Nd:YAG laser cleaning: Further developments on the yellowing issue

Carole Dignard

Abstract

Nd:YAG lasers operating at 1064 nm have been used now for over 15 years in conservation, most prominently in Europe to clean black pollution crusts from stone monuments and historic buildings. More recently systems are available that make use of the second and third harmonics (532 nm and 355 nm). In the last few years, there has been increasing concern that, although the Nd:YAG laser can often clean effectively without visible damage, it also often leaves behind a yellow appearance. This has been found on different substrates, including marble, limestone, plaster, paper, skin, cotton, wool, silk and feathers. The yellowing may have several concurrent causes: the presence of pre-existing yellow layers below the soiling; soiling residues; light scattering; and substrate damage leading to the formation of chromophores. The mechanisms of laser yellowing are far from being clearly understood, and at the moment, many investigations on the subject are still actively underway, as well as possible means of mitigating the yellowing, including the use of water (“steam” laser cleaning). This paper will provide a review of the most up to date information on this subject as presented at the Fifth International Conference “Lasers in the Conservation of Artworks” (LACONA V), which took place in Osnabrueck, Germany, in September 2003. It will include results of Nd:YAG laser cleaning tests on white, naturally soiled feathers carried out at the Canadian Conservation Institute in collaboration with the Los Angeles County Museum of Art.