Diagnosis of materials and effectiveness of Er:YAG laser cleaning as complementary treatment in a panel painting attributed to Lluís Borrassà (fifteenth century)

Abstract

A fifteenth-century panel painting by Lluis Borrassa (1360–1425) was the object of a restoration at the North Carolina Museum of Art. A multi-analytical approach based on Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy and gas chromatography–mass spectrometry (GC/MS) also coupled with a pyrolyzer was used to investigate the origin of the darkened coating obscuring the work of art in order to find a proper cleaning procedure. The results evidenced the presence of different restoration materials that constituted the superficial patina, which was difficult to remove by conventional solvent cleaning methods. The Er:YAG laser operating at 2.94 μm was tested as an innovative alternative method to the traditional solvent mixture and gels. Because of the presence of multilayers of over-painting and restoration materials, the Er:YAG laser also proved beneficial as a nondestructive and selective sampling tool. The organic material was collected by positioning a microscope glass coverslip (15 × 15 mm) on the painting surface, followed with laser ablation that removed the material, which condensed on the glass that was then submitted for the FTIR and GC–MS analyses. The acquired new knowledge of the characterized materials and optimization of the cleaning methods guided the conservators to choose the best restoration conditions and perform a safe and successful cleaning of the painting especially the blue pigment, which was covered with intractable restoration material. The Er:YAG was an efficient and safe method to remove these materials.