Micro-LIBS Maps Hidden Layers of Dunhuang Murals at the Micron Scale

The thousand-year-old murals of the Mogao Grottoes at Dunhuang, on the edge of China’s Gobi Desert, have long challenged conservators with a deceptively simple question: what exactly are they made of, and how are their pigments layered beneath the surface? A new study published in npj Heritage addresses that question with an analytical technique borrowed from physics laboratories rather than art history seminars. Researchers applied micro-laser-induced breakdown spectroscopy, or Micro-LIBS, to Dunhuang wall paintings, demonstrating that the method can both identify the mineral pigments used by ancient painters and map the thickness of individual paint layers at a spatial resolution measured in micrometres, all while leaving the fragile artwork essentially untouched.

Laser-induced breakdown spectroscopy works by focusing a short, energetic laser pulse onto a target. The pulse ablates a vanishingly small amount of material and, in doing so, creates a microplasma: a tiny, fleeting cloud of ionised atoms and electrons heated to thousands of degrees. As this plasma cools over microseconds, the excited atoms and ions emit light at wavelengths characteristic of their elemental composition. A spectrometer collects that emission, and by matching the observed spectral lines against known atomic transitions, analysts can determine which chemical elements are present in the sampled spot. The technique requires no vacuum chamber, no extraction of a sample, and no contact with the object beyond the laser pulse itself, which makes it unusually well suited to cultural heritage applications where even a pinhead-sized flake of removed paint is a loss.

What distinguishes the micro variant used in the Dunhuang study is spatial precision. Conventional LIBS setups often analyse craters tens to hundreds of micrometres across, which is adequate for bulk composition but too coarse to resolve the fine stratigraphy of a mural. Ancient wall painters at Dunhuang typically worked in layers: a rough earthen plaster support, a finer preparation ground, and then one or more paint layers, sometimes finished with varnishes or later overpaints added during successive restorations across dynasties. Each layer may be only a few micrometres to a few tens of micrometres thick. By shrinking the laser spot and carefully controlling pulse energy, Micro-LIBS can sample material from successively deeper strata within a single point, recording an elemental spectrum at each depth. Scanning the beam across the surface in a grid then converts a series of point measurements into a two-dimensional elemental map, and depth-resolved scanning adds the third dimension of layer thickness.

The researchers applied this capability to pigment identification first. Dunhuang murals are famous for their mineral palette: azurite and lapis lazuli for blues, malachite for greens, cinnabar and red lead for reds, orpiment and realgar for yellows, and lead white, gypsum, and calcite for whites and grounds. Each of these minerals carries a distinctive elemental signature. Copper lines reveal the presence of azurite or malachite; mercury lines betray cinnabar; arsenic lines point to orpiment or realgar; and lead lines indicate lead-based whites and reds. Because the spectra are element-specific rather than compound-specific, the authors combined LIBS results with the known mineralogy of the site and complementary spectroscopic information to assign pigments to particular pictorial elements, distinguishing, for example, different blue pigments that would look nearly identical to the eye but carry different conservation implications.

The second, and arguably more novel, contribution is the mapping of layer thickness. Depth profiling with LIBS relies on the fact that each laser pulse removes a shallow, roughly quantifiable slice of material. By firing a sequence of pulses at the same spot and monitoring how the elemental signal evolves from pulse to pulse, the analyst can reconstruct a vertical profile: the depth at which copper signal rises marks the top of a copper-based paint layer, and the depth at which calcium and silicon dominate marks the underlying plaster. Calibrating this ablation rate against reference materials of known thickness allows the researchers to convert pulse counts into micrometres. Applied across a scanned area, the method yields a thickness map of individual paint layers, revealing where painters applied pigment thickly, where it thins toward the edges of a brushstroke, and where later interventions added material on top of the original.

That kind of information has direct practical value for conservation. Dunhuang murals suffer from flaking paint, salt efflorescence, delamination of the plaster support, and discolouration of lead-based pigments that have darkened over centuries of exposure to humidity, temperature swings, and, in the past, pilgrim smoke and incense. Conservation treatments, from consolidation to cleaning, must be tailored to the actual structure of the paint system. Knowing whether a red passage is cinnabar or red lead matters because the two pigments respond differently to light and to solvents; knowing whether a darkened surface is original paint or a later overpaint determines whether cleaning is appropriate at all. A thickness map showing where layers are thinnest identifies the zones most vulnerable to further loss, allowing conservators to prioritise monitoring and intervention.

The micro-scale, minimally invasive character of the technique is central to its appeal. Traditional stratigraphic analysis requires taking a cross-section sample: a tiny wedge of paint and plaster removed from the wall, embedded in resin, and polished for examination under a microscope, often coupled with scanning electron microscopy or Raman spectroscopy. These methods deliver superb detail, but each sample is a permanent, if small, loss from an irreplaceable artwork, and the number of sampling points is necessarily limited. Micro-LIBS, by contrast, can in principle be applied in situ with a portable or fibre-coupled instrument, and the ablated material per measurement is so small as to be invisible to the naked eye. The trade-off is that LIBS identifies elements rather than molecular compounds, so it cannot by itself distinguish azurite from another copper carbonate, or cinnabar from other mercury sulfides. The study therefore positions Micro-LIBS as a rapid, wide-coverage first pass whose results can guide targeted, minimal sampling or complementary molecular techniques such as Raman or X-ray fluorescence mapping where compound-level certainty is required.

The Dunhuang setting amplifies both the difficulty and the significance of the work. The Mogao Grottoes, a UNESCO World Heritage site excavated into a cliff face over roughly a millennium beginning in the fourth century, contain some of the finest surviving Buddhist wall painting in the world, spanning more than four hundred decorated caves. The murals were executed by workshops working over centuries, and their materials reflect trade networks that brought lapis lazuli from Central Asia and mineral pigments from across China. Environmental conditions inside the caves, including fluctuating humidity driven by visitor numbers and desert climate cycles, continue to threaten the paintings. Any analytical campaign at such a site must balance the scientific value of new data against the imperative of preservation, and techniques that deliver rich information without contact or sampling are at a premium.

More broadly, the study illustrates a trend in heritage science: the migration of laboratory analytical techniques toward field-deployable, high-resolution, and increasingly three-dimensional tools. Where earlier surveys might have characterised a mural’s palette with a handful of samples, depth-resolved elemental mapping now offers a way to document stratigraphy across entire pictorial fields, capturing the variability of an artist’s hand and the accumulated history of restorations. For Dunhuang, whose murals record more than a thousand years of religious art, cultural exchange, and conservation practice, that means a fuller and less destructive record of what lies beneath the visible surface. For the field at large, it suggests that the microscopic architecture of painted heritage, long accessible only through destructive cross-sections, can increasingly be read in place, one laser pulse at a time.

Subject of Research: Pigment identification and paint layer thickness mapping of Dunhuang murals using micro-laser-induced breakdown spectroscopy

Article Title: Dunhuang murals at the micron scale: pigment identification and layer thickness mapping by Micro-LIBS

Article References: Zhang, G., Yin, Y., Han, W., Sikorski, M., Bai, X., Dong, C., & Sun, D. (2026). Dunhuang murals at the micron scale: pigment identification and layer thickness mapping by Micro-LIBS. npj Heritage Science. https://doi.org/10.1038/s40494-026-02981-y

Image Credits: AI Generated

DOI: 10.1038/s40494-026-02981-y

Keywords: Dunhuang, Mogao Grottoes, Micro-LIBS, laser-induced breakdown spectroscopy, pigment identification, layer thickness mapping, heritage science, wall paintings, conservation, depth profiling, cultural heritage, npj Heritage