Role of iron-rich clays on sintering of porcelain stoneware tiles

Porcelain stoneware traditionally contains low Fe2O3 (<1 wt%), limiting research on iron's effects beyond coloration to date. However, recent changes in the supply chain induced a shift towards resource efficiency and circular economy, which led to body formulations with Fe2O3 up to 2.5 wt%. This study investigates the impact of iron-rich clays on the firing behaviour of porcelain stoneware, focusing on sintering processes, Fe speciation, and physical properties of the vitreous phase. Results show that red clays influence vitrification paths and maximum densification temperature, as well as composition and properties of the melt. This occurs not only because the relatively high iron percentage (up to 1.7 wt%), but for a set of factors, like the overall mineralogical and chemical composition of batches. Iron turns to be crucial for the stability of ceramic bodies at high temperatures, since Fe3+ can partially reduce to Fe2+, releasing gases and contributing to bloating phenomena.