J. Cult. Heritage 1 (2000) S189–S198 © 2000 E ´ ditions scientifiques et me ´dicales Elsevier SAS. All rights reserved S1296- 2074( 00) 00181- 3/FLA Nd:YAG laser effects on inorganic pigments Antonio Sansonetti a , Marco Realini b * a Centro Studi Trivella, via Guicciardini 45, 20092 Cinisello Balsamo, Milan, Italy b Centro C.N.R. ‘Gino Bozza’, piazza Leonardo da Vinci 32, 20133 Milan, Italy Abstract – The effects of a Q-switched Nd:YAG laser irradiation on a group of supports, painted with six different inorganic pigments, was investigated. The pigments were chosen from among the most utilized on painted historical surfaces: red ochre, yellow ochre, chromium green, ultramarine azure, white chalk and carbon black, and they were distempered with two binders – linseed oil and gypsum – on primed wood panels. The pigments were characterized by means of X-ray diffraction and Fourier transform infrared spectroscopy. Chromatic characteristics, chemical composition and surface morphology of the painted layers were investigated before and after the laser irradiation. Three different fluences were used to detect the correlation between laser parameters and changes in painting layer characteristics. © 2000 E ´ ditions scientifiques et me ´dicales Elsevier SAS Keywords: laser / pigments / cleaning tests 1. Introduction During the last few years many laboratories have tested lasers to solve cleaning problems. The experi- ments have been carried out in order to test supports (different kind of stones, plasters, ceramics, metals, woods, glasses, etc.), different soiling layers (black crusts, varnishes, biological patinas, particulate, graffiti, fatty layers, etc.), and to front various clean- ing and logistic problems such as decayed support, inside works, carved and plane surfaces, etc. [1 – 3]. Different pigmented surfaces, both with natural or synthetic pigments, on which laser cleaning has al- ready been tested, are as follows: painting layers on wood with linseed oil as a binder, textiles, coloured glasses, coloured plasters, coloured synthetic varnish layers, coloured stones and coloured gypsum stuccoes. In most of the mentioned cases, surfaces were positioned in the interior. Hence they were covered by fatty thin soiling whose presence could be generated by candle smoke, human touch, particulate deposition; during cleaning operations some chro- matic variations, especially on red-coloured surfaces, were observed in some cases. This macroscopic changing has been observed by operators who used different laser apparatuses [4]. A significant example concerns a polychrome altar- piece built in the XVIII century in the Nicosia Cathedral (Nicosia is a small town in Sicily). This altarpiece, as many others in Sicily, has a white marble framework which is inlaid with polychrome tesserae. The tesserae are made of different materials to obtain a colourful effect: many of them are made with coloured glasses and part of them are made with some varieties of Sicilian jaspers, a precious quality of local siliceous microcristalline stone, which can be easily polished, and which can be found in red, yellow and grey hues; pigmentation is due to iron compounds in different oxidation states as in most of the red and yellow stones. During cleaning tests on the whole altarpiece palette, a chromatic variation was observed on the * Correspondence and reprints: marco.realini@polimi.it