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RESEARCH PRODUCT

Identification of Earth Pigments by Applying Hierarchical Cluster Analysis to Solid State Voltammetry. Application to Severely Damaged Frescoes

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description

Multivariate chemometric methods are applied for identifying earth pigments from square-wave voltammetric measurements performed at pigment-modified paraffin-impregnated graphite electrodes allowing for a separation between hematite-based earths, French ochres, Spanish ochres, siennas, umbers and green earths. This methodology is applied to the identification of the pigments in samples from the ceiling frescoes of Antonio Palomino (dated 1707) in the vaulted nave of the Sant Joan del Mercat church in Valencia (Spain). These frescoes suffered considerable damage by fire during the Spanish Civil War in 1936, resulting in severe chemical and chromatic alterations. Electrochemical data, supported by scanning electron microscopy/energy dispersive X-ray microanalysis and Raman spectroscopy, indicate that thermal stress induced the transformation of goethite-based yellow ochres into hematite and magnetite.