One of the reasons annealed copper is used for head gaskets is that it requires no sealant. Annealed copper gaskets when torqued correctly will squish into and around imperfections on the mating surfaces.
Once torqued down, (and after conditioning with a few heat cycles), the gasket becomes work hardened. When the engine cools, (due to the tightness of the head bolts,) the gasket will be further compressed and try to force itself even more into imperfections on the mating surfaces.This is the reason for re-torquing head bolts after break in, because the gasket becomes thinner, due to work hardening.
3-5 heat cycles from cold to operating temp, then to cold with a rest in between cycles, 5 is best.
If head gaskets leak it is usually caused by using them more than once, or from poor machining or by re-using components which are warped, and are in need of machining.
Because of poor fit up, sealants are needed to fill the spaces. Sealants are a poor excuse for poor workmanship.
When I went to school the correct procedure to anneal copper it to uniformly heat to cherry red and then quickly immerse in cold water. This will leave scale on the copper, which can be removed taking care not to get the copper too hot.
I would never recommend using a head gasket more than once, unless it is a matter of life and death.