Clayey materials for traditional bricks production in North-Eastern Italy through a combined compositional study: From firing dynamics to provenance

The compositional study of two main types of clayey materials outcropping nearby Padua (Veneto region, northeastern Italy) and bricks used in historical constructions of the city is here addressed. Mineralogically, the clayey materials are illitic-chloritc clays, both non-carbonatic and carbonatic/highly-carbonatic clays, that chemically correspond to clays with important contents of silica and/or iron and of calcium and/or magnesium, respectively. Two main type of historic bricks were produced: i) one using mixtures of illitic/illitic-chloritic clays with abundant quartz and of carbonatic clays and firing temperatures between 950 and 1000 ◦C, and ii) a second one made out of illitic-chloritic clays non-carbonatic and fired around 850–900 ◦C. The comparative analysis between the mineralogical changes occurred in the clayey materials with increasing temperatures and the mineral assemblages detected in the studied bricks have provided evidences about the mixture of raw clays, that could be in turn compositionally similar to those analysed. The development of aluminium and magnesium-calcium silicates and/or magnesium silicates during the firing was fostered by mixing such base clays, giving rise to very durable and highly calcareous bricks. Whereas titanium and the trace elements zircon, vanadium, chromium and zinc may entail markers of provenance of illitic-chloritic clays quarried in the area, the strontium may represent a geochemical fingerprint for constraining supply areas of carbonatic clays. The compositional analysis carried out through the combined use of X-Ray Powder Diffraction (XRPD) and X-Ray Fluorescence Spectrometry (XRF) has provided data regarding to the composition and provenance of the starting clays as well as procedures and firing dynamics adopted for the manufacturing of the traditional bricks in the city of Padua from Roman Times to Renaissance.