Patina: A microscopic feature of palaeo-environmental and geoheritage significance

Clifford, Penelope1, Semeniuk, Vic1,2

1Notre Dame University, Fremantle, Australia, 2V & C Semeniuk Research Group, ,

Patina is an ultra-thin crust of silica or silica and carbonate that is developed on glass and, while it best developed on anthropogenic glass, it provides important information on products and processes associated with weathering of glasses in general.  It is common in modern environments, though variable in expression dependent on environmental setting, and it has been recorded on pre-Mediaeval and Mediaeval artefacts.  Anthropogenic glass is geochemically unstable and, as such, it corrodes relatively rapidly (within years), generating a variety of weathering crusts of different thicknesses, and various internal structures.  The type of patina that is developed depends on the glass composition, the type of soil it is embedded in, the hydrochemistry of the soil water, climate setting, and whether the glass is located in an inland vadose zone or phreatic zone, a maritime coastal zone, or a submarine environment. The patina crusts are < 10 µm up to 100 µm, thickening with age. The solutional relationship of the patina to the glass varies from straight, undulating, irregular, to cuspate and, internally, shows structures of colloform to undulating lamination, parallel lamination, massive to mottled patterns, micro-brecciation, shrinkage cracks, and infiltrated dust-sized minerals, all reflecting and recording a history of solution and precipitation, and variation in climate. For vadose environments, the main agents in the patination is alternating wet and dry vadose conditions, and alternating acid and alkaline vadose conditions that result in precipitation of an amorphous silica ‘gel’ that forms silica laminae, its layer-parallel shrinkage, and the precipitation of calcite laminae.  While modern patina and historic patina have been documented from the various climate, hydrochemical, and pedogenic environments, the results are widely applicable to understanding and unravelling the weathering of natural materials such as obsidian, chert, and volcanic glasses – in this context, it conforms to the geoheritage category of ‘modern processes’ and provides a record of modern processes and products in the weathering of natural glass and glass-like materials.


Biography

Penelope Clifford graduated with Honours from The University of Western Australia, with a double major in Anthropology and Archaeology. Penelope is a practicing archaeologist, with strong interests in geology and geoheritage which she combines with the archaeological studies.  In collaboration with the V & C Semeniuk Research Group, within archaeology, Penelope has undertaken studies in landforms, habitats, stratigraphy, soils, hydrology, and palaeoenvironmental reconstruction of archaeological sites, and the geoheritage values of archaeological sites.  Currently, Penelope is researching patinated glass in terms of its microstructure, geochemistry, and developmental history and its use in studies of climate, soil processes, and hydrochemistry.

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