Controlling the effectiveness of the cleaning treatment of bronze using X-ray diffraction, The equestrian sculpture of "el Cid campeador" Seville.

Abstract

Copper and its alloys (bronze and brass) are materials which are widely used in sculptures andornamental pieces of cultural heritage due to their ability to develop stable protective patina in oxidant atmosphereconditions. However, depending on the environmental conditions, they can suffer the effect of corrosion due tourban pollution. One of the most common alterations is the so-called “bronze disease”, a cyclic corrosion causedby the presence of chlorides on their surface. The equestrian sculpture of “El Cid Campeador” was donated bythe Hispanic Society to the city of Seville (Spain). In fact, the Society’s founder, Archer Milton Huntington,and his wife, Ana Hyatt (author), donated the sculpture for the Ibero-American Exhibition of 1929. It is locatedbetween Prado de San Sebastián Gardens (“Jardines del Prado de San Sebastian”) and the Royal Tobacco Factory(“Real Fabrica de Tabacos”) in Seville, which is an urban area where atmospheric pollution is very high (SO2,NOx, VOCs, particles in suspension, etc.). Due to the severe degradation of the sculpture, in 2012 it was includedin a campaign for the conservation and restoration of public monuments promoted by Seville’s Town PlanningDepartment (“Gerencia de Urbanismo de Sevilla”. The methodology followed in studies prior to any interventionon the sculpture involved the chemical characterisation of the bronze alloy by means of XRF. Then, once themost abundant patinas had been identified according to their colour tone (brown, green, yellow, white), thecompounds were identified by means of XRD. During the process, it was possible to differentiate between stableand unstable compounds. After reviewing the available literature, four cleaning treatments were selected andapplied to the different patinas. Their efficiency was evaluated based on the decrease in the concentration ofunstable compounds using XRD semi-quantification. Finally, the stabilization of bronze was completed and aprotective layer of varnish (based on an acrylic resin with antioxidant additives) was applied