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, stored in the stone and the ceiling beams, and the furnace at the far end had been banked down and stilled for an hour without releasing much of it.
He stepped inside. There were cooled castings lined the shelf along the far wall, arranged by run order, with mold frames stacked for tomorrow nearby. The ceiling beams had gone darker across another hand's-width of surface since the first pour.
Aestrith was 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, differently than the usual way she kept it.
He crossed to where the castings were arranged.
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. He set it back and reached for the next piece, a pipe fitting with a smaller bore. Against the lamp's glow it looked consistent along its length, close enough to serve. He placed it back.
"The edges are fine," Aestrith said from the bench.
She hadn't looked up.
"They are," he said.
"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.
There was sweat at her temple, and a tightness around her eyes that hadn't been there this morning. "The furnace takes a few cycles to work up after you light it. The early frames suffer from that."
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?" Aestrith said.
"I sent her home, she was very pale." He looked at the far wall. "Probably sick."
Aestrith looked at him for a moment. Then she turned back to the bench.
He moved to the far end and opened the ledger to a different section. Those were pages he had been building separately from the current production notes, added to across several days.
He set the book flat on the bench between them.
She looked at the page.
The drawings there were different from his margin work. A large cast form at the center, a beam assembly drawn above it, pivoted at its middle, arms reaching to either side. A pump mechanism indicated below.
Multiple views of the form itself, dimensions estimated in brackets, the interior worked through from several sides.
"I've been looking forward to show you this," he said.
She observed the page without touching it. "What is it."
"An 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 a beat.
"The air pressing down from above drives the piston down into that space. The piston pulls one end of that beam down, and the other end comes up. Whatever's hanging from the other end gets pulled."
She was looking at 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," she said.
"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 comes back up on the beam's return."
She looked at the valve notation. "And the cold water supply."
"A separate tank. Refills from whatever water source is available."
She looked at that for a moment. "How many strokes a minute."
"A few. For this design the volume per stroke matters more than the speed."
He moved to the component list in the margin. "I've been thinking through what the foundry needs to produce. The easier pieces 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," she said.
He put his finger on it.
She looked at it. He let her look.
"The bore," she said.
"Yes."
"Its 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 indicated the dimension estimates in the brackets. "If it's oval by even a small margin anywhere along the run, the piston can't seal."
She looked at the dimensions. "This is an estimate."
"It is. I haven't figured out the numbers yet. They depend on what's possible on your end."
"A bigger chamber means more force," she said.
"A larger piece means a longer pour, longer cooling, more time for the bore to move." He looked at the estimate again. "There's a size that's good and still achievable. I need to know where your floor is before I can say whether I've found it."
She looked back at the sketch. He watched her work through it, finding the edges of it, checking where the thinking ran out.
"How much longer than the batch runs," she said.
"I don't know. The final dimensions matter, and the furnace temperature going in. I could estimate but the figure would be wrong by enough to matter."
He kept his voice 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 end and I can work back from it."
Aestrith looked at the sketch. Then at the lamp. Then at the far wall.
"This is more complicated than before," she said.
"I know."
"I know my limit in the batch configuration, but everything here is different. I can't explain to you something I don't have."
"Then we find out when we get there," he said.
She glanced back at him, a faint glint barely appearing on her eyes.
"After the simpler pieces," she said.
"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 surface edge clean.
"Fine," she said.
She didn't move toward the door yet. "The afternoon sessions are taking longer to come back from than they were before. You should have that before you plan the next pour."
He wrote a line in the margin. He had been watching the morning-after strain build across the sessions and letting it sit unaddressed. The rotation schedule running in his head needed to go into the records properly.
She looked at him for a moment. Then she reached for her coat.
He was still writing in the margin. The sketch forming there was more complex than anything the component notes warranted.
He finished the line and closed the book before she turned around.
They left the foundry together. The corridor outside was cooler, and the day's noise had calmed down of the building.
Godric was at the internal gate, eyes already on them as they came around the corner.
"Dunna's runner came to the gate an hour ago," Godric said when they reached him. "The first batch leaves the quarry at first light. The militia is ready."
Beorn looked at him.
"Good," he said.