From: Rabbi Moshe Sofer
To: Rabbi Shimon
The Chasam Sofer rules that dairy vessels purchased from a non-Jew require only hagalah, not libun. Since milk is a clear liquid, it creates bishul-type absorption rather than tiggun-type, so the principle of ke'bol'o kach polto is satisfied by hagalah alone. He also reinterprets the teshuvah cited by the Shach in YD 121:8, arguing that libun is only required when both tiggun-type absorption and the absence of a permitted intermediary combine.
Source: Shu"t Chasam Sofer YD §111