The Local Black Limestones Used to Make the Typical Black-And-White Alternation of the Pisa’S Romanesque Style

2020 IMEKO TC-4 International Conference on Metrology for Archaeology and Cultural Heritage Trento, Italy, October 22-24, 2020 The local black limestones used to make the typical black-and-white alternation of the Pisa’s Romanesque Style Marco Lezzerini1, Stefano Pagnotta2, Andrea Aquino1, Marcello Spampinato1 1 Department of Earth Sciences, University of Pisa, Via Santa Maria 53 – 56126 Pisa, Italy [email protected], [email protected] 2 Chemistry and Industrial Chemistry Department, University of Pisa, Via G. Moruzzi 13 – 56124 Pisa, Italy [email protected] Abstract – This paper documents the chemical, surface, but they suffer from colour fading, and assume mineralogical and petrographic characteristics, and a light-grey colour when exposed also for a not too long some physical properties of the black limestones that time to weathering processes. were exploited in the Middle Ages for building the The aim of this research is to characterize the black Pisa’s monumental architecture. The black limestones outcropping in the reliefs near Pisa, from a limestones are worthy of particular importance in chemical, mineralogical, petrographic and physical the medieval Pisa’s constructions because they are point of view, and to evaluate its use as building stones used to build important buildings (S. Maria Assunta in the Pisa’s city. Cathedral, Leaning Tower, Baptistery and several other city churches) and, in particular, the typical II. GEOLOGICAL SETTING black layers that characterize the Pisan architecture The Monti Pisani, also including the Monti d'Oltre are almost always made with these stones. The main Serchio, are a mountainous system of modest size that is local black limestones used as building stones in Pisa part of the Tuscan Sub-Appennine; they are located in belong to the Tuscan Nappe Unit: the Calcare a the central-northern part of Tuscany, and separate Pisa Rhaetavicula contorta formation, including the black from Lucca (Figure 1). limestone Portoro, outcropping in the Monte Pisano area, and the Calcare selcifero della Val di Lima formation, outcropping in the Monti d’Oltre Serchio area. I. INTRODUCTION In the framework of the research on the stones historically used as building materials in Pisa (western Tuscany, Italy) [1-4], this work deals with the black limestones belonging to the Calcare a Rhaetavicula contorta Fm, outcropping in the Monte Pisano, and the Calcare selcifero della Val di Lima Fm., outcropping in the Monti d’Oltre Serchio. The extensive use of these stones in Pisa occurred in the Middle Ages, when some important constructions were built [5-9]. For example, the black limestones together with the Monte Pisano marble were used to Fig. 1. Sketch map of the Monti Pisani area in 1:200000 make the typical black-and-white alternation of the scale (modified by [15]). Pisan Romanesque style in S. Maria Assunta Cathedral [10, 11], Leaning Tower [12], Baptistery [7], San Nicola [13] and San Paolo a Ripa d’Arno [9], and together with the Monte Pisano quartzites [14] in San Piero in As reported by Giannini & Nardi [16] and Rau & Vinculis. Tongiorgi [17], the lithotypes outcropping in the Monte These studied stones are black or dark grey on “fresh” Pisano area belong to the stratigraphic sequence of the 179 metamorphic units of Monte Serra and Santa Maria del following the EN standards [30-31]. The real density Giudice (Tuscan Metamorphic Units). Conversely, the (ρr) has been determined using a gas pycnometer formations belonging to the Tuscan Nappe (Tuscan non- (Ultrapycnometer 1000 by Quantachrome Corporation) metamorphic Unit) outcrop extensively in the Monti [30]. The measurements were performed on about 10 g d’Oltre Serchio and, locally, on the southern slopes of of very-fine-grained powders dried at 105 ± 5 °C for 24 the Monte Pisano as outcrops of limited extension. h under the following experimental conditions: ultrahigh The Tuscan metamorphic succession is here purity compressed Helium with outlet pressure of 140 represented by silicoclastic deposits from Paleozoic to kPa; target pressure, 100 kPa; equilibrium time, Carnian age, and carbonate rocks of Trias-Cenozoic age, automatic; purge mode, 3 minutes of continuous flow; mainly developed in the Unit of Santa Maria del maximum number of runs, 6; number of averaged runs, Giudice (northwest side of the Monte Pisano). In the the last three. The apparent density (ρa) has been Monte Serra Unit (southeast side of the Monte Pisano) determined by ratio between dry mass and volume of the post-Carnic successions are limited to small each sample. The specimens were placed in a stove at outcrops of marble, meta-limestones with flint and 60 °C until the dry weight was reached, (i.e. when the meta-radiolarites [17]. difference between two successive weighing at an The Tuscan Nappe, originally placed in a interval of 24 h is not greater than 0.1 % of the mass of paleogeographic area between the Apuan Alps zone the specimen). Then the specimens were immersed in towards the west and the Umbria-Romagna basin distilled water following [30]. The volume of the towards the east [18-20], is related to part of the late specimens was measured by means of a hydrostatic Triassic-early Miocene sedimentary cover and balance on water-saturated samples [32]; water stratigraphically is made up of, from the bottom: absorption at atmospheric pressure was determined evaporitic (late Triassic), carbonate (late Triassic-early according to [31] on cylinder specimens with an Cretaceous) and pelagic-turbiditic (Cretaceous-early approximative volume of 30 cm3. Total porosity has Miocene) successions [17]. been calculated according to (1) For the sake of clarity, the black limestones analysed in the present work belong to the Tuscan Domain and, P (vol. %) = 100 ∙ (1– ρa / ρr) (1) in particular, the Calcare a Rhaetavicula contorta and the Calcare selcifero della Val di Lima are deposits IV. RESULTS AND DISCUSSION related to the Tuscan Nappe of Upper Trias and From a field check of several important historical Middle/Upper Jurassic age [21-27], respectively. buildings in Pisa, we found the representative presence of RET in the church of San Paolo a Ripa d'Arno (Figure 2) and of SVL in the Cathedral of Pisa (Figure III. ANALYTICAL METHODS 3). Even if these materials are altered, it is quite easy to A total of 47 samples from two abandoned quarries recognize them thanks to their characteristic features have been collected and analysed: twenty-eight samples (Figure 4): the RET limestone present an alteration of Calcare selcifero della Val di Lima (SVL) were taken colour from light to dark gray with white calcite veins from the quarry Q1, about 1 km north of Vecchiano and and rarely yellowish marly veins; the SVL limestone eighteen samples of Calcare a Rhaetavicula contorta present an alteration patina from light to dark gray, but (RET) from the quarry Q2, opened in the south slopes unlike RET, it has black flint nodules and strips. of the Monte Pisano near Caprona (Figure 1). The analytical procedures used for this work are as follows: - chemical analysis by X-ray fluorescence [28-29] for the determination of major and minor compounds (Na2O, MgO, Al2O3, SiO2, P2O5, K2O, CaO, TiO2, MnO, Fe2O3); total volatile components were measured as loss of ignition at 950 °C on samples dried at 105 ± 5 °C; - mineralogical analysis by X-ray powder diffraction (XRPD) λ = 1.5406 Å, angle range 4-66 °2θ; - petrographic analyses by transmitted light microscopic observation of thin sections (Zeiss Axioplan microscope); - physical properties of the stones like real (ρ ) and r apparent (ρa) density, water absorption at atmospheric pressure, total and open porosity have been determined Fig. 2. Wall facing of the N side of the church of San 180 Paolo a Ripa d’Arno at Pisa, showing the presence of From the mineralogical point of view, the analysed RET limestones (black stones) and Breccia di Caprona samples are essentially made up of calcite and (white stones). subordinate quartz (Figure 5). Cal Qtz SVL Counts RET 5152535455565 2 Theta (WL=0.154 nm) Fig. 3. Wall facing of the transept S of the Santa Fig. 5. Diffractograms of representative analysed Maria Assunta Cathedral at Pisa, showing the presence samples (RET = Calcare a Rhaetavicula contorta Fm.; of SVL limestones with a typical black flint. SVL = Calcare selcifero della Val di Lima Fm.). The thin section observations allowed us to distinguish some petrographically distinctive features of the two rocks under examination: - RET limestone: presence of mudstone and wackestone with bioclasts, pellets and very rare ooids; - SVL limestone: presence of mudstone and wackestone, with pellets intraclasts, packstone, black flint and quartz. Fig. 4. Photographs of selected samples (the thickness of the samples is about 10 cm): on the left, a RET In table 2 the main physical properties of the analysed samples are reported. Both of analysed stones are sample with white thin intersecting veins of calcite; on 3 the right, a SVL sample characterized by typical characterized by real density like calcite (2.71 g/cm ), chromatic alteration (from black to gray) and by the low values of water absorption at atmosferic pressure, less than 0.5 wt% on average, and consequentely low presence of a centimetric black flint. porosity, respectively, 1.1 ± 0.5 by vol. for RET samples The samples analysed through XRF show the and 0.6 ± 0.2 by vol. for SVL samples. average chemical composition reported in Table 1. The average content of CaO for the analysed limestones is Table 2. Main physical properties of the black between 51.56 wt.% to 54.36 wt.%, while the values of limestones used as ancient building materials in Pisa constructions. SiO2 range from 0.38 to 5.96 wt% and those of MgO from 1.05 to 0.61 wt% for RET and SVL black limestones, respectively.

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