The Pietra di Vicenza microfacies: recognizing the provenance of building stones from the Cathedral and the Ghirlandina Tower (Modena, Italy).

The Cathedral and the Ghirlandina Tower in Modena (XI - XIV century), represent a masterpiece of the Romanesque art, included into the UNESCO World Heritage List. In recent years, some stone falling turned the attention to their need of urgent restoration. This in turn revealed the lack of modern detailed studies on the rock slabs used to cover the external walls of both monuments that were originally identified by Bertolani & Rossi (1971) and Bertolani (1984) .The systematic recognition of the rock slabs of the Ghirlandina Tower and of the façade and northern side of the Cathedral allowed to map nine different rock types, among which the so-called Pietra di Vicenza is one of the most prominent (Fig. 1).The commercial name Pietra di Vicenza indicates a set of Paleogene limestones with different mechanical and aesthetic properties (Cornale & Rosanò, 1994). The common trait is their provenance either from the Berici Mts. or from the southern Lessini Mts. (Vicenza).The historical documentation attests the medieval use of slabs deriving from despoliation of Roman monuments. However, part of the original slabs were replaced with different varieties of Pietra di Vicenza during subsequent restorations.In order to determine the original Roman stones, a preliminary survey has been carried out both on local museum collections (Capedri, 2005) and on recent archaeological findings in the Modena urban area. Then, the results have been compared with samples collected in several quarries in the Veneto area (Fig. 2).The palaeontological analyses, in particular the microfossil assemblages, allowed to distinguish four main Pietra di Vicenza microfacies:Microfacies 1 (Fig. 3)Well-sorted packstone, with abundant miliolids and coralline algae (commonly fragmented); rare specimens of Nummulites sp.Microfacies 2 (Fig. 4)Moderately-sorted packstone/grainstone, with abundant coralline algae (commonly crusts, less frequently fragments) and subordinate miliolids; some small Nummulites sp.Microfacies 3 (Fig. 5)Moderately-sorted to unsorted packstone, with abundant crustose coralline algae, small to medium-sized Nummulites spp., rare miliolids.Microfacies 4 (Fig. 6)Unsorted packstone, with small to large Nummulites spp. and Discocyclina spp.The analysis revealed a wide variety of fossil assemblages in the Oligocene limestones from the Berici Mts. and the southern Lessini Mts. According to our observations on the archaeological material, practically all the Pietra di Vicenza used by Romans belongs to microfacies 1 (Fig. 7). The latter has been found only in the basal part of the Oligocene limestones near Costozza (Figs. 8-9), where exploitation is documented since pre-Roman times. This village is located on the eastern margin of the Berici Mts., where the limestone strata lie close to the Bacchiglione River, which was probably used as waterway for transportation.Microfacies 2, although not found in the roman artifacts, is probably heteropic to microfacies 1.Microfacies 3 has been used during restorations of various ages including those repairing damages of World War II (1949). Restorations of 1898-1903 used only slabs belonging to the microfacies 4 (Fig. 10).BibliographyBertolani M. (1984) - Note sulla natura delle pietre usate nel Duomo di Modena in Lanfranco e Wiligelmo. Il Duomo di Modena. Edizioni Panini, Modena, pp. 298-305.Bertolani M. & Rossi A. (1971) - Osservazioni sul rivestimento lapideo della Torre Ghirlandina a Modena. in Deputazione di Storia Patria per le Antiche Province Modenesi, Modena, 93-101.Capedri S. (2005) - I materiali naturali utilizzati nei repertori del Museo Lapidario Estense, in Il Museo Lapidario Estense, catalogo generale, a cura di N. Giordani e G. Paolozzi Strozzi, Venezia, 509-513.Cornale P. & Rosanò P. (1994) - Le pietre tenere del vicentino - uso e restauro. Associazione Artigiani della Provincia di Vicenza, p. 176.