The heat hit him before the door finished opening. The smell of iron and hot stone came with it.
It had built across the afternoon in the foundry walls, trapped in the stone and ceiling beams. The furnace at the far end had been banked down and still for an hour without giving much of it back.
He stepped inside.
Cooled castings lined the shelf along the far wall in run order, with mold frames stacked nearby for tomorrow. The ceiling beams had darkened across another hand's-width since the first pour.
Aestrith stood at the bench.
She had her back to him, bent over something she was checking against the lamp. Without the coat. Her hair had been tied back for the work, not the way she usually kept it.
He crossed to the castings.
The first piece he reached for was a valve housing from the earlier run. He ran his thumb along the edge where the cross-section changed, checking the work. Then he set it back and picked up the next piece, a pipe fitting with a smaller bore. Against the lamp's glow it looked even along its length, close enough to serve.
"The components are fine."
Aestrith still hadn't looked up.
"They are."
"Wynn's getting the rhythm."
"He is." He turned toward her. "Did this batch run cleaner than yesterday's?"
"The middle of the run did. The first few frames are still so-so."
She turned then.
Sweat clung at her temple. There was a strain around her eyes that had not been there that morning.
"The furnace takes a few cycles to work up after you light it. The early frames suffer for it."
He nodded and moved to the bench.
He opened the ledger and wrote. The pull inconsistency across the crossbow batch traced back to the assembly process. Wynn needed a standardized order before the next production run.
He finished the note and stopped.
"Tam?"
"I sent her home. She was very pale." He glanced toward the far wall. "Probably sick."
Aestrith watched him a moment, then turned back to the bench.
He moved farther down and opened the ledger to another section. Those pages had been building separately from the production notes, added to over several days.
He laid the book flat between them.
She looked down at the page.
The drawings there were different from his usual margin work. A large cast form occupied the center. Above it sat a beam assembly pivoted through the middle, arms extending to either side. A pump mechanism had been marked beneath.
Multiple views of the form surrounded it, dimensions estimated in brackets, the interior worked through from several directions.
"I've been looking forward to showing you this."
She studied the page without touching it. "What is it?"
"An atmospheric engine."
He tapped the drawing. "It runs on steam and vacuum. The boiler heats water until it makes steam, then the steam fills the chamber from below. Then you inject cold water."
He paused.
"The air pressing down from above drives the piston into that space. The piston pulls one end of the beam down, and the other end comes up. Whatever's hanging from the other end gets pulled."
Her eyes stayed on the beam assembly. "So the air is the engine?"
"Yes."
"The steam is just for the setup."
"Right."
"How does the cold water get in?"
"A valve." He indicated the lower section of the drawing. "Open the valve, cold water enters, the steam condenses, the piston comes down. Close it, let the steam refill from below, the piston rises again with the beam's return."
She followed the valve notation. "And the cold water supply?"
"A separate tank. It refills from whatever water source is available."
She was quiet for a moment.
He shifted to the component list in the margin. "I've been thinking through what the foundry needs to produce first. Pipe sections. Valve housings. Bracket mounts for the beam pivot. Connection hardware. Wynn can run most of this alongside the crossbow production without pulling you off the current rotation."
"And the hard part."
He put his finger on it.
She looked at the mark and he let her.
"The bore."
"Yes."
"It's going to have the same problem as the crossbow components."
"Worse. The crossbow components are small enough that the density variation across any piece is manageable. The cylinder needs to be large enough to generate real force."
He tapped the dimensions in the brackets.
"If it's oval by even a small margin anywhere along the run, the piston can't seal."
She studied the measurements. "This is an estimate."
"It is. I haven't worked out the numbers yet. They depend on what's possible on your end."
"A bigger chamber means more force."
"A larger piece means a longer pour, longer cooling, more time for the bore to shift." He looked back at the estimate. "There's a size that's still workable. I need to know where your limit is before I can tell whether I've found it."
She kept looking at the sketch. He watched her follow it through, finding where the thinking held and where it stopped.
"How much longer than the batch runs?"
"I don't know. The final dimensions matter. So does the furnace temperature going in. I could estimate, but the figure would be wrong by enough to matter."
His voice stayed even.
"That's why I'm showing you the problem before I fix the design. The cylinder I can build is constrained by what you can influence. Tell me what the limit looks like from your side and I can work backward from it."
Aestrith looked at the sketch. Then the lamp. Then the far wall.
"This is more complicated than before."
"I know."
"I know my limit in the batch configuration, but everything here is different. I can't explain something I don't have."
"Then we find out when we get there."
She glanced back at him. A faint glint touched her eyes.
"After the simpler pieces."
"Yes."
"The valve housings, the pipe sections, the hardware. Then the cylinder."
"That's the idea."
She picked up one of the cooled fittings from the shelf and turned it once in her hands. The lamp caught the edge cleanly.
"Fine."
She did not move toward the door yet.
"The afternoon sessions are taking longer to recover from than they were before. You should know that before you plan the next pour."
He wrote a line in the margin. He had been watching the strain accumulate across the morning-after sessions and leaving it unaddressed. The rotation schedule in his head needed to make it into the records properly.
She watched him another moment, then reached for her coat.
He was still writing in the margin. The sketch forming there had grown more complicated than the component notes required.
He finished the line and closed the book before she turned around.
They left the foundry together.
The corridor outside was cooler, and the noise of the day had faded from the building.
Godric stood at the internal gate, already watching them as they came around the corner.
"Dunna's runner came to the gate an hour ago," he said when they reached him. "The first batch leaves the quarry at first light. The militia is ready."
Beorn looked at him.
"Good."