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My name is Antanas Melinis and I am currently a PhD Student at the Institute of Archaeology at University College London with a predominantly archaeological (UCL BSc Archaeology) and heritage science (UCL MRes Science and Engineering in Arts, Heritage and Archaeology) background.

Antanas Melinis1

Antanas Melinis1*, David Thickett2, Michael Charlton1 and Ian Freestone1
1 UCL Institute for Sustainable Heritage, University College London, London, UK
2 English Heritage, Swindon, UK
3 UCL Institute of Archaeology, University College London, London, UK [email protected]

Our understanding of the fundamental processes occurring at the interface of glass and its respective environment, is still far from complete. The manifestation of deterioration symptoms in vitreous materials depends not only on their chemical composition and storage or display conditions but also on the conditions they were exposed to before their accession into a heritage collection. As ancient glass corrodes through interaction with water that leaches out its alkali components, it is left with a hydrated, low-density, silica-rich surface layer. Objects may therefore continue to deteriorate in collections and storage unless the ambient RH is sufficiently low.

Unfortunately, excessively dry conditions may cause surface dehydration and shrinkage leading to ‘crizzling’- a damaging phenomenon defined by microcrack formation and flaking. The problem is further exacerbated in archaeological specimens, where the little remaining original glass risks structural failure as a result.

Observational studies have shown that a range of c. 35-45% RH is generally acceptable, although there is no data on the lowest tolerable RH of specific glass types as well as the consequences of displaying such vulnerable artefacts alongside materials with other requirements or corrosion by-products. The study will therefore focus on finding this point of balance by means of direct observation of early glass replicas subjected to various changes in environmental conditions. The elastic waves, released by the crizzling glass, are hoped to be picked up by acoustic emission (AE) equipment, while the development of the cracks themselves is to be tracked using photogrammetry in view of relating them to condition changes in real time. Selected objects which can be sampled are to be taken for analysis using optical microscopy and electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). The bulk chemical composition of the pristine glass will then be related to the composition and morphology of its corrosion layer in order to establish a dataset in which corrosion characteristics may be linked to the history of the object.

English Heritage has large numbers of glass artefacts in its collections and some of these are in a state of advanced decay. It is therefore anticipated that the project will provide updated guidelines for glass storage, informed by hydration rates, composition, and tendency to delaminate, to be used by English Heritage specialists as well as ancient glass custodians across other collections and institutions.

Figure 1 Crizzling as observed on glass objects as well as a magnified Medieval glass sample cross-section and surface.
Figure 2 Diagram of a basic acoustic emission testing setup (At: https://www.nde-ed.org/EducationResources/CommunityCollege/Other%20Methods/AE/AE_Intro.php)