A new underwater laser cleaning tool for large bronze artefacts

A group of twenty-eight fragments of monumental bronze statues from the Roman Age, recently underwent a series of conservation treatments at the Opificio delle Pietre Dure (OPD) in Florence, Italy. These fragments had never been investigated or cleaned before. During this restoration, supported by an extensive diagnostic campaign, an attempt was made to understand the conservation issues of each fragment and to identify the most suitable cleaning treatment in order to guarantee the removal of concretions and alteration products without damaging these precious artworks. In particular, two large scale fragments, depicting the two sides of a horse's maned neck, and identified as part of an equestrian monument, showed traces of gilding emerging from a thick layer of mineralization and sand encrustations. The bronzes were then subjected to cleaning tests by means of laser ablation to reveal any further remains of gilding. Among the different application methods of laser cleaning, irradiation in water has demonstrated its effectiveness in this case, offering operational advantages due to cavitation-mediated processes. Due to the size and shape of the artefacts (characterized by protruding details such as the mane, etc.), and the general state of conservation, it was impossible to submerge them completely or apply plastic films on their surface to create “pocket pools” filled with deionized water. In this paper we present a new tool for underwater laser cleaning application based on the modification of a water brush pen capable of creating an extremely localized water vesicle, avoiding placing any containment system on the surface. This method has proved to be very powerful in removing the mineralized layer without affecting the underlying gold leaf, leaving only a few traces of the deposits.