Unveiling the secrets of the Song Dynasty Qingbai porcelain: Coloring mechanism and non-destructive Terahertz testing

With the development of modern analytical instruments and technologies, optical and electrical characterizations serve as nearly non-destructive analysis methods, which have outstanding advantages in detecting cultural relics including ancient porcelains. In this study, the THz non-destructive testing accompanied by other optical and electrical techniques is applied to analyze the glaze and body of Qingbai porcelains from the Hutian kiln during the Song Dynasty, aiming at establishing the correlations among chemical composition, coloration, and dielectric properties of the porcelains. It reveals that the Qingbai porcelain body contains quartz, mullite, and perovskite, and is made from local Sanbao porcelain stone. The glaze layer was hypothesized to be a “calcium glaze” made from glazed fruit. Furthermore, samples from the Northern Song period exhibit a higher Fe[Formula: see text]/Fe[Formula: see text] ratio and a yellower glaze color compared to samples from the Southern Song period. Both Northern and Southern Song Qingbai porcelain glazes display a rounded and worm-like glass phase structure. The circular glass phase structure fulfills the Mie scattering condition, producing a white milky effect that enhances the brightness of the glaze surface. Additionally, the higher glass phase content and higher Fe[Formula: see text]/Fe[Formula: see text] ratio lead to an increase in the dielectric constant and dielectric loss of the Qingbai porcelains. Non-destructive testing based on terahertz time-domain spectroscopy reveals a decrease in dielectric constant and dielectric loss at THz frequency. Meanwhile, the corresponding terahertz image displays layered structures and defects in this cultural relic matrix similar to those observed under an optical microscope. This technique provides a convenient and effective method for evaluating and monitoring the preservation status of many cultural relics, such as porcelain, bronze, jade, and iron.