There's a bit of a paradox about stained glass and leaded windows being fragile.
The glass is, but the lead absorbs impacts rather well, and a leaded panel in a door will not shatter in the same way a single piece of glass (or even a double glazed panel) might.
I remember encountering one panel on a house in a less than glamorous part of Leeds that had had half a housebrick lobbed at it from a passing car: two pieces of glass were cracked, one (about one inch by three) was broken with a hole in it, and the lead was bent. A short term repair was effected with a couple of pieces of sellotape, and the final repair cost under £100. Another door in the same terrace had a single glass sheet pane, which, when similarly damaged by youngsters, cost more than twice that to replace.
This single sample piece has been accidentally dropped, and illustrates what happens very well:
- the lead keeps the glass in place and actually absorbs some of the impact, the glass being retained by the lead and cement. (If the cement is old and crumbly it won't be retained as well: this is a good reason to make sure panels are recemented when necessary). If the visual impact of the cracks are not a problem there may be no need to replace a piece damaged like this.