Multi-analytical investigation into the materials and techniques of paintings on Northern Wei Dynasty (398–494 CE) coffin planks excavated from Shanxi, China

Painted wooden coffins are important funerary objects bearing secular imagery and cultural information. While Egyptian and Chinese traditions of painted coffins are well documented, the Northern Wei Dynasty (386–534 CE), especially the Pingcheng Period (398–494 CE), has few excavated examples and limited scientific analysis of materials and techniques. The Zhijiabao site in Datong, Shanxi, yielded three painted coffin planks showing multiple colors and secular scenes but suffering degradation. The study aims to identify pigments and reconstruct production techniques using a minimally invasive analytical approach to inform conservation and contribute to Northern Wei art historical understanding. It addresses the gap in non-invasive analytical studies of Chinese painted coffins and seeks to reduce sampling volumes compared with prior work.

Previous research on Egyptian painted coffins has developed robust multi-technique protocols combining non-invasive and micro-invasive methods (e.g., multispectral imaging, Raman, IR, GC–MS), enabling identification of pigments, binders, and restoration phases, and inspiring construction of period-specific pigment libraries. In China, most coffin studies emphasized iconography, cultural interactions, and stylistic analysis across regions such as Hexi, Xinjiang (Han–Jin), Qinghai (Tibetan Empire/Tuyuhun), and Inner Mongolia (Liao). Scientific analyses of Chinese painted coffins have been limited, often relying on relatively large samples (>1 cm) and lacking reported non-invasive studies, potentially compromising artifact preservation. The Northern Wei Dynasty’s cultural amalgamation and the move to Pingcheng highlight the significance of technical studies; however, only a few painted coffins from this period have been excavated and their materials/techniques remain unclear. This work builds on advances in portable spectroscopy and microscopy to fill that gap by applying an integrated, minimally invasive analytical framework.

Study objects: Three coffin planks from a disturbed Northern Wei tomb at Zhijiabao Village, Datong, Shanxi (Plank A: 1.53 × 0.42 m; Plank B: 1.33 × 0.44 m; Plank C: 0.60 × 0.18 m). In-situ non-invasive analyses were performed at 20–23 points per plank. Four micro-samples were taken from edges/warped areas for lab analysis (SP1 green, SP2 blue, SP3 white, SP4 black with full cross-section). A simulated layered sample was prepared: underlayer orpiment+realgar mixture, top layer orpiment+indigo, to model observed green mixtures. In-situ non-invasive techniques: - Portable X-ray fluorescence (p-XRF; Niton XL3t 950): Ag target, 50 kV/40 µA, 30 s, mining mode; data via NDT 8.2. - Portable Raman spectroscopy (p-RS; Horiba HE785): 785 nm excitation, 50× objective, CCD cooled to −50 °C; 5 s integration, 2 accumulations; 100–1000 cm⁻1 range; sulfur calibration. - Fiber-optic reflectance spectroscopy (FORS; FX2000): 200–1100 nm, HL2000 halogen source, Y-type fiber; probe ~0.5 cm from surface, ~2 mm spot; 40–50 ms integration, 5 averages; whiteboard calibration. Laboratory micro-analyses: - Optical microscopy (OM; Leica DVM6): micro-morphology and cross-section stratigraphy. SP4 embedded in light-curing resin and polished with 500/800/1200/2000 grit papers. - Scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS; Phenom XL): backscattered mode, 15 kV, 10 Pa, no coating. - Confocal micro-Raman spectroscopy (micro-RS; Horiba LabRAM HR Evolution): 532 and 633 nm excitation, 50× objective; 5 s, 5 accumulations; 2 cm⁻1 resolution, ~1 µm spot; single-crystal silicon calibration; 100–2000 cm⁻1 range. Data processing used instrument vendor software; spectra plotted with OriginPro 2022. Comparative analyses included standard pigments and the simulated sample to interpret mixtures (especially green).

- Stratigraphy and technique: - Cross-sections show a yellow primer directly over wood, overlain by a black layer; each is ~10–12 µm thick. Most areas exhibit two layers; figure portraits sometimes have three layers. - Pigment identifications (multi-technique corroboration): - Red: Cinnabar (HgS). p-XRF detected Hg; p-RS peaks near ~254 and ~343 cm⁻1. Present at 18 measurement spots. - Yellow: Pararealgar (As4S4) identified by micro-RS (e.g., bands ~232, 273, 345 cm⁻1); As detected widely. Used as the primer. Yellow recorded at 13 spots. Realgar may also be present due to polymorphism and light conversion. - Green: Mixture of orpiment (As2S3) and indigo. FORS of in-situ points matched a simulated orpiment+indigo spectrum (absorption near 450 nm and ~660 nm; inflection shift). SEM-EDS detected As, S, C, N, O with no Cu, excluding copper greens. Raman bands characteristic of orpiment (e.g., ~202, 294, 311, 354, 383 cm⁻1) and indigo (e.g., ~546, 599, 1224–1253, 1574–1582 cm⁻1). Green recorded at 17 spots. - Blue: Indigo mixed with a lead-based white (likely lead white). SEM-EDS detected C, N, O, Pb (also minor Al, Si, Ca). Raman showed indigo bands; lead white Raman at ~1048 cm⁻1 was obscured by fluorescence, suggesting low content. Blue observed at 3 spots. - White: Gypsum (CaSO4·2H2O). EDS: Ca, S, O; Raman ~1008 and ~1133 cm⁻1 (sulfate bands). White at 6 spots. - Black: Carbon black. Raman D and G bands at ~1332 and ~1585 cm⁻1. Absence of phosphate band near ~960 cm⁻1 suggests not bone black. Black at 7 spots. - Production choices: - Use of a yellow arsenic sulfide primer appears characteristic for the Pingcheng-period Northern Wei context, paralleling a similar case (M229, Datong). - Green and blue colors achieved by mixing inorganic pigments with organic indigo dye, indicating deliberate color tuning. - Sampling and analysis scope: - Non-invasive measurements: 20–23 points per plank; 4 micro-samples analyzed; simulated layered sample validated green mixture interpretation.

The integrated non-invasive and micro-invasive approach effectively identified pigment palettes and layering strategies, addressing the study’s goal to reconstruct materials and techniques while minimizing sampling. The discovery of a pararealgar yellow primer under paint layers clarifies preparatory practices for surface leveling and color rendering, and suggests a Pingcheng-period stylistic or technical trait. Identification of green as an orpiment–indigo mixture and blue as indigo modified with lead white demonstrates purposeful blending of inorganic pigments with organic dye to achieve desired chroma and brightness, aligning with documented practices in other regions and periods but here evidenced for Northern Wei coffin painting. Elemental and molecular data (e.g., absence of Cu in greens; Raman/EDS markers) refine pigment attributions (excluding copper greens and bone black). These findings inform conservation strategies (e.g., sensitivity of arsenic sulfides and indigo to light and solvents, compatibility of consolidation layers with stratigraphy) and expand the technical corpus for Northern Wei art history, linking material choices to broader cultural exchanges and technological knowledge (e.g., established indigo production by Northern Wei era).

Six colors were characterized on the Zhijiabao Northern Wei coffin planks: red (cinnabar), yellow (pararealgar), green (orpiment+indigo), blue (indigo with likely lead white), white (gypsum), and black (carbon black). The paintings were executed over a yellow pararealgar primer; typical stratigraphy comprises two layers, with up to three layers in figure areas. The use of organic–inorganic mixtures for green and blue highlights sophisticated color design. The results provide a scientific basis for conservation and restoration and contribute new data for understanding Northern Wei painting materials and color practices. Future work could further resolve binder composition, quantify minor components (e.g., lead white in blue), explore broader corpus comparisons to test the generality of yellow priming in the Pingcheng period, and assess degradation pathways (e.g., realgar–pararealgar transformations, indigo photofading) to guide preventive conservation.

- Lead white in the blue mixture was inferred from Pb by EDS and contextual practice; its diagnostic Raman peak (~1048 cm⁻1) was obscured by fluorescence, likely due to low content and surface contamination. - While pararealgar was identified in the yellow layer, the coexistence or original use of realgar cannot be excluded due to their polymorphism and light-induced transformations. - The yellow primer’s specific cultural or symbolic meaning could not be determined with current data. - Sampling was intentionally minimized (only four micro-samples), which may limit detection of minor components or binders. - Findings derive from three planks from a single tomb; broader generalization to all Northern Wei coffins requires additional case studies.