The saltpeter smell reached him before the billet came into view. A sharp mineral note in the winter air, distinct enough to know what it was before the source appeared.

Batu followed it east through the camp.

Wei was outside when he arrived, standing at the billet’s entry with his arms folded against the cold. A compact man past forty, with the careful eyes of someone who had spent his working life measuring small differences.

His assistant was at the supply frame behind him, unwrapping ceramic containers from oilcloth.

They introduced themselves. Wei’s Mongolian was functional and direct.

"The proposal was a spring meeting at Samarkand," Batu said. "What changed."

Wei looked at him without apology. "The Ayas relay. Merchants carry better information than official couriers."

He paused.

"The western campaign was authorized with absolute Jochid authority. Sitting in Ferghana watching another man negotiate was the cautious choice. Coming north while the commission was still up to grab was the better one."

"Show me what you brought."

They moved to the supply frame.

Wei worked through the materials in order, starting with the incendiary side.

Three grades of compound, each in sealed ceramic containers. The first burned hot and fast, designed for fire arrow tips. The second burned slower and stuck to surfaces, better for ignition. The third was a tar-and-resin compound for keeping a surface burning rather than starting the fire.

"These are the simply batch," Wei said. "They do what they’re designed to do."

Then the propellant side.

Two iron fire tubes, short and heavy. Beside them, sealed ceramic containers of a different compound. The high-saltpeter mixture Wei had been developing on the Ferghana road.

"The ratio is higher than the Jin workshops used," Wei said. "More force and range on the tube."

He looked at the tubes. "It still disperses. The casing doesn’t hold the pressure long enough."

"Show me," Batu said.

They set up two targets.

A wooden post at forty meters for the tube. A timber target against a packed earth bank at thirty meters for the bomb, two upright posts and a horizontal beam from the construction effort, set up that morning.

Wei loaded the fire tube first. His assistant held the stand while he tamped the propellant charge and seated an iron ball at the tube’s open end.

He aimed it at the post and touched the ignition cord.

The crack was loud enough to carry across the camp.

The iron ball struck the post at forty meters and knocked it sideways from its footing. A small flame licked at the tube’s vent and died.

Batu walked to the post. A dent at the impact point, the wood compressed and splintered around it, the post warped from being knocked loose. He put his hand on it and felt how solid the rest of it remained.

He walked back.

The range was real. A post at forty meters was not the problem. The problem was a man standing behind a timber palisade, and the iron ball not reaching him with enough force to matter.

"Next," Batu said.

Wei placed a ceramic fire bomb at the base of the near post, the ceramic against the timber. He laid the ignition cord and stepped back.

Batu stayed where he was, further back than Wei expected.

The detonation was different from the tube. A flat crack that preceded the light, the sound reaching Batu’s ears before his eyes had finished processing the flash. Orange-white, contained within a second, then dark smoke rising fast in the cold air and spreading at its top.

Batu watched the frame.

The near post had a split running half a meter along the timber, the fiber separating at the impact point. The horizontal beam had dropped two finger-widths on that side, the joint giving.

The far post was untouched. A palm-sized fire was burning on the near post’s surface from compound that had splashed there. It burned without spreading and died in under a minute.

The smoke rose and thinned.

Wei was watching Batu’s face.

"It isn’t enough," Wei said. "I know it isn’t."

"The force goes in all directions," Batu said.

"Yes. More than half goes sideways and backward. To deal real damage on a fortification you need all of it going one direction."

Batu walked to the target. He stood in front of the near post and looked at the split in the timber, the visible line where the force had entered and the wood had given. Then he looked at the far post.

Untouched, because the force had scattered before reaching it.

He came back.

"I want to show you something," he said.

He sat on the supply crate and took out the felt pad. He picked up the stylus.

He drew a tube. A primitive cannon, thick-walled iron, closed at one end with a touch-hole, open at the other, an iron ball seated inside ahead of the propellant charge. The walls were not uniform. The closed end was four fingers of iron. The sides tapered toward the open end, which was one finger thick.

"When the propellant fires, the force drives forward and escapes through the front."

He drew the second sketch below the first. The cannon placed at a gate post’s base, the front against the timber, the closed end toward the engineer placing it. What the weld looked like after.

Wei had moved closer without noticing it.

Batu drew a third thing. A nozzle mounted on a wooden frame, able to turn, fed from a reservoir of tar compound, aimed at a timber wall from outside arrow range. The fire lance was the ancestor of this concept. It provided incendiary fire at range against a wall, rather than a single detonation against a single point.

"Two problems," Batu said. "Breaking a gate. And burning a wall from a distance."

Wei took the felt and held it in a way to read both sketches. He looked at the nozzle projector first. He could build it. The compound was already what that device needed and the feed mechanism was solvable.

The cannon was a different problem. The principle was sound. He had seen force find the line of least resistance before. The foundry work was the question. The closed end had to hold the full charge pressure without breaking, and the weld shown in Batu’s sketch required precision in the casting that was not always achievable.

He looked at the closed end’s proportions in the first sketch. His lips moved slightly and stopped.

"What does the closed end need to be to hold the full charge without cracking?"

Wei was still looking at the felt.