The Evaluation Of Laser Cleaning Of StoneSculpture

Abstract

A Q-switched Nd:YAG laser has been developed to clean polluted stone sculpture. Results are presented showing the controlled manner in which polluted layers are removed from the sculpture without damage to the underlying stone. The absorption of laser radiation at 1.06jam by a black encrustation has been shown to be three times that of a clean limestone surface. Measurements of the threshold laser fluence (energy density) for surface material removal show that there exists a range of fluence within which it is possible to clean the sculpture without damage to the stone surface. The amplitude of the shock pulse generated in air by the sudden ejection of material from the polluted surface has been monitored to show that as the stone is exposed the laser pulse has a negligible effect on the surface. INTRODUCTION The formation of unsightly and damaging black encrustations on stone monuments due to interaction with atmospheric pollution has become an all too familiar problem. Traditional cleaning techniques, such as particle abrasion and liquid jets, have often proved successful in restoring the outer appearance of a sculpture. However, by their very nature these techniques damage the underlying stone and often result in the loss of fine detail from a sculpture. Whilst working in Venice in 1972 it became apparent to Asmus and coworkers (Asmus [1]) that laser radiation is able to selectively vaporise layers of dirt from decaying marble sculpture. A Nd: YAG laser was later used in several conservation projects in Venice (Asmus et al [2,3,4]). However, at that time lasers were expensive and conventional cleaning techniques provided a more practical and affordable solution. At Loughborough, a Nd:YAG laser is being developed into a practical cleaning tool which can be used in a very precise and controlled manner to remove black encrustations from stone sculpture in various states of decay. Research has shown that a Q-switched Nd:YAG laser Transactions on the Built Environment vol 4, © 1993 WIT Press, www.witpress.com, ISSN 1743-3509 260 Structural Repair and Maintenance of Historical Buildings (wavelength 1.06 jum, pulse length 20 ns) is the most suitable form of laser radiation for the cleaning of sculpture (Cooper [5]). The technique allows removal of the encrustation without damage to the patina of the stone in a way not possible using conventional techniques. This paper presents results which demonstrate the very controlled nature in which dirt is removed from the sculpture whilst preserving the state of the underlying stone. EXPERIMENTAL 1. Absorption of Laser Radiation at 1.06 |im by Clean and Polluted Stone A matt or rough surface reflects radiation diffusely. An ideal diffuse reflecting substance scatters the incident radiation according to I(obs) = I(o) cos(oc) cos(P) where I(o) is incident intensity I(obs) is reflected intensity a is angle of observation P is incident angle of radiation This gives rise to the circular distribution of reflected radiation seen in figure 1.