by fire
熯 is a rare kanji whose standalone meaning is unclear. The given gloss "by fire" suggests an action or state involving fire, but the exact sense is uncertain.
The exact historical development of 熯 is uncertain. It combines 火 (fire) with a right side that resembles 漢 without the water radical, but the origin and function of this component are unclear.
The left side is 火 (fire). The right side looks like a complex arrangement of 口, 大, and 廾. Picture a fire being used to dry something out, as the kun reading かわく suggests drying.
For ゼン, imagine a Zen monk calmly tending a fire to dry herbs: Zen -> ゼン, and the fire 火 is the tool.