爐火純青

1. • the stove fire has turned bright green (allusion to Daoist alchemy) (idiom)

     lit.

2. • (of an art, a technique etc) brought to the point of perfection

     fig.

Character by character

• 爐(lú)stove
• 火(huǒ)fire
• 純(chún)pure
• 青(qīng)green