minerals Article Production Technologies of Ancient Bricks from Padua, Italy: Changing Colors and Resistance over Time Elena Mercedes Pérez-Monserrat 1, * , Lara Maritan 1 , Enrico Garbin 2 and Giuseppe Cultrone 3   Citation: Pérez-Monserrat, E.M.; Maritan, L.; Garbin, E.; Cultrone, G. Production Technologies of Ancient Bricks from Padua, Italy: Changing Colors and Resistance over Time. Minerals 2021, 11, 744. https:// doi.org/10.3390/min11070744 Academic Editor: Domenico Miriello Received: 18 June 2021 Accepted: 7 July 2021 Published: 9 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Geosciences, University of Padua, 35131 Padua, Italy; lara.maritan@unipd.it 2 Inter-Departmental Research Centre for the Study of Cement Materials and Hydraulic Binders (CIRCe Centre), University of Padua, 35131 Padua, Italy; enrico.garbin@dicea.unipd.it 3 Department of Mineralogy and Petrology, Faculty of Sciences, University of Granada, 18071 Granada, Spain; cultrone@ugr.es * Correspondence: elenamercedes.perezmonserrat@unipd.it Abstract: Representative and very uneven texturally bricks having yellow/beige or pale or dark red colors from the Renaissance walls (16th century) of Padua, Northeast Italy, were studied by means of colorimetric, petrographic (MOP), chemical (XRF), mineralogical (PXRD) and microstructural analysis (FESEM-EDS). Starting from the color measurements of the ceramic bodies, the manufacturing technologies and their influence on the physical behavior and durability of the bricks were established. The porous system was characterized by means of hygric tests and mercury intrusion porosimetry; the compactness and structural anisotropy were defined through ultrasound velocity; the uniaxial compressive strength was determined; and durability to salt crystallization and frost action of the bricks was assessed. Mg- and Ca-rich illitic clays fired at temperatures ≥900 ◦ C were used to manufacture the beige hue bodies, while the pale red bricks were made out with Ca- and Fe-rich illitic clays fired at 850–900 ◦ C. A lower carbonate content on the base clays and a lower firing temperature were the main causes responsible for the changing colors from beige to red hue. The increase of the red color was associated to higher silicate inclusions content and lower development of reaction rims around grains. The low sintering degree achieved yielded highly porous bodies with diverse porous systems, leading to differential physical performance and durability of the bricks that may turn out beneficial for the conservation of the historic walls. Keywords: archaeometry; brick fabrics; durability; porosity; sintering; urban coloring 1. Introduction When production technologies of ancient bricks are addressed, the color represents a macroscopic feature that provides approximative information of firing dynamics and compositional dynamics [1]. The final color of the ceramic bodies depends on the raw clays’ composition and on the degree of ceramic transformation during firing, which is mainly conditioned by temperature, time and redox environment inside the furnaces [2]. The colors of the ceramics may vary from white/off-white hue, indicating that Fe-free raw clays were fired in oxidizing conditions, to grey and black hues if the raw materials contained iron oxides and hydroxides or organic and/or Fe-bearing base clays fired in reducing conditions [3–5]. Between these two extremes, ceramic materials may display a wide range of colors and hues (from yellow, beige and red to dark brown and purple colors). Yellow/beige ceramic bodies result when carbonate rich clays (defined also as calcareous clays) are fired between 700 to 1000 ◦ C, yielding calcium pyroxenes and melilite phases [6,7]. To obtain light hue pastes, Al-species must be present, and a significant Si-rich glassy phase should have developed [8]. On the other hand, even in low concentrations, hematite might be responsible for the red colors of ceramic pastes [9]. Iron oxides, present in the raw materials and/or developed Minerals 2021, 11, 744. https://doi.org/10.3390/min11070744 https://www.mdpi.com/journal/minerals