Historic Churches 2019

BCD SPECIAL REPORT ON HISTORIC CHURCHES 26 TH ANNUAL EDITION 15 STAINED GLASS and its ENVIRONMENT Dan Humphries S TAINED GLASS adorns most of our ecclesiastical buildings and many of our historic secular buildings. As well as being objects of architecture, adornment, illustration and beauty that can tell us about the time and often place they were made, stained glass windows are also, critically, part of the building envelope. As such they must keep out wind, rain, bats, insects and draughts while also keeping in warmth. In short, the glazing has to keep the inside in and the outside out while usually being the thinnest, most thermally conductive and physically fragile part of that envelope. Indeed, stained glass has only a few millimetres of thickness with which to separate two climates that can be very different, most critically (in terms of the subject of this article) in temperature. WETTING OF STAINED GLASS Air contains water vapour. Inside a church moisture is drawn into the air from damp masonry, from the floors, from flower arrangements, parishioners’ breath, damp coats and a hundred other sources. The warmer the air, the more water vapour it can hold. If a church’s heating is left off all week and then switched to full power on the morning of a service, the warmer post-service air will be able to hold far more moisture than before. As it cools, all this surplus moisture will condense, with the most condensation occurring on the coldest surfaces. As a building with great mass, a typical church will warm and cool slowly compared to the external environment where temperatures will typically peak and trough every 24 hours, so temperatures can differ significantly between inside and outside even without any heating. There will be cooler temperatures outside than inside for most of the night and when this is the case, the stained glass, being an excellent thermal conductor will present a cool inner face to the interior of the church. As the cooling, sluggish air meets the face of the stained glass it chills further, potentially reaching dew point (at which water can no longer be maintained as vapour) and the moisture condenses on the inner face of the stained glass forming droplets. When the sun rises the following morning, it will hit the church on its south and east elevations, warming the glass quickly and causing the condensation to evaporate again. On the north side of the church however, the windows are not warmed by the morning sun. North- facing windows can stay wet for days during a condensation event in winter. As well as this wetting by condensation, the external face of stained glass is susceptible to wetting by precipitation – rain. CORROSION Most of us think of glass as waterproof but that assumption is not altogether true. Known as the gel layer, a zone at the very surface of the glass often just micrometres thick can be considered porous to water. Severe corrosion of a 16th-century glass surface caused by condensation (All photos: Dan Humphries)