Metal mordant binding mechanisms in historical silk revealed by spectroscopic and computational analysis

This study employs a multi-technique analytical approach to investigate two archaeological silk textiles in red and purple, focusing on the chemical speciation and coordination behavior of alum-based mordants. Dye components were identified using ultra-high-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UHPLC-QTOF-MS), and elemental composition was quantified via inductively coupled plasma mass spectrometry (ICP-MS). Based on these results, historically informed dyeing reconstructions were conducted using Rubia tinctorum and Lithospermum erythrorhizon in combination with potassium aluminum sulfate. X-ray photoelectron spectroscopy (XPS) was used to characterize the oxidation states and chemical environments of aluminum, confirming the predominance of Al³⁺ and its coordination with dye–fiber matrices. Trace levels of Al⁰ detected in archaeological samples were attributed to post-depositional transformation. To further clarify binding mechanisms, density functional theory (DFT) calculations were performed to identify the most stable coordination configurations and explore electron distribution and site selectivity. This integrated methodology offers molecular-level insight into mordant fixation chemistry, enabling the accurate identification of dyeing materials, guiding tailored conservation strategies, and informing the controlled reconstruction of historical dyeing techniques for museum display and cultural heritage restoration.