engleski

Characterization of patinas on recent bronze sculptures in Zagreb city centre

Artistic bronze sculptures are usually, after casting, artificially patinated in order to obtain a layer of black patina which is then protected by waxes or lacquers. With the time, protection is lost due to degradation of such organic coatings under the influence of UV light, moisture and atmospheric pollutants. Patinated bronze surface becomes then exposed to atmosphere. Intensity of bronze corrosion as well as dissolution or transformation of existing patina layer depend on corrosivity of the atmosphere as well as on the bronze and patina properties. In this work two bronze sculptures, exposed to outdoor atmosphere in Zagreb city centre were examined. The first one, sculpture of poet Marko Marulić (MM) by Vlade Radas was exposed to urban environment in 1999, while the second one “Igra srebra materina” (IS) by Lujo Lozica was exposed in 2015. Both were casted in the same foundry. Electrochemical impedance spectroscopy (EIS) measurements were conducted on several positions on each sculpture in order to relate the variation in exposure to sun and water condensation on electrochemical properties of patinated bronze. Measurements were conducted using agar based electrolyte according to setup developed by Ramirez et.al. [1] By analyzing the obtained impedance spectra, corrosion stability of both patina and underlying bronze was evaluated. The composition of patinated bronze in selected areas was determined by X-ray fluorescence (XRF), while the composition of patina alone was determined by FTIR spectroscopy and EDX measurements. Obtained results show that both sculptures are made of quaternary bronze with about 7% (IS) or 9% of Sn (MM). Spectroscopic studies of patina indicate mainly the presence of Cu2S and additionally formation of sulphate patina compounds at some points. Traces of the remaining organic coatings were not detected. EIS measurements show that the corrosion resistance varies, depending on the position of the sculpture where measurement was conducted. The biggest differences were observed between different points on newer sculpture IS.

patina ; bronze ; atmospheric corrosion ; electrochemical methods

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