Laser cleaning is a method used to remove unwanted material from the surface of objects. This can be dirt, coatings from earlier restorations, organic growths or corrosion. It has the advantages of speed and being highly selective - for example it can be used to remove dirt from stone without damaging the surface, even when the stone is extremely friable.
The Department of Conservation and Scientific Research has a dual wavelength (1064nm/532nm) Q-switched Nd-YAG laser for this purpose. It produces a highly directional beam of pure light in very short pulses. The dirt particles absorb the energy of the laser much more rapidly than does the material of the object being cleaned. These dirt particles become very hot and expand causing them to be ejected from the surface in the form of a vapour. Because the pulse duration of the laser is so short (under 10 nanoseconds or a hundredth of a millionth of a second) there is minimal conduction of heat on the surface and no damage is done to the artwork itself. When the laser beam hits a clean surface most of the light is reflected, stopping the cleaning action.
The laser is mainly used for cleaning stone sculpture (though it is not the only method used). We think of stone as a stable material but marble, limestone and sandstone, for example, will decay in certain environments. Ingrained dirt from burial or from atmospheric pollutants is difficult and slow to remove by more conventional cleaning methods. The laser is ideal for cleaning intricate carving or very large objects.
Lasers have also been used on a other materials such as terracotta, plaster, wood, bone, vellum and metal, and research is currently being carried out in the museum on how to optimise such work.