Spectral heat aging model to estimate the age of seals on painting and calligraphy

Abstract Seals, a common part of Chinese painting and calligraphy, play a key role in identification, collection, and other activities of painting and calligraphy. However, most of the seal age-analysis methods are still dependent on the experience and judgement of the experts. Thus, scientific evidence in this field is required. This study aims to explore the changes in the reflectance spectra by heat-aging on seal samples to establish a spectral heat-aging model that can be used to estimate the aging times of seals. An experiment was conducted to detect the reflectance spectral changes of sample seals on Chinese paintings and calligraphy after different heat aging times, and a model incorporating the spectral changes over aging time was built. After heat aging, the visible near-infrared spectra of the seal samples were acquired and analyzed. The results show that the cinnabar pigment contained in the samples exhibited no significant change in spectra. The spectral change bands of the rice paper and the seal-ink did not overlap, and the spectral changes of the seal-ink aging were found to be mainly come from the additive. Its sensitive bands that means those bands with distinct characteristics of reflectance spectra are concentrated in the spectral ranges of 1700−1800 nm and 2250−2400 nm. As the aging time prolonged, the additives of the seals were found to gradually undergo changes, whereas, the composition of the cinnabar pigment tended to be stable. Finally, a spectral heat-aging model was established using multivariable linear regression. The accuracy of this model was tested using 16 samples that were not used during training. The coefficient of determination (R2) value was found to be 0.83, and the correlation between the real and estimated aging times was 0.65. Results demonstrated that the visible near-infrared reflectance spectra changes could be used to estimate the age of seals on Chinese paintings or calligraphy.