Study on the Influence of Calcination Temperature of Iron Vitriol on the Coloration of Ancient Chinese Traditional Iron Red Overglaze Color

Abstract

Iron red, a traditional Jingdezhen overglaze color, is primarily colored with iron oxide (Fe2O3). In traditional processes, the main ingredient for the iron red overglaze color, raw iron red, is produced by calcining iron vitriol (FeSO4·7H2O). Analysis of ancient iron red porcelain samples indicates that the coloration is unstable, ranging from bright red to dark red and occasionally to black. Addressing this, the present study, from a ceramic technology standpoint, conducts a series of calcination experiments on industrial iron vitriol at varying temperatures. Utilizing methodologies such as differential scanning calorimetry-thermogravimetry (DSC-TG), Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy with X-ray energy dispersive spectrometry (SEM-EDS), and optical microscopy (OM), this research scientifically explores the impact of iron vitriol’s calcination temperature on the coloration of traditional Jingdezhen iron red overglaze color. The findings indicate that from room temperature to 550 °C, the dehydration of iron vitriol resulted in the formation of Fe2(SO4)3 and a minimal amount of α-Fe2O3, rendering the iron red overglaze color a yellowish-red shade. At 650 °C, the coexistence of Fe2(SO4)3 and α-Fe2O3 imparted a brick-red color to the iron red. As the temperature was elevated to 700 °C, the desulfurization of Fe2(SO4)3 produced α-Fe2O3, transitioning the iron red to an orange red. With further temperature increase to 750 °C, the particle size of α-Fe2O3 grew and the crystal reflectivity decreased, resulting in a purplish-red hue. Throughout this stage, the powder remained in a single α-Fe2O3 phase. Upon further heating to 800 °C, the crystallinity of α-Fe2O3 enhanced, giving the iron red overglaze color a dark red or even black appearance.