Reappraisal of red clays in porcelain stoneware production: Compositional and technological characterization

The disruption of the supply chain of ball clays from Donetsk basin, Ukraine, has demonstrated that relying on domestic raw materials is strategic and vital for the ceramic tile production. However, the use of local raw materials has in most cases led to the increase in the impurities, namely iron, content of porcelain stoneware batches. A case-study is presented about the Italian tile district located in the Sassuolo area. In this work, we undertook the full characterization of two red clays from local quarries that are iron-rich (~7% Fe2O3), thus leading to well-known issues in the ceramic tile production. The features of these samples were compared with those of a German ball clay containing ~3% Fe2O3. The complex mineralogical composition of the Val Rossenna varicolored clays and on the Monte Piano marls was unravelled, revealing that they are rich in illite-smectite (I–S) mixed layers and illite, and poor in chlorite, sulfates and organic matter. The effects of a partial substitution in a porcelain stoneware body of classic ball clays with the Italian and German red clays were investigated. The obtained bodies were fully characterized from milling to firing. The amount of the red clays was chosen to best evaluate the effect of iron on the technological properties of semifinished and finished products. The substitution of the classic ball clays with red clays did not introduce unsolvable bottlenecks in ceramic tile production. The addition of the German red clay resulted in a slight increase of the sintering temperature (compared to the benchmark), possibly related to the higher quartz content of this raw material. Conversely, the addition of the Italian red clay induced a reduction of the firing temperature (compared to the benchmark), likely due to the higher content of iron, feldspars and carbonates. The most significant variations concern the color of fired tiles, which results darker (compared to the benchmark) in all the tested bodies, for the presence of pyroxene, hematite, and iron dissolved in the vitreous phase.