[Nondestructive analysis and identification of pigments on colored relics by fiber optic reflectance spectroscopy]

Identifying pigments on colored relics is an important part of relic analysis and protection. In order to meet the special demands of protecting relics, taking into account the situation in China, we have refitted a domestic instrument to a fiber optics reflectance spectrophotometer with low price and good function to identify pigments without any damage. It consists of a tungsten lamp, a monochromator, an optical fiber, a shielding chamber, a photomultiplier tube, a reading meter, and a high voltage supply source. Through analysis of a great variety of Chinese ancient pigments, we have deduced three reflectance spectrum shapes of pigments, bell-shaped, s-shaped and oblique line-shaped. The blue or green pigments with bell-shaped spectra can be identified by the characteristic reflectance peak. The pigments with s-shaped spectra are usually red, yellow or brown. Although there are no characteristic reflectance peaks on the plots of the reflectance-wavelength, there is a peak in the first derivative (D) here. Then, the related pigments of this sort can be analyzed and identified with their derivative peaks. For the pigments with oblique line-shaped spectra, their reflectance and first derivative spectra are nearly linear and there are no peaks. The related pigments are usually white, black or gray. With our self-made fiber optics reflectance spectrophotometer, we analyzed the composition of pigments on colored pottery figurines and frescoes of Tang dynasty tombs in Xi'an by comparing their reflectance spectroscopic curves and characteristic reflectance peaks or first derivative peaks with those of standard pigments. The deep red pigment in sample 1#, the green pigment in sample 2#, and the orange and deep red pigments in sample 3# have been identified to be hematite with high purity, malachite and the mixtures of cinnabar/red lead and a little hematite, respectively. It has been indicated that the analytical results of relic pigments from this method are accurate and reliable, and verified by XRF analysis. Fiber optics reflectance spectroscopy is a new quick analytical technique to identify pigments on colored relics.