Why historic glass is more fragile than it looks

A stained glass window is not really a picture. It is a structure. Hundreds of small pieces of glass are held in a lattice of cast lead cames, stiffened by iron saddle bars, bedded into putty or cement and set into a stone reveal. The glass itself is astonishingly durable — medieval glass has survived eight centuries of weather — but everything holding it together has not. Lead corrodes and embrittles, solder joints crack, iron bars rust and expand, and the whole panel gradually sags under its own weight.

The result is that a window can look perfectly sound from the nave floor while quietly failing twelve metres above your head. Add condensation, decades of air pollution and a well-meaning repair carried out in 1968, and you have a conservation problem that will only get more expensive with time.

Reading the early warning signs

Arm yourself with a decent pair of binoculars and look properly, from inside and from outside where it is safe to do so. The following all deserve a note in your fabric log:

• Bulging or buckling panels that no longer sit flat in the opening. This is the single most serious sign and usually means the lead has lost its strength.
• White or brownish powder on the lead, a sign of corrosion; also look for thin, grey, crumbly cames that daylight shows through.
• Cracked or missing quarries, especially at the edges of a panel or near saddle bars, which often means the whole panel is moving.
• Rust staining running down the glass from iron bars, or a brown tide line where water has been tracking.
• Condensation on the inside face of the glass, particularly after services or on cold mornings.
• Loose or missing putty along the edges of the panel, and open joints between glass and stone.

What you can safely monitor yourself

You do not need to be a conservator to keep good records, and good records are what save windows. Photograph each window once a year, from the same viewpoint, in the same light, with the date noted. Note anything that changes between visits — a new crack, a little more sag, a fresh rust stain.

Around the window, check that rainwater goods work. Blocked gutters, overflowing hoppers and leaking downpipes are behind a great many failing windows. Look at the stonework too: hard cement pointing traps moisture in the wall and drives it into the glass and the ferramenta.

• Do keep a written and photographic log, and pass it on to whoever inspects the building.
• Do maintain gutters and drains, and improve ventilation where you safely can.
• Don't wash glass with household cleaners, vinegar or ammonia, and never use a pressure washer or abrasive pad.
• Don't apply tape, silicone sealant, epoxy or oil to cracks, lead or moving panels. These cause more damage than they prevent and are costly to reverse.
• Don't open, wedge or prop a window to "let it breathe".

The condensation problem

Moisture is the quiet enemy. Warm, damp air inside a building meets cold glass and condenses on it; the water then attacks the lead, the putty and the painted detail, and can leave permanent staining. Intermittent heating makes this worse, because the fabric is repeatedly warmed and cooled rather than held steady.

Improving gentle, steady ventilation and reducing sudden blasts of heat usually does more good than any treatment applied to the glass itself. Where protective glazing is proposed — external plain glazing or an internal isothermal system — it must be designed for that individual window, with proper ventilation, by someone who understands the microclimate. Poorly designed protective glazing can make condensation considerably worse.

When to call a conservator

Call in a specialist stained glass conservator if you see bulging or buckling, cracked glass in a panel that is clearly moving, broken or rusted saddle bars, heavy lead corrosion, or any panel that has dropped or shifted in its opening. Also call immediately after storm damage: make the opening safe with ventilated boarding fixed well clear of the glass, keep every fragment you can find in a labelled box, and do not sweep away broken glass.

For churches, speak to your inspecting architect, the relevant denominational advisory body and your insurer early, and check what consents are needed before work begins. Ask any conservator for a written method statement, a panel-by-panel schedule, and photographs before, during and after. Be wary of anyone who offers to take the window away and re-lead it without first surveying it properly. Re-leading destroys historic lead and original glass edges, so it should be a last resort, not a first quote.

Good repairs, poor repairs and the long view

Conservation works to a simple principle: do as much as necessary and as little as possible. Good practice means mending leads where they are sound, re-leading only deteriorated panels, matching new glass by hand, and using conservation-grade materials and non-ferrous saddle bars. It means recording everything.

Poor practice is easy to spot in hindsight: thick modern lead in an old panel, copper foil or bitumen repairs, dense cement that cracks the glass as it cures, epoxy used everywhere, and painted decoration over corrosion. If your building is inspected on a five-yearly cycle, use that rhythm to plan and budget for glass as you would for the roof or the stonework. Windows cared for steadily, in small stages, cost far less than a crisis — and they will still be glowing in another two hundred years.