Reading a bow in a hundred-year-old panel

Stand at the bottom of a window and look up along the plane of the glass, not at it. If the panel is bowing you will see the reflection of the opposite wall break into two, and where it breaks is where the panel has gone.

A bow of about fifteen millimetres over a panel is normal ageing and needs watching, not fixing. Thirty is a conversation with the fabric committee. Sixty is glass falling out within a decade, and by then the lead has work-hardened so thoroughly that the panel will not survive being straightened — it has to come apart.

The other thing to look for is the saddle bars. If the ties have been cut or have rusted through, the panel is carrying its own weight, and lead is not a structural material over more than about six hundred millimetres.