idiom · The literal stage in Daoist alchemy where the stove fire turns bright green, indicating the elixir has been successfully refined.

传说炼丹时，炉火纯青便代表大功告成。

idiom · Having reached the peak of perfection or mastery in a particular art, skill, or technique.

经过几十年的磨练，他的演技已经达到了炉火纯青的地步。