Reference optical characteristics of barium copper silicate pigments for non-invasive examination of painted cultural heritage

Barium copper silicate pigments are representative synthetic inorganic pigments used in ancient Chinese painted objects. Their unique crystal structures and optical responses have made them important research subjects in archaeometry and cultural heritage analysis. However, barium copper silicate pigments in archaeological samples are often associated with other mineral phases, binding materials, and weathering products. Establishing reliable reference data is therefore essential for distinguishing and identifying different barium copper silicate pigments. In this study, Chinese purple (BaCuSi₂O₆), Chinese dark blue (BaCu₂Si₂O₇), and Chinese blue (BaCuSi₄O₁₀) were synthesized by a hydrothermal method. X-ray diffraction (XRD), Raman spectroscopy, UV-Vis absorption spectroscopy, diffuse reflectance spectroscopy, steady-state photoluminescence excitation spectroscopy, and near-infrared emission spectroscopy were employed to establish comparable optical characteristic data for the three barium copper silicate pigments under identical experimental conditions. The results demonstrate that the three pigments exhibit distinguishable optical responses. The main absorption peaks were located at 578, 592, and 628 nm for Chinese purple, Chinese dark blue, and Chinese blue, respectively. Diffuse reflectance spectra revealed different reflectance behaviors among the three pigments, with Chinese blue showing the highest reflectance, followed by Chinese purple and Chinese dark blue. The corresponding photoluminescence excitation peaks were located at 581, 591, and 628 nm, while the near-infrared emission peaks appeared at 902, 917, and 954 nm, respectively. These differences indicate the influence of different crystal phases on visible-light absorption and near-infrared luminescence behaviors. Under visible-induced near-infrared luminescence imaging conditions, Chinese blue exhibited the strongest apparent luminescence response, followed by Chinese purple and Chinese dark blue. The reference pigments prepared in this study can provide materials for systematic investigations related to conservation and restoration, while the obtained spectroscopic data further enrich the optical characteristics of barium copper silicate pigments. In particular, the directly comparable reference optical data established under identical experimental conditions help address the insufficient systematic optical characterization data of barium copper silicate pigments, particularly Chinese dark blue, offering scientific support for the non-invasive identification and spatial localization of barium copper silicate pigments in painted cultural heritage objects.