The Lifespan of Stained Glass Windows
Stained glass windows are good for about a hundred years. Then it all goes down the drain, which takes about fifty years or so. This is important, because most of Toronto’s historic windows are past their expiry dates but have not yet disintegrated, as they will without help.
Some people find this strange, and think, but what about the ancient ones? In truth, the great cathedrals have been re-built many times. If your job is to maintain a whole cathedral of these, you have a workshop running full-time.
Glass, with luck, is an eternal material. Lead, solder, cement, and putty? Not so much.
Keep the Glass, the Rest is Trash.
It’s harsh, but that is the mantra for restoring windows. Museums might have more options, and maybe there’s a fine collection of lead and solder somewhere, but in practice we conserve the glass, and the pattern, and replace the rest.
When you look at century windows, they are far from flat. Although movement of the house can be a factor, usually the windows have done this to themselves.
When a window ages, every material gets bigger except the glass. The cement swells (as it fails). The leads swell (as they soften). The wooden frame swells, as humidity reaches into the old wood. A century is a long time for these materials.
In this image, the lead is deeply corroded. The cement is gone, washed away, but on the way it has etched the lead and left a black line of lead sulfates on the glass. The main leads themselves are chemically unstable and have developed a crust of soluble carbonates. All this, while little expansions have pushed the window halfway out of its frame.
Chemical damage, and deterioration of the base materials, makes up half the problem. The other half is mechanical damage. Sometimes glass takes a whack, and breaks. But sometimes it gets sneakier than that, and the glass is subject to long-term compressive force, which powders it. In this picture, the lower right border piece has taken a hit, but it disintegrated because it had already lost its internal cohesion to compressive stress. Only a century can do this, but a century really can do this. A century of stress can turn glass into powder.
Also, very commonly, the general distortion of a bending panel will start to break pieces. This is why the best old pieces have a border—to catch this. But this clever design has limits. Even if the glass does not break, the joint may bend enough to free one edge on one of the pieces. At that point, pieces may start to fall out.
Another issue is cleaning. Stained glass windows are hard to clean, and (for many) scary. They live in an in-between zone. They get treated as if they they were plaster or wood or painted surfaces, not like a glass plate you might wash in the sink. But they are glass. They do like being treated like glass.
It wasn’t a good century for air quality. Leaded gasoline, gas stoves, cigarettes… All those things are still there, embedded in the surfaces.
And then there are the putties, and the layers of paint—including lead paints. It’s been a long time since people routinely knew how to clean these things. Since then, the layers have built up.
When all these forces add up, it can be hard to believe in the outcome. But glass is an eternal material, and it loves to be clean. The rest is just solvents and patience, and steel wool, and baking soda. Scrubbing does work, eventually.
Painted glass, like the birds or church windows, is different. This stuff has oxides baked onto it, and is subject to chemical reaction. In general, painted elements are to be cleaned minimally. Maybe it will be strong enough for a mild chemical cleaning, or even for a soft scrub. But, if the surface is compromised by overcleaning, it may be powdery, in which case, never touch it. And, some colours may be powdery while others are stable. Unless I am sure, absolutely sure, I do not try to clean painted glass. When I do, it’s de-ionized water and cotton balls, until I am sure. Never touch a painted window with a commercial cleaner.
The good news is that almost everything can be fixed. Lead is just lead, lost pieces can be re-cut (within limits, because, the old glass is rare now).
The good news is that broken windows don’t have to be broken. They can be fixed.
There are limits. If you’re curious, please see What I Can and Cannot Do.