Minero-Petrographic Characterization of Chianocco Marble
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1 Type of the Paper (Article, Review, Communication, etc.) 2 Minero-petrographic characterization of Chianocco 3 Marble employed for Palazzo Madama façade in 4 Turin (NW Italy) 5 Francesca Gambino 1*, Alessandro Borghi 1, Anna d’Atri 1, Luca Martire 1, Martina Cavallo1, 6 Lorenzo Appolonia2,3, Paola Croveri2 7 1 Department of Earth Sciences, University of Turin, Via Valperga Caluso, 35, Turin, 10125, Italy; 8 [email protected] 9 2 Centro Conservazione e Restauro “La Venaria Reale”, Via XX Settembre 18, 10078 Venaria Reale (TO), Italy 10 3 Soprintendenza per i beni e le attività culturali della Regione Autonoma Valle d’Aosta, Piazza Narbonne, n. 11 3 - 11100 Aosta, Italy 12 * Correspondence: [email protected] 13 Received: date; Accepted: date; Published: date 14 15 Abstract: The study of ancient marbles plays an important role in the interpretation of an historical 16 and archaeological site and gives interesting information about building materials used in ancient 17 times and their trade routes. The present work focuses on Chianocco Marble that represents one of 18 the most important ancient white marbles for Cultural Heritage exploited in Piedmont region (NW 19 Italy) and employed for Palazzo Madama façade. A multi-analytical study based on petrographic 20 (optical and scanning electron microscopy), electron microprobe and stable isotope analyses was 21 carried out on these marbles in order to perform an archaeometric study. 22 Chianocco Marble was used in Turin during the baroque era by the Savoy architect Filippo Juvarra 23 (1678 - 1736) in historical buildings, as the façade of Palazzo Madama, the plinth of the façade of the 24 town Cathedral and the columns (now plastered) of the portico of Piazza San Carlo. This stone is a 25 dolomitic rock belonging to the Mesozoic cover of the Dora Maira Massif (Pennidic Unit). It shows 26 a vuggy fabric characterized by a vacuolar texture due to tectonic brecciation and subsequent 27 selective dissolution during subaerial exposure. This kind of research is useful to highlight the 28 importance of the use of local stones as building materials and to investigate stone materials for the 29 restoration and the maintenance of the historical buildings. 30 Keywords: Chianocco Marble; Heritage stone; Archaeometry, Western Alps, isotopic analysis, SEM- 31 EDS. 32 33 1. Introduction 34 Ancient buildings, artifacts and findings are mainly made of natural and artificial materials obtained 35 from geological resources. The development of Geosciences applied on Cultural Heritage has made 36 it possible to highlight how the study of the nature and origin of ornamental stones is predominantly 37 a geological matter, which cannot be solved without a geologic approach [1]. A proper 38 characterization of these materials requires minero-petrographic studies for defining their 39 provenance, conservation state, and application of good preservation strategies. 40 In Piedmont, and in particular in Turin, stone has always been largely used for both 41 constructions and decorations, becoming one of the distinctive elements of the local architectural 42 heritage: statues, city walls, floors, roofs, and other architectural elements, are often made of the many 43 varieties of rocks belonging to the different geological units of the Western Alps [2,3,4]. Often, the 44 selection of stone materials in architecture is driven by specific values and meanings attributed to the 45 different rocks; moreover, the use of specific lithotypes can be related to aesthetic values, technical Sustainability 2019, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sustainability Sustainability 2019, 11, x FOR PEER REVIEW 2 of 14 46 progress or even economic circumstances. Because of their easiness of lavorability, marbles have been 47 widely employed, from Roman times to the end of XVIII century, in valuable buildings [5]. 48 One of the most prestigious building in Turin is certainly Palazzo Madama (Figure 1a), an historical 49 and architectural complex located in the center of the town. It is an UNESCO World Heritage site and 50 at present is the seat of the city Ancient Art Museum. It is the testimony of two thousand years of 51 history: originally built by the Romans as a gateway to the town, the building became first a defensive 52 system, and then the symbol of the power until the sixteenth century, when it was replaced by the 53 Palazzo Reale as seat of the Dukes of Savoy. With King Carlo Alberto, politics also entered Palazzo 54 Madama: in 1848, the king placed the Subalpine Senate in the large hall on the first floor, destined to 55 become one of the places of politics in which Italy's unity was most strongly prefigured. Considerably 56 embellished under the regency of the two Royal Ladies also known as “Madame” (hence the name): 57 Maria Cristina of France and Maria Giovanna Battista of Savoy, the old medieval castle was retrained 58 by the work of Filippo Juvarra, who realized (1718-1721) the great façade which dominates the square 59 [6,7]. He chose the Chianocco Marble, a yellowish grey marble from the Susa Valley, for coating the 60 façade. The strong deterioration of this marble made necessary, over time, several restorations and 61 replacements by different stone materials recalling the original one but coming from different sources 62 [8,9]. As a consequence, many archaeometric studies carried out on the façade of Palazzo Madama 63 resulted to be contradictory and partially wrong in the attribution of the stones employed over the 64 centuries [10,11]. 65 For this reason, and because of many recentconservation issues, the Conservation and Restoration 66 Foundation “La Venaria Reale”, in collaboration with Fondazione Torino Musei and under the 67 supervision of Soprintendenza Archeologia, Belle Arti e Paesaggio per la Città metropolitana di 68 Torino promoted several technical and scientific investigations in order to develop a pilot project for 69 the overall conservation and future maintenance of the historical façade (Figure 1b). 70 The purpose of this paper is to provide a detailed petro-architectonic survey and a minero- 71 petrographic characterization of the Chianocco Marble also aimed to define the causes of its 72 degradation. 73 74 75 Figure 1. (a)View of the Palazzo Madama façade.; (b) Study site located in the central area of the 76 façade. 77 2. Geological setting 78 In the central portion of the Susa Valley (north‐western Italy), some marbles pertaining to the 79 meta‐carbonate cover of the Dora Maira Massif crop out. The Dora Maira Massif is a unit of 80 continental crust belonging to the Pennidic Domain of the Western Alps (Figure 2a), which was 81 pervasively deformed and metamorphosed during the Alpine orogeny, which occurred about 50 Ma 82 ago. The Dora Maira Massif is predominantly made up of gneiss and micaschists of Palaeozoic age Sustainability 2019, 11, x FOR PEER REVIEW 3 of 14 83 and rare slices of the original carbonate cover Triassic to Early Jurassic in age, which during Alpine 84 metamorphism became dolomitic marbles. The Alpine metamorphic cycle resulted in a first event 85 that developed under eclogitic conditions, during which peak pressure (P) and temperature (T) 86 conditions were reached, followed by a retrograde metamorphic event that developed under 87 greenschist facies conditions [12]. Historically, Susa Valley marbles have been distinguished in 88 “Foresto and Chianocco Marbles” on the basis of their extraction site [13,14]. Actually, they are two 89 different kind of rocks with different petrographic features resulting from different geological 90 processes. The Foresto Marble consists of massive whitish marbles whereas the Chianocco Marble 91 shows a vacuolar structure and a yellowish color. 92 93 Figure 2. (a) Geological setting of Piedmont region and location of Chianocco municipality; (b) 94 Location of the 5 quarry sites in Chianocco Municipality. 95 3. Materials and Methods 96 The support of the Earth Science Department of Turin to the study of the Palazzo Madama façade 97 consisted in the realization of an architectural-petrographic survey of the façade, in the 98 characterization of its lithotypes, in the diagnosis of the state of preservation, in the study of the 99 degradation causes and in the definition of a model of the evolution of the marble of the façade from 100 its formation to its employment. 101 For this kind of study related to the identification of building, monuments and artefacts made of 102 marble, a stone material most commonly traded in antiquity and showing few macroscopic 103 diagnostic features a multi‐analytical approach is essential. [15, 16, 17, 18]. 104 Starting from the architectonic relief of the façade, a mapping of stone materials in false color, named 105 “petro-architectonic relief” in this paper, was achieved. 106 Starting from the architectonic relief of the façade, a mapping of stone materials in false color, named 107 “petro-architectonic relief” in this paper, was achieved. The fragments detached from the façade were 108 catalogued and, from the data collected, the most representative samples were selected for detailed 109 studies. Sustainability 2019, 11, x FOR PEER REVIEW 4 of 14 110 In order to understand the properties of the material, the localization of the ancient quarries has been 111 essential. Five significant sites in Chianocco Municipality were individuated and sampling work has 112 been conducted. 113 Petrographic studies on uncovered thin sections (30 µm thick) were carried out by optical microscopy 114 and cathodoluminescence (CL) at the Earth Sciences Dept. of the University of Turin. 115 CL observations were performed on polished thin sections using a CITL 8200 mk3 equipment 116 (operating conditions of about 17 kV and 400 µA). 117 118 A Scanning Electron Microscope (JEOL JSM-IT300LV) equipped with an energy-dispersive X- 119 ray spectrometer (EDX), with a SDD (a silicon drift detector from Oxford Instruments), hosted at the 120 Earth Science Department of the University of Turin, was used for the determination of major 121 elements.