Laser cleaning of stone sculpture
This thesis is concerned with different aspects of laser cleaning of stone sculpture including the interaction of laser radiation with stone, evaluation and monitoring of the cleaning process and the development of a practical cleaning tool. Results show that a Q-switched Nd:YAG laser is the most suitable type of laser for the removal of black encrustations from marble and limestone surfaces. Measurements of the absorptivity of polluted and clean stone surfaces at 1.06 itm show that a typical black crust, resulting from atmospheric pollution, absorbs 2.5-3.5 times as much of the incident radiation as typical clean limestone and marble surfaces. This is shown to lead to the controlled removal of black crust material, by several thermal and mechanical mechanisms, without damaging the underlying stone. Measurements of ablation thresholds, by deflection of a probe beam, reveal that material is removed from a black crust at a fluence approximately half that required to remove material from a clean limestone or marble surface. Monitoring of the acoustic pulse generated during the laser beam/stone interaction is used to characterise the cleaning process. Results also show that cleaning is more efficient when a thin layer of water is applied to the surface prior to irradiation. Surface analysis techniques show that if cleaning is carried out below the damage threshold of the stone there is no modification to its physical or chemical structure. Cleaning of limestone sculpture from Lincoln Cathedral and terracotta sculpture from the Victoria and Albert Museum shows laser cleaning to be more refined than conventional cleaning techniques and to have considerable advantages in the area of health and safety.