That tarnish happens over time and many things can contribute to it. Moisture in the air. Temperature changes causing condensation is another. Handling the heads can help tarnish along. The lead itself can add to the problem if the lead is not pure. Re-heating lead multiple times can cause lead to discolor. The air itself can have high levels of acid or alkaline from industrial exhaust and that can oxidize lead over time if you live where industry is nearby. You best bet is to cast only what you intend to paint soon if you need to paint over very shiny head surfaces.