Gu led Yan and Sella to the maintenance shaft behind the furnace wall. The corridor narrowed as they went, and the floor held the furnace’s vibration like a drum. The manual gate was an iron wheel fixed to a stone channel. A crawlspace curved around it, just wide enough for a person to reach the inner brace.
Old inscriptions showed where the containment art directed force through the practitioner’s body. Gu traced the first sequence with one finger. At the last turn, the diagram had worn away beneath soot and a strip of cracked stone.
Sella opened the oldest maintenance book and found the missing line. “It runs under the wheel, then returns to the outer channel. The mark on the wall is the same.” She sketched the line on a slate and held it beside the inscription. “I can follow it from the control panel.”
The sketch had no scale, only turns and pressure marks. Sella and Gu compared the curves, then checked them against the channel cut into the stone. Gu remembered one bend from the original construction; Sella found the matching mark on the opposite wall. Between them, they reconstructed the path without pretending the missing diagram had been restored. She made a second sketch for the operators and marked the place where each call should come.
