Degradation of red lead pigment in the oil painting during UV aging

The degradation product and mechanism of red lead pigment (Pb3O4, minium) in pure powder and in modeled oil painting are dynamically tracked during ultraviolet light aging by the change of color, crystal structure, chemical composition, and surface morphology. After UV aging, the color of pure minium pigment gradually darkens from bright orange-red into black, but the color of model paint samples made by minium and tung oil binder changes into brownish and whitish color (Delta E* = 56.60). After aging, the crystal structure of Pb3O4 powder is proved to transfer into plattnerite (beta-PbO2) by X-ray diffraction (XRD) pattern, but the degradation product of Pb3O4 in model paint samples is identified as 2PbCO(3)center dot Pb(OH)(2) by the noticeable peaks at 24.8 degrees, 27.4 degrees, and 34.3 degrees in XRD pattern, and two new Fourier transform infrared peaks at 1393 cm(-1) and 682 cm(-1) attributed to the presence of CO32-, and a new peak at 1051 cm(-1) in Raman spectroscopy. During UV aging, the model paint samples' surface is verified gradually from wrinkled surface into nanoscale sheet-like structure and finally a regular hexagonal plate-like crystal structure, showing hexagonal crystals of hydrocerussit 2PbCO(3)center dot Pb(OH)(2). This reveals the interaction between organic tung oil binder and inorganic minium to accelerate the degradation of Pb3O4 pigment. Therefore, the degradation mechanism is deduced as that CO2 and H+ formed by oxidizing ester/carboxyl groups in tung oil reacts with Pb2+ to yield the white product of 2PbCO(3)center dot Pb(OH)(2). It is believed that this study will contribute much to the complete understanding of the degradation of minium pigment in oil paintings.