22 CONTEXT 177 : SEPTEMBER 2023 ALISON GILCHRIST Environmental protective glazing Environmental protective glazing (EPG), usually internally ventilated, is generally accepted to be the best approach to conserving vulnerable historic stained glass. For centuries, stained glass has decorated our prestigious public buildings, from churches and cathedrals to royal palaces. As artworks, stained glass windows may tell a story or teach us how to live a good life, they may commemorate important people or events, and they will often fill a building with the beauty of light and colour. Stained glass windows are not just works of art. They are also a vital part of the building envelope, providing a weather seal that separates the interior from the exterior. The material nature of stained glass means that it is intrinsically fragile; yet if well designed, well supported and protected, it is capable of surviving many centuries. The primary threats to stained glass windows arise from mechanical damage and from environmental conditions. Protection from mechanical damage Mechanical damage to stained glass often results from impact by hail, stones or other missiles, whether accidental or malicious. Protection against such damage has traditionally been provided using wire guards or grilles. Historically, guards have been made from iron or galvanised wire that eventually rust, or sometimes copper wire, which is durable but can stain the surrounding stonework green. The material most commonly used today is a welded stainless steel mesh, which can have quite a slender gauge, strong enough to provide good mechanical protection while still being fine enough for the guards to be reasonably unobtrusive. Guards are often powder coated to a dark colour, usually black or dark grey. Wire guards that are well made to fit within the stonework reveal of each opening, and securely fixed to the stonework with stainless steel clips and screws, offer effective protection that will last for many years. The main disadvantage of wire guards is the shadow lines that can sometimes be seen through the stained glass from the interior, particularly with clear or lightly painted glass. Wire guards are least intrusive when inset between the masonry tracery, but they can still cast a shadow, particularly on south-facing windows. Polycarbonate sheets being installed across the windows of the former Parliament Street Methodist Church in Nottingham (Photo: Jonathan Taylor)
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