An experimental study on the variability of the density, porosity and water properties of handmade bricks depending on their position in the kiln

The characterization of handmade bricks in cultural heritage buildings and vernacular construction is frequently constrained by the limited number of samples that can be extracted and tested. In the present case, the 245 bricks were produced from the same production batch and fired during the same kiln cycle, providing a comparatively controlled production assemblage. This study investigates whether, despite this common origin, their position within the kiln contributed to spatial variability in their physical and water-related properties. To this end, a complete layer of 245 handmade bricks from a kiln that had ceased operations decades ago in Melque de Cercos, Segovia (Spain) was analyzed, preserving its original spatial arrangement and a large portion of the bricks from the last firing. Non-destructive tests were conducted to determine dry density, open porosity, water absorption, and initial absorption rate, in accordance with the procedures established in EN standards. A statistical analysis was performed on the results obtained, and the value of each characteristic was correlated with the position in the firing chamber by creating color maps, which revealed substantial spatial variability in the characteristics related to position within the kiln. Additionally, nine bricks selected according to their position and dry density were characterized by XRF, XRD, and SEM. Bricks with higher densities are found near the firing zone, while bricks in the peripheral zones have lower densities. With open porosity and water absorption, the opposite is true: the highest values are found mainly at the periphery of the kiln, whilst the lowest values are found where the density is highest. Analysis of the box-and-whisker plots showed that density had the lowest dispersion, with a median of 1740 kg/m³ and a variation ranging from −1.6% to + 1.5%. Cold-water absorption and open porosity showed moderate dispersion, with median values of 15.0% and 26.0%, respectively; relative to their median values, cold-water absorption ranged from −4.4% to + 4.7%, whereas open porosity ranged from −3.2% to + 3.95%. The initial absorption rate showed high dispersion relative to its median of 1.98 kg/(m²·min), ranging from −16.2% to + 12.6%. When correlating the values obtained in the different tests, strong correlations were found between density and porosity (R² ≈ 0.73), as well as between density and water absorption (R² ≈ 0.85) confirming the internal consistency of the physical and hygric results and the existence of structured spatial variability. The integrated analysis identified differences in the raw materials and different relative degrees of thermal transformation, ranging from the preservation of illite to the formation of mullite and more coalescent microstructures, without a direct correspondence with density. The results demonstrate that traditional kilns produce highly heterogeneous materials, which has implications for material characterization, sampling strategies, and the conservation of brick-built cultural heritage and vernacular construction.