LANZHOU, Oct. 7 (Xinhua) -- Under visual observation, virtually no writing is visible on Jiandu, bamboo or wooden slips that ancient Chinese people wrote on before the invention of paper, but in near-infrared images, the outlines of the ink marks faintly emerge.
When switched to the ultraviolet band, subtle cracks and salt efflorescence on the surface of the slips, invisible to the naked eye, become clearly visible. This "magic" is performed by Dorje Rinchen, an office staff member of Gansu Jiandu Museum's science and technology conservation department.
The bamboo and wooden slips collected by Gansu Jiandu Museum were mostly unearthed from frontier fortress sites in northwest China dating back to the Han Dynasty. Made primarily of poplar, rose willow and pine, they are about 2,000 years old.
Buried for millennia in the extremely arid Gobi environment, the slips commonly suffer from such deterioration as salt efflorescence, insect damage, mildew stains and transverse cracks on their surfaces.
All these diseases directly hinder the advancement of text interpretation and research of the slips.
"To solve this problem, the museum introduced infrared imaging technology, which uses the characteristic of infrared wavelengths being strongly absorbed by ink but weakly absorbed by bamboo and wooden substrates, to restore and identify faded and hidden writing on the slips," said Dorje Rinchen.
"However, imaging and scanning in a single wavelength band can only restore parts of textual information and can hardly capture the comprehensive conditions of the slips."
To this end, starting in 2025, the museum introduced a multispectral imaging system. Equipped with high-sensitivity sensors, the system captures images across 32 channels, ranging from near-ultraviolet to near-infrared light, unfolding the slips along the spectral dimension and enabling their "panoramic" documentation.
"Light of different wavelengths differs in its ability to penetrate and reflect off ink, stains and deterioration," said Du Peng, deputy general manager of Tianjin Hengda Wenbo Technology Co., Ltd, the developer of the multispectral imaging system.
The multispectral technology can not only make faded writing on the slips "visible" but also, by comparing different wavelength bands, accurately distinguish original ink from later contamination and pinpoint the location and extent of deterioration, such as salt efflorescence and insect damage, said Du.
The system has carried out all-round image capture of more than 500 bamboo and wooden slips at the museum, generating 33 images for each slip, including the original.
"Jiandu are non-renewable organic cultural relics that slowly decompose under natural conditions," said Dorje Rinchen. Through this multispectral imaging technology, they have established comprehensive "medical records" for the slips, documenting their current condition and the deterioration they suffer.
"By collecting and comparing images again after a period of time, we can monitor the conditions of the slips," he said.
In addition to protecting bamboo and wooden slips, the multispectral imaging system can also be applied to cultural relics such as calligraphy, paintings and artifacts, as well as grotto murals and tomb murals, said Du.
The system developer expects to provide key information support for the multispectral archiving of cultural relics, extraction of hidden information and virtual restoration. ■
