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The present work focuses on the main phenomenological features of stone cleaning by lasers. They are the removal rate, degree of cleaning, and chromatic appearance of the treated surface associated with different conservation problems and laser parameters. A set of three different outdoor stone conservation problems were investigated here. The measurement of the ablation rates were carried out on encrusted stone artefacts and two sets of prepared samples in order to derive general behaviour through repeatable measurements. The analysis of the irradiation tests provided a quantification of the different efficiencies, degree of cleaning, and chromatic appearances associated with the fundamental harmonic of Q-switching, long Q-switching, and short free running Nd:YAG lasers (1,064 nm), as well as with the second harmonic of Q-switching sources (532 nm).

In previous laser cleaning tests and analyses carried out on fossil bones, we demonstrated the feasibility and effectiveness of the laser approach. Based on these results, we designed cleaning procedure employing fibre-optic-delivered Nd:YAG laser radiation, integrated with other conventional cleaning techniques, such as micro-sandblasting and chemical cleaning. The procedure has been optimised for application on important specimens, such as the fossil human "skull of Buia", recently discovered by a research expedition of the Department of Earth Sciences of the University of Florence and presently preserved in the Museum of Asmara (Eritre). Here, we report laser cleaning tests where this procedure has been applied on samples of other mammalian bones from Danakil depression and the site of matassino (Italy)

An optimised laser methodology was devised to clean gilded decorations found on hair and robes of the Santi Quattro Coronati, marble sculptures by Nanni di Banco. The optimisation of laser parameters was achieved through comparative cleaning tests by employing Nd:YAG (1064nm) and Nd:YAP (1340 nm) lasers systems. The different cleaning results were characterised by microscopy, spectral reflectometry, and FT-IR analyses. In particular, high removal control was achieved by short free running (SFR) Nd:YAP and Nd:YAG lasers that allowed gradual cleaning levels. Conversely, a single cleaning level was possible by Q-switching Nd:YAG laser, which also induced an undesired yellowish-orange appearance of the cleaned surface. Following the analysis of these results the whole cleaning treatment of gilded areas was performed using SFR Nd:YAG laser.

Photoablation is a physical process employed for the cleaning of artefacts. While several authors have reported investigations concerning the dependence of the effects induced on the material for different laser wavelengths, no similar investigations have been reported so far about the dependence on the pulse duration, in spite of its importance being recognised. In order to approach this issue and to optimise the interaction process on different materials, a new Q-switched Nd:YAG laser system with variable pulse width for cleaning applications was designed and built at the Quantum Electronics Institute in Florence. It provides an extended pulse duration in respect of the standard Q-switched regime, by means of interchangeable optical fibers in the resonator. The new laser allows to change the pulse duration in the range between tens and hundreds nanoseconds up to several microseconds. According to our experience the adjustment of the pulse duration in this range may avoid the possibility of strong mechanical forces and excessive heating in the substrate, as it has been observed with standard Nd:YAG laser systems. The laser may be proposed for an accurate cleaning in a variety of restoration problems and materials.

We investigated the optimisation of the laser cleaning parameters for the conservation of the Porta del Paradiso, gilded bronze masterpiece of the Florentine Renaissance. Photothermal and photoacoustic regimes involved in the ablation dynamics by Nd:YAG lasers with different pulse durations, were modelled on the basis of preliminary stratigraphic characterisation and irradiation trials. The physical analysis was aimed at understanding the nature of the ablative process and at estimating the thermal wave coupled into the gold film. This allowed the selection of a suitable pulse duration range to achieve a high discrimination level, thus optimising the cleaning result. The analytical description here reported also contains general results, which could be exploited in other conservation cases of gilded bronzes.

We determined the nonlinear Kerr refractive index (n2 of LiBaF3 and LiLuF4 undoped crystals, by means of Z-scan measurements at 1064 nm. These materials are suitable for Ce3+ doping to obtain broadband laser emission in the UV. These n2 values (2.0x10–16 and 1.5x10–16 cm2/W at 1064 nm, respectively) are important for the characterization of new optical materials in view of the development of high peak power laser oscillators and amplifiers.