05-cimmino et al - Dipartimento di Scienze della Terra

Per. Mineral. (2003), 73, 71-84
SPECIAL ISSUE 3: A showcase of the Italian research in applied petrology
http://go.to/permin
An International Journal of
MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,
ORE DEPOSITS, PETROLOGY, VOLCANOLOGY
and applied topics on Environment, Archaeometry and Cultural Heritage
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
ABSTRACT. — The presence of a great number of
different lithotypes contributes to the extremely
variable geological landscape of Liguria: its rocks
are one of the main reasons of the natural beauty of
this region and a mean used by people to
economically exploit their land and to express
themselves in art and architecture.
Many rock types are locally used, but in Italy and
abroad some particular stones («Ardesia», «Pietra di
Finale», «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 as
decorative tombstones, both indoor and outdoor, of
the historical buildings and in the local
craftsmanship.
RIASSUNTO. —La realtà geologica ligure assume
caratteri peculiari per la sintesi estremamente varia
di facies e di litologie e la roccia costituisce un
grande patrimonio, non solo perché caratterizza una
morfologia di incomparabile bellezza, ma anche
perché rappresenta il mezzo attraverso cui si è
espressa una cultura nell’uso del territorio e si è
realizzata e consolidata nel tempo un’attività
economica per la produzione di materiale lapideo a
varie destinazioni d’uso.
* Corresponding author, E-mail: [email protected]
Numerosissimi sono i litotipi utilizzati
nell’architettura, soprattutto a scala locale, anche se i
più conosciuti, in ambito regionale, nazionale ed
internazionale, sono rappresentati da varie pietre
(Ardesia, Pietra di Finale, Pietra di Promontorio,
Arenarie di M. Zatta o di M. Gottero) e da alcuni
marmi colorati (Rosso Levanto, Verde Polcevera e
Portoro).
In funzione della loro lavorabilità e durevolezza,
questi litotipi sono stati impiegati in passato nelle
opere monumentali e sono tuttora largamente
utilizzati per il restauro dei manufatti storici e per la
realizzazione di quelli moderni, sia in Liguria sia
altrove.
Per questi motivi è necessario approfondire gli
studi minero-petrografici sulle caratteristiche
primarie di questi marmi e pietre in modo da
acquisire una migliore conoscenza dei processi di
alterazione, soprattutto in climi di tipo mediterraneo;
in particolare, specifiche prove di laboratorio
possono contribuire all’identificazione dei
meccanismi chimico-fisici che comportano il
degrado dei materiali lapidei, consentendo una
corretta conservazione del patrimonio architettonico.
K EY W ORDS : stones (Ardesia, Pietra di Finale,
Pietra di Promontorio, Mt. Zatta or Mt. Gottero
Sandstones), coloured marbles (Rosso Levanto,
Verde Polcevera, Portoro), Liguria, conservation,
restoration, historical monuments
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F. CIMMINO, F. FACCINI and A. ROBBIANO
INTRODUCTION
From a geological point of view, Liguria is
considered one of the most complex areas in the
Italian peninsula and in the whole
Mediterranean region, because of its composite
tectonic evolution during the Alpine orogenesis.
Many rock types crop out in Liguria, but
only some particular stones («Ardesia», «Pietra
di Finale», «Pietra di Promontorio», Mt. Zatta
or Mt. Gottero Sandstones) and some coloured
marbles («Rosso Levanto», «Verde Polcevera»
and «Portoro») are very known.
It is possible to describe in a brief way the
geological profile of Liguria and then to identify
the outcrops of these materials, as it is shown in
the Figure 1 (Giammarino et al., 2002; Società
Geologica Italiana, 1991; 1994). From west to
east, the regional territory of Imperia is
characterized by the Delphinese-Provence Zone
and the Ligure-Piemontese Flyschs, where are
quarried the slates of the Argentina valley.
The Permo-Carboniferous crystalline
basement with Mesozoic sediments
(Brianzonese Zone) are typical of the regional
territory in Savona, where the «Pietra di
Finale» has been deposed in the Miocene.
In the western area of the Genova region
large strips of ophiolitic rocks metamorphised
in high-pressure conditions are visible (Voltri’s
Group and Sestri-Voltaggio Zone where
«Verde Polcevera» crops out).
In the eastern area, the CretaceousPalaeocene flyschs outcrop and include either
the «Pietra di Promontorio» either the Lavagna
Formation and in particular the Mt. Verzi
Slates Member either Mt. Zatta or Mt. Gottero
Sandstones. Typical ophiolitic strips,
responsible of the «Rosso Levanto» formation,
appear on the edge between Genoa and La
Spezia. The regional territory of La Spezia is
characterized either by Mesozoic carbonate
rocks, where it is possible to identify the
«Portoro», either by Tertiary sediments.
In this paper, the outcrop zones and the main
mineralogical, petrographic and physicalmechanical features (Table 1) are described for
these stones and coloured marbles: particular
attention is dedicated to their use and their
degradation characteristics, with the aim to
investigate the problems related to the
conservation and restoration of stones in the
historical monuments.
ARDESIE (SLATES)
Ligurian «ardesia» is a dark grey slate rich in
calcium carbonate, outcropping in Eastern
Liguria within the Val Lavagna Formation and in
particular to the Mt. Verzi Member, cropping out
in the Val Lavagna itself and, more SE, in the
hills of Cogorno (Genova); slaty lithofacies also
exist in the western part of Liguria, near Triora
(Imperia), linked to marly horizons, sometimes
very large, inside Delphinese-Provence flysch.
Main minerals are calcite (around 30-35%),
phyllosilicates (illite, white mica, chlorite)
quartz, minor dolomite, sulphides, graphite. The
structure of layered-lattice silicates is the reason
why «ardesia» can be easily splitted into thin and
light slabs of different thickness.
It was about X-XI century b.C. when the first
slabs of «ardesia» were quarried in Santa
Giulia hill, above Lavagna. In a notarial deed
kept in the State Archives of Savona is written
that in 1176 a.D. people from Recco (Genova)
bought slabs for the roof of Santa Maria church
in Savona (Savioli, 1998).
From XII to XIV century «ardesia» was
increasingly used in towns and villages of
Ligurian Riviera, but during the following
centuries this stone was used both in western
and in eastern Liguria, mostly for local
artworks and decorations (Terranova, 1996).
A peculiar use of the «ardesia» slabs is as
bearing for paintings, because slabs do not
warp as wood does: many examples of this use
can be found in Genova, both in University
Library and in old-town churches and painters
like Andrea Semino, Pellegro Piola, Bernardo
Strozzi, Giulio Benso and even Rubens, whose
paintings are now in Santa Maria in Vallicella
church in Rome, painted on «ardesia» slabs.
But the main use of «ardesia» is to cover
roof (Photo 1): the fundamental element is the
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)
«abbadino»; a thin and flat slab 3-4 mm thick.
«Ardesia» roofs are fireproof, long-lasting
(Cortesogno et al., 1998a; 1998b) and do not
need a strong carrying structure. Ligurian
square abbadini fit the roof structure mostly
used in Genova and surroundings, the one
called «the triple overlap + three» (Pandolfi,
1972). In old times «ardesia» was used also for
paving and flooring, and in the first half of XIX
century «ardesia» for paving was the first
Italian «ardesia» export item, used in cloisters,
arcades, pavements, cellars, shops, terraces,
storehouses, balconies. In houses it was used
indoor for paving the ground floors, while in
churches floors with «ardesia» octagons and
small marble squares were used.
At the beginning of XX century big slabs up
to 5 m 2 were produced, the so-called
«australians», asked to pave footpaths in
Australia. «Ardesia» was also used to build big
troughs called «trogoli»: containers for olive
oil so big to contain even 7.5 tons of oil.
«Trogoli» were better than jars because inside
their larger volumes it was more difficult for
bulk density
water absorption
compressive strength
compressive strength
after gelivity
flexural strength
impact test
thermal coefficient expansion
abrasion coefficient
determination
125-135
18-120
28-58
5-10
40-95
30-40
0.0055-0.0065 0.004-0.005
0.35-0.40
0.20-2.60
MPa
MPa
cm
mm/m°C
18,80-27,00
0.65-1.10
20-135
Pietra di
Finale
27,15-28,50
0.20-0.40
140-150
Ardesie
kN/m3
%
MPa
unit value
65-160
28-60
-
26,65-26,80
0.20-0.35
90-190
Pietra di
Promontorio
45-160
20-28
-
25,80-26,80
0.3-0.6
55-200
Mt. Gottero
Sandstones
27,30-28,80
0.15-0.60
95-200
Ophicalcites
75-185
10-70
45-80
0.0040-0.0070
0.40-0.90
TABLE 1
Values of the main physico-mechanical features of the different lithotypes
(original data and values indicate from Istituto Commercio Estero, 1982)
160-200
10-15
25-35
0.0050-0.0060
0.75-0.90
27,10-27,20
0.70-0.90
185-215
Portoro
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F. CIMMINO, F. FACCINI and A. ROBBIANO
Stones and coloured marbles of Liguria in historical monuments
Photo 1 – «Ardesia» roofs of some buildings in the oldtown of Genova; please notice the difference between the
original roof of the building in front and the one with
recently replaced «abbadini», low in the right
oil to freeze; «trogoli» were also used as water
reservoirs.
In house building it was used for door- and
window-sills, window- and door-lintels, doorpillars (Photo 2), while nowadays it is used to
make pieces of furniture and various tools such
as chimneys, wells, tubs, benches, blackboards.
The first documented use of «ardesia» slabs for
billiards dates back to 1928. «Ardesia» was
found to be excellent because it does not warp,
it has excellent elasticity values, absorption
capacity, impact strength and a very high
smoothness, due to the rock features and to the
processing systems.
No kind of «ardesia» is anyway good for
outdoor sculpture, because it is seriously
damaged and flakes off more and more as time
goes by: the «ardesia» bas-relieves made in
Lavagna in 1300 and lately in 1700-1800 are
now often in bad conditions, specially on the
75
Photo 2 – Main entrance of a historical palace, where
«Ardesia» has been utilised, Genova
worked surface and in the protruding points, and
the hue of the stone turns to light grey. Basrelieves of the same ages sculpted in western
Liguria (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, water
is the most dangerous for «ardesia» because it
dissolves calcite and reduces its solidity, can
remove clay minerals, modify chemical
composition and chromatic features. In recent
years also smog and acid rains pollution
contribute to decay and many Ligurian basrelieves in «ardesia» have been saved because
have been moved into St. Agostino Museum in
Genova.
PIETRA DI FINALE (STONE OF FINALE)
The «Pietra di Finale» crops out in midwestern part of Savona region, behind the sea-
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F. CIMMINO, F. FACCINI and A. ROBBIANO
coast from Capo Noli and Caprazoppa, in the
valleys 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 to
a great grey-pink limestone platform deeply
canyoned by torrents which divide the platform
in «islands».
Mineralogy and petrography studies have
evidenced that some lithotypes rich in fossils and
others more sandy are present. Particle sizes of
bio-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 most
sandy rocks, the terrigenous fraction is
commonly made of mono- and poly-crystalline
quartz 2 to 8 mm in size, detrital micas, oxides
soaking both matrix and cement, and clay
minerals: clasts are both rounded and rough.
Oxides inside the matrix cause the reddish hue
of the darkest variety of «Pietra di Finale».
Depending on the quarrying zone and on the
chromatic and physical features, can be
distinguished few varieties: a Bianco Doria,
soft rock quarried around Boragni; a Rosso
Verezzi, semi-hard rock of Verezzi; a Rosato di
Finale, hard rock from Castel Gavone and
Pianmarino.
«Pietra di Finale» is locally used since
Roman age: still well preserved is e.g. the
«Fairy Bridge», the first of the five Roman
bridges in Ponci Valley, near Verezzi and
Finalpia; it was built around 10 a.D. along «Via
Jiulia Augusta», the road to Gaul.
Among the historical buildings, it must be
mentioned St. Eusebio Church (XIV century),
San Sebastiano Church and the Five Belltowers Church (XV century) at Perti, the belltower of San Biagio Church in Finalborgo (XII
century) and the fascinating Diamond Tower of
Castle Gavone behind Finalborgo (Photo 3),
that was one of the most impressive fortified
residences of Del Carretto family; this tower
represents a charming evidence of how stone
was worked in northern Italy during
Renaissance (see also the Diamond Palace in
Ferrara). «Pietra di Finale» was highly
appreciated also in Genova, in the Galeazzo
Photo 3 – The Diamond Tower in «Pietra di Finale» of
Castel Gavone, Final Borgo (Province of Savona)
Alessi’s architecture during XVI and XVII
centuries, in Santa Maria Assunta di Carignano
Basilica, in Lercari-Parodi Palace in Via
Garibaldi and in Villa Cambiaso in Albaro.
After a standstill period, quarrying and
utilization of this stone restarted towards the
end of XIX century, when the «Pietra» was
used in bridges, viaducts, tunnel openings etc,
also outside Liguria (Torino-Savona railway)
and in many villas along the Riviera.
The top of quarrying activity, also
underground, was reached in 1924-1929, when
the «Pietra» was widely used in America for
mouldings and pillars of many villas built in
Neo-classical or in Palladian style. Other
important buildings of those years are, e.g., the
Cathedrals of Bengasi and Mogadisco, the
Museum of Vienna, the Winter Velodrome of
Paris, the Palace of Justice of Drauguignan, the
Rathaus of Hamburg, and in Italy the Fiat
Stones and coloured marbles of Liguria in historical monuments
Building in Turin, the Bocconi University in
Milan, Villa Ombrosa in Parma. In Genova, the
«Pietra di Finale» was used during XX century
for the openings of Giuseppe Garibaldi Tunnel at
Portello and in the years 1940-1950 for the
arcades of Piazza della Vittoria. In the second
half of the century this stone was used in the
main buildings of Piccapietra district, in the
head-office building of the Savings Bank of
Genova, in the University Clinic of San Martino
Hospital. Today it is mainly used to panel the
facades of new buildings, to restore old buildings
and, in lower quantity, for many different local
uses. It is a very good building material, with a
warm pinkish colour and a strong, well tested
weather-resistance; it is used therefore to make
blocks, jambs, pavements, steps, wainscots and
inner- and outer-coatings of buildings, as well as
to obtain architectural details, rims, decorations
(Photo 4) and even bas-relieves and sculptures
(Cimmino et al., 1995).
Photo 4 – Decoration in «Pietra di Finale» of a portal in the
church at Borgio Verezzi (Province of Savona)
77
During the decay of this stone, the carbonatic
binding phase is destroyed and salts are
produced, which can crystallise inside the
matrix with great volume variations; microcracks and pores are therefore produced. The
results of these transformations may be simple
surface alterations, exfoliation and detachments
or even deep holes. The decay due to acid rains
has been studied for all the rocks of the
Finale’s region, using both natural stone
samples and samples protected by fluorided copolymers (Contardi et al., 2000).
PIETRA DI PROMONTORIO
(STONE OF PROMONTORY)
This dark-grey bluish verging on black stone
belongs to the Flysch of Monte Antola, a
sedimentary sequence made by limestone,
sandstone and marly-limestone which crops out
along the seacoast and inland between Genova
and Chiavari. It is a marly limestone
(wackestone, wackestone-packstone and
packstone, with calcareous micritic binding
fraction), relatively compact, whose size ranges
between silt and clays. It may be schistose,
with high density of dissolution surfaces and
local accumulation of phyllosilicates, and it
may be compact with high amount of
terrigenous minerals showing few dissolution
surfaces but sometimes with micro-cracks
filled by spathic calcite.
The first reliable news about the utilisation of
the stones quarried in the very western part of old
Genova dates back to XII century, when between
1155 and 1159 the town walls were enlarged,
fearing the invasion of the Emperor Federico
Barbarossa (the first walls of Genova date back
to 849-899). The new walls stretched from Porta
dei Vacca, Porta Soprana (Photo 5) and Sarzano,
with a total length of nearly 2500 m. Few parts of
these walls still exist and many of them have
been lately restructured; belong to them the town
gates (Porta dei Vacca and Porta Soprana) and
the towers belonging to rich families: Torre degli
Embriaci and Torre dei Piccamiglio (XII
century), Torre degli Spinola (XIII century).
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F. CIMMINO, F. FACCINI and A. ROBBIANO
symbol of Genova) were enough old-fashioned,
but in spite of that it was possible to obtain
from those quarries the huge amount of very
big blocks necessary to build the underwater
foundations of many piers and wharves.
Before cutting the San Benigno hill, the same
quarries have been exploited for so many other
works, from the construction of all the banks
inside the port, included the one where
Magazzini Generali (General Storehouses)
stand in Molo Vecchio (Old Dock) and the
«Boccardo» and «Gadda» piers, to the Carlo
Felice Theatre, whose black-stone arcade links
up with the nearby arcade of Fine Arts
Academy building.
The best varieties of this stone are the finest,
most compact, even-grained ones; they have
often been utilised for many different
architectural uses (Photo 6) and for
decorations, especially for bas-relieves.
Photo 5 – Porta Soprana in «Pietra di Promontorio»,
Genova
The «Pietra di Promontorio» is very
important for medieval Genoese architecture
and was used in many buildings and churches:
the Commenda of Pré (1180), the Santo
Stefano (965) and St. Agostino (second half of
XIII century) churches, the Cathedral of San
Lorenzo (886-916), the Doria Palace and the
San Matteo church (1125). Also in 1125 the
Sottoripa arcades were built, with pillars and
arches in «Pietra di Promontorio». During
Renaissance and Baroque, palaces and
churches were usually built in plastered
masonry, often frescoed or stuccoed; but in
some cases also black-stone ashlars were used,
as it is possible to see in palaces along Via
Garibaldi and Via Cairoli.
The means used to ship the stone blocks
quarried in Promontorio (the hill that joined
Granarolo with Capo di Faro, where «la
Lanterna» – the Lighthouse – which is the
Photo 6 – «Pietra di Promontorio» and white marbles
ashlars in St. Lorenzo Cathedral (X century), Genova: note
the differential weathering of the «Pietra di
Promontorio»blocks
Stones and coloured marbles of Liguria in historical monuments
Chemical-physical features make «Pietra di
Promontorio» a lasting stone, but a
mineralogical and petrographic analysis of
some samples extracted along the town walls
has shown that the decay process is higher in
the external parts of the stone and lower inside
(Barrese et al., 1996); calcite dissolves and, as
a consequence, surface porosity increases.
Besides, the percentage of magnesium-rich
calcite increases near the altered rims, and this
fact may reduce the material cohesion during
sudden changes of temperature. Cohesion
reduction appears inside micro-cracks in the
most compact samples and along solution
seams in the schistose ones, increasing porosity
and accumulating clay minerals along the
dissolution surfaces (Beretta et al., 2001).
Chalk crystals have been found inside many
micro-cracks; they cyclically dissolve and recrystallise, repeatedly increasing the pressure
on the cracks, and this may take fragments off.
The presence of calcite rich in magnesium and
chalk together may be the cause of the evident
micro-cracking and micro-flaking phenomena
typical of the «Pietra di Promontorio» blocks in
walls and buildings.
Further mineralogical-petrographical and
physical-mechanical investigations on some
samples drawn from the medieval Torriglia
Castle, near Genova, have shown the decay
effects in the mountains, where daily and
yearly temperature ranges are high (Cimmino
et al., 2002). In particular, it has been observed
that samples subjected to humid conditioning
or to freezing processes reduce their average
resistance
to
simple
compression
approximately by !/4 with respect to the values
obtained in dry conditions.
MT. ZATTA OR MT. GOTTERO SANDSTONES
Among all the kinds of locally used
sandstone, the ones employed also out of
Liguria belong to Mt. Zatta or Mt. Gottero
Formation, widely outcropping in LigurianEmilian Apennine; these sandstones have
plane-parallel, sometimes weakly convolute,
79
laminations, their grain ranges from very fine
to coarse and are grey, brown-grey and dark
yellow in colour.
The rock has a granular, low-maturity
texture. In fine-grained samples, minerals tend
to line up along a preferred direction, with a
slightly iso-oriented texture.
The framework is composed of quartz,
plagioclase and k-feldspar. Mono- and polycrystalline quartz is more contained in
fragments 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-acid
rocks); muscovite, biotite and chlorite are poor.
Also very poor is the amount of calcitic
cement. Terrigenous constituents contribute for
more than 50% to the total rock volume,
therefore these sandstones can be considered
Terrigenous Arenites.
All the mineralogical-petrographical and
physico-mechanical results, both original
(Cimmino et al., 2003) and available in
literature (Di Battistini and Rapetti, 2003)
confirm the excellent qualities of Mt. Gottero
Sandstone.
Many historical evidences exist that
sandstone have been widely used during
centuries for foot-paths (Marchi, 1994) and
road paving in the main towns included in the
triangle whose vertices are the towns of
Genova, La Spezia and Parma.
Because of its easy workability and high
durability, sandstone has been used in the past
and is today taken into consideration for
restoration and replacing of paving, mostly in
towns and villages of its homeland. As a matter
of fact, during last decades many streets in old
towns had been covered with asphalt, so to
avoid the wear and tear of the stone, and as a
consequence its maintenance.
In recent years the cultural evolution of local
governors, the increased aesthetic sensibility
and the necessity to make historical villages
enjoyable through pedestrian ways have
brought people to re-discover the old-time
stone paving.
As a matter of fact in many cases, such as
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F. CIMMINO, F. FACCINI and A. ROBBIANO
during road works aiming to replace mains or
to renew street furniture, brand-new sandstone
paving has been realised (Photo 7), or
sometimes the old sandstone blocks still in
good conditions have been maintained together
with new blocks used to replace the old ones in
poor conditions.
About decay, alteration processes that affect
sandstone cause loss of cohesion among the
rock components, sand formation and
desquamation, the latter being linked above all
to the presence of structural and textural
discontinuities. In particular, decay affects not
only the easily alterable or water-sensitive
minerals, but also the binding fraction (cement
and/or matrix).
Therefore, the problems connected with
sandstone conservation are quite complex, also
because the restoration works are often
undertaken when the situation is extremely
serious. This happens mostly because
Photo 7 – Sandstone elements used in the paving of St.
Lorenzo Square, Genova
sandstone alteration usually appears all of a
sudden. In the first phase of alteration the
external part, which is the most exposed, shows
itself as a kind of coherent and even structure
that hides the altered part underneath, where
cohesion is lost. When in the end the exposed
surface falls down, the rock shows its true state
of bad conditions.
OPHICALCITES
Ophicalcites are rocks made by serpentinite
breccias sutured by calcite veins variously
interwoven, quarried around Levanto and
Framura (eastern Liguria) and in Polcevera
Valley (north of Genova) because of their
colours: red and green the former, white and
green the latter. They are used since 1200 as
ornamental materials (coloured marbles), at the
beginning as columns in religious buildings in
the old town of Genova, then for flooring.
Ophicalcites are tectonic breccias with gaping
fractures, whose sizes range from mm to dm,
filled with spathic calcite, fragments of rock
and micro-sparite calcareous mud. Near or
inside the fractures serpentinite is partially
replaced by carbonates and magnetite turns to
hematite, with a quite intense reddish hue. Pink
hue of various intensity is shown by microsparite mud and calcite is white («Rosso di
Levanto»). In the Sestri-Voltaggio zone, near
Pietralavezzara, fractures in the rock are almost
always thin and filled with calcite («Verde
Polcevera»).
«Rosso Levanto» mainly crops out in a
region whose boundaries are the villages of
Velva, Pavareto, Carro and Ziona northwards,
Mezzema, Deiva, Framura westwards,
Bonassola and Levanto southwards, Mount
Rossola and Mount Guaitarola eastwards.
According to petrographical and lithological
features, two main lithotypes exist: «Breccia di
Levanto» («Rosso di Levanto» s.s.) and «Breccia
di Bonassola» (Cortesogno et al., 1980).
The «Breccia di Levanto» occurs at the top
of serpentinites that represent the basis of
volcanic-sedimentary sequences and is
Stones and coloured marbles of Liguria in historical monuments
composed of highly fractured serpentinites
sutured by sparry calcite. The larger fractures
appear as true sedimentary lodes, with clasts
made of serpentinite and lower percentages of
gabbros and lody basalts. Circulation of
hydrothermal fluids causes carbonation of
serpentine and oxidation of the magnetite into
hematite. In serpentinite fragments the
oxidation is centripetal and reddens the
carbonatic elements, clasts, matrix and cement.
The name of «Breccia di Bonassola» defines
all the ophiolitic breccias of sedimentary origin
laying between the serpentinitic basement
which includes the «Breccia di Levanto» and
the overhanging sedimentary sequences. As a
function of fragment composition, one can
distinguish the «Breccia di Framura» (contains
mostly serpentinite) and the «Breccia di
Rossola» (contains mostly gabbros or basalts or
both).
Differences in petrogenesis mean differences
in trade. The most valuable variety, nowadays
almost exhausted, corresponds to the
sedimentary lodes of «Breccia di Levanto». In
them the size of the elements is proportionate,
red, white and green colours are brilliant and
the rock is compact and easy to work. A bit less
valuable is the «Breccia di Levanto» where
fractures are smaller, speckles are less evident
and hematite pigmentation is fainter. A
material with finer grain and small speckles is
extracted from sedimentary breccias («Breccia
di Bonassola»); it often shows clay-hematite
coats («mattonato», «brick-floor») that reduce
its durability and value.
The history of «Rosso Levanto» began in
ancient times, as testified by some Etruscan
funeral pieces of work (Rovereto, 1939). The
core of the production (both as tradition and as
commercial quality) has always been the coast
between Levanto and Framura; it increased
between 1880 and 1929 then decreased again.
Later, although the historical sites continued
their activity, quarrying activity moved to La
Spezia area and in the Sixties reached its top,
with many sites in the western and northern
parts of the area.
The stone was used in many Genoese palaces
81
and churches during Middle Ages and
Renaissance (altars, floors, facings and
coatings, columns – Pieri, 1950). The most
important piece of work where «Rosso
Levanto» has been widely used is the San
Lorenzo Cathedral in Genova: to realise its
columns (Photo 8), the «column quarry» was
opened in Levanto, and was re-opened again
during XVI century to build St. Ambrogio
church. In old Ligurian towns, mostly in
Genova and Chiavari, «Rosso Levanto» has
been often used also as external facings of
shops, luxury palaces, staircases of villas, etc.
«Verde Polcevera» consists of green
serpentinite breccias with white calcium
carbonate cement. It is quarried mostly at
Pietralavezzara (Campomorone), near Passo
della Bocchetta, in the hinterland of Genova; it
Photo 8 – Particular of a portal of St. Lorenzo Cathedral (X
century), Genova: note the presence of «Rosso Levanto»
(a), «Verde Polcevera» (b), White Marble (c) and «Rosso
Ammonitico» (d)
82
F. CIMMINO, F. FACCINI and A. ROBBIANO
usually shows a dark green background similar
to «Verde Alpi» and it is often sold with this
name. Two commercial varieties exist: first
quality «Verde Polcevera» with dark green
background and small white spots and branches
fading into green; second quality «Verde
Polcevera», richer in white parts and branches.
«Rosso Polcevera» can be considered one of its
varieties, quarried near Campomorone and near
the «Verde Polcevera» bodies. It is an
ophicalcite with bloody red and dark rust
gradations due to oxidation phenomena; its has
a mixed red and green background with very
effective varying veins.
«Verde Polcevera» is being used for
centuries both indoor and outdoor. In Genova it
has been used to panel main doors, banks and
shops, as well as the opening of Cristoforo
Colombo Tunnel in Dante Square.
From «Rosso di Levanto» and «Verde
Polcevera» it’s possible to obtain beautiful
polished slabs, easy to work and with good
strength, suitable for indoor uses; outdoor use
must be limited to the best qualities because of
selective decay, chalking, etc, affecting spathic
calcite above all.
PORTORO
«Portoro» crops out in the very eastern part
of Liguria, near La Spezia, between Mt.
Castellana and Mt. Muzzerone, on Palmaria
and Tino islands (whose quarries have also
provided the best commercial quality), at
Valdipino and on Mt. Bermego. The ancient
name of «Portoro» was «black and yellow
mixture» or «Portovenere»; «Portoro» locution
was born during Napoleone Bonaparte’s
empire, from the French «porte de l’or», while
English call it «Black and Gold».
The stratigraphic horizon of «Portoro» is
composed of dark micritic limestone, blackish
or seldom grey-violet, with typical yellow
veins intercalated with thin ochre-yellow marlclay levels.
«Portoro» consists of a black matrix, yellow
veins and violet veins; the matrix consists of a
very fine and regular micrite, strongly coloured,
whose colour intensity decreases in recrystallised zones; yellow veins contain limonite
and sulphides, whose colouring-matter is spread
in spots among dolomite crystals; violet veins
consist of dolomite mosaics where the most
coloured zones contain a hematite pigment.
In «Portoro» there is a very little amount of
debris and the organic substances, bitumen and
sulphides producing the black colour come
from anaerobe environments, probably dead
waters 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 buildings
and in palaces; Romans even used it in little
blocks to build the amphitheatre of Luni and to
pave the main urban roads, before using it as a
valuable ornamental marble. Its most valuable
variety, the «big-spotted» one, has been used in
the «Chiesa del Gesù» in Genova for the little
columns of the banisters (Photo 9). During XX
century it was also used in cemeterial
architecture and in local handicraft.
Intensive quarrying and exploitation of
«Portoro» re-started after World War II and
lasted up to all the Fifties, but today production
is very poor. Two commercial varieties of
«Portoro» exist, each variety being subdivided
into three qualities: large-veined «Portoro»
(«Portoro a macchia larga») and thin-veined
«Portoro» («Portoro a macchia fine»). The
large-veined «Portoro» has veins up to
decimetres in size connected by a branch-like
pattern: within this variety, Extra quality has
brilliant-gold yellow spots and very black
background, First quality («Portoro di Prima»)
less brilliant yellow veins and some calcite
white veins on black background, Second – or
Ordinary - quality («Portoro di Seconda» or
«Corrente») has mixed yellow and white veins
on grey background. The thin-veined «Portoro»
has thin gold-yellow veins interwoven inside
the whole mass: it is also subdivided into Extra,
First and Second – or Ordinary – qualities.
«Portoro» is mostly used in regions with a
hot weather (Sicily, Puglia, Provence, Northern
Stones and coloured marbles of Liguria in historical monuments
83
Photo 9 – Little columns made by «Portoro» in the banisters of Jesus’ Church, Genova
Africa) while in Northern Europe, where the
weather is colder, it has been replaced by more
weatherproof and bright marbles. As a matter
of fact, weathering agents cause a loss of
brilliance, therefore, few years after
installation, «Portoro» appears irreversibly
opaque, whitened and corroded. The use of
«Portoro» is therefore limited up there to
indoor paving and coatings, handicrafts, works
of art and slabs for furniture.
FINAL REMARKS
The main mineralogical, petrographic and
physical-mechanical features of some
particular ligurian lithotypes («Ardesia»,
«Pietra di Finale», «Pietra di Promontorio»,
Mt. Zatta or Mt. Gottero Sandstones, «Rosso
Levanto», «Verde Polcevera» and «Portoro»)
confirm good qualities of such stones and
coloured marbles.
In particular, the Slates, the «Pietra of
Promontorio» and the Mt. Zatta or Mt. Gottero
Sandstones, used also as building stones, have
high values of the compressive and flexural
strength, also after gelivity tests.
The «Pietra of Finale» has a mechanical
behaviour lightly lower, following its
mineralogical and petrographical features,
which suggest to be used as covering.
Also for the Ophicalcites and the «Portoro»
the compressive and flexural strength are
suitable to be used as decorative materials;
84
F. CIMMINO, F. FACCINI and A. ROBBIANO
nevertheless for the «Portoro» it is not
suggested the external utilization, due to the
water absorption coefficient too high.
Because of their workability and durability,
such lithotypes have been employed in the past
and they are today widely used both for the
restoration of historical monuments and for
modern manufacts. In particular, the Slates, the
«Pietra of Finale» in general all the coloured
marbles have been carefully utilised and
valorised out of Liguria and also in the foreign
countries.
As matter of fact, it is necessary to deepen
petrographic-textural studies on the primary
features of these stones and marbles, to reach a
better knowledge on their weathering
processes, especially for those ones used in
Mediterranean area; in particular, laboratory
tests are strongly needed, with the aim to
contribute to the identification of the physicochemical mechanisms causing the rock decay,
for a better preservation of the architectural
heritage.
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