Analysis of historic copper patinas 1: Influence of substrate on patina uniformity

摘要

The morphology and elemental composition of cross sections of almost ten copper samples have been explored. The samples have been exposed as copper roofs in a variety of environments and exposure times, the oldest at the Royal Summer Palace (Belvedere) in Prague (completed between 1557 and 1563) and the youngest at an approximately 100 year old building in the Old Town of Stockholm Other exposure sites include the Copenhagen Cathedral, the Helsinki Cathedral, the Drottningholm Castle outside Stockholm, the Mausoleum in Graz, the Basilika Maria Dreieichen in Rosenburg-Mold, Austria and the Otto Wagner Church in Vienna. All copper substrates contain inclusions of varying size, number and composition, which reflect different production methods and mineral compositions over the large time span of around 300 years between the oldest and the youngest copper material. The largest inclusions have a size of up to 40 micrometer, while most inclusions are in the size range from 2 to 10 micrometer. The most common element in the inclusions is O, followed by Pb, Sb and As. Minor elements include Ni, Sn and Fe. All patinas formed exhibit quite fragmentized structures, mainly exhibiting a bilayer structure with an inner part consisting of cuprite (Cu_2O) and an outer more or less porous part usually consisting of brochantite (Cu_4SO_4(OH)_6), sometimes also of antlerite (Cu_3(SO_4)(OH)_4). The thickness of these layers vary between the sites, with the brochantite/antlerite layer thicker than the cuprite layer in most cases. The extent of patina fragmentization seems to depend on the size of the inclusions, rather than on their number and elemental composition. Irrespective of elemental composition the inclusions in the substrate generally turn out to be electrochemically more noble than the surrounding copper, as judged from AFM-based Volta potential measurements. Detailed analysis by SEM/EDS of the interfacial regime between the copper substrate and the patina shows that the gradual transformation of the copper substrate to the patina is strongly influenced by the presence of inclusions. In particular larger inclusions acting as cathodes in the atmospheric corrosion process create conditions for local micro-galvanic effects which cause the fragmentization of the patina formed, primarily by the formation of more disrupted brochantite at the expense of formation of cuprite. Many of the inclusions can be seen in the patina layer largely unaffected by the transformation from copper to copper patina. In all, the results show a profound influence of larger-sized inclusions on the homogeneity and morphology of historic copper patinas, also on the thickness ratio between the cuprite and brochantite layers. This suggests marked variations in corrosion protection ability ofthe patinas formed during different centuries.