Multispectral imaging uncovers the hidden secrets of jiandu

LANZHOU — With the naked eye, virtually no writing is visible on jiandu — bamboo or wooden slips that ancient Chinese people wrote on before the invention of paper — but using near-infrared imaging technology, the outlines of the ink marks faintly emerge.

When switched to the near-ultraviolet band, subtle cracks and salt efflorescence on the surface of the slips, invisible under normal conditions, become apparent. This "magic" is performed by Dorje Rinchen, a staff member in 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 (206 BC-AD 220). 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 of these directly hinder text interpretation and research into the slips.

To solve this problem, the museum introduced near-infrared imaging technology. This wavelength of light is strongly absorbed by ink but only weakly absorbed by bamboo and wooden substrates, allowing writing that is hidden under normal conditions to be seen once again, says Dorje Rinchen.

"However, imaging and scanning in a single wavelength band can restore only part of the textual information and cannot capture the comprehensive conditions of the slips," he adds.

To solve this, in 2025, the museum introduced a multispectral imaging system. Equipped with high-sensitivity sensors, the system captures images across 32 wavelengths, 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," says Du Peng, deputy general manager of Tianjin Hengda Wenbo Technology, 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, says Du.

The system has taken comprehensive images of more than 500 bamboo and wooden slips at the museum, generating 33 images for each slip, including the original.

"Jiandu are nonrenewable organic cultural relics that slowly decompose under natural conditions," says Dorje Rinchen.

Through this multispectral imaging technology, they have established comprehensive "medical records" for the slips, documenting their current condition and the deterioration they have suffered.

"By collecting and comparing images again after a period of time, we can monitor their condition," he says.

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, says Du.

The system developer expects to provide key information support for the multispectral archiving of cultural relics, extraction of hidden information and virtual restoration.

XINHUA