At the eastern end of the fourth pine box-flume, where the boggy clay of Low Sike met the rising bedrock flank of Old Hill, stood a recessed archway of cyclopean gray basalt that looked like the sealed portal of an ancient barrow.
The outer arch itself had been raised by Harth’s founding guild masons four hundred years ago, its massive keystones dressed with the same clean, unmortared precision as the aqueduct gallery behind the River Lantern. Across the mouth of that five-foot arch, however, a later hand had mortared a solid plug of red vitrified kiln-brick and squared granite block, three courses deep and bound together with hard, iron-slagged military cement.
“Look at the trowel-struck joints on that plug, Jon,” Master Mason Marris Gault said, holding her tin reflector-lantern close to the dark red mortar seams while she ran her thumb over a stamped crow’s-foot mark in the corner brick. “That’s royal sapper pozzolana—lime mixed with roasted forge-slag and ox-blood. You can hit the face of a block like that with a twenty-pound sledge until your arms fall off and only bounce sparks off the glaze.”
Jon wiped the sweat from his brow with his forearm and leaned on his five-foot iron crowbar beside the nose of the timber flume. The air inside the recessed hill arch was stifling—a dry, baking heat radiated straight through the three-foot masonry plug, carrying the familiar metallic whine of the three Furnace rings spinning deep within the mountain root.
