Behaviour of the natural red pigment "grès de thiviers" in por-celanised gres

This work deals with the characterisation of the Grès de Thiviers (GT), a unique natural red pigment for traditional ceramics. The only occurrence of this raw material, composed of quartz and goethite, is a region in SW of France were it is exploited, enriched and exported all over the world. GT is mainly used as a bulk red pigment for white stoneware and its stability in firing is a direct consequence of its unique mineralogy and texture. In fact, despite of the nature and behaviour in temperature of the raw materials in the ceramic mixture, GT yields a final red colour to the fired ceramic body. The reason of this stability stems from the fact that goethite which transforms into hematite is the cement of quartz grains. In particular we studied the stability of GT in mixtures for porcelanised grès with totally different features (i.e. from highly refractory to poorly refractory). This work is aimed to demonstrate that the application of the mineralogical knowledge in synergy with the use of advanced techniques of material characterisation is capable to give a detailed conclusive picture of the physical-chemical and technological properties of materials of ceramic interest.