GEOSCIENCES AND CULTURAL HERITAGE

Portoro, the black and gold Italian ‘‘marble’’

Fabio Fratini1 • Elena Pecchioni2 • Emma Cantisani1 • Fabrizio Antonelli3 •

Marco Giamello4 • Marco Lezzerini5 • Roberta Canova6

Received: 22 December 2014 / Accepted: 31 March 2015 / Published online: 21 April 2015

� Accademia Nazionale dei Lincei 2015

Abstract Portoro is one of the most famous Italian black

limestones due to its characteristic golden-yellow veins on

a black background. It was used since Roman times,

mainly in the city of Luni. Since the Middle Ages, its use is

widespread in Genoa, and from the XVII century, it be-

came one of the most common stones in religious buildings

throughout Italy. At the end of the XIX century, its use has

spread abroad, particularly in Europe and USA. It was

extracted in several quarrying areas located near La Spezia,

but at present, only five quarries are active. This stone,

exposed to weathering, tends to bleach losing the appear-

ance of its golden streaks that determine its aesthetic ap-

peal. This research deals with the petrographic and

chemical characterization of the Portoro macchia larga

variety as well as the study of its chromatic alteration in

order to define guidelines for the most suitable use of this

stone and for restoration works.

Keywords Portoro � Limestone � Characterization �Chromatic alteration � Patina

1 Introduction

Black limestones, traditionally called Neri Antichi or Bigi

Morati by modern day Roman stonemasons (Brilli et al.

2010), were commonly used in antiquity, often in con-

junction with coloured stones/marbles, for architectonic

and sculptural elements such as capitals, columns, bases,

opera sectilia and statues. Numerous quarries were ex-

ploited for this material in various parts of the Mediter-

ranean basin (Brilli et al. 2010); most were used

exclusively on a local scale because of the poor quality of

the stone, whereas better quality stones from a few quarries

spread throughout the Mediterranean itself. Exploitation of

many of these quarries started in very early periods, gen-

erally reaching its maximum extraction phase during the

Roman Empire (Brilli et al. 2010 and references therein).

However, some of them had a ‘‘second commercial life’’ in

modern times, particularly during the Renaissance and the

Baroque periods, due to the appeal of their particular tex-

tures and colour patterns (i.e. black limestones with more

or less coloured veins).

The Portoro limestone, also called Mischio giallo e

nero, Portovenere marble, Black and Gold, Giallo e Nero

di Portovenere (Caselli 1924), is one of the most famous

and valuable Italian black limestones whose success in

modern times is mainly due to its unique texture

This contribution is the extended, peer reviewed version of a paper

presented at the session ‘‘Archaeometry and Cultural Heritage: the

contribution of Geosciences’’ held during the conference ‘‘The future

of the Italian Geosciences, the Italian Geosciences of the future’’,

organized by the Societa Geologica Italiana and the Societa Italiana di

Mineralogia e Petrologia, Milano, 10–12 September 2014.

& Elena Pecchioni

elena.pecchioni@unifi.it

1 CNR, Institute for Conservation and Valorization of Cultural

Heritage, Via Madonna del Piano 10, Sesto Fiorentino,

50019 Florence, Italy

2 Earth Sciences Department, University of Florence, Via G.

La Pira, 4, 50121 Florence, Italy

3 Analysis Laboratory Ancient Materials, DACC, University

IUAV of Venice, San Polo 2468, 30125 Venice, Italy

4 Physical Sciences, Earth and Environmental Department,

University of Siena, Via Laterina, 8, 53100 Siena, Italy

5 Earth Sciences Department, University of Pisa, Via Santa

Maria 53, 56126 Pisa, Italy

6 Viareggio, Lucca, Italy

123

Rend. Fis. Acc. Lincei (2015) 26:415–423

DOI 10.1007/s12210-015-0420-7

characterized by prevailing golden-yellow veins on a black

background. Quarried both on the promontory and on is-

lands in front of Portovenere (Liguria), it was already used

in Roman times, particularly in the city of Luni as slabs for

the cardo and decumanus (II century sec. BC), in little

blocks for the amphitheatre (I century BC) and also for the

realization of columns (one of them is in the Ar-

chaeological Museum of La Spezia) (Del Soldato and

Pintus 1985). Its use widely increased since the XII century

when the Republic of Genoa exploited it mainly for

building defensive structures (Pandolfi 1971; Cimmino and

Robbiano 2005) but also for the construction and decora-

tion of monuments, cathedrals and villas. Nevertheless, it is

in the Renaissance and in the Baroque times that the use of

Portoro, which was frequently paired with the less famous

Portargento, a white-veined black limestone quarried in

the same region (Beltrame et al. 2012), spreads all over

Italy as ornamental material (Bonci 2007): for instance, in

the Baptistery of the Church of Saint Mary in La Spezia, in

the Palace of the Marquis of Castagnola in Genoa, in

Portovenere in Jesus Church, Sts. Ambrogio and Andrea

Church, St. Siro’s Church and in the font of the St. Peter’s

Church (Fig. 1); in the St. Mark’s Basilica of Venice,

where post-antique squared and triangular tiles are visible

in the floor of the western arm of the narthex; in Rome, in

the St. Peter’s Basilica (a mask carved by Giacomo della

Porta in the deposition of Paul III), in the churches of San

Pietro in Vincoli (St. Peter in Chains), San Silvestro in

Capite (St. Sylvester the First), San Paolo fuori le Mura (St.

Paul outside the Walls), San Giovanni in Laterano (St. John

Lateran), San Lorenzo fuori le Mura (St. Lawrence outside

the Walls), Santa Maria Maddalena in Campo Marzio (St.

Mary Magdalene in Marzio Field), Sts. John and Paul and

San Luigi dei Francesi (St. Louis of the French); in

Palermo, in Jesus Church also known as Casa Professa,

etc.

At the end of the XIX century, its use has spread abroad,

especially in England, North America (Paramount Home

Theatre) and secondarily Belgium and France (Versailles,

Marly, Compiegne), where it was utilized for fireplaces,

coverings, plinths and panels for furniture (Pandolfi 1971;

Bonci 2007). The dam crossing the gulf of La Spezia was

realized partially with the blocks of Portoro coming from

Palmaria Island.

Similar but poorly known historical stones were the

Portoro quarried in medieval times in the Monti Pisani,

which was suggested to correspond to the Nero della

Duchessa stone (Cataldi et al. 2013) and the brecciate

varieties quarried in the Apuan Alps (Bartelletti and

Amorfini 2003) both used only locally.

Historically, other materials of similar aspect, but fre-

quently brecciate, were extracted in Piedmont (Portoro di

Nava) (Fiora and Alciati 2003; Fiora et al. 2003) and in

France (Portor de Saint Maximin in Var, Portor des Pyr-

enees and Portor des Pyrenees Bramevaque in the Pyrenees

region) (Fiora and Alciati 2007).

This material, as well as other carbonaceous black or-

namental stones (Canova 2002) exposed to the weathering,

tends to bleach with time losing the look of its golden

streaks and spots on the black background that determine

the particular aesthetic appeal (Fig. 2).

The aim of this study was to perform a mineralogical,

chemical and petrographic characterization of the black

portion of the Portoro macchia larga variety and to

define its chromatic alteration which develops with the

formation of a whitish patina, in order to define guide-

lines for the most suitable use of this stone and for

restoration works.

Fig. 1 Font in Portoro (Baroque age), inside the St. Peter Church in

Portovenere

Fig. 2 Hand specimen of Portoro displaying the colour contrast

between the internal black stone and the bleached surface

416 Rend. Fis. Acc. Lincei (2015) 26:415–423

123

2 Geological setting and varieties of Portoro

Portoro crops out in the mountain chain that closes to the

west the gulf of La Spezia, from the village of Biassa to the

north, to Palmaria, Tino and Tinetto Islands to the south.

This lithotype is related to the stratigraphy and structure of

the area, constituted by a reversed NW–SE anticline with

Tyrrhenian vergence. Such anticline is followed to the east

by the La Spezia graben and the Lerici-Montemarcello

promontory (Ciarrapica and Passeri 1980). Such structures

are the consequence of the deformative phases that affected

the Tuscan Nappe from Late Miocene (Federici and Raggi

1975). Moreover, systems of direct and transcurrent faults

with direction NW–SE, NE–SW and ENE–WSW succes-

sive to the Late Miocene deformative processes are also

present (Carter 1991). Particularly, the Portoro formation

is delimited at the bottom by the ‘‘La Spezia Formation’’

(which is in turn divided into two members, ‘‘limestones

and marls of Monte Santa Croce’’ and ‘‘Portovenere

limestones’’) and at the top by the ‘‘Monte Castellana

Dolomites’’ (upper Rhaetian–Hettangian).

The main layer of Portoro suffered stretching that re-

duced, in many areas, its thickness that is at maximum

6–7 m. It consists of a dark grey to black nodular limestone

with white- and yellow-coloured dolomitic vein patterns

(Abbate et al. 2005) (Fig. 3).

A more detailed observation of the Portoro bed (which

crops out always overturned) makes it possible to evidence

the following levels, named, from top to bottom (over-

turned sequence): Scalino marmorizzato, Scalino, Banco,

Sottobanco or Zoccolo, Sottozoccolo (Pandolfi 1971; Chelli

et al. 2005; Ciarrapica 1985) (Fig. 4):

• Scalino marmorizzato is a fine-grained black-grey

limestone that, when very rich in dolomite, is called

Tarso and it is not commercialized;

• Scalino is constituted by at least five golden-yellow

veins on an absolute black background. The thickness

of this level is between 0.8 and 1 m; in every six veins,

there is a quite straight vein (sometimes, it can be found

in the Banco);

• Banco shows a black background with at least seven

golden-yellow veins, some of them wider than the

previous ones. The thickness of this level is

1.20–1.50 m;

• Sottobanco or Zoccolo is characterized by narrower

yellow veins and diffused white veins. The thickness of

this level is 1.20–1.50 m;

• Sottozoccolo is thicker than the previous one and with

more diffused white veins;

The described sequence is that typical of Portoro mac-

chia larga (large spotted), the most famous and prized,

which is separated from the underlying Portoro macchia

fine (narrow spotted), by the Nero e Bianco di La Spezia

(also named Portargento), a level that shows completely

dominant white veins. The Portoro macchia fine is con-

stituted by very fine straight or irregular veins and is less

valuable. Therefore, within the Portoro macchia larga, the

first quality that can be found is the Scalino, characterized

by an absolute black background with golden-yellow veins,

while a gradual fading is observed going towards the bot-

tom of the Portoro bed (Sottozzoccolo) where less valuable

varieties are present (Pieri 1966).

From the commercial point of view, both the Portoro

macchia larga and Portoro macchia fine can be distin-

guished in four qualities for a total of eight typologies

according to the intensity of the colour of the background

and of the veins (Cimmino et al. 2003), today not all of

them available on the market.

According to Ciarrapica and Passeri (1980), the Portoro

limestone has been classified as a mudstone (Dunham

1962), and the black colour is due to the dispersion of very

minute particles of carbonaceous matter, sometimes con-

centrated in veins and stylolites. Locally, some veins of

secondary microsparitic calcite, often isoparallel, are pre-

sent. Authigenic grains of quartz are sometimes observed

dispersed in the carbonatic mass or concentrated in thin

layers. The pigmentation is particularly evident in the

Scalino facies.

The veins of different colours show the following

composition:

– the golden-yellow veins are characterized by limonite

and sulphides present among dolomite crystals;

– the white veins are formed by coarse-grained dolomite

crystals.

Sometimes are present also purplish veins consti-

tuted by mosaics of dolomite crystals coloured by

haematitic pigment dispersed or concentrated into red

stylolites.

3 The quarrying areas

Portoro was extracted in several quarries located near La

Spezia (Liguria region, NW Italy), precisely on the

promontory of Portovenere and in Palmaria and Tino Is-

lands, located in front of Portovenere, where the extraction

began in the open air and continued underground with the

technique of the abandoned pillars (Fornaro 1999; Cim-

mino et al. 2006).

In the XIX century, Cappellini (1864) reports the loca-

tion of thirty quarries existing in the area of La Spezia, one

of which on Tino Island and five on Palmaria Island

Rend. Fis. Acc. Lincei (2015) 26:415–423 417

123

Fig. 3 Geological map of the

area where Portoro outcrops are

present (modified after Abbate

et al. 2005)

418 Rend. Fis. Acc. Lincei (2015) 26:415–423

123

(Fig. 5). In the XX century, the quarrying activities on the

islands continued with ups and downs. Indeed, at the

beginning of the century, the activity in Palmaria was

remarkable (at least 10 quarries); at the end of the 1930s,

the extractive industry faced a crisis which partially re-

covered after the Second World War. Subsequently, be-

cause of operational difficulties due to both environmental

constraints and depletion of stone veins of good quality,

the quarrying activity on the islands slowly declined up to

cease at the beginning of the 1980s when the quarry of

‘‘Caletta’’, located in front of Tino Island, was closed.

Since 1997, Portovenere together with Palmaria and Tino

Islands are part of the UNESCO World Heritage sites and

since 2001 constitute the Natural Regional Park of

Portovenere.

Concerning the amount of production, 1959 was the year

of greater production, with 18,024 tons extracted; in the

following years, the production stabilized around 6000 tons

per year (Giordano 1969) and towards the beginning of the

seventies raised to about 10,000 tons per year (Pandolfi

1971).

Currently, only five quarries are active: the quarries of

‘‘Cavetta’’ and ‘‘Anime’’ in the municipality of Por-

tovenere, the quarries ‘‘Castellana I’’ by Falconi

Domenico, ‘‘Castellana’’ by Portoro BCC and ‘‘Santa

Croce’’, in the locality of Santa Croce, all in the mu-

nicipality of La Spezia. The consequent fall in produc-

tivity exposed this ‘‘marble’’ to the competition of similar

commercial materials from abroad, cheaper and with huge

production like Portoro Leonardo from Namibia, Portoro

Santo Domingo and a variety from China (Fiora and Al-

ciati 2007).

Only the knowledge and awareness of the original Ital-

ian Portoro (namely the kind and disposition of the veins,

their composition and the background colour) may help in

Fig. 4 Lithostratigraphic

sequence of the Portovenere

limestones (modified after

Chelli et al. 2005)

Fig. 5 Abandoned quarry of Portoro in Palmaria Island

Rend. Fis. Acc. Lincei (2015) 26:415–423 419

123

recognizing its value with respect to other lithotypes

commercialized with the same trading name.

4 Materials and analytical methods

The research has been carried out on ten samples with a

whitish chromatic alteration taken from the ‘‘Castellana

I’’ quarry, located in the municipality of La Spezia

belonging to the Portoro macchia larga variety and

particularly from the Scalino level. On the black por-

tion of the internal unweathered part of the stone (be-

low the patina), the following analyses have been

carried out:

– mineralogical composition was carried out on pow-

dered samples using a PANalytical X’Pert PRO

diffractometer with monochromatic CuK a1 radiation,

operating at 40 kV, 30 mA, investigated 2hrange = 3�–70�, equipped with X’ Celerator multi-

revelatory and High Score data acquisition and inter-

pretation software. Analyses were carried out on the

bulk rock and also on the residue after acid attack with

hydrochloric acid (2 % w/v) and on the fraction\4 lmextracted by sedimentation according to the

Stokes’law;

– major elements composition was carried out by X-ray

fluorescence (XRF) on pressed powder pellets, using a

Philips PW 1480 wavelength dispersive XRF spec-

trometer with Rh anode. The procedure for the

correction of the matrix effect and the calculation of

the percentages (all elements are expressed as a

percentage rounded to the second decimal) according

to the method of Franzini et al. (1975) was followed;

total volatile components (H2O? and CO2) were

determined as loss on ignition (LOI) at 950 �C on

powder dried at 105 ± 5 �C;– determination of the CaCO3 content was carried out

with a Dietrich-Fruhling calcimeter;

– quantitative analysis of total carbon and organic carbon

was carried out using a GC-CHN Elementar Analyzer

FISONS NA 2000 gas chromatograph with a thermal

conductivity detector at the Laboratory of CHN—Mass

spectrometer of the Institute for Marine Geology of

CNR—Bologna: the total carbon was determined from

the untreated sample powder, while the organic carbon

was determined from the analysis of the powder treated

with hydrochloric acid;

– in order to investigate the composition of the organic

substances, FT-IR (Fourier Transform Infrared Spec-

troscopy) analyses through the Golden Gate were

carried out on untreated powders, on insoluble residues

after attack with hydrochloric acid, on the residual

powder after heating at 600 �C and on extracts in

different solvents (chloroform, hexane/methylene chlo-

ride 70/30, benzene/ethanol 2/1).

On the bulk sample inclusive of the patina, the following

analyses were performed:

– petrographic analyses was carried out on thin sections

with a ZEISS Axio Scope.A1 polarized microscope

equipped with a 5 megapixel camera resolution and a

dedicated AxioVision image analysis software, in order

to study the texture of the stone and the aspect of the

surface in cross thin sections;

– morphological and microchemical analyses on strati-

graphic sections perpendicular to the surfaces of

alteration were carried out with a scanning electron

microscope Zeiss EVO MA 15, coupled to an analytical

system (EDS OXFORD INCA 250). The following

standards were used for the semi-quantitative analyses:

albite, MgO, Al2O3, SiO2, wollastonite, MAD-10

feldspar, Ti and Fe.

5 Results and discussion

The XRPD mineralogical analyses of the internal un-

weathered part of the stone (below the whitish patina)

show the exclusive presence of calcite with traces of

dolomite and quartz. The analysis of the insoluble residue

after hydrochloric acid attack shows the presence of

quartz and phyllosilicates (micas and clay minerals); the

analysis of the fraction \4 lm makes it possible to rec-

ognize the types of clay minerals: illite, kaolinite and

chlorite (Table 1).

The results of the XRF chemical analyses of the major

elements are reported in Table 2.

The XRF data show mainly high values of CaO, in

agreement with the CaCO3 data obtained through cal-

cimetry and with the abundant presence of calcite (XRPD).

The low amount of MgO (XRF) is referred to the presence

of dolomite in traces (XRDP). Generally, the presence of

MgO and dolomite is to be put in relation with

Table 1 Mineralogical data obtained on ten samples of Portoro

(internal unweathered part of the stone, insoluble residue after acid

attack, fraction\4 microns)

Samples Composition inner

stone

Insoluble

residue

Fraction B4 lm

Portoro Calcite xxx

Dolomite tr

Quartz tr

Quartz xx

Phyllosilicates x

Illite xxx

Kaolinite x

Chlorite tr

xxx very abundant, xx abundant, x present, tr traces

420 Rend. Fis. Acc. Lincei (2015) 26:415–423

123

dolomitization phenomena that in the variety Scalino are

very low, justifying the particular dark colour of the rock.

Also, the XRF data of SiO2 are low and must be put in

relation with the low content of quartz.

Besides, the good correlation between the values of CO2

obtained by calcimetry compared with data of calcination

obtained through loss on ignition which refers particularly

to the loss of CO2 (Table 3) suggests only a possible

presence of organic matter, certainly below the 1 %, ana-

lytical detection limit of both methods.

The analyses performed through GC-CHN Elemental

Analyzer (after attack in hydrochloric acid) show an av-

erage value of organic carbon of 0.081 ± 0.005 C %,

therefore a low percentage, but sufficient to determine the

dark colour when widespread in the fine carbonatic matrix.

The FT-IR data determined on the bulk samples and on the

residue after acid attack on the extracts in hexane/methy-

lene chloride and benzene/ethanol have not identified any

organic substance.

The petrographic analysis on cross thin section shows

that the stone has the typical appearance of a mudstone,

with a micritic texture and a partially heterogeneous aspect

due to the presence of dispersed little dark dots with

Fig. 6 Cross thin section of the weathered surface of Portoro

observed in transmitted light (xpl): the surface shows a thin layer of

recrystallized calcite (20 lm thick). Below this recrystallization layer,

at a depth of about 100 lm, a 80-lm-thick layer rich in dark dots is

present

Table 2 XRF data (wt%) and calcimetry (CaCO3 contents) of ten Portoro samples (internal unweathered part of the stone)

Samples SiO2 TiO2 Al2O3 Fe2O3TOT MnO MgO CaO Na2O K2O P2O5 L.O.I. CaCO3g

PT1 0.78 0.01 0.39 0.15 bdl 1.66 53.30 0.01 0.10 bdl 43.70 97.90

PT2 0.65 0.02 0.33 0.18 bdl 1.50 53.50 0.00 0.10 bdl 43.90 98.40

PT3 0.58 0.01 0.29 0.10 bdl 1.16 54.00 0.01 0.10 bdl 44.30 98.50

PT4 0.75 0.01 0.38 0.08 bdl 1.80 53.30 0.03 0.10 bdl 43.60 98.45

PT5 0.64 0.02 0.32 0.07 bdl 1.28 54.60 0.00 0.10 bdl 43.20 98.75

PT6 0.58 0.01 0.29 0.15 bdl 2.06 53.80 0.06 0.00 bdl 44.10 98.50

PT7 0.66 0.01 0.33 0.14 bdl 1.82 53.40 0.01 0.10 bdl 43.30 98.60

PT8 0.89 0.01 0.45 0.14 bdl 1.90 52.80 0.04 0.10 bdl 42.20 98.30

PT9 0.75 0.02 0.38 0.13 bdl 2.00 53.20 0.05 0.20 bdl 43.10 98.40

PT10 0.90 0.01 0.45 0.13 bdl 1.80 53.00 0.04 0.20 bdl 43.20 98.60

�X 0.72 0.01 0.36 0.13 bdl 1.70 53.50 0.03 0.11 bdl 43.46 98.44

r 0.12 0.00 0.06 0.03 bdl 0.30 0.50 0.00 0.06 bdl 0.59 0.22

�X average value, r standard deviation, g gas for calcimetry, bdl below detection limit, Fe2O3TOT total iron expressed as Fe2O3

Table 3 Comparison between LOIf and CO2g

Campione LOIf CO2g LOIf–CO2g

Portoro mean values 43.46 ± 0.59 43.29 ± 0.20 0.17 ± 0.05

f data from fluorescence, g data from gas calcimetry

Fig. 7 Cross thin section of the weathered surface of Portoro

observed with SEM (BS image) showing the presence of a more

porous layer below the surface

Rend. Fis. Acc. Lincei (2015) 26:415–423 421

123

dimensions of 20–30 lm whose rim is not well defined

(organic substance). The surface, where the whitish chro-

matic alteration is present, shows a thin layer of recrys-

tallized calcite (20 lm thick). Below this recrystallization

layer, at a depth of about 100 lm, a layer 80 lm thick, rich

in dark dots, is present (Fig. 6). The presence of this layer

could be generated by imbibition and evaporation cycles

able to mobilize the organic substance under the surface.

According to these data, the chromatic alteration seems

to be due partly to the presence of a layer of tiny calcite

crystals which increase roughness, therefore favouring the

scattering of light (bleaching effect) and in part to the

depletion of pigmenting substances.

The SEM analyses performed on the same cross thin

section show the presence of a more porous layer below the

surface with interconnected porosity decreasing without

discontinuity inward (Fig. 7); the thickness of this layer is

approximately 100 lm. The microchemical analysis (EDS)

shows a similar composition between the surface and the

internal unweathered part of the stone (Table 4). As a

matter of fact, from the chemical point of view, it is im-

possible to recognize secondary calcite precipitation from a

support of the same composition.

6 Conclusions

Portoro, exposed to weathering, tends to bleach with time

losing the appearance of its golden streaks and spots that

determine its aesthetic appeal. The analyses carried out in

order to characterize the Portoro and to investigate the

chromatic alteration showed that the stone is constituted by

a calcitic matrix dark in colour due to the widespread

presence of a pigmenting substance. The literature data

indicate the organic pigment as the most frequent dark dye

of limestone rocks. Nevertheless, the analyses carried out

in order to recognize the presence of the organic substance

have only partially confirmed these data; as a matter of

fact, the GC-CHN Elementar Analyzer revealed only small

quantities (\1 %) of insoluble organic carbon (but

evidently sufficient to give an homogeneous dark colour

when dispersed in the fine carbonatic matrix), and the FT-

IR analysis was not able to detect soluble organic materi-

als, particularly as bituminous substances.

With respect to the whitish chromatic alteration of the

Portoro after exposure to weathering, we evidenced the

development of the following stratigraphic sequence (from

the surface):

– a thin layer of fine-grained recrystallized calcite

(20 lm thick);

– a 100-lm-thick porous level depleted in organic

substance;

– a 80-lm-thick layer enriched in dark dots, probably

made of organic substance.

These data allow us to explain the chromatic alteration

which is due to a double effect: in part to the presence of a

layer of tiny calcite crystals whose roughness favours the

scattering of light with a consequent decrease in the colour

saturation (bleaching effect) and in part to the depletion of

pigmenting substances just below this layer.

These data allow us to confirm that the use of this

lithotype as decorative material is not suitable in outdoor

conditions because of its chromatic alteration. As a matter

of fact, this stone material can be used preferably indoor or

outdoor but in zones not directly exposed to the action of

atmospheric agents, in order to prevent marked bleaching

effects.

As concluding remarks, the knowledge of the charac-

teristics of the Italian Portoro is fundamental to help in

recognizing and maintaining its value with respect to

commercial materials from abroad, named Portoro but far

from the aesthetic aspect of this historical Italian one.

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‘‘Portoro’’ surface 0.90 ± 0.15 1.10 ± 0.60 7.00 ± 1.50 89.00 ± 2.00 1.18 ± 0.40 0.65 ± 0.20

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