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

Abstract Murals are invaluable but fragile cultural heritage assets. Accurate characterisation of paint material composition and layer thickness is essential for conservation, yet conventional methods are either invasive or suffer from signal interference. Here, we show that micro-laser-induced breakdown spectroscopy (Micro-LIBS)—with optimised parameters and minimally destructive under controlled conditions—can precisely identify pigments and layer thickness in a detached fragment (MG-01) from the Dunhuang Mogao Grottoes. Using a 2.0 mJ laser pulse and 1.2 μs acquisition delay, we identified atacamite (Cu₂(OH)₃Cl) for green, red lead (Pb₃O₄) for red, and gypsum (CaSO₄·2H₂O) for the white ground layer, confirmed by Raman spectroscopy. By monitoring (CuI, PbI, CaII) emission lines as a function of laser pulses, we reconstructed the painting sequence: ground—red—green, with individual layer thicknesses consistent with cross-sectional microscopy. These results confirm that Micro-LIBS is a reliable, high-spatial-resolution and minimally invasive tool for the stratigraphic analysis of ancient murals.