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.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.