engrave, inlay, carve, exhaust
琱 centers on working a surface with precision: engraving, carving, or inlaying designs into a material. The 'exhaust' sense is a rare extension, possibly from the idea of using up material through detailed work.
琱 combines 王 (jade) with 周, which likely provides the sound. The character originally referred to engraving or carving jade, a meaning that extended to other materials.
The left side 王 (jade) suggests a precious stone, and the right side 周 looks like a tool with a frame and a mouth-like opening. Imagine a craftsman using a fine tool to engrave intricate patterns into a jade surface.
For チョウ, picture a master engraver named Chou carefully carving a jade piece: Chou -> チョウ, and his chisel dances across the stone.