A team of skilled restorers and scientists have been working for years at the conservation of magnificent monuments that adorn the Square of Miracles (Piazza dei Miracoli) in Pisa.
The first project involved the worldwide-famous Leaning Tower. In this case the surface conservation status was strongly influenced by the inclination of the tower, which accentuates the deterioration linked to natural aging of the materials and to the conservation history of the Tower.
Studies and tests had shown that the major conservation problems were related to erosion and disintegration of the stone and to the presence of gypsum and limestone deposits. In the area under the slope, however, the particles which are not washed away by the self-shielding effect of the Tower itself, are deposited and cause the formation of black crusts. Diffuse cracks, fractures and detachments of fragments are evidence of crushing phenomena related to the inclination suffered by columns and capitals.
The most appropriate methodologies for the restoration have been identified on the basis of the investigations results and direct experience gained in a project yard that took place in 2000, with the contribution of the European Union.
Eleven cleaning systems, 5 products for consolidation, 18 types of mortars were considered appropriate and necessary for the conservative intervention. Among the cleaning methods, laser technology has been used successfully on the most damaged and exfoliated parts of the capitals for a gradual and safe removal of the black crusts without damaging the fragile substrate affected by cracks and gaps.
Subsequently, the efforts of restorers focused on the preservation of the Cathedral. The last actual restoration of the monumental Cathedral, aside from the renovated façade for the Jubilee of 2000, dates back to the forties. A total restoration using advanced technologies has been carried out. The first phase started in 2013 and ended up in 2015. The intervention started with a pilot project that healed a portion of the basin apse next to the transept of the Blessed Sacrament, then the restoration has been extended to the entire building which was in a state of high degradation characterized by biological attack and aggressive deposits, moreover black crusts and missing parts were found in the most decorated parts. These decorated parts (such as capitals) were all laser cleaned: the laser allowed the fast and safe removal of the thick black crust even from very modelled and curled decorations.
The conservation problem in Pisa shows a clear example of the potentials of laser cleaning of stone. Lasers are nowadays seen as an additional instrument in the restorer’s toolbox. The main advantages of the use of lasers for the removal of black crusts from stone surfaces are the following: no chemical species are required, the removal is very progressive so the restorer can decide the most appropriate level of cleaning, the cleaning is extremely precise because it involves only the area lit by the laser beam, which may be accurately limited as needed and last, but not least, if the laser is properly set, the process of cleaning automatically stops after the black crust removal because of the different reflectivity of the marble. After the pioneering work made by J. Asmus since 1970 [1,2] it has been shown by many studies [3,4] that the Nd:YAG lasers offer the best compromise of selectivity and efficiency, being also rugged enough to work in difficult conditions such as open air or on scaffoldings. It has been proven that, beside the wavelength, the pulse duration is a crucial parameter for the optimization of the laser-stone interaction [5,6]. In the 90s a novel fiber-coupled Nd:YAG laser emitting pulses of 20µs, the so-called Short Free Running (SFR) temporal regime was introduced : this pulse duration, intermediate between the very short pulses (Q-switched regime, 10ns pulse duration) and very long pulses (Free running regime, 500µs pulse duration) was proved to provide unique gradualness and self-termination performances without damaging the stone substrate. This new SFR system was massively applied in restoration works of historical façades [8,9,10], ancient archaeological artworks  and stone reliefs  and Renaissance masterpieces such as the Prophet Abacuc by Donatello, the Fonte Gaia in Siena by Jacopo della Quercia, panels of the Giotto’s campanile in Florence and many other works. Most of the discussion, which accompanied the early stage of laser cleaning application, concerning possible side effects due to laser irradiation of stones gradually reduced along the last decades thanks to systematic phenomenological and process-optimization studies that allowed to define the operative fluence ranges ensuring effective discrimination between the layer to be removed and the substrate underneath to be uncovered .
Furthermore, also the well-known problem of the yellowish appearance associated with Q-switching Nd:YAG cleaning of white stones  has been thoroughly investigated [15,16] and practically solved. Some studies showed that this chromatic appearance was due to the partial removal of pigmented stratifications and/or to penetration of organic substances through the outer layer of the stone.
The removal of alteration layers from stones can often be achieved by using the laser only but in many cases the combination of the laser with traditional chemical or mechanical methods may be needed or preferable. The laser cleaning of the capitals of both the Leaning Tower and the Cathedral in Pisa was performed after the imbibition of the black crust in water for one night to soften the crust itself and make it easier to be removed by laser. In general, the efficiency of the laser removal is increased if the surface to be treated is wet with water before the irradiation. This wetting produces two main effects: a refractive and an ablative one. The refractive effect leads to a decrease of reflectance for the wet sample, due to a change in the refractive index between the material and the air (less radiation is reflected away). Moreover, the voids in the material are filled and this causes a reduction of the optical diffusion effect (higher penetration). On the other side, for the ablative effect the fast vaporization of water penetrating into microcracks speeds up the material ejection.
In conclusion, the conservation of the monuments in Piazza dei Miracoli in Pisa represented one of the last successful application of laser cleaning for the removal of black crusts from stone substrates. The optimization of the main laser parameters, supported by rigorous scientific investigations, is of crucial importance to spread the use of this innovative technique in the world of conservation of Cultural Heritage.
[by Alessandro Zanini and Laura Bartoli, Conservation Technologies Department]
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