the vitreous glaze on china, porcelain, etc.
泑 centers on the smooth, glassy coating applied to ceramics: the vitreous glaze on china or porcelain.
泑 combines 氵 (water/liquid) with 幼, which likely contributes the sound. The character is used for the liquid-like glaze on ceramics, but the exact historical development is uncertain.
Water 氵 is mixed with something young 幼 and fresh to create a smooth, glassy glaze that coats porcelain.
For ユウ, imagine a potter saying 'you' as they hand you a beautifully glazed cup: you -> ユウ, and the glaze shines.