On the Conservation of the Baptistery Doors in Florence

Corrosion investigation of fire-gilded bronze involving high surface resolution spectroscopic imaging

“…As for many others gilded monuments, such as the horses of Saint Mark's basilica in Venice [7], the growth of corrosion products at the gold/bronze interface (with the formation of characteristic corrosion craters) led to blistering or break-up and loss of the gold layer [6,8]. The corrosion products detected beneath the gold layer in the case of Ghiberti's Paradise Door mainly consisted of copper oxide, basic sulphates and copper carbonates [9], with lesser amounts of chlorides and nitrates [10]. Furthermore, due to the action of acid rains, copper salts also dissolved and diffused above the fire-gilded surface, where brochantite, antlerite, paratacamite and copper hydroxyl nitrate [11] were identified by X-Ray Diffraction and IR spectroscopy.…”

“…The presence of several corrosion products, including hygroscopic compounds at the gold/bronze interface, makes fire-gilded bronzes quite unstable and complex systems [9,12,13], requiring specific restoration and conservation procedures [8]. In particular, cleaning procedures based both on chemical and on laser methods were applied [11,14,15].…”

Atmospheric corrosion of fire-gilded bronze: corrosion and corrosion protection during accelerated ageing tests

“…In this case, as in many others such as the horses of Saint Mark's basilica in Venice [5], the growth of corrosion products at the gold/bronze interface lead to blistering or breakup and loss of the gold layer [6,7]. The corrosionproductsbeneaththegoldlayer, detected in thecaseofGhiberti's Paradise Door,weremainlycopperoxide, basicsulphates and copper carbonates [8], with minor amounts of chlorides and nitrates [9]. However, copper salts also diffused above the gilded surface, where mineralogical analyses by X Ray Diffraction and IR spectrometry revealed the presence of brochantite, antlerite, paratacamite and copper hydroxyl nitrate [10].…”

“…Galvanic currents were found to increase at increasing RH and in the presence of hygroscopic corrosion products. These tests and polarization curves recorded on bronze and gold revealed that gilded bronze artworksare quite unstable systems [8,11,12], whose restoration and conservation are difficult tasks tackled by the application of different strategies. In particular, specific cleaning procedures based on chemical or laser cleaning methodologies have been used [10,[13][14][15].…”

“…Both preventive techniques, such as microclimate control [8,17]) and use of coatings and inhibitors [11,18,19], were investigated. In order to help microclimate control, specific sensors based on the measurement of galvanic currents in gold/bronze couples were found useful for finding the most suitable climatic parameters to guarantee negligible corrosion rates [20].…”