This was because the earliest gunpowders were not very explosive, because the chemistry of the powder was not understood, and because the art hadn't been developed yet. Gunpowder exploded when charcoal and sulfur burned extremely rapidly, the combustion enabled by a rich source of oxygen - namely nitrate salts, later called saltpeter. The most common source of nitrates was bat droppings from caves. In the early years, this guano was not refined at all, simply added to the mixture.
But the great discovery of the fourteenth century was that gunpowder exploded better when it was ground extremely fine. This process was called "incorporation," and if properly done, it yielded gunpowder with the consistency of talcum powder. What happened during the endless hours of grinding was that small particles of saltpeter and sulfur were forced into microscopic pores in the charcoal. That was why certain woods, like willow, were preferred; their charcoal was more porous.
Marek said, "I don't see a sieve. Are you going to corn it?"
"No." Johnston smiled. "Corning's not discovered yet, remember?"
Corning was the process of adding water to the gunpowder mixture, making a paste that was then dried. Corned powder was much more powerful than dry-mixed powder. Chemically, what happened was that the water partially dissolved the saltpeter, allowing it to coat the inside of the charcoal micropores, and in the process, it carried the insoluble sulfur particles inside, too. The resulting powder was not only more powerful but also more stable and long-lasting. But Johnston was right; corning was only discovered around 1400 - roughly forty years from now.
"Should I take over?" Marek said. Incorporating was a lengthy process; sometimes the grinding went on for six or eight hours.
"No. I'm finished now." The Professor got to his feet, then said to Sir Guy, "Tell my Lord Oliver that we are ready for his demonstration."
"Of Greek Fire?"
"Not precisely," Johnston said.
In the late afternoon sun, Lord Oliver paced impatiently along the massive wall of the outer perimeter. The battlement was more than fifteen feet wide here, dwarfing the row of cannon nearby. Sir Guy was with him, as well as a sullen Robert de Kere; they all looked up expectantly when they saw the Professor. "Well? Are you at last prepared, Magister?"
"My Lord, I am," the Professor said, walking with two of his bowls, one under each arm. Marek carried a third bowl, in which the fine gray powder had been mixed with a thick oil that smelled strongly of resin. Johnston had told him not to touch this mixture on any account, and he needed no reminding. It was a disagreeable, reeking goo. He also carried a bowl of sand.
"Greek Fire? Is it Greek Fire?"
"No, my Lord. Better. The fire of Athenaios of Naukratis, which is called `automatic fire.' "
"Is that so?" Lord Oliver said. His eyes narrowed. "Show me."
Beyond the cannon was the broad eastern plain, where the trebuchets were being assembled in a line. They were just out of shot range, two hundred yards away. Johnston set his bowls on the ground between the first two cannon. The first cannon he loaded with a sack from the armory. He then placed a thick metal arrow with metal vanes into the cannon. "This is your powder, and your arrow."
Turning to the second cannon, he carefully poured his finely ground gunpowder into a sack, which he stuffed into the cannon mouth. Then he said, "Andr, the sand, please." Marek came forward and set the basin of sand at the Professor's feet.
"What is that sand for?" Oliver asked.
"A precaution, my Lord, against error." Johnston picked up a second metal arrow, handling it gingerly, holding it only at each end and gently inserting it into the cannon. The tip of the arrow was grooved, the grooves filled with thick brown acrid paste.
"This is my powder, and my arrow."
The gunner handed the Professor a thin stick of wood, glowing red at one end. Johnston touched the first cannon.
There was a modest explosion: a puff of black smoke, and the arrow flew onto the field, landing a hundred yards short of the nearest trebuchet.
"Now my powder, and my arrow."
The Professor touched the second cannon.
There was a loud explosion and a blast of dense smoke. The arrow landed alongside a trebuchet, missing it by ten feet. It lay in the grass.
Oliver snorted. "Is that all? You will forgive me if I have-"
Just then, the arrow burst into a circle of fire, spitting blobs of flame in all directions. The trebuchet immediately caught fire, and men on the field ran forward, carrying the horses' water bags to put it out.
"I see," Lord Oliver said.
But water seemed to spread the fire, not quench it. With each new dousing, the flames leapt higher. The men stepped back, confused. In the end, they watched helplessly as the trebuchet burned before them. In a few moments, it was a mass of charred, smoking timbers.
"By God, Edward and Saint George," Oliver said.
Johnston gave a small bow, smiled.
"You have twice the range and an arrow that alights itself - how?"
"The powder is ground fine and so explodes more fiercely. The arrows are filled with oil, sulfur and quicklime, mixed with tow. Touching any water makes them catch fire - here it's the dampness of the grass. That is why I have a basin of sand, should the slightest bit of the mixture be upon my fingers and start to burn from the moisture of my hands. It is a most delicate weapon, my Lord, and delicate to handle."
He turned to the third basin, near Marek.
"Now, my Lord," Johnston said, picking up a wooden stick, "I pray you observe what follows." He dipped the stick into the third bowl, coating the tip with the oily, foul-smelling mixture. He held the stick in the air. "As you see, there is no change. And there shall be no change for hours, or days, until
" With the theatricality of a magician, he splashed the stick with a small cup of water.
The stick made a hissing sound, began to smoke, and then burst into flames as the Professor held it. The flame was a hot-orange color.
"Ah," Oliver said, sighing with pleasure. "I must have a quantity of this. How many men do you require to grind and make your substance?"
"My Lord, twenty will do. Fifty is better."
"You shall have fifty, or more as you will," Oliver said, rubbing his hands. "How quickly can you make it?"
"The preparation is not lengthy, my Lord," Johnston said, "but it cannot be done in haste, for it is dangerous work. And once made, the substance is a hazard within your castle, for Arnaut is certain to attack you with flaming devices."
Oliver snorted. "I care nothing for that, Magister. Make it now, and I shall put it to use this very night."
Back in the arsenal, Marek watched as Johnston arranged the soldiers in rows of ten, with a grinding bowl in front of each man. Johnston walked down the rows, pausing now and again to give instructions. The soldiers were grumbling about what they called "kitchen work," but Johnston told them that these were, in his words, the herbs of war.
It was several minutes later when the Professor came over to sit in the corner with him. Watching the soldiers work, Marek said, "Did Doniger give you that speech, about how we can't change history?"
"Yes. Why?"
"It seems like we're giving Oliver a lot of help to defend his castle against Arnaut. Those arrows are going to force Arnaut to push his siege engines back - too far back to be effective. No siege engines, no assault on the fortress. And Arnaut won't play a waiting game. His men want quick scores - all the free companies do. If they can't take a castle right away, they move on."
"Yes, that's true ."
"But according to history, this castle falls to Arnaut."
"Yes," Johnston said. "But not because of a siege. Because a traitor lets Arnaut's men in."
"I've been thinking about that, too," Marek said. "It doesn't make sense. There are too many gates in this castle to open. How could a traitor possibly do it? I don't think he could."