Historic Churches 2019

22 BCD SPECIAL REPORT ON HISTORIC CHURCHES 26 TH ANNUAL EDITION It is important to design EPG to allow for good maintenance. It should give easy access to the interspace to allow for cleaning and clearance of the drainage. For important stained glass, an advantage of framing stained glass may be quick dismounting in an emergency, but the fixtures and fittings must be chosen with care to ensure this does not increase the risk of theft. (This system was put to the test once at Canterbury Cathedral when the stonework of the Great South Window failed suddenly and catastrophically, and the stained glass was saved.) PROS AND CONS OF EPG By protecting the stained glass from direct weather impacts, and in particular from ultraviolet light (the exterior glass can be chosen to reduce UV penetration) EPG can help stained glass conservators avoid extensive repair, giving them the opportunity to use more sensitive and reversible materials such as epoxy glues to repair glass which would not be suitable if the stained glass needed to act as a weather barrier. Indeed, EPG may sometimes be the only way to preserve in situ a window made with highly light-sensitive or water-sensitive materials (such as fibreglass). In some circumstances, it can also help prevent impact damage; and it is largely reversible. On the other hand, EPG can also be one of the trickiest interventions to manage, principally because of its impacts on the wider building. The precise look of the single sheet of stained glass in its original position in a window is critical to the aesthetics of the building, both on the interior and the exterior, and it will have been considered with great care by the original architects and glaziers. The nature of EPG means it will also usually require a small degree of permanent change to the fabric (for example, new glazing grooves cut into the stone surrounds, or moving of the metal supports). These impacts can be mitigated but doing so successfully requires all stakeholders to consider a wide range of options before agreeing on the best approach. The aim must be to arrive at an agreed view on the significance of the window, and the contribution it makes to the significance of the building; and on how these might both be sustained and enhanced. Together with technical requirements, this means that optimum design for EPG may well vary from window to window, even on the same building. This is eloquently illustrated in the case study by Dan Humphries on page 15. There are other risks as well. Single stained glass windows allow a low level of natural ventilation, and this can be very important for the condition of the building: increasing air tightness with EPG may therefore have unintended consequences. There is also a risk that, in diverting condensation away from the stained glass, it may increase condensation events elsewhere (notably on the ferramenta). DECIDING WHETHER OR NOT TO USE EPG As with all good practical conservation, EPG is not an intervention that would be used on stained glass in good condition ‘just in case’. It may be the right solution if: • despite building repairs and adjustments to the climate- control systems, environmental problems continue to have serious impacts on the window; or • the stained glass is very fragile and/ or historically important, and will be compromised technically or aesthetically by repair methods that would render it water- tight and wind-resistant; or • the technique of the stained glass Condensation tray and drain Protective glazing, shown here with modified leading Stained glass in new frame Apertures top and bottom to allow air flow Original glazing groove EXTERIOR INTERIOR Lead flashing AIR FLOW Ventilation gap Interspace Stained glass Cut-away diagram of a typical EPG system with the original stained glass brought inwards and a new leaded light in the original glazing location (Diagram: Historic England/Iain McCaig) Microbial growth on medieval stained glass rendering its original image completely illegible (Photo: Leonie Seliger)