Laser cleaning of historical limestone buildings in Bordeaux appraisal using cathodoluminescence and electron paramagnetic resonance.
BACKGROUND
Most historical buildings in Bordeaux city are made of limestone. This yellowish-white rock is rather porous and highly sensitive to pollution. As a consequence of local weathering conditions, these buildings present a dark appearance due to the development of a superficial dark grey to black crust.


METHODS
For the last decade, a campaign has been underway to clean these buildings. Eleven techniques of surface treatment have been used, including laser beam technology. As a contribution to the study of laser beam effects on stone buildings, two analytical methods have been used on clean versus unclean surfaces: Cathodoluminescence (CL) and Electron Paramagnetic Resonance (EPR), in addition to SEM-EDX and XRD.


RESULTS
The black crust is composed of different types of particles: carbon porous micro-particles of industrial origin, atmospheric dust due to the erosion of soils and rocks, alumino-silicate particles from urban pollution; all these particles being cemented by gypsum.


CONCLUSIONS
As far as heritage conservation is concerned, the laser surface treatment not only preserves the original patina of the stone, but also leaves surface smoothness unaltered.


CONCLUSIONS
CL and EPR data confirm that lasers--with highly controlled parameters--only get rid of the black crust and, thus, reveal the underneath layer, the so-called patina. This patina shows no luminescence, whereas the limestone on which it has grown shows a bright orange emission of CL. This indicates CL to be a fast and easy way to provide a high quality control for the restoration of polluted ancient stones.
Chapoulie
Rémy
R
CRP2A (Laboratory of Physics for Archaeology), CNRS UMR 5060 IRAMAT, Université Michel de Montaigne Bordeaux 3, Maison de l'Archéologie, 33607 Pessac, France. chapoulie@u-bordeaux3.fr
Cazenave
Sandrine
S
Duttine
Mathieu
M
D000397:Air Pollution; D001108:Architecture; D002119:Calcium Carbonate; D003252:Construction Materials; D004578:Electron Spin Resonance Spectroscopy; D005602:France; D007834:Lasers; D049449:Luminescence
D002119:Calcium Carbonate