Chinese painting is a very old art form, and it is usually produced on paper or silk. The paintings present different characteristics, based on the rules defined for sketches, pigment usage, and painting style, depending on the period, location, and cultural context, as well as the painting habits and intention (Zhang 2016). The technical investigation of the painting techniques used in these works helps to inform our knowledge of the influence of the artists on the artistic production, history, and even archaeology of the period and its subsequent influence.
For conservators, it is important to understand the materials used in an original work, including the colorants, binders, and preparatory layers. Analysis to identify the painting materials should be carried out before or during the treatment.
Some of the analytical methods used, such as polarized light microscopy (PLM), scanning electron microscopy–energy-dispersive x-ray spectroscopy (SEM–EDX), and transmission-mode mid-infrared spectroscopy, require a small sample to be taken. However, techniques with sampling involved were not possible in this study due to the fragile condition of the ancient paper, silk, and pigments in question.
To study painting techniques, nondestructive methods such as hyperspectral imaging (HSI) and macro-X-ray fluorescence (MA-XRF) are used. However, MA-XRF is unable to identify traditional Chinese blue and green pigments, including azurite, malachite, and atacamite, because they all have copper in their molecular structure. While the reflectance spectra of the three kinds of pigments at 2000–2500 nm are different due to the different vibrational overtones of their molecules, HSI can reveal the preparatory sketches, pigment compounds (Delaney et al. 2010, Liang 2012, Cucci et al. 2016), and provide quantitative pigment maps nondestructively, which is a great step forward in the analytical field.
To identify all of the pigments simultaneously, an automatic hyperspectral scanning system with a spectral range of 400–2500 nm was built for large-scale paintings (maximum size of 2 × 2 m) by the Palace Museum in collaboration with the Aerospace Information Research Institute, Chinese Academy of Sciences, which has great experience in developing HSI equipment and image processing.