Natural Earth Pigments as Geological Polymineral Systems: A Structured Approach to Provenance and Degradation Assessment in Heritage Science

Natural earth pigments are polymineral materials whose present composition may differ from the source because of selection, grinding, heating, mixing, binder incorporation, and later alteration. This study develops a structured method for separating these contributions and for stating which provenance and degradation claims are supported by available evidence. The material is represented by geological, technological, and degradation components linked to elemental, mineralogical, spectroscopic, microscopic, and colour observations. Provenance is expressed as probabilistic compatibility across source levels, whereas degradation assessment ranks classes of environmental exposure rather than reconstructing a unique history. Published data from Roussillon yellow ochres and commercial and historical green earths are re-analysed as proof-of-concept checks, and a literature-based Herculaneum application illustrates degradation reasoning. The ochre analysis shows that mineralogical and colour relations remain directionally stable under robustness checks and that wet sieving can shift measured composition without eliminating family-level compatibility. The green-earth analysis shows that commercial labels and bulk chemistry alone do not reliably establish celadonite–glauconite family membership, whereas structural and vibrational evidence provides stronger discrimination. The Herculaneum case demonstrates how thermal events and subsequent exposure can alter ochre states. The study provides a practical diagnostic sequence and defines the present limits of inference.