Cade gaped, leaning precariously to get a closer look. He swept a fingernail across the clay brick of the rim, only to find a crystalline substance beneath his fingernail.

“Codex, what other way was potassium nitrate harvested, historically?”

“Saltpeter men were known throughout Britain and France in the eighteenth century. By royal decree, these men could enter any premises to harvest saltpeter that had formed on the walls and soil—such was the demand for gunpowder at the time. Saltpeter efflorescence typically formed on the walls of sewers and stables, where the combination of urine and excrement allowed the crystals to form on the surrounding surfaces over time.”

Cade felt the hairs on the back of his neck stand up. This was their solution. Abaddon had given the Romans a year because that was how long they would have taken to make the saltpeter. But over the years they had been abandoned here, they had been producing it in a more … organic way.

That was why the new contenders had only been given a week.

They had the sulfur. They had the wood to turn into charcoal. Now … they had the saltpeter. The only question was, would it be enough?

“Codex, what are the basic ratios of charcoal, sulfur, and potassium nitrate to make gunpowder?”

“Seventy-five percent potassium nitrate, fifteen percent charcoal, ten percent sulfur,” came the reply.

Cade grunted in frustration. The primary ingredient was also the substance they lacked the most.

But now another idea came to him. This was not the only place to have sewers. If they were lucky, Jomsborg would have its own sewage system, long dried and empty, but potentially loaded with crystals too.

The only thing was, they could not rely on that. They would have to harvest what they could here.

“Cade,” a voice called.

Quintus stumbled down the path from the fort, rubbing his bleary eyes.

“Quintus, come see,” Cade called.

The young legionary hurried over, and Cade wrapped him in a hug.

“It’s the solution to all our problems!” Cade said.

Quintus extricated himself and peered into the hole, before wrinkling his nose and turning away.

“Very … nice,” he said politely, then paused. “I am confused.”

Cade laughed, realizing that Quintus had been fast asleep when they had seen the cesspit on their way to the Sulphur Queen, and they had detoured around it on their way back.

“Do we still have some of that rope on the sleds?” Cade asked.

Now Quintus seemed utterly baffled, but he followed his friend as Cade hurried back to where they had left the sleds and began to knot a rope around himself, tucking the loop beneath his armpit.

“Think you can hold my weight?” Cade asked.

“Are you okay, Cade?” Quintus asked. “Maybe you need to sleep.”

“No time,” Cade said breathlessly, hurrying back to the pit. “I want to prove this can work by morning.”

He approached the rim of the cesspit, staring down at the horrible, gloopy mass at the bottom. Cade really, really hoped the rope wouldn’t break.