What is yuuyaku?

釉薬・ゆうやく【yuuyaku】
glaze【gléɪz】

Also called uwagusuri. A glassy liquid for covering unglazed pottery and porcelain. It is for decoration, making pottery durable, giving a glossy shine, and becoming waterproof.

 

The English translation is glaze, which comes from gla (=gla/gli/glu/gle). Like glass, glad, and glacier, gla has the etymology of smoothly shining. You feel the brilliance in the word, yuuyaku, from gla(ze).

参考
〈「工芸」英訳ガイドライン〉（ ザ・クリエイション・オブ・ジャパン）