Inclusion coating technique for ceramic pigments

The inclusion of coloured inorganic phases inside transparent and refractory matrixes is studied in order to obtain ceramic pigments thermally and chemically stable during the firing of ceramic supports. In this work a new heterocoagulation/spray-drying two step process was set up and optimized to form coatings of inert matrixes on iron oxide (hematite) surface. Hematite and inert matrix water suspensions were separately prepared and the colloidal stability evaluated through measurements of zeta potential with an electroacoustic technique. Once the experimental conditions for the maximum surface charges attraction were established, the pigment and inert matrix suspensions were mixed each other and the heterocoagulation achieved. The heterocoagulated mixtures were spray-dried and the resulting included pigments applied to the ceramic bodies. The chemical-physical characterization of spray-dried powders was performed (ICP, SEM, XRD) in order to to appraise the process efficiency. The technological performances of included pigments were evaluated into different ceramic matrices fired at typical working temperatures (980-1200°C). The chromatic appearance (CIELab parameters) of the resulting products was evaluated by DRS. An improvement of hematite stability for low firing applications was obtained starting from amorphous silica as matrix. An intense red colour resulted in the glazes fired at 980°C where included pigment was applied in comparison with light grey colour resulting by the application of hematite alone or silica/hematite dry mechanical mixture (blank). In order to obtain a pigment stable for higher firing temperatures more refractory ceramic matrixes were also investigated and some preliminary results presented.