External or Protective Glazing:
Most churches in Ireland, unlike here in South Africa, have clear external glazing fitted on the outside of their stained glass windows. I believe external glazing was introduced for mainly one or both of the following reasons. Firstly when central heated buildings became the norm and better insulated double glazed windows followed, and secondly because churches could get away with covering leaky stained glass windows. This was done by completely covering the stained glass, or in some cases the external glass was fitted right up against the stained glass, sealing in condensation. Temporarily it did its job, but on the long run, the trapped heat and moisture created a small greenhouse effect. Between the north facing layers of glass, it was common to find moss, alga and even in some cases ferns. On the other hand, the south facing windows had a problem of trapped heat. Sometimes you couldn’t keep your hand on the inside of the stained glass for longer than three seconds. So in a nutshell, external glazing speeds up the destructive effect on the lead and even the glass… unless it is vented.
You may ask why bother at all with external glass? Does it have any function at all? In order to preserve the leaded window, there must be an air flow between the stained glass and the external glazing. This is introduced by venting the bottom and top to create the ‘chimney effect’. With the condensation and trapped heat taken care of, the exterior glass could protect the stained glass from the elements and act as a barrier from the odd vandal throwing a stone.
As with many other churches, we were commissioned by St. Cronan’s Church, Balla Co. Mayo to go around the church, remove and clean the inside of the external glass, replace any of the cracked or broken panes and to introduce the system of ventilation.
Stainless steel grills are also widely used around Ireland, and their primary use is to protect stained glass windows from vandalism. The only downfall of using grills is when you inside the church looking out , they are visible behind the glass. Because the older grills are made from steel, they corrode and leave nasty rusty streaks on the stone work. They were replaced with black powder coated stainless steel grills.