Raman Study Of Ming Porcelain Dark Spots: Probing Mn-Rich Spinels

We report on the analysis of dendritic crystals present in the dark spot areas of blue-and-white Ming porcelains. Using Raman spectroscopy and scanning electron microscopy equipped with energy dispersive spectrometer, we were able to evidence Mn-rich spinels. Small Mn-based spinel crystals being highly sensitive to laser-induced heating, we identified proper working conditions, avoiding excessive heating and damage. Local temperatures during Raman measurements were deduced from Stokes/anti-Stokes intensity ratios. By analysing the behaviour of the A(1g) mode of Mn-rich spinels, we were able to identify phases ranging from jacobsite-like to hausmannite-like compounds. Polarized Raman scattering was shown to provide a convenient means of discriminating between these two types of phases, as they display different symmetries. Inhomogeneities within the dendritic network, as evidenced by micro-Raman measurements, highlight its gradual formation during the cooling phase of firing, depending of Fe3+ and Mn3+ availability. The presence of Mn-based spinels is consistent with the use of a local Chinese Mn-rich cobalt ore during the middle Ming Dynasty.