Non-destructive identification of iron oxide crystals in 15th century A.D. Chinese imperial red overglaze decoration

Red overglaze decoration is one of the earliest and most important colorful overglaze decorations in China, which had a remarkable influence on the successful firing and development of polychrome overglaze porcelains. The uncontrollability of red overglaze decoration led to a certain proportion of defective porcelain products in the Ming Dynasty (1368–1644 CE). To investigate and identify the coloring effect of iron oxide crystals in red overglaze decoration, three groups of samples with different color appearance were non-destructively analyzed and observed by OM, spectrophotometer, XRF, Raman spectroscopy, XRD and SEM. The results show that the hue of red overglaze decoration was caused by the different crystal forms of iron oxide. Hematite (α-Fe₂O₃) displays red color, while maghemite (γ-Fe₂O₃) and magnetite (Fe₃O₄) yield orange-red and dark red colors. With the similar raw materials and manufacturing processes, the presence of maghemite and magnetite are related to subtle differences in processing raw materials and firing them.