MULTIANALYTICAL APPROACH FOR THE STUDY OF BRONZE AND GILDED BRONZE ARTEFACTS
Creators: name: BRAMBILLA, LAURA
Publisher: name: Universit� degli Studi di Milano
Year: 2012

description: This work focuses on one of the most common metal alloy used in the field of art: bronze. 
Bronze was discovered about 3500 years BC and was used in the antiquity to create tools, weapons, armour, and later coins, medals, sculptures and other works of art. Bronze objects were harder and more durable than stone or copper ones and easier to work than the iron ones. These characteristics contributed to render bronze a material largely widespread through millenniums. 
Of great interest are also the colourings that outdoor bronze exhibit. In fact, when exposed to atmosphere, copper and its alloys form a thin layer of corrosion products, called patina. The naturally formed patinas have different colours depending on the surrounding environment and its chemical composition: urban, rural, marine. A patina on a bronze artwork not only protects the metallic substrate, but also enhances the aesthetic of the artistic objects. Coloured patinas form spontaneously on copper alloys by very slow controlled corrosion either in the presence of moisture, carbon dioxide, and oxygen, or other chemical species present in the atmosphere. Patinas may also be created artificially by artists for aesthetic and protective reasons. Investigation of the corrosion behaviour of copper patinas is of utmost importance for understanding the atmospheric deterioration of bronze monuments as well as objects of cultural heritage. During this work, a multidisciplinary approach has been applied in order to thoroughly understand the degradation mechanisms of the patinas of corrosion products that naturally form over bronzes surface. Particular attention has been paid to the transformations connected to relative humidity variations.
To further adorn and make more precious the artefacts, bronze statue were also gilded. The gilding of a bronze statue allows to obtain an object that has the same aspect of a statue made of pure gold, but that is less expensive and simultaneously presents certain resistance characteristics. Examples of gilded bronze artworks can be found in the Italian artistic patrimony, such as, for example, the horses of St. Mark in Venice, the statue of Marcus Aurelius in Rome and the Porta del Paradiso, the eastern portal of the Baptistery in Florence. 
The latter has been the starting point of this work. This important gilded bronze of the Italian Renaissance was damaged during the flood that devastated Florence in 1966 and since then a long process of restoration and characterization of the artwork started and it is still going on. 
The study of gilded bronze artworks is a difficult topic in the field of cultural heritage due to the complex structure (tri-layer) of the material. The structure, in fact, is composed by a metallic substrate (bronze), the gold and, between the two metals, a layer of corrosion products formed during time as a consequence of degradation processes. The presence of hygroscopic salts between the gilding and the bronze may lead to the detachment of the gold layer as a consequence of volume variation related to changes in the crystalline forms of the copper salts composing the patina. 
At the beginning of this work, the ?Porta del Paradiso? was at the last stage of a long process of restoration carried out by the Opificio delle Pietre Dure and the door was almost ready to be finally unveiled to the public. At that stage of the research it was necessary to identify the best exposure option for this precious and unfortunately very unstable masterpiece of the Renaissance. The optimal display option for the ?Porta del Paradiso? had to take into account two important factors: first, it was necessary to choose an environment that could assure the best preservation condition for the restored artwork and therefore able to block corrosion; second, a solution that could provide an enjoyable viewing experience for the public was also expected. 
To fulfil these requirements, three different showcases were tested: a) a closed showcase purged by nitrogen; b) a closed showcase with low and controlled relative humidity; c) an open showcase with controlled microclimate that should create the same local environmental conditions as the showcase with low RH, with the big advantage of avoiding a glass panel between the object and the visitor. 
To evaluate the resulting conditions of conservation imposed by the three different showcases, qualitative and quantitative studies were necessary. Unfortunately the techniques commonly used to obtain quantitative data regarding corrosion phenomenon from metallic artefacts (i.e. polarization resistance and electrochemical impedance spectroscopy) are not suitable for bimetallic artefacts such as the ?Porta del Paradiso?. 
Initially some replicas of the ?Porta del Paradiso?, with the same bronze alloy and the same gilding technique of the door, were produced by the OPD. It was then necessary to age these samples in order to obtain a system that reproduces the actual condition of the artwork. For this reason a new methodology was developed during this work for the electrochemical artificial ageing of gilded bronze specimens. Unfortunately, due to the unavoidable short circuit between gold and bronze, these aged samples were not suitable to obtain quantitative data concerning the corrosion rate of the door. An alternative solution appeared to be the realisation of galvanic sensors, used as replicas of corroded gilded bronzes, that were developed and used to measure the macrocouple current of the system ?Bronze-Patina-Gold?, that is directly correlated to the corrosion rate by Faraday?s law. 
The most critical steps during the development of the galvanic sensors are: a) the preparation of an artificial patina of corrosion products and b) the gilding methodology. The realisation of an artificial patina proved to be a very crucial point, because the patina has to fulfil a series of important and specific requirements, such as: 1 ? composition similar to the real cases, 2 ? adequate thickness, 3 ? good homogeneity, 4 ? good adhesion to the substrate, 5 ? adequate electrical resistance. 
The main constituents of natural patinas have been identified after a thorough literature review. Then a procedure for the realisation of artificial patinas with similar composition was developed. Different kind of patination techniques available in literature (such as chemical and electrochemical patination) have been tested to obtain the artificial layer of corrosion products. Unfortunately none of them allowed to obtain a patina with all the required characteristics. A new methodology was then developed for the patina used to realise the galvanic sensors. This method is inspired by the so-called ?applied paste? and consists in spreading on the metallic surface a specific mixture of copper salts and water .This procedure allows to control precisely all the parameters previously mentioned. 
Concerning the second critical step, i.e. the gilding procedure, two different methodologies have been tested: sputtering and gold leaf application. The best one revealed to be the application of a gold leaf. 
A first set of galvanic sensors was realised and used for the solution of the ?Porta del Paradiso? case. In that occasion they proved to be a very powerful tool and allowed the identification of the best display option for the doors. However these sensors presented some evident problems of durability and reproducibility. The methodology of preparations of the galvanic sensors has subsequently been completely revised and improved leading to the obtainment of new galvanic sensors, named ?gold leaf galvanic sensors?, that proved to be much more durable and reproducible. The galvanic sensors can be used both for monitoring the corrosion rate of real objects and for testing new conservation or cleaning procedures.