 1 - Mapping of natural and man-made building materials employed in the main façades of Lllngarini and Alliata di\lillafranca palaces.

6 R. ALAIIVIO, E. AZZARO, R. GIARRUSSO, P. MARESCALCHI and G. MONTANA

Fig. 2 - (a) Ugo delle Favare palace (XVII century): solllble salt efflorescences and black crusts; (b) Alliata di Villqfrancapalace (XVIII century) brownish-black films on Billierni limestone; (c) Alliata di Villafranca palace (XVIII century)examples of black crusts and differential degradation in the biocalcarenite; (d) Lungarini Palace: detachment of the XVIIIcentury plaster bringing to light the older one (XVII century) decorated with a painted «diamond» ashlar-work.

century) and probably the used calcareniteshould be of different type from the one used inthe Iower order. Unfortunately this aspect, atpresent, cannot be supported by Iaboratoryanalysis due to difficulty of sampling.

In this building, from the examination of thestate of the defects emerges a generaI picture inwhich the hardest degradations are related tohumidity (capillary infiltrations) and thereforeto the formation of soluble salts, as well as toenvironmental pollution and biological factors.Recognition of the defects by thephotogrammetric relief has been processed in agraphical table according to therecommendation ICR/CNR Normal 1188,allowing an immediate and synchronic reading(fig. 4). In generaI, most of the pathologies arevery similar to those already described for the

baroque palaces. Nevertheless, some othersalteration/degradation forms have been noted:

- patina: all the wall surface of the mainfaçade is covered by an ocher patina whichalters the originaI color of the stone material.This is evident in the parts subjected towashing processes, in which it can be noted theoriginaI color of the stone;

- loss of material: it occurs in some parts ofthe façade, mainly in those jutting out of thecornices and moldings of the columns;

- washing-effect: Draining of l'ain water fromthe cornices and the protruding parts causes thewashing away of the ocher coloring in the wallsurface;

- efflorescences: this expression ofdegradation is caused by the evaporation of thewater which permeates the squared blocks of

Mapping and characterization oJstone materials and their alteration/deterioration ... 7

Fig. 3 - Image of the main façade of the church of Santa Maria dei Miracoli (XVI century) showing the different types ofbiocalcarenite (upper and lower orders) and the ocher film.

Santa Maria deiMiracoli

.... EffiorescenceSuperficial deposi t

Biological patina

g:~ Black crust

• VegetationLime mortar

Concrete

Ilmll Integrations

----Lesions

Metallic pins

Fillet gaps

Scuppers

Gaps

Washed areas

Fig. 4 - Graphic restitution of the photogrammetic relief of the church of Santa Maria dei Miracoli showing the state ofdefects.


biocalcarenite, with consequent crystallizationof salts towards the external surface; it is foundin some zones under the cornice of thearchitrave, over the two pilasters and in theframe of the left window.

Natural stones: notes on traditional use andcompositional characteristics

The Pleistocene biocalcarenite, in antiquitynamed Kiddan by Arab conquerors (that is tosay «golden stone»), is the most used buildingmaterial in the monumental architecture ofPalermo and of the whole western Sicily. Thisextensive employment strongly depended onthe trouble-free quarrying and shaping of thelithotype as well as on the large distribution ofits outcrops, generally located along the coastalareas. In the plain of Palermo these porousrocks, with a thickness of 20-30 m, cover byunconformity Mesozoic limestone anddolostones as well as tertiary flyschoid deposits(Caflisch, 1966; Ruggieri, 1973). Theirpeculiar depositional process is clearlyresponsible for the variability in color, grainsize, degree of cementation, abundance ofdetrital accessory minerals and rock fragments.The heterogeneity of the textural, structuraland, in some extent, compositionalcharacteristics is sometimes found even in thesame quarry, both laterally or vertically alongthe stratigraphic sequence. Different qualitiesof building stone were therefore exploited to beadapted to the various architectural employs. Inthe urban Palermo, at present days, are rarelyvisible the remains of ancient quarries whichactually were dozens and dozens (Cipolla,1929; La Duca, 1964; Todaro, 1988; Alaimo etal., 1998; Montana and Scaduto, 1999). Sincethe first half of the XVI century started theexploitation of the areas located in the extraurban territory which continued unceasinglyuntil the first decades of the XX century. Thefirst phase of cultivation of a calcarenite quarrygenerally took pIace by means of undergroundgalleries (or mucate, according to the Arabicdenomination) in order to go after theproductive rock-Iayers. Then, this system ofexploitation was followed by cultivation under

the open sky, which allowed lower costs andmaterial waste (Todaro, 1988). The lands to beexploited were subdivided in smaller fields ofrectangular shape, whose traces are still visiblein the territory (fig. 5). Mining and the shapingof the blocks were carried out during summerin order to allow the porous stone to naturallydry. The squared blocks were usually leftexposed two years to the atmospheric agentsfor being able to recognize and discard themost vulnerable ones (Cipolla, 1929).

As already seen in the previous paragraph, inthe four studied monumental buildings, thelocal biocalcarenite constitutes by far the mostrepresentative lithotype, used both for theprospects and for ornamental elements.Nevertheless, the presence of diffuse blackincrustations or films and patinas interfere withthe appraisal of the macroscopic differences(color, degree of cohesion and prevailing grainsize) between the stone' s variety used in thewalls and the one employed in the carved parts.

After the observation of thin sections underthe polarizing microscope, the calcarenitesused in Lungarini palace, Ugo delle Favarepalace and in the church of S. Maria deiMiracoli are alI classifiable as grainstone(Dunham, 1962). They showed quitecomparable compositional and texturalfeatures, (fig. 6). The mean/prevalent grain sizeranges between 0,4 mm (Lungarini palace) andl,O mm (Ugo delle Favare); maximum grainsize is comprised between l,O mm (S. Mariadei Miracoli) and 3,0 mm (Ugo delle Favare).Concerning with the bioclasts, in thesevarieties, the fragments of calcareous algaeresulted always prevailing, followed byrelati vely less quantities of foraminifera,lamellibranch, bryozoan and echinoderms;fragments of algae are often impregnated ofiron oxides which are responsible of the slightyellowish color of the stone. Detritalminerals/lithoc1asts resulted quite sporadic,represented by Mesozoic limestones,dolostones, with micritic or microspariticstructure and mono-crystalline quartz. Primaryfine grained matrix (micrite mixed with c1ayminerals) is poorly represented (only scarce

Mapping and characterization ofstone materials and their alterationldeterioration 0.0 9

Fig.5 View ofthe remains an ancient quarry ofbiocalcarenite in the surroundings ofPalermo (Aspra).

residues enclosing or filling the spaces betweenthe larger particles). An interstitial cementmade of sparry calcite is relatively moreabundant even though it cannot be consideredcopious. It is also characterized by crystalsshowing the typical acute scalenohedral formand by small druse aggregate, filling the largerpore spaces. The total macroporosity has beenvalued to be around the 20%. Intergranularpores deriving from the selective dissolution oforiginaI bioclasts are prevalent. The walls ofthe pores often result covered by a thin layer ofmicrite.

This composition has been confirmed by thepowder XRD patterns which showed the clearpredominance of calcite with only smallquantities or trace of dolomite, quartz and clayminerals.

The variety of biocalcarenite used in themain façade of Alliata di Villafranca palaceresulted quite different. It can be classified as

packestone (after Dunham, 1962). Therefore,the microcrystalline primary matrix isrelatively more abundant with respect of theabove described varieties (fig. 7). Theprevalent grain size has been recognizedaround 0,2 mm (fine sand). Moreover, it isricher in de tritaI mineralsllithoclasts likelimestone fragments, mono-crystalline quartzand chert grains, up to 10% (area).

Finally, some words should be spent aboutthe well cemented and hard Mesozoiclimestone, locally named grigio di Billienti,which has been often used for plinths andcolumns. It belongs to the lower Lias - upperTrias and crops out in the mounts surroundingPalermo toward south. It is characterized by arich fossil fauna and a dark gray color withplagues of fine black or yellowish material (fig.8). Whitish calcite veins and concretions arealso frequento The more famous andappreciated variety, above all in baroque age,

lO R. ALAIMO, E. AZZARO, R. GIARRUSSO, P. MARESCALCHI and (

Fig. 6 ~ Palarizing microscape image af the biacalcarenitetype used in the façade af Lungarini palace (crossed nical;scale bar = 0,3 mm).

Fig. 8 Macrascapic appearance af the brecciated varietyafthe Billiemi gray limestane.

shows a typical brecciated texture, constitutedof elements with variable dimensions from acentimeter to various decimeters (fig. 9). Theparticular durableness and resistance to theatmospheric agents, if considered together withthe nice appearance (in the several tonalities ofgray) and, above all, the thickness of the stratawhich favored the exploitation of enormousmonolithic blocks, have privileged theextraordinary diffusion of this lithotype in thebuilding practice (specially as decorative stone)of Palermo in the past centuries (G. Montana &V. Gagliardo Briuccia, 1998).

Plasters: local tradition, raw l1wterials andcomposition

X-ray diffraction, optical microscopy andSEM/EDS analyses allowed to define the

Fig. 7 - Palarizing mi,type used in the façadl(crossed nical; scale b,

Fig. 9 - Transmittedvariety af the Billiel71i !

nature of sand astudied plastencomposition satialready determibuildings of PalelThe same sequencobserved in thewhich resulted pmore ancient la:composed by anstone substratumaround 10 mm,abundant and poo60%); it follows aof thickness) corraggregate (40-60Ssorted, with medilmm); the sequenc

Mapping and characterization (~lstone materials and their alteration/deterioration 0.0 Il

level (1-2 mm), containing a relatively smalleramount « 5-20%) of fine aggregate grains «0.2 mm). Only few compositional differenceshave been established between plastersmanufactured in the XVII and XVIII century.AlI the analyzed samples showed a similarcomposition of the sandy aggregate. In generaI,a slight predominance of siliceous materials(up to 50-65 % of total aggregate) has beenoted with respect to components of carbonatenature (45-50%). The siliceous constituentsare: policrystalline and mono quartz(prevailing), chert, feldspar, quartzarenite andradiolarite grains (fig. 10b and fig. 10c).SporadicalIy fragments of powdered potteryhave also been noted. The carbonatecomponents are lithoc1asts deriving from thePleistocene biocalcarenite as well as Mesozoiclimestones and dolostones. Even as regardscomposition of the binder matrix stronganalogies between plaster manufactured indifferent ages can be pointed out. By X-raypatterns and SEMIEDS imaging is evident thata magnesium-rich lime has been used (fig.10d). It has been c1early manufactured fram thecalcination of magnesian carbonate rocks (bothdolomites and dolomiti c limestones), ischaracterized by the presence, together withcalcite, of magnesite and, secondarily,hydromagnesite. Brucite-composed white c10tshave been frequently observed (up to severalmillimeters in size), which are consistent with abad carbonation. In fact a magnesium richcarbonate rocks if heated at about 900°C, willproduce CaO together with MgO, the most partof which is sintered. In this case, the hydrationreaction (in the course of lime slaking) takespIace with lower efficiency in comparison withpure CaO.

The comparison of the mineralogical-petrographic data with the lithology of therocks cropping out in the surroundings ofPalermo and the consultation of ancientmanuscripts, books and notarial acts, let todeduce the exploitation areas of raw materials.The composition of the aggregate essentiallyreflects the contribute of the Numidian Flyschformation (Upper Oligocene-Lower Miocene)

as regards siliceous fragments, while thecalcareous component derives from Mesozoicrelieves and Pleistocene biocalcarenites. Sandaggregate compositional sorting put forward afluvial sand, therefore the supplying sitesshould be chosen between the watercourses ofthe Palermo surroundings whose drainagebasins draw on the above mentioned geologicalformations. In a notarial act dated back to the1654 is evidently requested that the sand formaking up the plaster must come from theSperone, a locality along the shoreline whichthree centuries ago was only about 3 kilometersfar from the city walls. However, the finalstretch of the Oreto, another stream nearby theold walls of Palermo, should be considered assand supplying site.

As regard the binder, its composition isconsistent with the Mesozoic dolostones(Fanusi formation) widely cropping out in therelieves which surround the western side of thecity. Moreover, chemical analysis of this roc1e(Montana, 1997-a) demonstrate a very lowconcentration of Si02 and A120 3 (both around0.2 wt%) and, consequently, an hydraulic index(weight % Si02+A1203+Fe203/CaO + MgO)even lower than 0.1, therefore appropriate foran air lime binder. In these lands, which in timepast belonged to the Benedictine abbey of S.Martino, it developed a widespread craftproduction of lime. Ruins of several ancientlime kilns are still visible and dozens oforiginaI documents, notarial acts of the 17th and18th centuries expressly require the «SantoMartino» lime for the manufacture of plasters,mortars and stucco works (Montana, 1997-a).

Alteration/degradation products

Precipitation affects stone primarily in twoways: dissolution and alteration. In exposedareas of the buildings roughened surfaces canbee observed, due to removal of material andloss of carved details. Thealteration/deterioration pathology affectingboth the descri bed biocalcareni te andlimestones consists of brownish-blackishencrustations and films. Both showed a similarmineralogical composition: calcite and gypsum


Fig. lO (a) Reflected light micrograph of the XVIII century plaster from Lungarini palace showing a typical stratigraphysequence composed of three layers: (A) inner and coarser level; (B) intermediate level; (C) thinner finishing level (scale bar= 2.0 mm); (b) polarizing microscope image of the inner layer of the XVIII century plaster (crossed nicol; scale bar = 0.6mm); (c) polarizing microscope image of the intermediate layer of the XVIII century plaster (crossed nicol; scale bar = 0.5mm); (d) SEM micrograph of the XVIII century plaster from Alliata di Villafranca palace showing the adherence of thebinder to the aggregate grains; qualitative chemical composition carried out by EDS point analysis (upper right) is referredto the binder matrix.

Mappìng and characterìzatìon ojstone materìals and theìr alteratìonldeterìoratìon .,. 13

turned out to be the most representative phasesby XRD and SEM/EDS analyses. This blackcrust, although it can form anywhere onexposed carbonate stone surfaces; nevertheless,it only remains on protected surfaces which arenot directly washed out by rain-water. Gypsumcrystals form networks that trap partic1es of dirtand pollutants, so the crust looks black. Sooneror later the black crusts swell up and thecrumbled stone become exposed. SEMinvestigations revealed some ultra-structuraldifferences between the outer and the innerparts of the crusts. In the latter single grains arecoalescing and porosity is reduced, while in theexternal part, where particulate matter depositsprevails, the aggregate is always more porousand, specialIy in the thickest black crusts, it isheterogranular. Soluble salt, such as gypsumand halite (the latter deriving from marineaerosols) are mainly present in sub-efflorescences (fig. 11). Their cyc1ic growth,carried by humidity and water migration in

pores space produce a strong disaggregatingeffect. They preferentially form under thecornices which are subjected to stagnation andevaporation of water infiltrating from the upperpart. Sometimes alkaline soluble salts mayform due to the use of incompatible recentintegration materials (Portland cement basedmortars).

Intergranular decohesion due to thecrystallization of soluble salts inside pores is themain deterioration effect in the studied plasters.In some cases the effects of degradation areparticular1y heavy and interest the entirestratigraphic sequence, above alI in the lowerparts of the masonry (capillary action). Thewater imprisoned in the porous stonesubstratum migrates more slowly towards theoutside due to the smaller permeability of theplaster, favoring soluble salt crystallization and,consequently, deterioration phenomena(detachments). XRD and SEM/EDS analysesestablished what has been already seen on other

Fig. Il - SEM micrograph of the black crust developed on the biocalcarenite from Ugo delle Favare palace; EDS areaanalysis is also showed (upper right).


external plaster samples collected from othershistorical buildings of Palermo (Alaimo et al.,2000-b). Magnesium sulphates (epsomite,hexahydrite and kieserite), calcium sulfate(gypsum) and sodium chloride (halite) are thepredominant mineralogical phases (fig. 12).Gypsum seems to slightly prevail in respectwith magnesium sulphates in the sub-efflorescences. The process which originatesuch a deterioration effect starts from thedeposition of atmospheric particu1ate matter andsea spray on the external wall surface coveredof plaster. As well known, either acid pollutantsor salts become chemically reactive when incontact with precipitation and/or condensationwaters. The main effect are dissolution andsulphation of the calcareous binder matrix aswell as formation of soluble salts efflorescenceand subefflorescence. It should be emphasizedthat, in the specific case of Palermo, the sourceof sulphate ions seems to be only to a lesserextent of anthropogenic origin (e.g. emission of

S02) but has to be related to the great influenceof sea spray and to windborne gypsum particlescarried by the southern winds (Alaimo et al.,1989). The predominance of magnesiumsulphates is to be related to the widespread useof magnesian lime in the manufacture ofPalermo's plasters from the XVII to the XIXcentury, as already underlined. In fact, [S04F-enriched percolation water reacting withmagnesium carbonate in the binder matrix areliable to precipitate calcite and solublemagnesium sulphates (soIubiIity of epsomite =720 glI at 20°C). In fact calcite is by far IesssoIubIe than magnesite, respectiveIy 0,014 glI(20°C) and O, Il glI.

CONCLUSIVE REMARKS

Acquisition of data on the natural or artificialbuilding materiaIs used in the monumentalconstructions of the historical center of

Fig. 12 - SEM micrograph showing deliquescent crystals of NaCl (halite) growth in the binder matrix (composed ofmagnesite and calcite) of the XVIII century plaster collected from the façade of Lungarìnì palace; EDS area analysis isshowed in the upper right part of the image.

Mapping and characterization ofstone materials and their alterationldeterioration o •• 15

Palermo with the relief of the state ofconservation and a thematic cartographyrelative to the most important prospects, isexpected to supply a useful instrument forprogramming restorations. The realization of adatabase regarding the compositionalcharacteristics and types of degradation of usedbuilding materials in the architecture practiceof the ancient city would concur to theplanning of restoration interventions, withtechnologies and products custom-made for thespecific requirements. At the same time, thestudy of the mechanisms of alterationldegradation, for every single case study, hasthe scope to adjust the research towardsspecific procedures and materials to be used forsubstitutions, integrations or reconstructions.

Like already pointed out in the introduction,one of the more arduous problems to solve inthe course of the planning of an architectonicrestoration consists in the quantitativedefinition of the «compatibility» between theoriginaI materials and those to be used in theintegrations. In some cases, the choice ofproducts technologically considered in theforefront, which are not, however (in thespecific case), the most suitable ones, mightseriously compromise the restorationintervention (especially in terms of duration)and also cause worsening. In the cases studied,concerning with the historical center ofPalermo, it has been shown that the stonesubstratum on which the most ancient layers ofplaster adhere, it is constituted of the localbiocalcarenite. This rock type generallypossesses a very elevated open-porosity,ranging from the 25% to beyond 40% (Alaimoet al., 2000-c). Being Palermo a coastal citywith a temperate humid climate andcharacterized by an intense vehicular traffic,problems associated with soluble saltsdeposition and migration (essentiallyalkaline/earth-alkaline sulfates and sodiumchloride), should be checked, specially inpresence of such a porous lithic substratum.

Therefore, the possibility to determine thepetrographic and physical characteristics ofnatural and man-made building materials as

well as to replicate the receipts of to ancientplaster and, if it is the case, improve them inthe light of current knowledge, it seems toassume a great importance in the safeguard ofcultural heritage.

ACKNOWLEDGMENTS

This research has been financially supported by1999 MURST-Cofin grant.

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