Wood fired clay oven – build log

Update November 16th, 2018

Adding pictures to the bottom of the old log doesn't work (like usual, picture-handling is a big fk'n mess here at hackaday.io). So here's the update:

I found a fitting piece of 1.5mm thick aluminium, drilled a hole and cut a 75mm diameter hole with my jigsaw.

The rosette is mounted with twelve cup rivets.

Stick-out of the stove pipe is around 60mm.

Mounted to the oven with cup-rivets. It's sturdy as hell. You can see the longer stove pipe just shoved down onto the stick-out pipe. Fits like a glove, happy with it!

Most of the year the oven won't be used, so I decided to not fixate the whole stove pipe to the oven.

Those thin, aluminized steel pipes are tapered. So I cut a 250mm piece from the 1000mm pipe. This piece is mounted into the oven and the long piece is just shoved onto it when I use the oven. Nice!

For the fastening to the oven I used a piece of left over aluminium sheet and printed a drill/cut guide which was glued on with PVA/wood glue:

The tabs are then bent by 90°: 

As the last log showed, the 60 to 70mm thick clay doesn't preserve the heat very well. It needed insulation badly.

I wanted at least 50mm or so of insulation. A small format metal sheet (2000x1000mm) wouldn't be long enough then. So I chose the next format of 2500x1250mm. This gives the opportunity to use 80mm of insulation: yay!

I got my metal sheet (2mm thick) and used a roller at the company to roll it into a cylinder of 740mm diameter with 30mm overlap:

Next step was to do the cutouts for the four posts:

I wanted to use rock wool instead of glass wool and soon regretted that decision.

Rock wool is everything but easy to bend, so we had to cut lots of strips which were a mess to get around the oven! Wire was used to hold it in place.

Above picture shows the second layer. Overlap was not as planned but should work nonetheless.

The metal cylinder was easily bent open by about 400mm and put over the insulation. Two straps were needed to get the already drilled and tapped holes aligned and bolted together again.

Insulation worked great. You can now still touch the outside when the oven is a max temperature (around 500°C, 932°F).

To close the back, I chose not to buy another sheet but to use the strips left over:

They were cut to length, roughened and glued together with 10mm overlap. After the glue cured, cup rivets were used to keep it from coming apart ever again :)

Those create a water and airtight connection.

This is how it looks now. Best thing is I made use of the whole metal sheet this way. Almost zero waste. Nice!

Still to be done: closing the front of the oven. I need to cut a ring for that but have no sheet metal left. We'll see how to solve that.

Baking in an wood fired clay oven is not like in your kitchens oven. Its really a lot of work and the learning curve is quite steep too.

As every oven is unique in its properties, one needs to go through a lot of trial and error to find the right "values" for:

• type, size and amount of wood
• duration of heating till the ovens mass is saturated
• time the oven needs to "even out" the temperature
• ...

So I just couldn't take any more waiting. The oven was not insulated yet but I needed to bake my first pizza/bread in it. So, let's fire it up:

So, after heating it up for around 2.5 hours, I thought the right temperature was reached.

Pizza dough was prepared the day before, so we got everything ready and tossed the first pizza into the oven:

Hmpf :(

The heat just wasn't right. Without insulation the oven was just not able to hold the heat of around 350°C (660°F) long enough.

Pizza was okay, but I wanted a crisp bottom, charred in some spots and a crisply crust. Not this time...

But the insulation should help with that, right? You'll see soon!

Clay/loam is a natural stuff. This means it does not behave exactly like you would expect it from standardized mixtures like store-bought fire clay.

So, after the first big fire in the oven, many cracks and separations formed and needed to be repaired. I prepared a good amount of very rich repair clay with lots of straw/hay/sand in it:

After these fixes the oven was left to dry/cure for a few days. It hasn't cracked much more after those fixes.

Until log 6 the oven had no real front. No door, no hinges, just an opening made from flimsy metal (drums are made from around 0.8 to 1mm thick sheet metal).

I measured everything again and came up with the following dimensions of the front face:

So, I sat down and did some CAD'ing and ordered the parts in 8mm thick standard mild steel (S355JR, 1.0045).

I have no pictures of the single parts but here it is mounted to the front:

The frame is bolted to the drum with screws (which got a fair amount of copper paste before tightening them):

The handle to open and close the door is still a bit on the botchy side of things, but it works:

This is by the way one of the exceptions to building the oven from the cheapest/scrap parts. I wanted the front to look and feel professional, so I bought it plasma-cut and sandblasted. It was around 25€ I think.
But I used the inner cutout as the hatch (open in the above picture), which is not ideal because it has 3mm space all around, thus not closing exactly and wasting some heat...

Edit 17.12.2018 – found a few additional pics. Added here:

This abomination of an "editor" just isn't capable of letting me add pics in between already existing pics without deleting the pic above and/or under. This bug annoys the hell out of me, reported it numerous times, nothing's happening. Anyway, here are some additional pics:

Original log:

October 2017 was quite the warm month. Perfect for building without sweating too much. But at the time the dome was done, it wasn't as sunny anymore. I decided to force-dry the thing:

I put a tray of burning BBQ coal in and just let it heat up a bit.

Finale! Let it burn burn burn!!! It was a really strong fire. Huge flames shooting out of the oven. Really hoped no-one calls the fire dept. Luckily everything went ok :)

To get the dome exactly like planned I constructed the two identical faces in CAD and printed them on 4 A4 pieces of paper, glued them onto some 18mm thick spruce planks and cut them out with a jigsaw:

I then screwed wooden slats to both faces to create some kind of trough:

This mould fits perfectly this time. You can see the shitty one in the lower right corner. It got destroyed later and used as fire wood :)

There's only one good method to in-cooperate all the ingredients thoroughly: stomping it like there's no tomorrow:

This time the ratio was much better. We used chicken wire to give it some additional strength:
The mix was still quite rich but worked perfectly:

This is the result of a few hours stomping on mud like crazy and slapping it against the soon-to-be oven to create a nice dome:

Here you can see how the drum sits on the frame. The two (not painted) bars can rotate by a few degree.

That's it for part five of this series. Stay tuned for the fiery part six. What do you think is the best way to remove the wooden form? :)

So, this is where we encountered the first real set-back. But let's see what I planned to do:

This wobbly thing is what I thought would suffice to create the clay/loam dome.

We prepared everything and put the mould in place:

The chamotte bricks are not yet pit in place in the above picture. It was just to test the fit. So, we made a huge batch of mud/sand/straw and began building the dome:

What happened was the following:

The mould gave way because it was much too soft and wobbly. We added pieces of wood to no avail...

Second mistake was the clay. It was much too rich and gave way under its own weight. We gave up after an hour or so because there was no improvement to our situation.

I had to rethink the mould...

So this log is a real quickie.

I got 4 washed concrete panels which have exactly 1x1m when laid down as a square.

I had to level the ground before that and used a rubber mallet to get everything level afterwards.

One problem is rain. A heavy rain shower later it looked like this:

I'll have to put a drainage to avoid this.

This is the frame I found in the metal dumpster. I modified it a bit to fit my needs. Picture above doesn't show the final version anyway. You'll see how I modified it in the next posts!