Wei was still looking at the felt when Batu answered him.

"We pour the molten metal all at once."

Batu searched through his fragmented memories to explain. "If you make it in pieces, it will break where the parts meet once the pressure increase. After the pour, bury it in sand and let it cool there. Don’t pull it out while it’s still hot. The whole casting needs to cool evenly or it will crack. When you test it, start with half the charge you think it can take. The first failure will show you where the weakness is."

Wei set the felt down slowly. He went through each part of the advice against what he already knew before he responded.

"I know about pouring it in one go."

He said. "The joins are always the first place to break in every tube I’ve seen crack. I’ve been working toward that, but I hadn’t committed to it at this size."

He looked at the taper in the sketch, the thick closed end drawn at four fingers, the open end at one. "The cooling is what I’ve been stuck on for months. The closed end stays hot longer than the open end. That difference is where it cracks, when one side starts to shrink and the other is still hot."

"Sand slows it down," Batu said.

Wei picked up the felt again. "I see. If I bury the mold after the pour and let it cool over several days instead of hours, it should cool more evenly. I’ve done that with smaller pieces and it helps. I haven’t tried it with this much difference in thickness, so I can’t be sure, but the idea’s worth a try."

He set the felt down and looked at the projector sketch instead. "The projector is considerably easier. I don’t need foundry for that. The compound I have is right, and the frame and nozzle are simple wood and metal work. I can build it with what the camp already has."

"How long," Batu said.

"Four to six weeks for a working version of the projector."

Wei glanced at the cannon sketch again. "This one is three to four months for a first version, counting the failed attempts. If the first casting cracks, I have to make the mold again, and that adds time. The first attempt will probably go wrong regardless of how much we plan."

He said it without apology or exaggeration. The honest number.

Batu received both timelines and moved past them. The spring campaign opened on its own calendar. The projector mattered before Ryazan. The cannon served what came after, if it was ready by then.

"What do you need," he said.

"A furnace that can handle this much molten iron."

Wei was already thinking through it. "My work so far has been smaller, and what I have now isn’t enough for something this size. I heard about Rus metalworkers in the workshop district, can they help with the pour?"

"They can work alongside you," Batu said.

Wei nodded once. "Then I need space beside the furnace for sand beds deep enough to cover the mold completely. And a testing area well away of the city. At least two hundred meters between the test position and the nearest building. If the first cannon fails when fired, the blast needs open ground, not the workshop district."

He paused.

"I’ll need you to supply me with more saltpeter. What I brought is enough only for now."

Batu noted each item. Khulgen would handle the materials.

"There’s a second craftsman coming in spring."

He said. "The Kashgar man. He can work alongside you on this."

Wei looked at him with the expression of a man who had heard something he didn’t want to hear. "I’m not sharing my formulas with another craftsman."

He started to rant like an old man. "What I’ve developed took years on the road and before that in the workshops. You bring another man in here and he watches how I work, the ratios and the grades and how the compounds are mixed. How can I be sure he won’t steal all my work."

Batu locked gazes. "Your formulas are yours. The Kashgar man is coming because he may have experience with cast iron at a larger scale than you’ve worked. If he doesn’t know anything you don’t already know, that will be clear inside a day and he moves on. If he does know something useful, you want him working on this."

Wei understood the distinction. He wasn’t satisfied with it, but he understood it.

He made a short sound and looked back at the casting sketch.

Batu stood. "Khulgen’s office will arrange the foundry setup, the saltpeter supply, and the testing area. The metalworkers go to you when you’re ready for the pour. I’ll send word about the Kashgar man when the date is confirmed."

He left the felt sketches on the supply crate and walked out.

The eastern section of the camp had a calmness different from the main camp’s rhythm. The paths here were not worn yet, the ground unmarked by days of foot traffic the way the older sections showed. The engineer corps billet was behind him. The metalworker section was west.

He walked toward the main camp and let the cold hit his face without reacting to it.

The fire projector would exist in six weeks if Wei’s estimate held, before the army went out to Bulgar.

The cannon would come three to four months after the foundry work began, or it would need another attempt and come later, and the campaign would account for whatever it got. The Kashgar man would add something to the casting problem or he wouldn’t.

The spring campaign would open on its own calendar. None of what had been started this morning pushed it back. What it changed was the campaign after the first season, when the army moved past timber earthworks into stone and reinforced gates, and a door that had to be forced could instead be broken from a place the defender hadn’t prepared for.

He walked and did not add to what had already been done.