The eastern district of Sarai was different from the market district in smell before anything visible. The coal smoke first, and then under it the particular biting mineral sharpness of sulfur that had been burning and cooling and burning again.

The workshops appeared past the last residential buildings on the eastern street, one in a particular, a long structure with a high vent from which a thin pale thread of smoke rose and spread flat at its top was Batu’s destination.

A testing ground stretched east from the workshop’s rear door across open flat ground, a timber target set at distance.

Wei was at the compound storage racks when Batu came through the entrance, working a sealed ceramic container from the lower position with both hands, and he did not hurry the task because of the arrival.

He set the container on the rack’s second shelf and turned. The look he gave Batu was the one he gave every visitor to this space, in the form of a brief accounting of why they were here and how long it would take his time.

"You’re back," he said.

"Evidently." Batu confirmed.

Wei pointed at the timber rack against the far wall without speaking.

The new prototype cannon was there, whole.

The thick closed end sat solidly on the rack’s lower beam, the long iron body tapering forward toward the open aperture, and nowhere along its length was there a crack. The middle section was intact and even, no fault line running through it, no variation in the surface that would tell a watching eye where the iron had given way because the iron had not given way at all.

A man Batu did not recognize was working near the furnace, making adjustments to the base stones with attention. He had the compact build of a worker who had spent years lifting heavy objects, and his coat was different in cut from anything Wei’s other men wore.

"The Kashgar man?" Batu said.

Wei’s expression did not change exactly, but something in it confirmed it. "His knowledge of the design between the closed end and the taper produced a modification to the cannon. The new prototype was stable because of that modification."

He said it the way someone named a fact about a past weather event, accurately, completely and with no intention of taking about it.

Then he looked at the cannon.

"We should go outside."

The testing ground in summer was dry and flat, the grass short and pale from the heat.

The cannon sat on its carry frame, a heavy timber structure with iron wheel fittings that took four men to move, positioned at the near edge of the open ground with the aperture facing east toward the target frame at a hundred and fifty meters.

Two of the Rus metalworkers stood at the cannon’s side, and the Kashgar man had followed them out and now stood a short distance from the group, watching.

Wei loaded the cannon with the patience.

The propellant charge went in first, tamped firm with the long rod. Then the iron ball, seated above it.

Then Wei pressed his palm flat against the middle section of the body and held it there for three seconds, reading the iron the way Ahmad read stone in a streambed.

"Back away," he said.

The metalworkers stepped away from the cannon’s line.

Batu stood where Wei had indicated, far enough that the shockwave would not find him, close enough to see everything.

Wei touched the ignition cord.

There was a large noise.

A single deep crack that the open eastern ground took and carried east in a rolling wave, and for one second the air had everything the explosion had created at once. The smoke came from both the touch-hole and the aperture in two directions simultaneously, as if the cannon was exhaling with its whole length at once.

The iron ball crossed the distance in less time than the eye could fully track, a streak through the summer air, and the target frame at a hundred and fifty meters went sideways off its footing with a single hard crash that echoed back across the open ground a beat after the cannon’s noise had already passed it.

One of the horses tied at the workshop wall broke its tether and ran east before catching itself at the fence line, ears back, not sure which direction the danger had come from.

Wei was already walking toward Batu. He had not looked at the target.

"Thoughts?" Batu said.

"It works." Wei snorted with pride.

He glanced once toward the fallen target frame. "At a hundred and fifty meters, against that kind of timber construction, it snaps it clean At a siege you are likely further way. The force at two hundred and fifty meters is considerably weaker but enough to break down a gate in multiple shots."

"The reloading time?"

"Three minutes, four if the crew is unfamiliar about the propellant tamping."

That would be conservatively five shots every twenty minutes. Batu considered how much damage could be caused to a city gate in the period of time, and then looked at the fallen target at the end of the range for a moment.

"The carry frame is too heavy on uneven ground," he said.

Wei looked at him steadily.

"I’m aware."

"It needs to be fixed before next spring."

Batu stared at Wei, "I want a sled variant simple enough that my men can disassemble and reassemble the mount at a river crossing without losing pieces in the mud."

Wei was quiet for a moment.

He had not designed for that. His thinking had been about the cannon itself, the casting, the propellant, the range. The question of moving it across flooded spring ground and still having it work at a Rus city gate was a problem he only then realized of its existence.

"I’ll need the Kashgar man for the sled fitting," he said at last.

It was not a concession, just an accurate accounting of what the task required.

"Khulgen will set terms with him," Batu nodded.

They moved back to the workshop’s covered side and Wei went through the rest of what he needed in a methodical note.

One compound production facility first, a dedicated building on the eastern district for the saltpeter refinement and an open-air section for the charcoal side of it.

The saltpeter was a limited material. With a workshop in Sarai at full capacity, he could put the campaign’s compound requirements in storage before winter and the army would not have to ratio containers in the field again the way it happened at Bulgar.

The fire lance projectors came next.

He had built two, and he could build three more while the Rus metalworkers handled the frame and fitting work. Three more projectors meant five total in the field, enough to work pressure a siege from multiple directions.

And two cannons for the next spring.

The new prototype was done, sitting on its rack. Another one of the same prototype could be ready before winter if the foundry work started within the month.

Batu listened to all of it, and when Wei was done, he said, "One thing about the compound, grind it finer than what you’ve been making now. It will burn harder and cleaner, which gives the shot more force forward instead of wasting it out the sides. What you have works, but it can still be stronger."

Wei looked at him.

He did not ask where that knowledge came from, because the question would require an answer he was not sure he was allowed to hear.

"Show me what you mean by finer," he simply said.

"I’ll write it down." Batu said.

Wei frowned, then averted his gaze to the cannon on its carry frame, at the target lying sideways at the end of the range, at the sled problem waiting for the Kashgar man.

"Everything will be done before spring."

It was both a confirmation and a statement of what he was about to spend the rest of the season doing.

"Very well." Batu accepted the commitment.

He walked back through the workshop and out toward the district, leaving the man already moving toward the compound storage with the look he had just been handed a set of problems and was reservedly excited for it.