After lunch, Bartlett and I followed the stone-boat procession for half a furlong to another clearing with two long rows of brick hemispheres ahead of us, each a little over twenty feet wide and about ten feet high with maybe a three feet long and six feet tall entryway with double iron doors. A quick count showed eighteen in each row, some with thin streamers of white or bluish smoke coming from a small opening in their roof.

As we approached the nearest of the hemispheres, a tall black unicorn stallion with a seemingly perpetual frown walked over. Bartlett said, “Spuds, this is Mazz. Well, he’s really Mazzini, but everyone calls him Mazz or Mazzy. He’s the head of our charcoal production team and one of the most innovative burners around. I’ll leave you with him until dinnertime.” Bartlett trotted back in the direction of the forestry teams.

Mazz nodded curtly at me and we bumped hooves. He said in a rumbling bass voice, “Bounder said you might be talking to us charcoal burners. What do you already know about charcoal burning?”

“Very little,” I said. “Just that ponies use kilns or woodpiles to heat wood for days, and somehow it turns into charcoal.”

Mazz snorted. “Well, it really is that simple, if you had to give a ten-second explanation. For the one-minute explanation, heating wood without oxygen results in a process called pyrolysis, not the combustive burning you see in a fireplace. Pyrolysis of wood gets rid of some volatile stuff like water, wood alcohol, wood acid, and wood tars. That leaves charred carbon pieces that burn hotter and cleaner than dry wood while being much less dense. Those burning properties make it essential for forging and smithing tasks, as well as being a useful component in steelmaking. So the trick is, we need a way to heat dry wood without exposing it to enough oxygen to cause combustion instead of pyrolysis.”

He gestured at the rows of brick hemispheres. “These are charcoal batteries. A battery is a series of kilns for burning wood into charcoal, usually at least a dozen of them because of burning and cooling times. A team of four ponies should be able to handle a single battery of fourteen to eighteen kilns; we prefer to set up three batteries in one location and assign a dozen or so ponies – creatures,” he corrected himself, “to work it. One master, with two to four journeymares to help supervise another seven to ten general forestry jacks and maybe a couple of apprentices. We’ve already got two batteries set up here; we’ll be building a third once the next shipment of materials arrives by train.”

“Let me walk you through the process for a single battery. We’ll need to start in the middle of it, literally, because of where we are in the cycle. Let’s head over here, where they’re most of the way through with loading. We fit about thirty tons of dry wood in there, and that obviously takes some time to get inside.” Mazz led me over to the sixth kiln, which was being loaded with logs from a pair of stone-boats.

Three ponies were carrying logs in through the kiln’s short arched doorway tunnel, with a pair of outward-swinging metal doors wide open. I could hear two more ponies working inside, and spotted the glow of a unicorn’s field through the doorway. As one of the stone-boats was emptied, another heavily loaded one arrived behind a pair of muscular Earth pony mares. The mares unclipped themselves from the harness of their stone-boat, proceeded to the empty one to clip themselves in, and began dragging it back to the wood-drying sheds that I had seen not long ago.

“We use some long, smaller sticks to form a base – usually some of the branches that we get from the arborist teams – then fill the kiln up almost all the way with logs,” said Mazz. “That way, the logs don’t directly touch the ground. We just leave a single empty column in the middle, right below the ‘eye’ – the hole in the top of the kiln. That column’s about a foot and a quarter wide, same width as the eye, with a few vertical wooden poles to provide some structure.

“Immediately around that empty column, leaning against the poles, go a couple layers of bucked pieces of firewood. Starting maybe two feet out from the center, just outside the firewood against the poles, go big, wide logs, fat trunks that were unsplit. The hottest temperatures in the kiln are near the center. If we put the big trunks on the outer part near the kiln wall, they won’t get hot enough to pyrolize all the way through. By the time we get five feet from the center, we’ve got to use small trunks and split logs. Seven feet out, it’s all split logs.

I remarked, “I wouldn’t have expected there to be so much thought going into how to lay out wood inside the kiln! It’s impressive how much detail you put into this!”

Mazz wryly rumbled, “All it takes is one creature getting a good idea every decade and having it spread, and you’re going to make quite a few advancements over the centuries. Another advancement is, there’s ten small holes, each about hoof-sized, spaced around the bottom of the kiln with little shallow trenches leading to the empty column. Those provide a small amount of air intake, while the eye is for exiting smoke. It’s an easy, durable design. Mostly, it’s just bricks with a mud mortar, and those two steel doors. We don’t even need to use steel doors; old-time ponies could mud-brick up the entrance and crack it open each use if they didn’t have sufficient metal.

“Looks like they’re maybe five minutes from finishing loading. After that, we’ll close the doors, pour a pint of cooking oil onto each of a dozen fascines, then have a pegasus drop the fascines in through the eye, stacking them atop each other with maybe a foot of clearance to the eye. Once that’s done, we get a different pegasus to go up there with a lit stick and toss it into the eye, igniting the fascines.

“Charcoal burning was invented by earth ponies. Back when teams were entirely earth ponies, they’d just have to toss a lit stick into the eye, then wait. It’d be too dangerous to try to climb up onto the kiln while it’s firing. But with pegasi, they can fly twenty, thirty feet above the kiln and look down into the eye to see how the fascines are burning. If the burning column looks like it’s running low, they can just toss two or three more fascines in through the eye.

“Thing is, we want pyrolysis, not combustion. So we want to heat things up, but have as few of the logs combust as possible since we want to turn those into giant lumps of charcoal. Sure, we know that some logs will be set alight by the fascines, and some of that has to happen for sufficient heating. That’s what the bucked firewood just around the center is for. But by tossing in those extra fascines every so often, we’re burning fewer logs by keeping that central column literally burning hot with sticks, and making sure that the large trunk pieces get turned into charcoal.

“Once the fire in the central column’s been going a few hours and some of the other firewood around it has begun to combust, there’s been enough heat given out that the kiln walls really start warming. It takes some time for that double layer of bricks to heat up, but once it does, it’ll stay hot for a damn long time untouched.

“We’ve even been experimenting with using slightly different wood arrangements inside the kiln, see if we can find a better way to get that initial heating up going,” Mazz concluded.

I watched as the loaders finished stacking wood inside the kiln, then doing a final inspection. “What do you mean by a different wood arrangement?”

Mazz gestured at one of the small holes at the base of the kiln. “Well, we want to get the whole pile of wood up to temp as fast as we can, with as little combustion as possible. So I decided to see what happens if we didn’t just burn from the central column outward.

“Most ponies are happy to just keep doing what works. Pah! Once they’re journeymares or close to it, I don’t want anyone on my crew to mindlessly follow a procedure just because! I want them to improve the process, not just use the process! Keeping that central column fueled with fascines is something that we started trying once I got my first pegasus worker, and it’s meant that we can use a thinner layer of firewood around the column and put more big trunks in to pyrolize.

“My team leaders and I talked it over with Lemmy – that is, Lemon Hearts – a couple years ago, and decided to see what happens if we intentionally set some fascines near the air holes so they could combust, and more rapidly heat up part of the kiln from the base. Get it so we can load more big wood pieces and require less bucking and sawing.”

The loaders finished their inspection and trotted out as I asked, “Any results so far? Two years seems like quite a bit of time.”

“It’s definitely longer than what most people might expect! But we need to carry out, at minimum, a hundred firings to get sufficient data. It’s too easy for a single firing, or even a few firings, to be affected by something else, especially as we’ve worked in three different places the past couple years. We’ve got the batteries here testing our last set of configurations at this location, and we’re finally about to resolve this. The frontrunner so far is to have a half-dozen fascines each at four of the ten air holes; and it’s given a shade over three and a half percent more charcoal yield than only having an initial burn in the central column.”

As a pegasus apprentice colt began carrying up and dropping fascines into the kiln’s eye, Mazz grumbled, “And don’t think three and a half is piddling! Thirty tons of wood go in, standard yield was a bit under nine tons of lump charcoal and now we’ll get about nine-and-a-quarter. Profit margins can be thin in this business, so averaging an extra ten or eleven sacks of lump charcoal with each burn helps give us a safety net.”

We watched as the colt moved away and a pegasus filly flew over to drop a torch into the kiln’s eye. As she backed off, we could see an occasional lick of flame come up out of the eye. Within a minute, I could hear the crackle of igniting wood. Within a few more, I saw a thin stream of white smoke flow up from the eye.

“Just a little white smoke so far,” said Mazz. “If you kept watching, you’d see that smoke stream get both wider and denser. By denser, I mean you’d barely be able to see through it to the other side. White smoke means that we’re getting combustion, and the temperature in the kiln’s slowly going up. It gets hot enough, any remaining moisture from the dried logs gets steamed away. Some other components of the wood, like the wood acids and vinegars and alcohols, also end up getting vaporized. Ideally, the wood alcohol and vinegars also ignite to raise the temperature even more, rather than going up with the smoke.

“Let’s move a couple kilns further up. Now, this one here’s got some changing color in the smoke. Look how it’s not all white anymore, it’s like a blue and white swirl. And we go one more up and it’s all blue smoke. Blue smoke is great! It means we’re getting pyrolysis with very little combustion and the wood is turning into charcoal.”

Mazz led me up to the final kiln in the battery. “Once the blue smoke stops, pyrolysis is finished and the wood inside has reached maximum carbonization. This one’s been burning for six days and the smoke just stopped an hour ago. Now we’ve got to wait for the kiln and its contents to cool down enough for us to safely remove the charcoal. During the early days of Earth pony operated kilns, they had to wait days and days. Now though...” he gestured at two pegasi approaching with small rainclouds.

“Rain cooling?” I asked.

“A very light rain, to start. We first close up the kiln’s eye and all other airholes, and then we have the clouds produce barely a misty drizzle. We want a slow and steady lowering of the temperature, not just dumping a flood onto the kiln and damaging bricks via thermal shock. We’re going to walk back down to the fourth kiln – the one right before the one you saw loaded and fired – next. You’ll notice larger, steadier rains coming down on the first three kilns as we cool them off.”

As we walked back down the battery, I did in fact notice the increased amounts of rain over the first three kilns. The third in particular was a steady, ongoing shower, and as I watched, a pegasus came over with more raincloud material to emplace. But the fourth was dry, and some of the crew of ponies I had seen loading logs into the sixth kiln were now wearing masks over their muzzles and loading filled burlap bags onto stone-boats. Mazz went over to a small supply shed and came out with a mask for me.

“Put this over your muzzle. We’re going to take a look at what they’re doing inside the kiln.”

“I’ve got a question. If you’re using rain cooling, the outside of the kiln is going to be damp with water. Won’t it take some of the heat to steam that off and get the bricks to temperature?” I asked.

“You’re a sharp one!” Mazz barked. “It does indeed! But that’s another experiment that was done a long time ago, nearly a century even. And repeated every so often in different climes. The general result is that when we’re in a timber-rich area, the extra loads we can get after saving time in rain cooling easily outweigh the extra energy and time to redry the kiln, especially with pegasi-set breezes to help the drying.”

As we went in, I noticed that the air inside the kiln was still noticeably warmer than the chill outside. Two earth ponies with snow shovels attached to their fetlock tool-bands were scooping up charcoal and tossing it onto a diagonally-mounted mesh frame, where the large pieces would slide down into a sack held up by the telekinetic field of a unicorn. When the sack was full, the unicorn placed it aside for others to carry it out to the waiting stone-boats.

As the charcoal burners continued to empty the fourth kiln of the split logs turned lump charcoal, I noticed that a small amount of black detritus remained on the ground or fell through the mesh. Still more of it shook off the large pieces of lump charcoal when the sacks were placed onto the ground. I expected that a similar process would occur when they loaded the sacks onto the stone-boats or unloaded the charcoal at the storage sheds near the steam injectors.

“What do you do with those little charcoal pieces that fall or crumble off when you’re moving the big lumps?” I asked Mazz.

“The charcoal fragments? We call those fines, and usually about ten percent of the charcoal produced in the brick kilns is fines. Say nine-tenths of a ton, each burn. Maybe another five percent become fines in the process of transport. And nearly everything from the metal retorts – we’ll look at those later – comes out as fines, because we use those to process bark and twigs. So call it a bit over twenty percent inefficiency of what we get from burning,” Mazz groused.

“Charcoal fines are the annoyance of everypony who works producing or transporting charcoal, and have been for hundreds of years. Twenty percent lost is a big hit to the profit margins! Since fines range in size anywhere from a shooter marble down to tiny pieces that are just about dust, they’re too small to burn in any conventional fashion. Try to light a pile of fines and you’ll just end up with heated air currents sending charcoal and carbon dust into the air and all over everything. Few ponies are willing to buy fines! We do manage to get use out of them with a little more work, even if it’s in a less efficient and versatile manner.

“The first part of the job is unicorn work, usually. The fines sweeper doesn’t need to have good or even average field strength, but their field dexterity will need to be decent. Once the lump charcoal is cleared out of a kiln or drying shed, the sweeper goes in and telekinetically sweeps up all the fines and puts them into barrels. The goal is to recover about 80% of the fines in the kiln. They’ll do the same with the fines that accumulate on the bottom of a wagon or cart. A unicorn using their field is a lot faster than an earth pony or pegasus using a broom and dustpan, and we learned the hard way not to let pegasi try to use wind to funnel it all up!

“As for what we do with fines? Follow me,” Mazz said as he led us out of the kiln.

Author's Note

Click here for the notes to Chapter 5.