Characterization of Pietra Ollare artifacts: archeometric and petrographic approaches.

Conz E., Cantù M., Rebay G., Riccardi M. P.

Dipartimento di Scienze della Terra e dell’Ambiente, Università degli Studi di Pavia

Pisa, 17 settembre 2013

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• The Italian term «Pietra ollare» (from the latin olla: pot) refers to basic and ultrabasic metamorphic rocks that belong to greeschist, blueschists and eclogite facies. These rocks are very common in Alpine ophiolites.

Pietra ollare…what does it mean???

Mechanical properties

• Good workability• Fire resistant• Food preserving• Refractory

Handmade(Iron Age)

Lathe: wall thinning of containars(Augustus Age)

IV century AD Increase of production in Aosta Valley

V century AD Developing of pietra ollare production and trade

Early Middle Ages Improving of pietra ollare working techniques

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A literature review…• Several studies about the characterization of «Pietra ollare» manufacts

have been carried out since 70’s.

• Tiziano Mannoni (Mannoni, Pfeifer, Serneels, 1987) was the first to investigate the petrography and systematically classify «Pietra ollare» archeological findings, defining a classification with 11 different categories (from A to L) that are still use today.

• Some comparative petrochemical studies of archeological findings and stones from main production centers of the Alps have been performed.

Archeological findings from Vaude and Lemie archeological sites (Torino, Italy), photos by Maurizio Rossi

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Mannoni categories (Mannoni, Pfeifer, Serneels, 1987)

Group A Serpentinoschists

(rare in Aosta Valley)

Group BCoarse grain

talc-carbonate-bearing rocks containing amphiboles

Group ETalcoschists

containing amphiboles

Group H Metagabbros

Central Alps

Ligurian Alps

Group FFine grain Cloritoschists

Group G Coarse grain Cloritoschists

Aosta Valley, Western Swizerland, Piedmont

Group L «Prasiniti» albite + chlorite metabasites(rare in all the Alps)

Group I Amphiboloschists

Group K Olivine schists

Group CCoarse grain

talc-carbonate-bearing rocks

Group DFine grain

talc-carbonate-bearing rocks

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Valli di Lanzo pietra ollare quarries and archeological sites

Tectonic sketch map of Northwestern Alps (Modified from L. PELLETIER, O. MÜNTENER, 2006, p. 113). Valli di Lanzo and location of pietra ollare quarries.

1) Bric dou Sarvaj quarry, Ala di Stura, Val d’Ala (Photo by Laura Vaschetti, 1995)

Archeological sites

Quarries

2) Rio Gorgia quarry, Lemie, Valle di Viù. Stone blocks (Photo by Laura Vaschetti, 2011)

3) Rio Gorgia quarry, Lemie, Valle di Viù. (Photo by Gisella Rebay, 2013)

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Archeological findingsVA/45VA/46VA/81VA/28VA/68VA/82

Vessel shards. Late roman-early medievalsettlement of San Carlo Canavese, Vaude.

LE/86+95

Vessel shard. Late roman-early medieval workshop of Lemie (Valle di Viù).

CA01CA02CA03

108:791:1108:5120:14

Semi-finisched whorl and vessel shards. Medieval mining settlement of Usseglio (Val di Viù).

Samples from Bric dou Servaj quarry. Ala di Stura (Val d’Ala).

CA06Sample from Rio Gorgia quarry, Lemie (Valle di Viù).

06/07/1

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Provenance and classification problems…• All specimens (archeological findings and

stones) belong to the group F of the Mannoni classification.

• All the studied specimens are fine grained green chlorite-schists.

• The diffusion of lithologies corresponding to group F throughout the Alps does not allow to univocally associate findings to quarries.

Hand sample analysis

So what?

Geochemical studies

• Mineral and texture characterization• Description of the mineral phases• Morphological classification of foliations

• Geochemical analyses (Major and trace element) of representative mineral phases

Archeological studies

• Charachterization of aesthetic and techical properties of the artefacts

• Investigation of local production centers

Petrographic studies

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Mineral and texture characterization

VA46 2 mm

chl

amph

1 mmVA82

chl

ttn

grt

px

amph

S1

S1

S2

S1

chl

ttnLE/86+95 2 mm

1 mmCA01grt

chlS1

CA01 Bric dou Servaj, quarry, Ala di Stura LE/86+95 Workshop of LemieVA46, VA82 Settelment of Vaude

ep

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Micro structural analyses

Mineral D1 Post-D1

chl

gr

Op

CA01Mineral D1 D2 Post-D2

chl

Op

amph

ep

VA46

Mineral D1 Post-D2

chl

Op

amph

px

gr

ttn

VA82

Mineral D1 D2 Post-D2

chl

Op

ttn

LE01

chl

gr

op

amph

ep

px

ttn

Relative age diagrams

chlorite

garnet

opaque

amphibole

epidote

pyroxene

titanite

CA01 ≅ VA46 ≅ LE01 VA82 ???

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Geochemical analyses(major elements)

Whole rock major element analyses carried out by ICP-ES

Binary diagrams of major elements of chloriteschists findings and quarry samples. (Mannoni’s collection outcrops data compared with literature data of medieval artifacts and millstones).

Martin et al., 2008

Santi et al., 2009

Antonelli et al., 2012

Zoppi, 2012

M6M12

M3

Samples from St. Marcel Valley

Fidings from Medieval Tuscan archeological sites

Roman millstones from Aquileia

Mannoni’s collection Aosta Valley quarries samples (Cervinia, Châtillon, S. Vincent, S. Jaques, Champorcher, Gressoney, Ayas, Arnad, etc.)

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Geochemical analyses(minor elements)

Martin et al., 2008

Santi et al., 2009

Antonelli et al., 2012

Zoppi, 2012

Whole rock trace and minor element analyses carried out by ICP-MS

Binary diagrams of minor elements of chloriteschists findings and quarry samples. (Mannoni’s collection outcrops data compared with literature data of medieval artifacts and millstones).

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MetodologyHand sample

analysis

Lithotype identification

Petrographicanalysis

Archeological analysis

ColorGrain sizeMineral phases (volume percentage)

ColorGrain sizeMineral phases (volume percentage)

+Cristal shapeTextureMicrostructural analyses

ShapeSizeConservation stateTypological studyChronologyWorking tracesUse tracesReferences

Archeological finding characterization

Geochemical analysis

Mayor and trace elements of mineral phases, selected by the micro-structural position, with SEM-EDX and LA-ICPMS

Geochemical characterization

Possible findings-quarries correlation

Sampling in possible provenance quarries

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Database

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1) Mannoni’s classification is a useful

tool but not enough detailed

4) It’s possible to associate a finding to a quarry in a local

production system when quarries and settlements are within few km

distance

5) The correlation between petrographic, geochemical and archeological analyses

could be a possible way

3) Classification and provenance studies, that have been carried out since today, do not allow to univocally associate findings to

quarries

2) “ Pietra ollare” lithotypes are very

common in Western and Central Alps.

Conclusions

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Thanks for your attention!!!

elisa.conz@unipv.it

Pietra ollare TeamSaint Jaques,

Val d’Ayas, agosto 2012