At the end of 2019, the Palace Museum participated in a special program on “Disease Prevention and Evaluation Techniques for Typical Fragile Organic Cultural Relics in Collection” in the National Key R&D Program of China led by Jingzhou Cultural Relics Preservation Center. The program’s proposal on the subject “Research on In-Situ Evaluation Method for Typical Diseases and Deterioration Degree of Precious Organic Cultural Relics in Museum Collection” was successfully approved.

Deterioration is one of the key challenges for the conservation of organic cultural relics, but so far, the commonly used techniques are unable to realize non-destructive evaluation and prediction on the degree of deterioration, which has caused many difficulties in preventive conservation and preservation state evaluation of cultural relics. Regarding diseases that frequently occur on valuable organic cultural relics but can’t be easily observed by the naked eye, such as paper and textile fiber deterioration, pigment and dye fading, and lacquer film deterioration, this subject mainly aims to establish an in-situ evaluation method for the degree of deterioration of organic cultural relics. The project is divided into four subsidiary subjects based on the research tasks: "Research on the Evaluation of Paper Aging Degree Using Hyperspectral Imaging Technique", "Development of Portable Hyperspectral Imaging In-Situ Scanning System", "Research on the Evaluation Method for Lacquer Film Deterioration Based on Micro-Area UV Aging / Thermal Decomposition-Gas Chromatography / Mass Spectrometry", and "Research on the In-Situ Evaluation Method for the Micro-Aging and Fading of Pigments and Dyes in Paintings, Calligraphy and Textile Cultural Relics".

In the first subsidiary subject “Research on the Evaluation of Paper Aging Degree Using Hyperspectral Imaging Technique”, 216 simulative samples of paper and silk paintings and calligraphy are prepared. Based on the preliminary experimental results, 110 paper aging samples are selected for tests on viscosity and degree of polymerization, and hyperspectral data is collected simultaneously. Based on the tests, a preliminary evaluation model for the degree of paper aging is established using the degree of polymerization as the characterization index. Based on the preliminary research and practical evaluation, the HSI technique is more suitable for evaluating paper aging. Therefore, this research focuses on using the HSI technique to evaluate the degree of paper aging and will apply this method to paper used for paintings and calligraphy relics in the next step of work.

Figure 1. Simulative sample of painting and calligraphy substrates made with traditional crafts and techniques

In the second subsidiary subject “Development of Portable Hyperspectral Imaging In-Situ Scanning System”, the structure of the existing HSI scanning system is modified, upgraded and optimized to enable in-situ hyperspectral data collection for larger surfaces and vertical hanging cultural relics. The upgraded HSI scanning system can quickly collect hyperspectral data for cultural relics like large paintings and textiles through a 2D optical-mechanical platform, providing hyperspectral analysis data for the study of the deterioration degree of paintings and textiles in this subject.

Figure 2. Portable hyperspectral imaging in-situ scanning system

The third subsidiary subject “Research on the Evaluation Method for Lacquer Film Deterioration Based on Micro-Area UV Aging / Thermal Decomposition-Gas Chromatography / Mass Spectrometry” uses standard samples of lacquer and tung oil to study the deterioration mechanism of the two substances under the influence of light and temperature. The study identifies the characteristic components and their change patterns that represent the degree of deterioration of lacquer and tung oil, and eventually applies the research results to evaluating the degree of deterioration of lacquer film on lacquerware.

Figure 3. Standard samples of lacquer and tung oil film, and chromatogram of lacquer film aging products produced during the micro-area UV aging process

The fourth subsidiary subject “Research on the In-Situ Evaluation Method for the Micro-Aging and Fading of Pigments and Dyes in Paintings, Calligraphy and Textile Cultural Relics” prepares a series of typical traditional dyed silk samples, and conducts in-situ online light aging studies on these samples as well as mock faded samples using a micro-aging device. With this research, a preliminary database for micro-aging of dyes is established. The research uses the CIE L*a*b* color space to establish the fade model for typical textile dyes, and explores the color fade rates and change patterns of the dyed textiles from different aspects, including color difference, color saturation difference, color parameters, and fade patterns, so as to tell the color change trend of textiles. On this basis, the research takes the dyed textile fragments from different excavation spots of the Keyakekuduke Watchtower Site in Xinjiang Uygur Autonomous Region as the example to explore how this technique supports the in-situ fade analysis for actual archaeological textile samples, which can be an important reference for in-situ fade research on similar cultural relics.

Figure 4. Micro-aging testing equipment (left) and some dyes’ fading curves (right)

Figure 5. Photobleaching behaviors of Phellodendron. (a) Characteristic reflectance spectra. (b) Changes in color parameters (L, a, b). (c) Color changes in CIE L*a*b* space. (d)-(f) Fade models of color parameters (L, a, b).