In Situ Identification of Pigment Composition and Particle Size on Wall Paintings Using Visible Spectroscopy as a Noninvasive Measurement Method

Abstract

Noninvasive examination methods of chemical composition and particle size are presented here based on visible spectroscopy to achieve the identification and recording of mineral pigments used on ancient wall paintings. The normalized spectral curve, slope and curvature extracted from visible spectral reflectance are combined with adjustable weighting coefficients to construct the identification feature space, and Euclid distances between spectral reflectance from wall paintings and a reference database are calculated in the feature space as the discriminant criterion to identify the chemical composition of mineral pigments. A parametric relationship between the integral quantity of spectral reflectance and logarithm of mean particle size is established using a quadratic polynomial to accomplish the noninvasive prediction of mineral pigment particle size used on ancient wall paintings. The feasibility of the proposed methods is validated by the in situ nondestructive identification of the wall paintings in the Mogao Grottoes at Dunhuang. Chinese painting styles and historical evolution are then analyzed according to the identification results of 16 different grottoes constructed from the Sixteen Kingdoms to the Yuan Dynasty.

Analysis of the spectral reflectance and color of mineral pigments affected by their particle size

“…Other kinds of mineral color block samples have similar phenomena and have their own nonlinear monotonically decreasing characteristics. The analysis results in Table 2 and Figure 8 are consistent with the conclusions of Equation (8) and Inequality (16).…”

“…In the whole visible spectral waveband, the radiation of each wavelength is mixed by this weight function to ultimately determine the hue of the opaque object. Equation (8) shows that the weight function will also change with the particle size. Thus, the hue changes when the weight function changes.…”

“…In Figure 7A to F, six kinds of mineral pigments have their own curve variation characteristics with particle size. Because different kinds of mineral pigments have different absorption characteristics and refractive indices, their physical parameters are different in Equations (8) and (12). Note that the reflectance factor curves of the same kind of mineral pigment with different particle sizes have different magnitudes and slightly different shapes.…”

“…In recent decades, several studies have been conducted to investigate the relationship between the particle sizes and spectral reflectance or color of mineral particles. [3][4][5][6][7][8][9] These studies have found that changes in the particle size lead to changes in the color appearance and spectral reflectance of the mineral particles. However, to the best of our knowledge, only a portion of these studies relates to the colorful mineral pigment particles used in oriental mural or rock paintings.…”

“…Therefore, the particle size of mineral pigments is an important parameter for scientists and artists. In recent decades, several studies have been conducted to investigate the relationship between the particle sizes and spectral reflectance or color of mineral particles . These studies have found that changes in the particle size lead to changes in the color appearance and spectral reflectance of the mineral particles.…”