Stones and coloured marbles of Liguria in historical...

ABSTRACT. — The presence of a great number ofdifferent lithotypes contributes to the extremelyvariable geological landscape of Liguria: its rocksare one of the main reasons of the natural beauty ofthis region and a mean used by people toeconomically exploit their land and to expressthemselves in art and architecture.

Many rock types are locally used, but in Italy andabroad some particular stones («Ardesia», «Pietra diFinale», «Pietra di Promontorio», Mt. Zatta or Mt.Gottero Sandstones) and some coloured marbles(«Rosso Levanto», «Verde Polcevera» and«Portoro») are very known.

These materials are today widely used asdecorative tombstones, both indoor and outdoor, ofthe historical buildings and in the localcraftsmanship.

RIASSUNTO. —La realtà geologica ligure assumecaratteri peculiari per la sintesi estremamente variadi facies e di litologie e la roccia costituisce ungrande patrimonio, non solo perché caratterizza unamorfologia di incomparabile bellezza, ma ancheperché rappresenta il mezzo attraverso cui si èespressa una cultura nell’uso del territorio e si èrealizzata e consolidata nel tempo un’attivitàeconomica per la produzione di materiale lapideo avarie destinazioni d’uso.

Numerosissimi sono i litotipi utilizzatinell’architettura, soprattutto a scala locale, anche se ipiù conosciuti, in ambito regionale, nazionale edinternazionale, sono rappresentati da varie pietre(Ardesia, Pietra di Finale, Pietra di Promontorio,Arenarie di M. Zatta o di M. Gottero) e da alcunimarmi colorati (Rosso Levanto, Verde Polcevera ePortoro).

In funzione della loro lavorabilità e durevolezza,questi litotipi sono stati impiegati in passato nelleopere monumentali e sono tuttora largamenteutilizzati per il restauro dei manufatti storici e per larealizzazione di quelli moderni, sia in Liguria siaaltrove.

Per questi motivi è necessario approfondire glistudi minero-petrografici sulle caratteristicheprimarie di questi marmi e pietre in modo daacquisire una migliore conoscenza dei processi dialterazione, soprattutto in climi di tipo mediterraneo;in particolare, specifiche prove di laboratoriopossono contribuire all’identificazione deimeccanismi chimico-fisici che comportano ildegrado dei materiali lapidei, consentendo unacorretta conservazione del patrimonio architettonico.

KEY WORDS: stones (Ardesia, Pietra di Finale,Pietra di Promontorio, Mt. Zatta or Mt. GotteroSandstones), coloured marbles (Rosso Levanto,Verde Polcevera, Portoro), Liguria, conservation,restoration, historical monuments

Per. Mineral. (2003), 73, 71-84 http://go.to/perminSPECIAL ISSUE 3: A showcase of the Italian research in applied petrology

Stones and coloured marbles of Liguria in historical monuments

FRANCA CIMMINO1*, FRANCESCO FACCINI2 and ANDREA ROBBIANO2

1 Dipartimento per lo Studio del Territorio e delle sue Risorse, Università degli Studi di Genova, corso Europa 26, 16132 Genova, Italy

2 Studio Associato di Geologia tecnica e ambientale, corso Garibaldi 58/5, 16043 Chiavari (GE), Italy

* Corresponding author, E-mail: [email protected]

An International Journal ofMINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,ORE DEPOSITS, PETROLOGY, VOLCANOLOGYand applied topics on Environment, Archaeometry and Cultural Heritage

INTRODUCTION

From a geological point of view, Liguria isconsidered one of the most complex areas in theItalian peninsula and in the wholeMediterranean region, because of its compositetectonic evolution during the Alpine orogenesis.

Many rock types crop out in Liguria, butonly some particular stones («Ardesia», «Pietradi Finale», «Pietra di Promontorio», Mt. Zattaor Mt. Gottero Sandstones) and some colouredmarbles («Rosso Levanto», «Verde Polcevera»and «Portoro») are very known.

It is possible to describe in a brief way thegeological profile of Liguria and then to identifythe outcrops of these materials, as it is shown inthe Figure 1 (Giammarino et al., 2002; SocietàGeologica Italiana, 1991; 1994). From west toeast, the regional territory of Imperia ischaracterized by the Delphinese-Provence Zoneand the Ligure-Piemontese Flyschs, where arequarried the slates of the Argentina valley.

The Permo-Carboniferous crystallinebasement with Mesozoic sediments(Brianzonese Zone) are typical of the regionalterritory in Savona, where the «Pietra diFinale» has been deposed in the Miocene.

In the western area of the Genova regionlarge strips of ophiolitic rocks metamorphisedin high-pressure conditions are visible (Voltri’sGroup and Sestri-Voltaggio Zone where«Verde Polcevera» crops out).

In the eastern area, the Cretaceous-Palaeocene flyschs outcrop and include eitherthe «Pietra di Promontorio» either the LavagnaFormation and in particular the Mt. VerziSlates Member either Mt. Zatta or Mt. GotteroSandstones. Typical ophiolitic strips,responsible of the «Rosso Levanto» formation,appear on the edge between Genoa and LaSpezia. The regional territory of La Spezia ischaracterized either by Mesozoic carbonaterocks, where it is possible to identify the«Portoro», either by Tertiary sediments.

In this paper, the outcrop zones and the mainmineralogical, petrographic and physical-mechanical features (Table 1) are described forthese stones and coloured marbles: particular

attention is dedicated to their use and theirdegradation characteristics, with the aim toinvestigate the problems related to theconservation and restoration of stones in thehistorical monuments.

ARDESIE (SLATES)

Ligurian «ardesia» is a dark grey slate rich incalcium carbonate, outcropping in EasternLiguria within the Val Lavagna Formation and inparticular to the Mt. Verzi Member, cropping outin the Val Lavagna itself and, more SE, in thehills of Cogorno (Genova); slaty lithofacies alsoexist in the western part of Liguria, near Triora(Imperia), linked to marly horizons, sometimesvery large, inside Delphinese-Provence flysch.Main minerals are calcite (around 30-35%),phyllosilicates (illite, white mica, chlorite)quartz, minor dolomite, sulphides, graphite. Thestructure of layered-lattice silicates is the reasonwhy «ardesia» can be easily splitted into thin andlight slabs of different thickness.

It was about X-XI century b.C. when the firstslabs of «ardesia» were quarried in SantaGiulia hill, above Lavagna. In a notarial deedkept in the State Archives of Savona is writtenthat in 1176 a.D. people from Recco (Genova)bought slabs for the roof of Santa Maria churchin Savona (Savioli, 1998).

From XII to XIV century «ardesia» wasincreasingly used in towns and villages ofLigurian Riviera, but during the followingcenturies this stone was used both in westernand in eastern Liguria, mostly for localartworks and decorations (Terranova, 1996).

A peculiar use of the «ardesia» slabs is asbearing for paintings, because slabs do notwarp as wood does: many examples of this usecan be found in Genova, both in UniversityLibrary and in old-town churches and painterslike Andrea Semino, Pellegro Piola, BernardoStrozzi, Giulio Benso and even Rubens, whosepaintings are now in Santa Maria in Vallicellachurch in Rome, painted on «ardesia» slabs.

But the main use of «ardesia» is to coverroof (Photo 1): the fundamental element is the

F. CIMMINO, F. FACCINI and A. ROBBIANO72

«abbadino»; a thin and flat slab 3-4 mm thick.«Ardesia» roofs are fireproof, long-lasting(Cortesogno et al., 1998a; 1998b) and do notneed a strong carrying structure. Liguriansquare abbadini fit the roof structure mostlyused in Genova and surroundings, the onecalled «the triple overlap + three» (Pandolfi,1972). In old times «ardesia» was used also forpaving and flooring, and in the first half of XIXcentury «ardesia» for paving was the firstItalian «ardesia» export item, used in cloisters,arcades, pavements, cellars, shops, terraces,

storehouses, balconies. In houses it was usedindoor for paving the ground floors, while inchurches floors with «ardesia» octagons andsmall marble squares were used.

At the beginning of XX century big slabs upto 5 m2 were produced, the so-called«australians», asked to pave footpaths inAustralia. «Ardesia» was also used to build bigtroughs called «trogoli»: containers for oliveoil so big to contain even 7.5 tons of oil.«Trogoli» were better than jars because insidetheir larger volumes it was more difficult for

Stones and coloured marbles of Liguria in historical monuments 73

Fig. 1 – Geological map of Liguria (redrawn from Giammarino et al., 2002; Società Geologica Italiana, 1991; 1994)


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oil to freeze; «trogoli» were also used as waterreservoirs.

In house building it was used for door- andwindow-sills, window- and door-lintels, door-pillars (Photo 2), while nowadays it is used tomake pieces of furniture and various tools suchas chimneys, wells, tubs, benches, blackboards.The first documented use of «ardesia» slabs forbilliards dates back to 1928. «Ardesia» wasfound to be excellent because it does not warp,it has excellent elasticity values, absorptioncapacity, impact strength and a very highsmoothness, due to the rock features and to theprocessing systems.

No kind of «ardesia» is anyway good foroutdoor sculpture, because it is seriouslydamaged and flakes off more and more as timegoes by: the «ardesia» bas-relieves made inLavagna in 1300 and lately in 1700-1800 arenow often in bad conditions, specially on the

worked surface and in the protruding points, andthe hue of the stone turns to light grey. Bas-relieves of the same ages sculpted in westernLiguria (Taggia, Triora, Pieve di Teco - Imperia)are in better conditions because «ardesia»quarried in Argentina Valley is less cleavable.

Among the possible reasons of decay, wateris the most dangerous for «ardesia» because itdissolves calcite and reduces its solidity, canremove clay minerals, modify chemicalcomposition and chromatic features. In recentyears also smog and acid rains pollutioncontribute to decay and many Ligurian bas-relieves in «ardesia» have been saved becausehave been moved into St. Agostino Museum inGenova.

PIETRA DI FINALE (STONE OF FINALE)

The «Pietra di Finale» crops out in mid-western part of Savona region, behind the sea-


Photo 1 – «Ardesia» roofs of some buildings in the old-town of Genova; please notice the difference between theoriginal roof of the building in front and the one withrecently replaced «abbadini», low in the right

Photo 2 – Main entrance of a historical palace, where«Ardesia» has been utilised, Genova

coast from Capo Noli and Caprazoppa, in thevalleys of Pora, Aquila and Sciusa creeks,around the villages of Gorra, Verezzi, Calvisio,Calice, Eze, Boragni, Carbuta, Rialto, Orco,Feglino and in the Manie Plateau. It belongs toa great grey-pink limestone platform deeplycanyoned by torrents which divide the platformin «islands».

Mineralogy and petrography studies haveevidenced that some lithotypes rich in fossils andothers more sandy are present. Particle sizes ofbio-clasts range from 0.3 cm (many Echinoidea)up to 5 cm (fragments of Bivalvia and Briozoa).Usually it is a bio-calci-rudite. Inside the mostsandy rocks, the terrigenous fraction iscommonly made of mono- and poly-crystallinequartz 2 to 8 mm in size, detrital micas, oxidessoaking both matrix and cement, and clayminerals: clasts are both rounded and rough.Oxides inside the matrix cause the reddish hueof the darkest variety of «Pietra di Finale».

Depending on the quarrying zone and on thechromatic and physical features, can bedistinguished few varieties: a Bianco Doria,soft rock quarried around Boragni; a RossoVerezzi, semi-hard rock of Verezzi; a Rosato diFinale, hard rock from Castel Gavone andPianmarino.

«Pietra di Finale» is locally used sinceRoman age: still well preserved is e.g. the«Fairy Bridge», the first of the five Romanbridges in Ponci Valley, near Verezzi andFinalpia; it was built around 10 a.D. along «ViaJiulia Augusta», the road to Gaul.

Among the historical buildings, it must bementioned St. Eusebio Church (XIV century),San Sebastiano Church and the Five Bell-towers Church (XV century) at Perti, the bell-tower of San Biagio Church in Finalborgo (XIIcentury) and the fascinating Diamond Tower ofCastle Gavone behind Finalborgo (Photo 3),that was one of the most impressive fortifiedresidences of Del Carretto family; this towerrepresents a charming evidence of how stonewas worked in northern Italy duringRenaissance (see also the Diamond Palace inFerrara). «Pietra di Finale» was highlyappreciated also in Genova, in the Galeazzo

Alessi’s architecture during XVI and XVIIcenturies, in Santa Maria Assunta di CarignanoBasilica, in Lercari-Parodi Palace in ViaGaribaldi and in Villa Cambiaso in Albaro.

After a standstill period, quarrying andutilization of this stone restarted towards theend of XIX century, when the «Pietra» wasused in bridges, viaducts, tunnel openings etc,also outside Liguria (Torino-Savona railway)and in many villas along the Riviera.

The top of quarrying activity, alsounderground, was reached in 1924-1929, whenthe «Pietra» was widely used in America formouldings and pillars of many villas built inNeo-classical or in Palladian style. Otherimportant buildings of those years are, e.g., theCathedrals of Bengasi and Mogadisco, theMuseum of Vienna, the Winter Velodrome ofParis, the Palace of Justice of Drauguignan, theRathaus of Hamburg, and in Italy the Fiat


Photo 3 – The Diamond Tower in «Pietra di Finale» ofCastel Gavone, Final Borgo (Province of Savona)

Building in Turin, the Bocconi University inMilan, Villa Ombrosa in Parma. In Genova, the«Pietra di Finale» was used during XX centuryfor the openings of Giuseppe Garibaldi Tunnel atPortello and in the years 1940-1950 for thearcades of Piazza della Vittoria. In the secondhalf of the century this stone was used in themain buildings of Piccapietra district, in thehead-office building of the Savings Bank ofGenova, in the University Clinic of San MartinoHospital. Today it is mainly used to panel thefacades of new buildings, to restore old buildingsand, in lower quantity, for many different localuses. It is a very good building material, with awarm pinkish colour and a strong, well testedweather-resistance; it is used therefore to makeblocks, jambs, pavements, steps, wainscots andinner- and outer-coatings of buildings, as well asto obtain architectural details, rims, decorations(Photo 4) and even bas-relieves and sculptures(Cimmino et al., 1995).

During the decay of this stone, the carbonaticbinding phase is destroyed and salts areproduced, which can crystallise inside thematrix with great volume variations; micro-cracks and pores are therefore produced. Theresults of these transformations may be simplesurface alterations, exfoliation and detachmentsor even deep holes. The decay due to acid rainshas been studied for all the rocks of theFinale’s region, using both natural stonesamples and samples protected by fluorided co-polymers (Contardi et al., 2000).

PIETRA DI PROMONTORIO

(STONE OF PROMONTORY)

This dark-grey bluish verging on black stonebelongs to the Flysch of Monte Antola, asedimentary sequence made by limestone,sandstone and marly-limestone which crops outalong the seacoast and inland between Genovaand Chiavari. It is a marly limestone(wackestone, wackestone-packstone andpackstone, with calcareous micritic bindingfraction), relatively compact, whose size rangesbetween silt and clays. It may be schistose,with high density of dissolution surfaces andlocal accumulation of phyllosilicates, and itmay be compact with high amount ofterrigenous minerals showing few dissolutionsurfaces but sometimes with micro-cracksfilled by spathic calcite.

The first reliable news about the utilisation ofthe stones quarried in the very western part of oldGenova dates back to XII century, when between1155 and 1159 the town walls were enlarged,fearing the invasion of the Emperor FedericoBarbarossa (the first walls of Genova date backto 849-899). The new walls stretched from Portadei Vacca, Porta Soprana (Photo 5) and Sarzano,with a total length of nearly 2500 m. Few parts ofthese walls still exist and many of them havebeen lately restructured; belong to them the towngates (Porta dei Vacca and Porta Soprana) andthe towers belonging to rich families: Torre degliEmbriaci and Torre dei Piccamiglio (XIIcentury), Torre degli Spinola (XIII century).


Photo 4 – Decoration in «Pietra di Finale» of a portal in thechurch at Borgio Verezzi (Province of Savona)

The «Pietra di Promontorio» is veryimportant for medieval Genoese architectureand was used in many buildings and churches:the Commenda of Pré (1180), the SantoStefano (965) and St. Agostino (second half ofXIII century) churches, the Cathedral of SanLorenzo (886-916), the Doria Palace and theSan Matteo church (1125). Also in 1125 theSottoripa arcades were built, with pillars andarches in «Pietra di Promontorio». DuringRenaissance and Baroque, palaces andchurches were usually built in plasteredmasonry, often frescoed or stuccoed; but insome cases also black-stone ashlars were used,as it is possible to see in palaces along ViaGaribaldi and Via Cairoli.

The means used to ship the stone blocksquarried in Promontorio (the hill that joinedGranarolo with Capo di Faro, where «laLanterna» – the Lighthouse – which is the

symbol of Genova) were enough old-fashioned,but in spite of that it was possible to obtainfrom those quarries the huge amount of verybig blocks necessary to build the underwaterfoundations of many piers and wharves.

Before cutting the San Benigno hill, the samequarries have been exploited for so many otherworks, from the construction of all the banksinside the port, included the one whereMagazzini Generali (General Storehouses)stand in Molo Vecchio (Old Dock) and the«Boccardo» and «Gadda» piers, to the CarloFelice Theatre, whose black-stone arcade linksup with the nearby arcade of Fine ArtsAcademy building.

The best varieties of this stone are the finest,most compact, even-grained ones; they haveoften been utilised for many differentarchitectural uses (Photo 6) and fordecorations, especially for bas-relieves.


Photo 5 – Porta Soprana in «Pietra di Promontorio»,Genova

Photo 6 – «Pietra di Promontorio» and white marblesashlars in St. Lorenzo Cathedral (X century), Genova: notethe differential weathering of the «Pietra diPromontorio»blocks

Chemical-physical features make «Pietra diPromontorio» a lasting stone, but amineralogical and petrographic analysis ofsome samples extracted along the town wallshas shown that the decay process is higher inthe external parts of the stone and lower inside(Barrese et al., 1996); calcite dissolves and, asa consequence, surface porosity increases.Besides, the percentage of magnesium-richcalcite increases near the altered rims, and thisfact may reduce the material cohesion duringsudden changes of temperature. Cohesionreduction appears inside micro-cracks in themost compact samples and along solutionseams in the schistose ones, increasing porosityand accumulating clay minerals along thedissolution surfaces (Beretta et al., 2001).

Chalk crystals have been found inside manymicro-cracks; they cyclically dissolve and re-crystallise, repeatedly increasing the pressureon the cracks, and this may take fragments off.The presence of calcite rich in magnesium andchalk together may be the cause of the evidentmicro-cracking and micro-flaking phenomenatypical of the «Pietra di Promontorio» blocks inwalls and buildings.

Further mineralogical-petrographical andphysical-mechanical investigations on somesamples drawn from the medieval TorrigliaCastle, near Genova, have shown the decayeffects in the mountains, where daily andyearly temperature ranges are high (Cimminoet al., 2002). In particular, it has been observedthat samples subjected to humid conditioningor to freezing processes reduce their averageresistance to simple compressionapproximately by !/4 with respect to the valuesobtained in dry conditions.

MT. ZATTA OR MT. GOTTERO SANDSTONES

Among all the kinds of locally usedsandstone, the ones employed also out ofLiguria belong to Mt. Zatta or Mt. GotteroFormation, widely outcropping in Ligurian-Emilian Apennine; these sandstones haveplane-parallel, sometimes weakly convolute,

laminations, their grain ranges from very fineto coarse and are grey, brown-grey and darkyellow in colour.

The rock has a granular, low-maturitytexture. In fine-grained samples, minerals tendto line up along a preferred direction, with aslightly iso-oriented texture.

The framework is composed of quartz,plagioclase and k-feldspar. Mono- and poly-crystalline quartz is more contained infragments of rock and, together with feldspars,forms more than 50% of the framework itself.Fragments of metamorphic rocks (micaschists,gneiss) prevail over volcanic ones (middle-acidrocks); muscovite, biotite and chlorite are poor.Also very poor is the amount of calciticcement. Terrigenous constituents contribute formore than 50% to the total rock volume,therefore these sandstones can be consideredTerrigenous Arenites.

All the mineralogical-petrographical andphysico-mechanical results, both original(Cimmino et al., 2003) and available inliterature (Di Battistini and Rapetti, 2003)confirm the excellent qualities of Mt. GotteroSandstone.

Many historical evidences exist thatsandstone have been widely used duringcenturies for foot-paths (Marchi, 1994) androad paving in the main towns included in thetriangle whose vertices are the towns ofGenova, La Spezia and Parma.

Because of its easy workability and highdurability, sandstone has been used in the pastand is today taken into consideration forrestoration and replacing of paving, mostly intowns and villages of its homeland. As a matterof fact, during last decades many streets in oldtowns had been covered with asphalt, so toavoid the wear and tear of the stone, and as aconsequence its maintenance.

In recent years the cultural evolution of localgovernors, the increased aesthetic sensibilityand the necessity to make historical villagesenjoyable through pedestrian ways havebrought people to re-discover the old-timestone paving.

As a matter of fact in many cases, such as


during road works aiming to replace mains orto renew street furniture, brand-new sandstonepaving has been realised (Photo 7), orsometimes the old sandstone blocks still ingood conditions have been maintained togetherwith new blocks used to replace the old ones inpoor conditions.

About decay, alteration processes that affectsandstone cause loss of cohesion among therock components, sand formation anddesquamation, the latter being linked above allto the presence of structural and texturaldiscontinuities. In particular, decay affects notonly the easily alterable or water-sensitiveminerals, but also the binding fraction (cementand/or matrix).

Therefore, the problems connected withsandstone conservation are quite complex, alsobecause the restoration works are oftenundertaken when the situation is extremelyserious. This happens mostly because

sandstone alteration usually appears all of asudden. In the first phase of alteration theexternal part, which is the most exposed, showsitself as a kind of coherent and even structurethat hides the altered part underneath, wherecohesion is lost. When in the end the exposedsurface falls down, the rock shows its true stateof bad conditions.

OPHICALCITES

Ophicalcites are rocks made by serpentinitebreccias sutured by calcite veins variouslyinterwoven, quarried around Levanto andFramura (eastern Liguria) and in PolceveraValley (north of Genova) because of theircolours: red and green the former, white andgreen the latter. They are used since 1200 asornamental materials (coloured marbles), at thebeginning as columns in religious buildings inthe old town of Genova, then for flooring.Ophicalcites are tectonic breccias with gapingfractures, whose sizes range from mm to dm,filled with spathic calcite, fragments of rockand micro-sparite calcareous mud. Near orinside the fractures serpentinite is partiallyreplaced by carbonates and magnetite turns tohematite, with a quite intense reddish hue. Pinkhue of various intensity is shown by micro-sparite mud and calcite is white («Rosso diLevanto»). In the Sestri-Voltaggio zone, nearPietralavezzara, fractures in the rock are almostalways thin and filled with calcite («VerdePolcevera»).

«Rosso Levanto» mainly crops out in aregion whose boundaries are the villages ofVelva, Pavareto, Carro and Ziona northwards,Mezzema, Deiva, Framura westwards,Bonassola and Levanto southwards, MountRossola and Mount Guaitarola eastwards.

According to petrographical and lithologicalfeatures, two main lithotypes exist: «Breccia diLevanto» («Rosso di Levanto» s.s.) and «Brecciadi Bonassola» (Cortesogno et al., 1980).

The «Breccia di Levanto» occurs at the topof serpentinites that represent the basis ofvolcanic-sedimentary sequences and is


Photo 7 – Sandstone elements used in the paving of St.Lorenzo Square, Genova

composed of highly fractured serpentinitessutured by sparry calcite. The larger fracturesappear as true sedimentary lodes, with clastsmade of serpentinite and lower percentages ofgabbros and lody basalts. Circulation ofhydrothermal fluids causes carbonation ofserpentine and oxidation of the magnetite intohematite. In serpentinite fragments theoxidation is centripetal and reddens thecarbonatic elements, clasts, matrix and cement.

The name of «Breccia di Bonassola» definesall the ophiolitic breccias of sedimentary originlaying between the serpentinitic basementwhich includes the «Breccia di Levanto» andthe overhanging sedimentary sequences. As afunction of fragment composition, one candistinguish the «Breccia di Framura» (containsmostly serpentinite) and the «Breccia diRossola» (contains mostly gabbros or basalts orboth).

Differences in petrogenesis mean differencesin trade. The most valuable variety, nowadaysalmost exhausted, corresponds to thesedimentary lodes of «Breccia di Levanto». Inthem the size of the elements is proportionate,red, white and green colours are brilliant andthe rock is compact and easy to work. A bit lessvaluable is the «Breccia di Levanto» wherefractures are smaller, speckles are less evidentand hematite pigmentation is fainter. Amaterial with finer grain and small speckles isextracted from sedimentary breccias («Brecciadi Bonassola»); it often shows clay-hematitecoats («mattonato», «brick-floor») that reduceits durability and value.

The history of «Rosso Levanto» began inancient times, as testified by some Etruscanfuneral pieces of work (Rovereto, 1939). Thecore of the production (both as tradition and ascommercial quality) has always been the coastbetween Levanto and Framura; it increasedbetween 1880 and 1929 then decreased again.Later, although the historical sites continuedtheir activity, quarrying activity moved to LaSpezia area and in the Sixties reached its top,with many sites in the western and northernparts of the area.

The stone was used in many Genoese palaces

and churches during Middle Ages andRenaissance (altars, floors, facings andcoatings, columns – Pieri, 1950). The mostimportant piece of work where «RossoLevanto» has been widely used is the SanLorenzo Cathedral in Genova: to realise itscolumns (Photo 8), the «column quarry» wasopened in Levanto, and was re-opened againduring XVI century to build St. Ambrogiochurch. In old Ligurian towns, mostly inGenova and Chiavari, «Rosso Levanto» hasbeen often used also as external facings ofshops, luxury palaces, staircases of villas, etc.

«Verde Polcevera» consists of greenserpentinite breccias with white calciumcarbonate cement. It is quarried mostly atPietralavezzara (Campomorone), near Passodella Bocchetta, in the hinterland of Genova; it


Photo 8 – Particular of a portal of St. Lorenzo Cathedral (Xcentury), Genova: note the presence of «Rosso Levanto»(a), «Verde Polcevera» (b), White Marble (c) and «RossoAmmonitico» (d)

usually shows a dark green background similarto «Verde Alpi» and it is often sold with thisname. Two commercial varieties exist: firstquality «Verde Polcevera» with dark greenbackground and small white spots and branchesfading into green; second quality «VerdePolcevera», richer in white parts and branches.«Rosso Polcevera» can be considered one of itsvarieties, quarried near Campomorone and nearthe «Verde Polcevera» bodies. It is anophicalcite with bloody red and dark rustgradations due to oxidation phenomena; its hasa mixed red and green background with veryeffective varying veins.

«Verde Polcevera» is being used forcenturies both indoor and outdoor. In Genova ithas been used to panel main doors, banks andshops, as well as the opening of CristoforoColombo Tunnel in Dante Square.

From «Rosso di Levanto» and «VerdePolcevera» it’s possible to obtain beautifulpolished slabs, easy to work and with goodstrength, suitable for indoor uses; outdoor usemust be limited to the best qualities because ofselective decay, chalking, etc, affecting spathiccalcite above all.

PORTORO

«Portoro» crops out in the very eastern partof Liguria, near La Spezia, between Mt.Castellana and Mt. Muzzerone, on Palmariaand Tino islands (whose quarries have alsoprovided the best commercial quality), atValdipino and on Mt. Bermego. The ancientname of «Portoro» was «black and yellowmixture» or «Portovenere»; «Portoro» locutionwas born during Napoleone Bonaparte’sempire, from the French «porte de l’or», whileEnglish call it «Black and Gold».

The stratigraphic horizon of «Portoro» iscomposed of dark micritic limestone, blackishor seldom grey-violet, with typical yellowveins intercalated with thin ochre-yellow marl-clay levels.

«Portoro» consists of a black matrix, yellowveins and violet veins; the matrix consists of a

very fine and regular micrite, strongly coloured,whose colour intensity decreases in re-crystallised zones; yellow veins contain limoniteand sulphides, whose colouring-matter is spreadin spots among dolomite crystals; violet veinsconsist of dolomite mosaics where the mostcoloured zones contain a hematite pigment.

In «Portoro» there is a very little amount ofdebris and the organic substances, bitumen andsulphides producing the black colour comefrom anaerobe environments, probably deadwaters near a dolomite reef, represented by«Calcare Massiccio» whose basement is«Portoro» itself.

«Portoro» was used already in I century a.D.as ornamental stone in many religious buildingsand in palaces; Romans even used it in littleblocks to build the amphitheatre of Luni and topave the main urban roads, before using it as avaluable ornamental marble. Its most valuablevariety, the «big-spotted» one, has been used inthe «Chiesa del Gesù» in Genova for the littlecolumns of the banisters (Photo 9). During XXcentury it was also used in cemeterialarchitecture and in local handicraft.

Intensive quarrying and exploitation of«Portoro» re-started after World War II andlasted up to all the Fifties, but today productionis very poor. Two commercial varieties of«Portoro» exist, each variety being subdividedinto three qualities: large-veined «Portoro»(«Portoro a macchia larga») and thin-veined«Portoro» («Portoro a macchia fine»). Thelarge-veined «Portoro» has veins up todecimetres in size connected by a branch-likepattern: within this variety, Extra quality hasbrilliant-gold yellow spots and very blackbackground, First quality («Portoro di Prima»)less brilliant yellow veins and some calcitewhite veins on black background, Second – orOrdinary - quality («Portoro di Seconda» or«Corrente») has mixed yellow and white veinson grey background. The thin-veined «Portoro»has thin gold-yellow veins interwoven insidethe whole mass: it is also subdivided into Extra,First and Second – or Ordinary – qualities.

«Portoro» is mostly used in regions with ahot weather (Sicily, Puglia, Provence, Northern


Africa) while in Northern Europe, where theweather is colder, it has been replaced by moreweatherproof and bright marbles. As a matterof fact, weathering agents cause a loss ofbrilliance, therefore, few years afterinstallation, «Portoro» appears irreversiblyopaque, whitened and corroded. The use of«Portoro» is therefore limited up there toindoor paving and coatings, handicrafts, worksof art and slabs for furniture.

FINAL REMARKS

The main mineralogical, petrographic andphysical-mechanical features of someparticular ligurian lithotypes («Ardesia»,

«Pietra di Finale», «Pietra di Promontorio»,Mt. Zatta or Mt. Gottero Sandstones, «RossoLevanto», «Verde Polcevera» and «Portoro»)confirm good qualities of such stones andcoloured marbles.

In particular, the Slates, the «Pietra ofPromontorio» and the Mt. Zatta or Mt. GotteroSandstones, used also as building stones, havehigh values of the compressive and flexuralstrength, also after gelivity tests.

The «Pietra of Finale» has a mechanicalbehaviour lightly lower, following itsmineralogical and petrographical features,which suggest to be used as covering.

Also for the Ophicalcites and the «Portoro»the compressive and flexural strength aresuitable to be used as decorative materials;


Photo 9 – Little columns made by «Portoro» in the banisters of Jesus’ Church, Genova

nevertheless for the «Portoro» it is notsuggested the external utilization, due to thewater absorption coefficient too high.

Because of their workability and durability,such lithotypes have been employed in the pastand they are today widely used both for therestoration of historical monuments and formodern manufacts. In particular, the Slates, the«Pietra of Finale» in general all the colouredmarbles have been carefully utilised andvalorised out of Liguria and also in the foreigncountries.

As matter of fact, it is necessary to deepenpetrographic-textural studies on the primaryfeatures of these stones and marbles, to reach abetter knowledge on their weatheringprocesses, especially for those ones used inMediterranean area; in particular, laboratorytests are strongly needed, with the aim tocontribute to the identification of the physico-chemical mechanisms causing the rock decay,for a better preservation of the architecturalheritage.

REFERENCES

BARRESE P., BERETTA C., CIMMINO F., MANNONI T.and ROBBIANO A. (1996) — Il degrado dellaPietra di Promontorio. Plinius, 16, 30-31.

BERETTA C., CIMMINO F. and PIAGGIO M. (2001) —Il degrado del calcare marnoso del M. Antola inantichi monumenti del Genovesato: studiopetrografico-applicativo . In: «Le PietreOrnamentali della Montagna Europea»,Associazione Georisorse e Ambiente, 147-155.

CIMMINO F., FACCINI F. and ROBBIANO A. (2002) —Caratteristiche fisico-meccaniche di un materialelapideo ligure (calcare marnoso di M. Antola)utilizzato come pietra da costruzione in unmanufatto emergente di pregio monumentale.G.E.A.M., 107, 25-29.

CIMMINO F., FACCINI F. and ROBBIANO A. (2003) —L’utilizzo delle Arenarie di M. Zatta nellepavimentazioni dei centri storici delle zone diprovenienza (Appennino Ligure-Emiliano). FISTGEOITALIA, 326-327.

CIMMINO F., ROBBIANO A. and PRIMAVORI P. (1995)— La pietra di Finale. Marmo Macchine, 124,168-183.

CONTARDI V., FRANCESCHI E., BOSIO S., ZANICCHI

G., PALAZZI D., CORTESOGNO L. and GAGGERO L.(2000) — On the conservation of architecturalartistic handwork of the «Pietra di Finale». J.Coult. Heritage 1, 83-90.

CORTESOGNO L., GAGGERO L. and REPETTO G.(1998a) — Petrography, properties and defects ofLigurian Slates. Plinius 20, 87-89.

CORTESOGNO L., GAGGERO L. and REPETTO G.(1998b) — The Ardesie Formation in the NorthernApennine: stratigraphic and structural setting inthe perspective of quarrying and of technicalproperties. Plinius 20, 89-91.

CORTESOGNO L., GALBIATI B. and PRINCIPI G.(1980) — Le brecce serpentinitiche Giurassichedella Liguria orientale. Arch. Sc. Genève, 33,185-200.

DI BATTISTINI G. and RAPETTI C. (2003) –ARENARIA — Pietra ornamentale e dacostruzione nella Lunigiana. Silva Ed., Parma, 269pp.

GIAMMARINO S., GIGLIA G., CAPPONI G., CRISPINI L.and PIAZZA M. (2002) — Carta Geologica dellaLiguria, scala 1.200000. Litografia ArtisticaCartografica Ed., Firenze.

ISTITUTO COMMERCIO ESTERO (1982) — Marmid’Italia - Guida tecnica. F.lli Vallardi Ed., Milano2° vol.

MARCHI P. (1994) — Pietre di Liguria. Sagep Ed.,Genova, 383 pp.

PANDOLFI D. (1972) — L’industria dell’ardesia inLiguria. L’industria Mineraria, 23, 595-612.

PIERI M. (1964) — I marmi d’Italia. Hoepli Ed.,Milano, 480 pp.

ROVERETO G. (1939) — Liguria geologica. Mem.Soc. Geol. It., 2, 743 pp.

SAVIOLI L. (1988) — Ardesia, materia e cultura.Sagep Ed., Genova, 222 pp.

SOCIETÀ GEOLOGICA ITALIANA (1991) — Guidegeologiche regionali. Alpi Liguri. BE-MA Ed.,Milano, 295 pp.

SOCIETÀ GEOLOGICA ITALIANA (1994) — Guidegeologiche regionali. Appennino Ligure-Emiliano.BE-MA Ed., Milano, 382 pp.

TERRANOVA R. (1996) — Le cave di ardesia inLiguria: geologia, coltivazioni, lavorazioni eutilizzazioni. Mem. Soc. Geol. It., 51, 1165-1177.


Stones and coloured marbles of Liguria in historical...

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