Primitive Pottery Wood Firing Updraft Kiln

Every summer I spend two weeks 'off grid' in a cabin by a lake in Jura, France.

Last year I finally realized that beneath the first layer of grey mud we have to tread on to reach deep water, the more compact layer is in fact white clay.

This summer, my grand scheme was to extract this clay and to build a kiln to fire it.

Here is how I did it.

I am a woodworker and designer, you can check out here : @atelier.arcadie what I do the rest of the year !

Supplies

- BBQ Steel Grill x2

- Flat stone x1

- Small rocks x a few

- Clay x a lot

- Wind x a lot

- Water x a bucket

- Forest sticks x quite a few

Step 1: Foundations

Location : It all starts with a good location, the windier, the better since wind will help you reach higher temperature.

So take your time exploring your surroundings and choosing a good spot as you will not be able to move the kiln !

I did not build real foundations, I just chose an area with a compact ground and oriented the door of the kiln to let the wind blow in.

Step 1 : I spread a rather thick layer of clay on the ground using the grill as a template and I let that first layer sit for the night.

Clay : As I mentionned at the begninng, I started this project when I realized that I had a endless source of usable white clay. I just had to shovel the clay out of the bed of the lake and, it was ready to use for that kings of project, just a few reed roots and tiny shells. I still had to gather a large quantity for this project as it is compressed. As you can see in the picture with the upside down boat that is about 5 meters, I then had to go get some more once and almost double that quantity. In retrospect I should have looked for some kind of straw or gravel to temper my clay and avoid cracks that I will mention later.

Step 2: Structure

Step 2 : I placed small rocks half embedded in the bottom of the kiln to give some strenght to the structure and link the levels to eachother.

BBQ Steel Grill : I bought it for 10 euros at the local departement store., it has a diameter of 37 cm ( a little less than 15 inches). In retrospect I should have taken a thicker grill as the barrels quickly went Red hot and lost their shape and spacing, also they did mark the bottom of the greenware.

Step 3: Building Up

Step 3 : I compressed and shaped by hand another layer of about 8 cm high (3 inches) and let it sit for the night, I used a small stick as a gauge to check the height of the layer at different points. For the thickness of the walls I aimed for 10 cm (8 inches) for a total diameter of 60 cm (about 23 inches),

I then layed another level and started the first fire to dry it faster. The drawback is that the cracks are bigger than when the clay dries overnight without fire.

Step 4: Adding a Stone Lintel

Step 4 : I layed a third level, dried it with a fire, and then placed the flatstone accross the opening as a lintel for the 'door' of the firebox.

I also added two more stones in that layer as consoles to hold the grill.

You need quite a large firebox to reach the temperature your clay needs to fire, so don't be afraid to build it large and high !

My door is 25x25 cm (10x10 inches), it could be bigger.

Flat stone : Just a regular stone from the lake, I had to 'cut' it to dimension using with another rock. The rock became hotter than the outside of the clay, so I had a thermal bridge effect drawing more heat from the inside of the kiln than the walls. Maybe the kiln would be better without the stone, and just a skeleton of wood as a lintel

Step 5: Firing Capacity and Crafting of the Lid

Step 5 : I added two layers over the grill. They are the walls of the actual firing volume capacity since the greenware (uncooked pottery) will sit onto the grill. If you want to fire high pieces add two or more layers.

I also crafted a lid, to close the kiln while firing. It is made of another metal grid with filled with clay, the handles of the grill came in very useful. The lid had to be removable to load and unload the kiln. I could also be flat stones or bricks.

Step 6: Extracting Usable Clay(s)

On my way to the Jura, I found two sources of clay :

- one was on the road, where the road 'cut' some hills and you could see the geological layers. In between some layers you could see red earth powder pourring and accumulating into little mounds. I took a bucket of that, thinking that this was primary clay, which is clay has just been formed by nature, crumbling rock into the smallest of particles, stone dust.

- the other one was near a swamp there was a field with dried up grey/black ground showing large cracks, I took some of the nuggets that were formed by the cracks, this was alluvial clay, meaning particles that were carried along by water and deposited on the side of the river or where rain accumulates.

And then in the lake itslef I had the white clay that just needed some filtering (I guess... lacustrine clay).

So for the red powder I just mixed it with water, sifted it to get rid of small rocks and plant remainings. The idea is to also get rid of the heavier particles that set in the bottom of the bucket and only keep what can float in the water and color it. In the end I filled an old fashioned bed sheet with red water as you can see in the first picture and let the water drip from the bag and the clay dry up inside it.

For the grey nuggets I pounded them to powder and then did the same as for the red, and for the white it was already wet so juste some sifting before the bag.

The grey and red clay had the same texture in the end, very sticky and moldable. The white was less sticky and more prone to crack

I went on with some basic modeling to test the firing :

Small beads in each clay and small beads in mixed white/red, white grey clays

A Small pinch bowl with the red clay

some experimentation with the piston I had to plan for maybe try some lowtech 3D printing !

Step 7: Firing and Celebrating

Unfortunately I only had time to do one firing session before going back to the city, here is how it did unfold

I let my pieces dry overnight and then dried them some more on top of the kiln while I was pre-heating the kiln.

- 9 to10 am : I preheated the kiln with a small to medium fire, turning the pieces regularily to dry them some more.

- 11 am : I put the greenware on the grill

- 11 to 11:45 I gradually added lumber in the kiln until the firebox was full of embers.

- 11:45 am. I 'closed' the firebox with a stone (that could still let Oxygen in. I also added some clay to the lid to close some more the kiln and get a long and slow decrease of the temperature.

- 5:30 pm : I was able to hand pick my pieces from the grll.

2 out of the 3 of the clays I harvested and tested that day did fire, the red and the white fired, and would be resistant to water, but the black simply got a mildly hard shell from the firing but a softcore that still would be marked by a nail.

It has been a incredible experience, my only regret is to not have be able to monitor the heat of the kiln, to know what temperature I reached, when, and to be able to improve the kiln build, this is for next year I guess !

A big thank you to Andy Ward and this video which has been the primary source of this instructable !