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.
Highlights
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]
This study aims to highlight the important role of clay bricks in the built heritage of the city to betopart theofcitizen awareness for thefor preservation of suchofa such defensive complex and its and be of part the citizen awareness the preservation a defensive complex immediate surroundings
Despite that high porous bodies are more susceptible to decay by salts action [42], the preliminary grouping on 2color measurements of loss historic bricksM1_4.8, rather stopThe in weight increasing alreadybased in cycle and the small material in sample well-preserved sampledopen fromporosity the Renaissance walls of the the largest city of Padua hasofentailed the which has the highest (PO ≈ 52%)
Summary
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 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 [3,4,5]. 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). To obtain light hue pastes, Al-species must be present, and a significant Si-rich glassy phase should have developed [8]
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