炉火纯青

lú huǒ chún qīng

skill brought to perfection

Character by character

炉lú
furnace

火huǒ
fire

纯chún
pure

青qīng
blue

他的书法已经到了炉火纯青的地步。

His calligraphy has reached true mastery.

Tone: positive (praise).

Daoist alchemists knew the elixir was ready when the furnace flame turned pure blue.

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