I think you're right Lozza, I'm a fitter and turner by trade and I remember when I was an apprentice, and doing a lot of mainteince fitting, I was always told that when you wanted to reuse a copper washer, you heated it to dull red and quenched it in water to anneal (soften) it so it can be used again.
Work hardening is a broad term used to express the change of character a material exhibites after being used, with copper being so soft, a few applications of compression and heat are enough to work harden it and change its charictistics.
The charasticts of copper in a head gasket application is to provide minimal sqeeze or loss of dimension and yet make a good seal. If it's used too many times, you should also make sure it has not deformed and formed ridges or lips that could cause leaks or has compressed below standard thickness. For example, check out the copper washers under the M10 socket head bolts that hold nearly all Japanese forks together, they nearly always deform to the extent that they're sometimes difficult to get out of the slider. I always file them flat and sometimes reduce the OD so they can once again fit in the counter-bore then re-anneal before refitting.
Like with everything where you're using / rebuilding old machinery, best result are achieved is in the fitting detail adopted in reassembly.
Peter
