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Short Bio: Francesca Gherardi is a senior materials scientist in the Investigative Science team at Historic England, UK. After receiving a PhD in Materials Engineering from Politecnico di Milano, Italy, she worked as a research fellow in several international projects on the set-up of innovative conservation methodologies. Her research interests are the analysis of artworks, archaeological objects and buildings, and conservation treatment strategy development.
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ABSTRACT
A Multianalytical Approach to Study Degradation and Alteration Aompounds in Archaeological Artefacts and Wall Paintings: Case-Studies and Challenges
Dr. Francesca Gherardi1*
1Historic England, United Kingdom
Multianalytical investigations on heritage artefacts are carried out to determine the materials and construction technology, provenance, and dating, and to study their state of preservation.
Portable instruments and surface analytical techniques are often not suitable to study objects made of different layers, as the X-rays may penetrate different layers and the results will contain chemical data from all of them.
To study the stratigraphy of an object, their morphology, and their composition, small samples, which are representative of the artefact, are taken and used to prepare cross-sections. After the observations are made by optical microscopy, the cross-sections are analysed by scanning electron microscopy combined with an energy dispersive spectrometer (SEM-EDS) in order to study the samples at higher magnifications and to characterise their composition.
In archaeological copper-alloy artefacts recovered from the Rooswijk shipwreck (1740), SEM-EDS unravelled the nature of corrosion layers developed on the objects, together with information about their thickness, porosity, and distribution on the original substrate. EDS mapping detected the distribution of chemical elements in each layer, identifying leached ions from the bulk which formed corrosion layers.
In the medieval wall paintings at St. Mary’s Church, Kempley (UK), SEM-EDS was used to investigate the darkening of the original lead-based pigments due to the interaction with oxygen and with atmospheric pollutants, the presence of varying amounts of chlorine in green copper pigments, and the fading and alteration of arsenic pigments used in very thin paint layers.
The data collected from these case studies has allowed for further knowledge about the degradation of heritage materials and guided the conservators in the selection of the most appropriate cleaning procedures for the removal of reactive compounds which can promote further corrosion, and to monitor the selectivity and effectiveness of conservation treatments.