The fire core sat trapped in a sullen, smoky twilight.
Inside the heat-proof bosh, the heat had plateaued at seventeen hundred degrees Fahrenheit. It was hot enough to char timber and soften lead. But six hundred degrees below the liquidus heat needed to melt high-carbon pig iron.
Through the quartz peep-holes of the copper tuyeres, Master Borin Bronze-Lung peered into the burning zone with bloodshot eyes.
“The air is killing the fire,” Borin wheezed, wiping black soot from his forehead. “Every time the big pistons push fourteen hundred cubic feet of cold river draft into the tuyeres, the blast chills the coke. It’s like pouring ice water into a boiling kettle.”
Giles Croft stood on the foundry terrace with a sheet of squared drafting parchment spread across the lid of an empty kaolin barrel.
His pencil moved with sharp, sharp-angled care, sketching a snake-like cluster of cast-iron pipes enclosed in a brick oven chamber.
“The Neilson heat-powered paradox,” Giles said, not looking up. “In an open hearth, more air produces more heat. But in a confined blast fire stack, the sensible heat carried away by the expanding cold air gas exceeds the heat energy generated at the burning front. We’re feeding the fire with air at sixty degrees Fahrenheit. To reach smelting enthalpy, we must preheat the blast.”
Lady Beatrice of Dunwall stood beneath the timber canopy of the survey shack, sipping a mug of hot chicory tea offered by Sister Teresa. Her eyes were fixed on the forty-foot chimney. There, the sluggish black smoke continued to roll.
“My lord Croft,” Beatrice called out, her voice crisp with skepticism. “Your great chimney has been grumbling for three hours, yet not a single drop of metal has issued from its foot. In the capital, when a potion-maker’s crucible stalls, the apprentices start packing their bags before the court guard arrives.”
“I’m not a potion-maker, Lady Beatrice,” Giles replied calmly, folding his sketch. “And this isn’t a crucible. It’s a mass-balance heat exchange loop. Borin, where are the cast-iron drain tube bends from the distillery salvage?”
Borin blinked, dropping his fire gauge against his chest. “In the scrap bay behind the forge. Twelve U-shaped iron tubes, eight inches across. We cast them for the salt boilers last winter.”
“Fetch them right away,” Giles commanded. “And tell Duncan to lay a brick burning box around the main blast pipe hub.”
The foundry yard became a wild beehive of activity.
The joint between the cold-air main and the hot-blast stove needed an airtight, heat-resistant seal that wouldn’t char under five hundred degrees of preheated air.
Master Borin Bronze-Lung prepared the rust cement himself in a battered iron kettle, stirring a pungent grey paste of iron borings, sharp flux salt, flowers of sulfur, and water.
“The old dwarf-glue,” Borin said, slapping a trowel of the steaming paste into the cast-iron socket. “When the sulfur and sal ammoniac bite into the iron filings, the iron rusts at ten times its natural speed. The rust expands inside the socket as it oxidizes, wedging the joint so tight that neither air nor steam can force a path through it. By nightfall, this collar will be solid as a forged casting.”
Duncan Stone-Splitter and his masons laid a second skin of common red brick around the outside of the stove chamber, leaving a two-inch air gap filled with crushed melted rock waste wool.
“Heat padding,” Giles explained to Toby, who was jotting measurements in his survey notebook. “The heat inside the burning box is nearly nine hundred degrees. If we don’t insulate the outside walls, blasting heat loss through the brickwork will fade thirty percent of the recovered heat energy before the air ever reaches the tuyere nozzles.”
Toby nodded, his pencil flying across the paper. “Every degree of preheat that stays in the blast is a degree the peat coke doesn’t have to provide.”
“Exactly,” Giles said. “Sensible heat delivered directly into the burning zone bypasses the activation energy threshold of the fuel. It’s pure heat-powered leverage.”
Master Duncan Stone-Splitter and twelve masons threw together a box-shaped brick chamber, ten feet long and six feet wide, directly between the back-and-forth blower outlet and the fire tuyeres.
Inside this chamber, Borin and Caleb mounted the twelve cast-iron U-tubes in a linked snake-like bank, sealing the joints with a fireclay-and-iron-filings mastic.
“Now for the fuel,” Toby Karr said, running up with a brass wrench. “Do we haul more peat briquettes to fire this new stove, Lord Giles?”
“We don’t spend a single ounce of fuel,” Giles said.
He pointed his wooden ruler at the top of the blast fire stack. There, a thick, yellow-tinged gas was venting into the air.
“The exhaust gas leaving the throat is twenty-five percent carbon monoxide,” Giles explained. “It was generated in the reduction zone when the dropping coke reacted with oxygen. That gas is combustible. Instead of venting high-heat fuel into the sky to give Inquisitor Ostrander an excuse to complain about soot, we divert it.”
Master Caleb climbed the iron access ladder of the main stack. He carried an eight-inch sheet-iron bypass chimney pipe.
With six swift hammer blows, he knocked out a brick plug in the upper stack wall and clamped the iron chimney pipe directly over the sky-slit.
A heavy counter-weighted damper diverted the hot exhaust gas down the outside pipe, funneling the carbon monoxide directly into the brick stove chamber below.
“Throw a match into the stove chamber, Borin,” Giles ordered.
Borin tossed a burning brand through the stove lighting port.
WHOOMPH.
The trapped carbon monoxide gas ignited with a clean, roaring blue-violet flame that wrapped around the cast-iron U-tubes in a dazzling bath of fire.
The iron pipes inside the chamber began to glow. First dull crimson, then bright cherry red, and finally a glowing, semi-clear orange.
Inside those glowing iron tubes, fourteen hundred cubic feet of fresh river air pumped by the waterwheel was forced through the snake-like maze.
The cold blast didn’t enter the fire tuyeres at sixty degrees anymore.
It entered at four hundred and fifty degrees Fahrenheit. A white-hot, roaring gale that blasted directly into the heart of the peat coke bed.
The effect was instant and terrifying.
Inside the fire crucible, the dull orange twilight vanished.
The burning front blew up with a sound like an approaching express train. A deep, ear-splitting, underground roar that set the timber narrow high walkways trembling and sent shakes through the soles of every boot on the terrace.
The sluggish black smoke at the top of the stack vanished.
In its place erupted a clean, roaring plume of pale golden gas forty feet long, lighting up the granite bluffs and casting razor-sharp shadows across the river wharves.
Borin Bronze-Lung threw his leather goggles over his eyes. Then it pressed his face against the quartz check port of the north tuyere.
He recoiled as if struck by a fist.
“Ancestors of the mountain,” Borin screamed, clutching his bald scalp with both soot-stained hands. “The filament! The fire gauge filament is gone! It vanished into the white light.”
Giles stepped up beside him, adjusting his dark-tinted glass glasses. “Read the scale, Borin.”
“Two thousand six hundred and fifty degrees,” Borin choked out, his voice cracked with awe. “The coke isn’t burning, Lord Giles... It’s dissolving! The limonite ore is melting like butter on a hot griddle! The iron is raining! It’s raining fire into the crucible.”
Lady Millicent Croft walked along the stone low roof wall overlooking the foundry terrace, her tally-book open and her brass nib clicking against the thick parchment.
“By preheating the blast to four hundred and fifty degrees,” Millicent stated, turning to Lady Beatrice, “our calculated fuel consumption drops from two point eight tons of peat coke per ton of metal down to one point eight tons. That’s a thirty-five percent reduction in running overhead.”
Lady Beatrice watched the shimmering heat waves rising from the padded stove with sharp, evaluating eyes.
“And what does that thirty-five percent mean in coin, Lady Millicent?” Beatrice asked.
“It means that raw pig iron tapped from this hearth will cost three silver shillings and fourpence per hundredweight to produce,” Millicent replied without looking up. “The southern merchant syndicates currently wholesale their iron in Dunwall for eleven shillings and sixpence. Even after paying fourpence a ton in transit tolls along your Duchess’s turnpikes, we can undersell the southern guilds by more than fifty percent and still retain an eighty percent gross margin on every cast bar.”
Beatrice slowly drew a breath of the crisp autumn air. Her knuckles tightened on the handle of her riding crop.
“Duchess Rowena won’t merely purchase your iron, Lady Millicent,” Beatrice said. Her voice dropped into a solemn, calculating murmur. “She will want an exclusive supply promise. If Highmarch controls the spreading of Grey Crag iron, we control the arms of every baron from the northern rifts to the western sea.”
“Grey Crag doesn’t sign exclusive covenants,” Millicent replied with calm, office calm distance. “We sign purchase orders with fifty percent cash deposits in standard royal gold. The market, Lady Beatrice, is entirely indifferent to your territorial ambitions.”
Lady Beatrice stepped out from beneath the timber canopy, her glasses sliding down her nose.
She looked at the glowing stack, then at the roaring brick stove, her cool high-born poise fully shattered.
“Two thousand six hundred degrees,” Beatrice murmured, her hand grasping her fur collar. “From peat mud and cold river water... By the gods of Highmarch, he has harnessed the core of the world.”
Down along the river wharves, the crew of the Synod’s black galley tumbled onto the deck in sheer panic.
Judge Malric Ostrander stood by the gangway, his face lit up in the pale golden glare of the fire throat half a mile away.
His iron aspergillum slipped from his fingers, clattering against the deck planks with a hollow ring.
“The white flame,” Ostrander whispered, his gaunt face pale with holy terror. “A mortal man has ignited the star-fire upon the marsh... Without prayer... Without incense... Without holy decree.”
His clerk stared at him with wide, trembling eyes. “Father... Is it a monster?”
Ostrander didn’t answer. He slowly stepped back into the shadow of the cabin, his hands shaking inside his black woolen sleeves.
Up on the charging gallery, Toby Karr was jumping up and down. He waved his survey slate in the air.
“Hearth level is rising, Lord Giles.” Toby shouted over the roar of the blast. “I lowered the sounding rod! The molten bath is twelve inches deep! The melted rock waste is floating clean and clear above the metal.”
A crisp, musical ping rang in Giles’s ears, and the system screen flared with bright amber text.
Giles pulled his pocket watch from his waistcoat, checked the time, and snapped the brass case shut.
“Sound the tap-bell, Toby,” Giles said, turning toward the foundry floor. “Clear the sand beds and put on your leather gaiters. We tap the melted rock waste first, then we pour the iron.”