# Exploring Residual Clays for Low-Impact Ceramics: Insights from a Portuguese Ceramic Region
## 3 Results and Discussion
Samples D1-2 contained carbonates, which decompose between 600 and 800 \({}^{\circ}\)C. The lower LOI values in samples C1-2 (7-8%) were consistent with lower clay mineral and carbonate contents, with higher proportions of non-volatile phases, such as quartz. Samples B1 and E1 showed higher Ba content, which can be beneficial in ceramic production by acting as a mild flux, and may enhance glaze adherence in glazed products (Table 4) [50]. Strontium, which may substitute for Ca in plagioclase or carbonates, showed higher values in samples with calcite or dolomite, which can influence color and melt behavior in glaze formulations. The elements Cr, Ni, and V, likely linked to detrital or residual Fe-Mg phases (e.g., smectites), showed their highest values in A1 and A2, reflecting the smectite and Fe-bearing clay minerals [51]. These are typically inert in red ceramics but may influence fired color, particularly under reducing conditions. Zinc, which can be linked to clay minerals or carbonates, was most enriched in samples D1-2 and E1, potentially indicating weathering or a contribution from minor hydrothermal activity [52]. The Rb and Y concentrations correlate with feldspar and mica contents, e.g., high Rb in E1 and B1, corresponding with increased illite and K-feldspar content, suggesting residual K-mica weathering [53]. The Pb levels were low to moderate, with slightly higher values in samples B2 and D2, but below thresholds for ceramic materials. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
**ID** & **As** & **Ba** & **Cr** & **Cu** & **Nb** & **Ni** & **Pb** & **Rb** & **Sr** & **V** & **Y** & **Zn** & **Zr** \\ \hline A1 & 5 & 415 & 214 & 54 & 12 & 42 & 6 & 85 & 50 & 122 & 16 & 87 & 132 \\ A2 & 6 & 357 & 286 & 50 & 13 & 45 & 8 & 63 & 74 & 118 & 16 & 78 & 189 \\ B1 & 5 & 572 & 172 & 47 & 14 & 60 & 26 & 108 & 65 & 112 & 18 & 121 & 87 \\ B2 & 5 & 417 & 177 & 38 & 15 & 52 & 32 & 86 & 83 & 116 & 16 & 107 & 77 \\ C1 & 7 & 407 & 139 & 55 & 16 & 46 & 10 & 77 & 107 & 101 & 17 & 99 & 194 \\ C2 & 14 & 450 & 138 & 44 & 15 & 37 & 19 & 70 & 114 & 96 & 16 & 97 & 164 \\ D1 & 11 & 277 & 41 & 42 & 5 & 8 & 17 & 35 & 131 & 168 & 14 & 153 & 70 \\ D2 & 5 & 365 & 141 & 50 & 12 & 89 & 27 & 74 & 116 & 163 & 16 & 126 & 86 \\ E1 & 5 & 556 & 171 & 26 & 20 & 39 & 12 & 115 & 104 & 135 & 24 & 154 & 170 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Chemical composition of samples’ trace elements (in mg/kg). \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**ID** & **Al2O3** & **CaO** & **Fe2O3** & **K2O** & **MgO** & **MnO** & **Na2O** & **P2O5** & **SiO2** & **TiO2** & **LOI** \\ \hline A1 & 22.59 & 0.57 & 9.71 & 1.55 & 2.24 & 0.10 & 0.44 & 0.06 & 52.13 & 0.89 & 9.04 \\ A2 & 19.73 & 1.30 & 9.25 & 1.10 & 2.65 & 0.08 & 0.65 & 0.05 & 55.41 & 1.02 & 8.62 \\ B1 & 27.61 & 0.30 & 12.17 & 2.26 & 0.91 & 0.06 & 0.48 & 0.11 & 44.95 & 0.90 & 9.93 \\ B2 & 27.68 & 0.74 & 11.04 & 1.49 & 2.14 & 0.07 & 0.38 & 0.05 & 44.01 & 0.85 & 11.18 \\ C1 & 21.59 & 1.07 & 8.38 & 2.17 & 1.40 & 0.03 & 1.63 & 0.05 & 55.54 & 0.91 & 7.06 \\ C2 & 21.80 & 1.34 & 8.65 & 2.17 & 1.41 & 0.04 & 1.05 & 0.04 & 54.65 & 0.92 & 7.69 \\ D1 & 17.72 & 2.91 & 14.70 & 0.73 & 4.53 & 0.21 & 2.60 & 0.20 & 46.86 & 1.45 & 8.06 \\ D2 & 19.89 & 3.20 & 11.60 & 1.70 & 5.31 & 0.15 & 1.07 & 0.12 & 46.78 & 1.26 & 8.82 \\ E1 & 20.83 & 1.88 & 10.73 & 3.24 & 3.75 & 0.15 & 1.30 & 0.13 & 49.12 & 1.23 & 7.42 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Chemical composition of samples’ major elements (in %; LOI—loss of ignition). The Atterberg limits LL, PL, and PI allow the evaluation of the workability and water affinity of clayey materials, with a higher PI indicating greater plasticity and shrink-swell potential [54]. The samples with the highest plasticity indices were A1-2, B2, and C2 (Table 5), with A2 exhibiting the highest LL and PI, reflecting its dominant smectite content (71%, <0.002 mm) and fine granulometry (>50%, <0.002 mm).