Optical devices provide unprecedented insights into the laser cleaning of calcium oxalate layers

Year: 2016

Authors: Striova J., Fontana R., Barucci M., Felici A., Marconi E., Pampaloni E., Raffaelli M., Riminesi C.

Autors Affiliation: INO-CNR, Largo Fermi 6, 50125 Firenze, Italy; Opifico delle Pietre Dure di Firenze, V.le F. Strozzi 1, 50122 Firenze, Italy; Università degli Studi di Firenze, Viale Morgagni 40/44, 50134 Firenze, Italy; ICVBC-CNR, Via Madonna del Piano 10, Sesto Fiorentino, Italy

Abstract: Calcium oxalates are insoluble colorless or whitish salts constituting noble patina, on both natural and artificial stone artworks’ surfaces, the presence of which is extremely valued. The oxalates are not considered detrimental to the substrate, however, being often accompanied by other substances such as gypsum, silicates, and pigmented particles. They may form very adherent, relatively thick and colored layers creating disfiguring effects and hindering legibility of the pictorial surface. For this reason it may be appropriate to diminish their thickness, but patina’s partial preservation is particularly required calling for extremely gradual and controllable cleaning approach. Thinning of calcium oxalate patina from a detached 16th century fresco (from Sansepolcro) was performed through the use of laser (Nd:YAG and Er:YAG) systems and chemical means (Carbogel loaded 5 wt.% of tetrasodium EDTA). Optical coherence tomography (OCT), providing-a non-invasive stratigraphic cross-section of the examined surface, allowed to distinguish the oxalate from the underlying original layers and therefore to have an overview about its distribution, to numerically evaluate patina’s thickness range and to provide the information on the amount of the material both removed and left on the artwork’s surface. Laser scanning conoscopic microprofilometry allowed for a high-density sampling of the artwork’s surface providing a three-dimensional model of the surface pattern. The obtained 3D models were used to estimate the amount of material removed and to compare them with those provided by OCT. The successful exploitation of the proposed exceptional cleaning monitoring methodology may be seen as an innovative and valid support for the restorers in the conservation of mural painting or other surfaces covered by oxalate layers and may pilot more targeted, cautious and respectful cleaning intervention. (C) 2015 The Authors. Published by Elsevier B.V.

Journal/Review: MICROCHEMICAL JOURNAL

Volume: 124      Pages from: 331  to: 337

More Information: The research leading to the prototype of confocal microscope was funded by the EU Community\’s FP7 Research Infrastructures Programme under the CHARISMA Project (Grant Agreement 228330 , WP9, Task 9.1). In the framework of the project, the instrument was modified and transformed in a confocal OCT device. The authors would like to thank Prof. Piotr Targowski for valuable discussions, especially regarding the improvement of our OCT system. Authors JS and MR would like to acknowledge the financial support provided by INSIDDE project (Grant Agreement no. 600849 ) in frame of FP7-ICT-2011.8.2 ICT.
KeyWords: Calcium oxalate; Laser cleaning; Monitoring; Mural paintings; Noble patina; Optical coherence tomography
DOI: 10.1016/j.microc.2015.09.005

Citations: 27
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