I've been annealing copper washers by heating
them - usually on a stove element - and just leaving them to cool down ... for at least 40 years, i have rarely had any instances where this did not produce the required seal. Done it on nortons, triumphs, matchlesses, harleys, every type of car, old commer trucks, and now on boat diesels. It's not a particularly good engineering solution to getting a seal in a fuel
system IMO but it's one we have to live with on older engines so it is a required skill.