Fixing a hole with Red Clay: repairing charcoal kilns in Niiharu

Last winter, Charcoal Making Club of Niiharu Lovers struggled to bake bamboo charcoal. We lit the fire at 6:00 for the charcoal kilns, and they could not be hot enough at 20:00. Later, when we opened the kilns, we found out the bottom of the metal barrels had huge holes which made it impossible for the kilns to cook bamboos efficiently. This summer, Charcoal Making Club did a major overhaul for kilns by replacing the broken metal barrels with high spec oil drums provided with the courtesy of one of the world majors. There are many designs for modern charcoal kilns in Japan. (Photos can be seen here.) Niiharu’s kilns are made of a drum of 57cm of diameter and 90cm tall, buried in red clay. The bottom of a drum has a hole of 11cm diameter. The bottom wall of the drum also has 4 holes each of which is connected with a pipe of 5cm diameter. The top of a drum is a lid with a hole for a Ø3cm pipe. The drum is then put on a U shaped concrete gutter as its Ø11cm hole is over the trough. The drum and gutter are held by red-clay loaded in a space surrounded by four 150cm*120cm metal walls. The clay holds the drum and 4 pipes by piling up the square tightly to the rim of the drum. When we see the kiln from a side, it looks like a metal box situated on a clay mount with 4 pipes sticking out from the top.

A sectional view of Niiharu’s charcoal kiln

It’s a view from the above of the kiln.

The Lovers took out the metal barrels
from the structure.

The new kilns.
I didn’t know there were several kinds of metal barrels.
My seniors said the new barrels had thicker wall
so that it can keep the heat well.

The science of charcoal making is, when we add heat on logs slowly but continuously in an almost sealed-up space, they start to heat up themselves and release volatile substances their cells hold. If we keep burning them by firewood, they will eventually become ashes which are the materials, such as metals, that cannot evaporate with the energy created by camp fires. In contrast, if we stop adding external energy, but create tightly sealed environment for the heated logs, they continue to burn themselves slowly with very limited supply of oxygen and discharge unstable oils from their body as much as possible. Though, without new fire supplied, the closed structure eventually cools down and the self-combustion of logs inside also stops. The logs burned in this way end up as a chunk of carbon. They are now charcoals which are incompletely combusted logs in a shut room. When a charcoal is very close to pure carbon, they do not contain volatile element much. Unlike firewood, they can burn for a long time without creating flame. In other words, a charcoal kiln intends to let the logs free the unstable elements completely during combustion. Niiharu’s kilns are designed to do that just by burning firewood in the concrete gutter. As this is for weekend forest volunteers, it’s not for cooking wood charcoal for days, but for one day making with bamboos. Even though, the inside of the kilns must reach around 700ºC that made the previous metal drums porous, and the concrete slab that sealed the U-shaped gutter with the metal drum crumbled when we removed the red clay to take out the metal drums. The power of fire and heat …

The difference between completely burned woods and charcoal.
One of my senior Lovers showed me how different they were.
Completely burned woods looked like a charcoal,
but they were very fragile and easily became ashes
just by rubbing them between the fingers.
In contrast, our bamboo charcoal maintained its form
and emitted metallic sound when we gently tapped them.

The dug-out metal barrel, upside down.
It was surely worn out.

The side wall of the barrel.
No wonder the kilns could not heat itself up well.

I now know the pipes at the bottom wall of a drum were not welded but inserted to the hole and stabilized by clay. The pipes are for volatile substance inside the bamboo free to evaporate. The red clay is typical soil in Kanto region especially where the volcanic ashes from Mt Fuji fell. I asked if we can just burrow up the land wherever allowed in the Niiharu Forest to replenish the stock. “Huh? Of course not. Niiharu is rich forest with lots of fallen leaves and the other organic materials. When we use the top soil, those organic materials will be burned during the charcoal cooking, and the mud cannot seal the kilns tightly.” So, we’ve been to a place in the forest where we can harvest the clay beneath the top soil. We have to DIG deeply. I also noticed the difference in red clay between “before” and “after” of charcoal cooking. When we dug out the drums from the structure, the “clay” was hard by tapping, but could crumble easily into very fine grains with a strike of spade. We needed face masks to operate to avoid coughing. In contrast, freshly procured red clay is dense. By adding water, we can make mud balls easily. I understand why a desert is desert … The tierra of desert was exposed to direct sun light continuously. It must have turned into the “after” clay of Niiharu’s kilns. I guess the organic richness of the soil is definitely greater in Niiharu than those places in desert in China …

Red clay in Niiharu

We dug a hole to harvest the clay.

You may say there are lots of red clay
in our charcoal making hut.
Actually, we used up almost all of them during the repair.
My seniors said this winter we have to procure more
before baking charcoals in order to seal the kiln.

Hmmm, it may be difficult to see in this photo
the difference between the “fully-baked clay” and newly procured one
... I hope you sense the texture of before-baked clay
in the bucket by comparing the baked clay surrounding it.

The red clay was in full-operation during the renovation. After replacing the crumbling drums and slabs, the mud balls were applied to seal the gaps and holes of the metallic materials, to act as a plaster for bricks and U-shaped gutters to stay together, and to attach tightly the pipes at the bottom wall of a drum. Hmmmmm … playing with mud balls in kindergarten must be something in order to learn the basic technology of human settlement. Hey, moms in the world, don’t stop your kids to play in mud. Certainly their garment could be damaged beyond repair, but that’s an important experience to understand human history! The mud-pasted structure is next stabilized by piling dry red clay within the 4 metal walls. … In the end we worked reticently to fill the once emptied space. Digging the thing out, and digging the thing in. Zen ...

The walls were temporarily removed to work in the kiln.

The worn-out walls were replaced by new metallic panel.
But …

Lovers noticed the newly procured panels have holes.
So, they were sealed by mud balls.

“Hey, I’m a city boy of Yokohama,
and have never done indecent things of
soiling my pants when I was a kindergartener.”
“Well, so you have to do it now.”

Preparing mud balls

Plastering the bricks was also with mud balls.

Before setting the barrel

First, situate the marble …

Then, seal the space with the clay,

Put the marker for the Ø11cm hole,
and seal the remaining gaps with more clay …

And set the metal barrel.
“Watch out for the direction of the 4 holes.
They must be on the diagonal line of the space.”

Next, we inserted the pipes
and adjusted the direction of the kiln in diagonal.
It’s because when we bake the charcoal,
long bamboo chimneys will be attached to the pipes
which go out from the holes of the walls of the hut.
The diagonal direction of the pipes is determined
by the locations of the side holes.

An operating kiln with chimneys should look like this.

At this stage, we made it sure
the mouth of the pipes inside the barrel smooth.
Otherwise, when we load bamboos to be baked,
the ledge created by a pipe impedes packing.
When we cook charcoal,
inside of the barrel should be tightly filled as much as possible
so as to make the circulation of the heat efficient.

Now the pipes were provisionally stabilized with a hemp line.

Then the gaps between the pipes and the barrels were
sealed by mud balls from outside.
All in all, sealing the space inside of the barrel is very important.

Now cover the bottom of the barrel with red clay a bit
and stomp it over to make the closure for sure.

And pile the clay up, up, up …

Done!

Now the new charcoal kilns have pretty red and blue lids in Niiharu. We hope this winter we can finish our baking within 8 hours. Fingers crossed.

If you find a problem in Niiharu Forest, please make a contact with

Office for the Park Greeneries in the North 北部公園緑地事務所

Yokohama Municipal Government Creative Environment Policy Bureau 横浜市環境創造局

Phone: 045-311-2016 (I guess in Japanese only)

FAX: 045-316-8420 (I hope there is somebody who can read English …)

Niiharu Administrative Office / Satoyama Exchange Center 新治管理事務所・里山交流センター

Phone: 045-931-4947

Fax: 045-937-0898

Email: info@niiharu.jp

http://www.niiharu.jp/