Orange Layered Patinas on Limestone Surface in the Batalha Monastery (Portugal): Characterization and Decay Patterns

Abstract

Samples of orange patinas found on a limestone balustrade and an ornament of the Batalha Monastery have been investigated by X-ray micro-diffractometry (µ-XRD) and Low-Vacuum Scanning Electron Microscopy coupled with Energy Dispersive Spectrometry (LV-SEM + EDS). Aim of the study was to determine the composition of the layered patinas, assess whether they were been intentionally applied or naturally formed, and study their degradation patterns. Preliminary results revealed that the orange patinas on the balustrade and the ornament showed different compositions and appearance, suggesting distinct formation pathways. Orange layers on the ornament which suffers salt decay and delamination nowadays, mainly consisted of gypsum with hematite as a minor component, implying the possibility of an intentional application of a mixture of ochre and lime as tint plaster. Orange patinas on the balustrade show the presence of Ca-oxalates, abundant weddellite and minor whewellite, with minor hematite suggesting the yellowish/orange color to be due to Ca-oxalate patinas imbedding soil dust airborne particles. Such patina was possibly formed naturally either by the chemical attack due to atmospheric air pollutants from traffic exhausts emissions or by bacterial activity. No delamination was observed, abrasion is the major decay phenomenon on the balustrade sample. A comparison was made between this patina and the so called “scialbatura”, a surface yellowish coating often found by conservators on limestone and marble in ancient monuments in the Mediterranean region.