Meindert Sprang wrote:
snip
Correct. It is a well known physical property of copper. It hardens when it
is heated.

Meindert

Nope, that's not right. Heating copper (also brass, aluminum, and most
other non-ferrous metals) and cooling it (the quicker the better
generally) anneals it and restores the workability.

That is why reusable copper spark plug washers, head gaskets, etc., are
heated to restored their sealing qualities. They get work hardened in
use and might not seal at a specified clamping force. The sealing
qualities can be restored by annealing.

Jack

--
Jack Erbes in Ellsworth, Maine, USA - jackerbes at adelphia dot net
(also receiving email at jacker at midmaine.com)