a sagger is merely a box to put pots in when firing. In coal fired kilns the ash contaminated the glaze so pots needed protection. Made from fireclay. Woodfired kilns same, but without a sagger often the contamination was desired and beneficial.

Yorkshire term for being amazed/excited (no direct synonym) was thralled presumably from "enthralled" or a legacy that enthralled left behind. And nesh meant soft.