Investigation on the Crystal Structures of Hematite Pigments at Different Sintering Temperatures

Hematite is one of the most stable oxide phases and has been used as a rock-art pigment material in the prehistoric era. Hematite in the form of ochre has been widely used in rock art, burial rites, cosmetics, and decorations on pottery surfaces. Hematite has several hues, ranging from red to black. The variety of hues might arise due to the differences in physicochemical properties of hematite such as crystallinity, crystallite size, morphology, and electronic structure. The differences in physicochemical properties in hematite pigments might be originated from the differences in the pretreatment technique used before application. Herein, we have conducted a thorough study to investigate the evolution of the crystal structure of hematite under various temperatures. We aim to determine the temperature needed to change the hematite color and uncover the transformation of crystallographic properties as a function of sintering temperature. The hematite was synthesized using the precipitation method with Fe (NO3)3.6H2O as a precursor and ammonia (NH4OH) solution as a precipitating agent. The study of the hematite structural evolution was carried out by varying the sintering temperatures from 600 to 900 °C and analyzing the products after sintered using X-ray diffraction technique. The results showed that hematite hue was gradually darkened as the sintering temperature increased as a consequence of higher crystallinity and larger crystallite size of hematite crystal. This study confirms that the origin of color changes in hematite is due to the heating treatment of hematite material.