Nano encapsulation of hematite into silica matrix as a red inclusion ceramic pigment

The hematite as a natural and non toxic red inorganic pigment has been known since prehistoric times but color shade of hematite becomes unstable with temperature and need to protect with a suitable matrix. The best red shades are obtained by the inclusion of hematite in silica matrixes. A co-precipitation method has been applied in order to improve the inclusion efficiency of hematite into silica crystals; iron sulfate was used as Fe precursor and precipitating agent was ammonia. Effects of the single flux agent (NaF and NaCl) and binary flux agent (NaF·NaCl), have been studied. Continuous changes in color were measured by comparing L*-a*-b* values of the heated samples. TEM analysis on heat treated powders shows some hematite single crystals that were occluded with silica crystals successfully. Due to its chemical and thermal stability, the hematite/silica pigment may be considered as a suitable red pigment for ceramic manufacturing by fast firing cycles.

► Spherical hematite nano particles were provided with diameter of approximately 20-30 nm by ammonia as a precipitant agent and without mineraliser.
► The dimensions of the hematite particles depend on the mineralizers used.
► The use of mineralizers effects the silica crystallization and promotes tridymite phase formation instead of crystobalite.
► The stronger red shade is obtained by using two mineralizers, especially in the ratio NaF·2NaCl at calcination temperature as lower as 800 ̊C.
► In situ formed nano hematite is single crystals by mentioned method.