Tracking the chemical life histories of spruce resin-beeswax mixtures through degradation experiments using reflectance-mode FTIR

Within residue analysis, experimental degradation studies provide a critical bridge between pristine, modern-day reference materials and the altered residues encountered archaeologically. Here we report on outcomes of a 3-year burial experiment involving spruce resin-beeswax adhesives recovered from cave, loess, sandy, forested, and surface contexts. Reflectance-mode Fourier transform infrared spectroscopy (FTIR) was used to document chemical trajectories across nine specimens using pristine reference materials as baselines. Results show that signals from beeswax are generally resilient, while those from spruce resin are more unstable and frequently weakened or replaced by carboxylate bands. These observations are compatible with progressive hydrolysis and the formation of metal carboxylates, or soap salts. Carbonate and hydroxyl bands further reflect environment-associated inputs, though patterns are inconsistent within depositional categories, potentially reflecting micro-environmental heterogeneity. A pilot principal component analysis (PCA) provides exploratory visualization of environment-associated patterning under specific pre-processing strategies but is sensitive to data treatment, underscoring the risk of potential overfitting given the scope of available data. Collectively, these findings illustrate how FTIR can detect both preserved and transformed adhesive signatures, while also emphasizing the need for non-deterministic interpretive frameworks that account for taphonomic variability. The approach and caveats outlined here aim to strengthen the methodological foundation for residue analysis in archaeology by demonstrating how experimental degradation datasets can refine spectral interpretation of archaeological adhesives.