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Figure 1. Flow chart of the interdisciplinary and multi-analytical study carried out on the original bricks that shape the Renaissance walls (16th century) of the city of Padua, Northeast Italy.

Figure 1 Flow chart of the interdisciplinary and multi-analytical study carried out on the original bricks that shape the Renaissance walls (16th century) of the city of Padua, Northeast Italy.

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Figure 2. Padua’s Renaissance walls. (a) The irregular perimeter of the Renaissance walls (red) in- cluded the Medieval walls (orange)—today largely disappeared—and the fluvial system that crossed the city (light blue)—nowadays somewhat changed-. The white lines are the main passageways that articulated the Medieval city according to the corresponding Medieval gates; (b—d) brick fabrics on the facing South in 2019; (e) intervention on the wall accomplished during 2020-2021.
Figure 2. Padua’s Renaissance walls. (a) The irregular perimeter of the Renaissance walls (red) in- cluded the Medieval walls (orange)—today largely disappeared—and the fluvial system that crossed the city (light blue)—nowadays somewhat changed-. The white lines are the main passageways that articulated the Medieval city according to the corresponding Medieval gates; (b—d) brick fabrics on the facing South in 2019; (e) intervention on the wall accomplished during 2020-2021.
Figure 3. Appearance of the bricks on fresh surface. Very diverse textures and three main colors—yellow/beige, pale an: dark red- were observed. The colors of the ceramic bodies reveal the use of at least two types of raw clays and the significantly different texture and macroscopic features were mainly the consequence of the diverse manufacturing processes, suggesting that clayey materials from diverse areas were used and different preparations were adopted. The flux textures can be due to the heterogeneity of the base clay or may suggest the intentional mixing of clays, also for the optimization of the available raw clays. Moreover, the presence of clay lumps indicates that the clayey materials were not properly kneaded and/or seasoned [30].
Figure 3. Appearance of the bricks on fresh surface. Very diverse textures and three main colors—yellow/beige, pale an: dark red- were observed. The colors of the ceramic bodies reveal the use of at least two types of raw clays and the significantly different texture and macroscopic features were mainly the consequence of the diverse manufacturing processes, suggesting that clayey materials from diverse areas were used and different preparations were adopted. The flux textures can be due to the heterogeneity of the base clay or may suggest the intentional mixing of clays, also for the optimization of the available raw clays. Moreover, the presence of clay lumps indicates that the clayey materials were not properly kneaded and/or seasoned [30].
Table 1. Average and Std of lightness (L*), chromatic coordinates (a* and b*), chroma (C*) and hue angle (h°) of the ceramic bodies.
Table 1. Average and Std of lightness (L*), chromatic coordinates (a* and b*), chroma (C*) and hue angle (h°) of the ceramic bodies.
Figure 4. Ceramic bodies hue from the plotting of the chromatic coordinates (a* and b*) and changing colors with increasing of the red component. 1 Despite the yellow hue shown by the brick M1_4.8, this sample was considered belonging to the beige hue bodies (group A) throughou the results and discussion section.
Figure 4. Ceramic bodies hue from the plotting of the chromatic coordinates (a* and b*) and changing colors with increasing of the red component. 1 Despite the yellow hue shown by the brick M1_4.8, this sample was considered belonging to the beige hue bodies (group A) throughou the results and discussion section.
1 LOI = loss on ignition. Table 2. Chemical analysis of the major and minor oxides (in wt %) and zircon trace element (in ppm) of the ceramic bodies determined by XRF.
1 LOI = loss on ignition. Table 2. Chemical analysis of the major and minor oxides (in wt %) and zircon trace element (in ppm) of the ceramic bodies determined by XRF.
Figure 5. Ternary diagrams Si0,—Al,O3—CaO + MgO (left) and CaO—Fe,03-—MgO (right) with the plot of the composition of the bricks, identified according to their color. Sample numbered according to Tables 1 and 2.
Figure 5. Ternary diagrams Si0,—Al,O3—CaO + MgO (left) and CaO—Fe,03-—MgO (right) with the plot of the composition of the bricks, identified according to their color. Sample numbered according to Tables 1 and 2.
might yield to leucite (KAISi2O¢) precipitation. Therefore, zeolite formation also points out the important presence of calcium carbonate in the base clays and may induce some color changes, turning the pastes lighter. Figure 6. PXRD patterns of the beige, pale red and dark red hue ceramic bodies, according to the increasing of the red component (a* chromatic coordinate). Mineral abbreviations: quartz (Qz), k-feldspars (Kfs), illite (Ilt), albite (Ab), anorthite (An), diopside (Di), gehlenite (Gh), forsterite (Fo), hematite (Hem), ferrobustamite (Fbst), calcite (Cal), aragonite (Arg), analcime (Anl) and leucite (Lct), after Whitney and Evans [75].
might yield to leucite (KAISi2O¢) precipitation. Therefore, zeolite formation also points out the important presence of calcium carbonate in the base clays and may induce some color changes, turning the pastes lighter. Figure 6. PXRD patterns of the beige, pale red and dark red hue ceramic bodies, according to the increasing of the red component (a* chromatic coordinate). Mineral abbreviations: quartz (Qz), k-feldspars (Kfs), illite (Ilt), albite (Ab), anorthite (An), diopside (Di), gehlenite (Gh), forsterite (Fo), hematite (Hem), ferrobustamite (Fbst), calcite (Cal), aragonite (Arg), analcime (Anl) and leucite (Lct), after Whitney and Evans [75].
Figure 7. Dendrogram obtained from the cluster analysis of PXRD data according to Euclidean distance and average linkage method on position of peaks. Firing temperature, determined according to the mineralogical association, is also reported, as well as the color of the ceramic bodies.
Figure 7. Dendrogram obtained from the cluster analysis of PXRD data according to Euclidean distance and average linkage method on position of peaks. Firing temperature, determined according to the mineralogical association, is also reported, as well as the color of the ceramic bodies.
Figure 8. Photomicrographs in plain-polarized light (above) and crossed-polarized light (below) of the beige, pale and dark red hue ceramic bodies, ordered according to the increase of the red component (a* chromatic coordinate). Three general petro-fabrics were stated: Al (M1_4.8, M4_5.2, M2_1.1 and M2_4.6), A2 (M4_4.1, M3_3.1, M3_5.3 and M3_1.4) and B1 (M4_4.3, M1_4.7 and M4_3.1). Samples M3_4.1, M2_4.3, M3_4.0 and M3_2.1 do not belong to any of these general petro-fabrics. Images were taken at the same magnification.
Figure 8. Photomicrographs in plain-polarized light (above) and crossed-polarized light (below) of the beige, pale and dark red hue ceramic bodies, ordered according to the increase of the red component (a* chromatic coordinate). Three general petro-fabrics were stated: Al (M1_4.8, M4_5.2, M2_1.1 and M2_4.6), A2 (M4_4.1, M3_3.1, M3_5.3 and M3_1.4) and B1 (M4_4.3, M1_4.7 and M4_3.1). Samples M3_4.1, M2_4.3, M3_4.0 and M3_2.1 do not belong to any of these general petro-fabrics. Images were taken at the same magnification.
The beige hue bodies were achieved an early stage of sintering (Figure 9a,c) despite the high temperatures reached (>900 °C). As silicon, aluminum, calcium and magnesium were i In fac ncorporated into the high-temperature phases, the melting process was blocked [76]. , the micromass was found rather porous than the expected after MOP observations. It is important to highlight that although the carbonates induce the blocking of the sintering at hig partic h temperatures, they foster the development of a very stable microtexture [77]. Dark es with a melilite composition reaction rims and composed by Mg-rich silicate crystals, more abundant in petro-fabric Al (Figure 9a,b), can be noted. A Ca-Al-silicate matrix is extended through some areas of the micromass, and Ca-pyroxene crystals are crysta ized within (Figure 9c,d). Figure 9. HRSEM-BSE images and EDX spectra performed on the beige hue (Al and A2 petro-fabrics), pale red (B1 petro-fabric) and dark red bodies. (a) General view of the microstructure of petro-fabric Al, with Mg-rich dark particles and
The beige hue bodies were achieved an early stage of sintering (Figure 9a,c) despite the high temperatures reached (>900 °C). As silicon, aluminum, calcium and magnesium were i In fac ncorporated into the high-temperature phases, the melting process was blocked [76]. , the micromass was found rather porous than the expected after MOP observations. It is important to highlight that although the carbonates induce the blocking of the sintering at hig partic h temperatures, they foster the development of a very stable microtexture [77]. Dark es with a melilite composition reaction rims and composed by Mg-rich silicate crystals, more abundant in petro-fabric Al (Figure 9a,b), can be noted. A Ca-Al-silicate matrix is extended through some areas of the micromass, and Ca-pyroxene crystals are crysta ized within (Figure 9c,d). Figure 9. HRSEM-BSE images and EDX spectra performed on the beige hue (Al and A2 petro-fabrics), pale red (B1 petro-fabric) and dark red bodies. (a) General view of the microstructure of petro-fabric Al, with Mg-rich dark particles and
Figure 10. Free water absorption (1), forced water absorption (2) and drying (3) of bricks belonging to petro-fabrics Al, A2 and B1. Weight variation (AM/M) versus time (in h). Each curve represents the mean of two measurements.
Figure 10. Free water absorption (1), forced water absorption (2) and drying (3) of bricks belonging to petro-fabrics Al, A2 and B1. Weight variation (AM/M) versus time (in h). Each curve represents the mean of two measurements.
Real density va ues are quite similar in all the samples (from 2.5 to 2.8 g/cm?) and mainly related with silicates and carbonates occurrence (quartz = 2.7 g/cm?, k-feldspar = 2.6 g/cm°, calcite = 2.7 g/cm? and dolomite = 2.9 g/cm?). The larger real densities of the beige hue bodies may correspond to t he high-temperature phases, with greater densities (melilite = 2.9 g/cm’, anorthite = 2.8 g/cm? and diopside = 3.3 g/cm). The more porous are the bodies the higher is the difference between both density values. In fact, samples M1_4.8 and M2_1.1, having the highest open porosity values (P, > 50%), display the lowest apparent density values.
Real density va ues are quite similar in all the samples (from 2.5 to 2.8 g/cm?) and mainly related with silicates and carbonates occurrence (quartz = 2.7 g/cm?, k-feldspar = 2.6 g/cm°, calcite = 2.7 g/cm? and dolomite = 2.9 g/cm?). The larger real densities of the beige hue bodies may correspond to t he high-temperature phases, with greater densities (melilite = 2.9 g/cm’, anorthite = 2.8 g/cm? and diopside = 3.3 g/cm). The more porous are the bodies the higher is the difference between both density values. In fact, samples M1_4.8 and M2_1.1, having the highest open porosity values (P, > 50%), display the lowest apparent density values.
1 As values measured in M1_4.8 and M4_4.1 bodies have resulted rather different than those from the other samples belonging to the corresponding petro-fabrics (A1 and A2), in order to not distort the average such values have been excluded from the calculi. able 4. Vp;, Vp2 and Vp3 = velocities of ultrasonic wave propagation in the three orthogonal directions (m/s), Vp = mean elocity (m/s), structural (AM) and relative (Am) anisotropies (%) of the bricks. Mean values for the petro-fabric are Iso reported.
1 As values measured in M1_4.8 and M4_4.1 bodies have resulted rather different than those from the other samples belonging to the corresponding petro-fabrics (A1 and A2), in order to not distort the average such values have been excluded from the calculi. able 4. Vp;, Vp2 and Vp3 = velocities of ultrasonic wave propagation in the three orthogonal directions (m/s), Vp = mean elocity (m/s), structural (AM) and relative (Am) anisotropies (%) of the bricks. Mean values for the petro-fabric are Iso reported.
Figure 11. Mercury intrusion porosimetry curves, accumulative (in red) and pore-size distribution (blue) of the beige, pale red and dark red hue ceramic bodies, expressed as volume intrusion of mercury, versus pore radius (in micrometers).
Figure 11. Mercury intrusion porosimetry curves, accumulative (in red) and pore-size distribution (blue) of the beige, pale red and dark red hue ceramic bodies, expressed as volume intrusion of mercury, versus pore radius (in micrometers).
After the salt crystallization test, some material loss is visually observed, mainly in the Al bodies and mostly located in the corners and edges of the cubic samples. The lower increase of weight is recorded in the Al bodies and some A2 bodies, and the B1 bodies display rather similar weight rising (Figure 12a). The weight increasing between cycles 2 and 9 chiefly corresponds with the salts crystallized within the pores. Then, the weight began to fall, primarily due to material loss—or it was almost stabilized with a very subtle decline—mostly because of the crystallized salts that remain inside the bodies. Figure 12. Variation of weight in bricks belonging to petro-fabrics Al, A2 and B1 during durability tests. (a) Salt crystal- lization and (b) Freeze-thaw. Weight variation (AM/M) versus time (in number of cycles, each cycle corresponds to 24 h). Each curve represents the mean of two measurements.
After the salt crystallization test, some material loss is visually observed, mainly in the Al bodies and mostly located in the corners and edges of the cubic samples. The lower increase of weight is recorded in the Al bodies and some A2 bodies, and the B1 bodies display rather similar weight rising (Figure 12a). The weight increasing between cycles 2 and 9 chiefly corresponds with the salts crystallized within the pores. Then, the weight began to fall, primarily due to material loss—or it was almost stabilized with a very subtle decline—mostly because of the crystallized salts that remain inside the bodies. Figure 12. Variation of weight in bricks belonging to petro-fabrics Al, A2 and B1 during durability tests. (a) Salt crystal- lization and (b) Freeze-thaw. Weight variation (AM/M) versus time (in number of cycles, each cycle corresponds to 24 h). Each curve represents the mean of two measurements.
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MineralogyConstruction TechnologyCeramic Analysis (Archaeology)Architectural Heritage Conservation
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