On Jul 25, 2:20*am, Clive wrote:

Yes. * Whilst a lead plug looks to be entirely of lead, it is in fact
mainly steel with a bored core

"Lead plug" isn't the most helpful term anyway.

They're not lead, they're tin (with obsessive purity standards,
depending on your regulatory body)

They're not steel bodies, they're a cuprous alloy so that they don't
rust in - they are removed and replaced fairly frequently.

The "plug" isn't solid either. It's (an improved design, although not
universal) a brass plug thickly soldered in by the tin plug around it.
The idea is that once the plug softens, this core is blown out and
then the whole plug is open. If it's a literal "lead plug", as for the
older ones, then a small pinhole melts first and (if not too
overheated) then jet of escaping steam can be enough to chill this and
re-freeze it, so the plug never opens fully.