Rediscovering the palette of Alentejo (Southern Portugal) earth pigments: provenance establishment and characterization by LA-ICP-MS and spectra-colorimetric analysis

Abstract

Colored earth pigments sourced from Alentejo, Portugal, can be geologically categorized as either weathered carbonate rocks (terra rossas), schist units, or weathered iron ore deposits. The material was used until the mid-1900s by local residents as an ingredient in their traditional lime wash paintings and possibly in the production of artistic murals across the Alentejo region since pre-historic times. An integrated methodology incorporating laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and spectra-colorimetric analysis (CIELAB coordinates and reflectance curves), complemented by XRD, WDXRF, SEM-EDX, optical microscopy, and granulometric analysis, was used to characterize thirty-one Alentejo colored earths in an effort to correlate provenance with pigments properties. Data obtained from elemental analysis (major and trace) revealed a generic and similar elemental “fingerprint” that unable their distinction according to geographic provenance. Samples of weathered iron ore deposits derived from explored iron, copper, and sulfur mines are more easily discriminated using the chalcophilic (“sulfur-loving“) elements. Color analysis revealed a range of hues; olive-yellow to dark reddish-brown owing mainly to differences in the type and proportion of the color component present, independent of the accessory mineral.