Historic Churches 2019

20 BCD SPECIAL REPORT ON HISTORIC CHURCHES 26 TH ANNUAL EDITION PROTECTIVE GLAZING Deciding whether or not to use EPG Robyn Pender E NVIRONMENTAL PROTECTIVE glazing (EPG) systems are increasingly being used to protect our most important and most vulnerable stained glass windows from decay. If well executed these systems can provide the protection required to enable fragile examples to be retained in situ. On the other hand, they can be challenging interventions, entailing alterations that can harm the character and historic significance of the windows. To be sure that EPG is the right solution, and if so that it is optimally detailed, a wide range of contributory factors need to be taken into consideration. THE NEED FOR EPG Stained glass windows are not just beautiful works of art: for a millennium they have been a favoured way of decorating important buildings, especially churches and cathedrals, and so are of the greatest historic significance. Windows are also part of the building envelope separating the interior from the exterior, and indeed are its most fragile and least massive component. This makes stained glass particularly vulnerable to environmental deterioration. Well- executed glazing with well-made glass and well-fired paint may nevertheless survive centuries in almost perfect condition; but if it is technically flawed (for example, if the glass is chemically unstable or the painted decoration incorrectly applied or insufficiently fired) then environmental deterioration can be serious. Problems include: • glass corrosion, loss of painted decoration, and growth of algae and other micro-organisms due to condensation, and in some cases rainwater • corrosion of metal supports, including the rusting of ferramenta, with the consequent cracking and loss of stonework • wind pressure causing distortion and collapse • projectile damage (both deliberate and accidental). For stained glass windows suffering from serious environmental problems, there are very few options that allow them to be safely preserved in situ over the long term. Since the drop in the price of glass in the 19th century, one treatment has been to place a second glazing plane on the weather side of the window. Early experiments along these lines were fraught with problems including interstitial condensation and algal growth, but ventilating the space between the glazing planes was found to greatly improve results, and is now an essential feature of modern ‘environmental protective glazing’ (EPG). Despite research, the functional criteria for ventilating EPG were not well David Griffiths installing the modified stained glass into an EPG scheme at Canterbury Cathedral (Photo: Leonie Seliger)