Marketed in an ochre-coloured powder, it is added to the bodies in percentage levels that vary from approximately 2 to 9 % giving pink-red colourings. On average, Grès de Thiviers contains 85/90% of quartz (SiO2) and 10/15% of Fe2O3, in the form of Goethite (FeO(OH)).
From close SEM-XRD examination and from Rietveld analysis, it appears that the yellowish Goethite is surrounded by micro-grains of quartz which, during firing, protect it from any possible chemical aggression of the alkaline environment of the bodies, leading to its transformation into Hematite (red), still surrounded by quartz. This means the colour red that is created is particularly stable.
Oxides of natural and synthetic iron do not develop equivalent red colourings because the latter are not protected by the quartz and are attacked and solubilised by the surrounding vitreous matrix, binding themselves to elements present in the glass.