Conserving Stained Glass Using Environmental Protective Glazing

Author(s): Tobit Curteis, Leonie Seliger

Because stained-glass windows form part of the building envelope – separating the internal and external environments – they are uniquely vulnerable to aggressive environmental deterioration. On the exterior, rainfall, wind and pollution can cause structural and chemical deterioration of the glass and the leading; on the interior, condensation can cause irreversible loss of paint and other applied decoration. Unfortunately, our ability to improve the environmental conditions to which historic glass is subjected is limited. One of the few interventions available that can provide protection whilst keeping the historic glass in situ is secondary glazing, in the form of Environmental Protective Glazing. The term ‘Environmental Protective Glazing’ (EPG) describes secondary glazing systems where the principal purpose is reducing the impact of microclimatic conditions on historic glazing (as opposed to systems intended primarily to provide protection from impact). EPG has been used in one form or another since the 19th century, but while considerable design developments have taken place over that time, there have remained many lacunae in our understanding of how the system actually works. The confusion over technical aspects of EPG is highlighted by its popular name, ‘isothermal glazing’; as this Research Report shows, the success of EPG actually depends on temperature differences. Historic England is regularly consulted about the merits and justifications of EPG systems for stained glass conservation. Since the remit of the organisation is to care for the historic environment as a whole, we do have concerns that, if the design of the EPG system is not approached with sufficient care and attention, the benefits to the glass can sometimes be at the expense of other historic elements, including the appearance of the window in its setting, and the exterior views of the building. EPG is not a universal panacea: as ever with conservation, the undoubted gains must be balanced against the negative impacts. The decision about whether EPG is the right choice in a particular situation will depend not only on the nature and seriousness of the deterioration, but on the significance of the glass, the window, the building, and the setting. In 2011, Historic England's Building Conservation and Research Team initiated the research reported here, with a fundamental aim: to establish whether EPG is robust enough to allow flexibility in design choices that could to minimise harm. In other words, how might modifications of the basic design of EPG affect its effectiveness?

Report Number:
Research Report
Glass Building Building and Landscape Conservation Glazing Protection


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