Provenance of Materials Employed in the Construction of Orvieto Cathedral (Umbria, Italy)
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Per. Mineral. (2000), 69, 2, 143-163 http://go.to/permin An InternationalJournal of � � PERIODICO di MINERALOGIA MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, established in 1930 ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, Archaeometl)' and Cultural Heritage Provenance of materials employed in the construction of Orvieto Cathedral (Umbria, Italy) BEATRICE MORON!* and G!AMPIERO Pou Dipartimento di Scienze della Terra, Universita di Perugia, Piazza Universita, 1-06100 Perugia, Italy Submitted, January 2000 -Accepted, April 2000 ABSTRACT. -A multivariate analytical approach evidenziano I' esistenza nel monumento di tre was carried out in order to characterize travertine diverse litologie di «basaltina», definite «a grana and «basaltina» which are the main building stones fine», «a grana grossa» e «a grana molto fine» in employed in the construction of Orvieto Cathedral. base alia dimensione dei fenocristalli di leucite, e di Results indicate the existence in the monument of tre litologie di travertino, stromatolitico, fitoermale e three different lithotypes of «basaltina», «fine detritico, definite in funzione dei caratteri tessiturali. grained», «coarse-grained» and «very fine-grained» L'approccio multivariato si e rivelato un potente distinguished on the basis of the size of leucite mezzo nella verifica delle fonti storiografiche in phenocrysts, and three distinct lithotypes of merito alia provenienza dei materiali. Il confronto travertine, stromatolitic, phytohermal and detrital, tra i campioni del Duomo e campioni omologhi distinguished on a textural basis. prelevati da antiche cave e zone di estrazione ha The multivariate approach was a powerful tool in dimostrato che tutti i litotipi provengono da un' area checking historical sources on the provenance of the piuttosto ristretta situata tra Orvieto e Bagnoregio. Il materials. Comparisons between samples from the confronto tra campioni omologhi prelevati da Cathedral and homologous samples from ancient diverse zone del Duomo realizzate in epoche quarries and zones of excavation revealed a differenti ha invece evidenziato che le localita di provenance from a quite narrow zone, between estrazione della «basaltina» e del travertino sono Orvieto and Bagnoregio, for all lithotypes. rimaste immutate nel tempo. Comparisons, between homologous samples from different zones in the Cathedral shows that the areas KEY WORDS: Building stones, petrograp hy, of excavation of «basaltina» and travertine did not geochemistry, mineral chemistry, quarries, change with time. provenance, Orvieto Cathedral. RIASSUNTO. Un approccio analitico multivariato e stato sperimentato nella INTRODUCTION caratterizzazione del travertino e della «basaltina» in opera nel Duomo di Orvieto. I risultati ottenuti Orvieto Cathedral is a masterpiece of Gothic Italian architecture. Like many contemporary * Corresponding author, E-mail: [email protected] monuments in Central Italy, it is characterized 144 B. MORON! and G. Pou by a typical bichromy obtained using two The historical data on provenance (Riccetti, different building materials: travertine and 1988; Riccetti, pers. comm., 1992) were «basaltina», a local name for a leucite-bearing checked by accurate geological and rock of tephritic to phonolitic composition. The geomorphological study, then seventy fresh external walls of the church and the circular samples of travertine and «basaltina» were windows of the apse are made of alternating collected in outcrops from documented or «basaltina» and travertine blocks, and the inner recognizable ancient quarries. Exact walls of the apse and chapels of «basaltina» identification of ancient quarrying sites was not blocks only. Building of the Cathedral began in so simple, since most of the place-names 1290 and lasted about three centuries. mentioned in the archives do not appear in Therefore different zones of the Cathedral have present-day maps, so that in most cases different ages (fig. 1). samples were taken from different sites within A large number of samples from the suspected quarries; the criterion of Cathedral and from ancient quarries were macroscopical similarity with samples from the carefully characterized from the textural, Cathedral was adopted in the choice of chemical and mineralogical points of view samples. (Moroni, 1991; Piasco, 1991; Querci, 1995; Most of the sampling sites lie in a quite Giunchi, 1997) through a multivariate narrow zone on the right bank of the Paglia analytical approach. Results of the comparative river, between Orvieto and Bagnoregio, in the examination of homologous samples from eastern part of the Vulsini volcanic area which different zones in the monument and from belongs to the «Roman Magmatic Province» quarries are reported in this work, with the aim (Peccerillo and Manetti, 1985). Stratigraphy is of verifying the existence of any changes in the characterized by two pyroclastic complexes with provenance of the materials used at different an interbedded ignimbrite formation (Faraone time and of interpreting the building events. and Stoppa, 1989); the whole magmatic series lies on a Plio-Pleistocene sedimentary substratum. Distinct lava flows of variable SAMPLING AND METHODS extent alternate with tuffs in the pyroclastic complexes, together with small travertine lenses A total number of about two hundred and of lacustrine sedimentation. A few travertine fifty travertine and «basaltina» samples were samples were also collected in other localities taken from various zones inside and/or outside mentioned in the historical sources as sites of the church (fig. 1). They consist of cores and provenance of building materials, such as chips: cores were drawn by drill, chips were Cetona and Titignano. The exact location of collected by hammer and chisel. outcrop samples is shown in fig. 2. The method of sampling depended on the All samples from the Cathedral and from state of conservation, which is quite variable in outcrops were accurately characterized by the various zones of the monument: generalized optical microscopy, X-ray fluorescence ageing with cracking and exfoliation spectroscopy (XRF) and scanning electron characterizes materials from outside, whereas microscopy coupled with EDS microanalysis very localized processes of exfoliation may be (S.EM-EDS). XRF was performed on pelletized observed on the SSE rose window and upper powdered samples, major elements (except wall inside the apse (Poli et al., 1993; Mm·oni MgO and Na20, by atomic absorption) were and Poli, 2000). In any case, deterioration determined by the full matrix correction mainly or exclusively involves «basaltina» and method of Franzini and Leoni ( 1972) and trace is due to physical or mechanical causes. Chips elements following the Kaye' s (1965) method. were taken from weathered material; cores SEM-EDS microanalysis (15KV beam current, represent sound material. 60" count time) of clinopyroxene, plagioclase Provenance of materials employed in the construction of Orvieto Cathedral (Umbria, Italy) 145 ······::::tf:::: Jl . 11 , 11 •' -1290-1308 I •' t 1308-1310 I 4 --. , 1328-1334 I '•.. I .. 1 1 11 �1350-1365 •' I' -- 4 �1408-1444 I I I; I I 1 I 1 �@&J11513-1532 I I I IJ 11 - sample ,, locations ea ..._ I I I; I: I, I . , I 11 I 11 ... .. .11 :I , I I 11 ..•' 11 -- .. ,· I ,, , . 11 t I ,I Fig. I - Chronological plan of Orvieto Cathedral, with sampling sites. 146 B. MORON! and G. Pou FB VFB ST PT SARTEANO • M. Cetona DELLA PIEVE M. ORVIETO eR Corbara Lake Tordimonte fb • Pod.e La Petrara e BAGNOREGIO 0 10 km Fig. 2 - Sketch-map of studied area with location of sampling sites and corresponding lithologies. Phytohermal travertine was also sampled in Cb, and phytohermal and detrital travertine in TO (not shown, due to lack of space). Provenance of materials employed in the construction of Orvieto Cathedral (Umbria, Italy) 147 and Fe-ore was performed on polished thin clinopyroxene and the presence of sanidine in sections obtained from samples, but only CB suggest that it is more differentiated pyroxene analyses are reported in this work, for than FB, whereas the predominance of their greater archaeometric significance. A total clinopyroxene in VFB is evidence of a less number of about two hundred pyroxenes differentiated magma. representative of the different lithologies and Clinopyroxene shows distinguishing features provenances were analysed. XRF and SEM in the different lithologies. It is euhedral to EDS data were treated by SIMCA multivariate subhedral, with irregular borders, pale to dark statistical analysis (Bisani et al., 1982a, b), a green in colour, sometimes with opaque principal components analysis method in which inclusions and concentrically or sector-zoned in empirical modeling is applied to a data matrix the FB and CB samples, but euhedral, with in order to group data into classes with sharp edges, prismatic to acicular in shape, different principal component values, giving colourless or olive-green in colour in the VFB information, on the structure of classes and samples. Plagioclase is euhedral, frequently relationships between data and classes. concentrically zoned with a cloudy core due to incipient sericitization. Leucite shows typical RESULTS polysynthetic twinning and contains small clinopyroxene and opaque inclusions; it is «Basaltina» generally fresh or mildly fractured. Petrography. Three different lithologies of Some FB and CB samples from the apse are «basaltina» were recognized on a macrosopic characterized by typical alteration zones as scale: i) a grey,