Laser cleaning of stucco's fragments from an early middle age bas-relief

A complete conservation program on a set of stucco fragments coming from a middle age bas-reliefs has been planned by Direzione Regionale per i Beni Culturali e Paesaggistici della Lombardia with the scientific support of ICVBC-CNR. The particular conservation history of the fragments produced pulverized surfaces and substantial problems occurred. Chemical and mechanical cleaning systems have been tested without successful results due to stucco gypsum composition and to its state of conservation. Laser cleaning has been much more promising since the beginning of the tests, operated with Quanta System Palladio Nd:YAG, both at wavelengths of 1064 and at 532 nm. The correct fluence to ensure the best balance between harmfulness and effectiveness has been defined using non-destructing methods (Reflectance Colorimetry and Stereo-Microscopy) and micro-destructive analyses (Polarised Microscopy). The most useful fluences have been found in the range 450-475 mJ/cm(2), as to diverse soiling thickness, using 1064 wavelength. Laser cleaning allowed the discovery and safeguard of yellowish patinas and of very small finishing remnants; on this remnants, in some cases with polychromy, cleaning with an Er:YAG laser has been tested with the aim to preserve them without any discoloration. Harmfulness criteria had the priority over effectiveness granting a good conservation of stuccos patinas and finishing.