“What are you on about?” Earnshaw snapped. “Are you saying the two men who died in this hallway killed themselves? Were they bombers? Did their bomb go off prematurely?”

“What bomb? Where are your eyes, man? There was no bomb. At least not the type of which you are thinking. If this were a black powder explosion, there would be remnants of the kegs that held the powder. There would be at the very least a hole in the floor and in the ceiling as well.” Rhynwick pointed down and then up before poking once again through the mangled cart and twisted shovel blades.

“If there was no bomb, what caused the blast?” Daniel asked.

“Dust.”

Daniel looked at Ed and then the engineer. “Dust?”

“Yes. The conditions were perfect.” Striding down the hallway, then spinning on his heel and returning, the young man’s eyes measured the space. “Didn’t you know flour dust was more combustible than black powder?”

As a point of fact, Daniel hadn’t.

Rhynwick’s shoulders slumped at their blank expressions. He rubbed his forehead, leaving a black smudge behind. When he looked up, his face had fallen into patient lines.

“Flour is a starch. Which means it burns easily. Any starch, when in powder form, exposes far more surface area to the oxygen in the air than it does when piled or sacked or stored in a barrel.”

“Oxy-gen? What’s that?” Earnshaw asked, suspicious.

“It’s a gas in the air you breathe. Discovered in 1774 by Joseph Priestley. Necessary to support your life, and also, in a pure form, combustible.”

Daniel remembered why he’d studied art and history at university and not science.

“If you do not have oxygen, a fire cannot burn. It’s why when you put a jar over a candle, the flame goes out.” Rhynwick spread his hands, as if asking them to come along this simple journey of logic with him.

“I’ve heard of mills exploding before, but I thought machinery caused it. Something catching fire in the works,” Ed said.

“Though machinery can catch fire, explosions in mills are almost always caused by flour dust in the air igniting. It only takes a pinch of flour per cubic foot of air to combust. A particle of flour burns instantly, igniting the ones around it, flashing quickly. That’s why this fire did not burn long. There cannot have been too much flour in the air, because the blast was fairly contained. It went outward from here, fast and hard enough to kill the two men who caused it, though they wouldn’t have known it. The men were shoveling, which stirred flour into the air. A single spark from a shovel hitting the stone wall would have been enough to set things in motion.”