Radiation ablation for the Isis temples at Philae, Egypt

George Stout investigated radiation ablation as a means of erasing white paint stripes (applied to facilitate realignment) from Philae Temple sandstone blocks after their reassembly on the island of Agilkia. Forty-two years ago state-of-the-art laser technology offered very few options in terms of diversity in wavelengths and pulse lengths. Laser divestment in art conservation had only been demonstrated for black sulfate encrustations on white calcareous stones. However, a CO2 IR (10.6 mu m) TEA laser from an inertial-confinement thermonuclear fusion program was available and had been employed to ablate white tufa deposits from brown Cyprian pottery shards. Consequently, Stout adapted the heat-transfer analytical modeling of a concurrent laser divestment project in Venice to white paint upon brown sandstone, characterizing the Philae situation to guide TEA-laser divestment tests on the white-striped Isis stones. In contrast to results obtained with radiation from Nd:YAG, Nd:glass, and ruby lasers, the mid-infrared radiation from the TEA laser yielded clean ablation of the white paint from the sandstone: free of etching, discoloration, or paint-ash residues. This favorable outcome was consistent with Stout's heat-transfer calculations and was evidently a consequence of both the thermal and optical properties of the paint and the Philae stones. Whereas far more versatile laser systems are now available, the Philae experiment did reveal the potential of self-limiting radiation divestment in the damage-free removal of refractory coatings of high albedo. This article presents a historical account of these early experiments with laser divestment.