Building a woodfired oven with clay or mud
After having built a oven for myself, i think it's a nice to have piece of work in your backyard.
Be warned, it takes quite some time to build, but also to use it. It's no substitute for your Microwave... it's rather in the slowfood class, even if you can bake a pizza in 2-3 minutes.
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Step 1: What Type and Size Do You Want?
Depending on what you want to bake and the space available, you can adjust the size of your oven. The amount of material needed doesn't scale linear. My oven needed 4 times the material of a small but usable oven.
In numbers that was 600 kg of clay powder, around 950kg of sand.
In my case, the size was selected because my main use of it is to bake bread and i wanted to be able to use the cookie pans from my electric oven. I also selected the thickness of my thermal layer to keep the heat long enough to bake multiple batches of bread, without the need to reheat.
As a general guideline, i wouldn't go smaller than 40cm/16inches inner diameter. Mine has 75cm/30inches. The thickness of the thermal layer should be no less than 15cm/6inches. Mine has 20cm/8inches.
At some point you need to decide whether you want a chimney or not. I had the chance to use a oven with a chimney, but decided against it for my oven. The main advantage of a chimney is to take the smoke out of your face. The main disadvantage in a simple design, lots of heat will go out the chimney instead of heating up your oven. It also adds complexity to your design. If you need to add a chimney, because your fire doesn't burn cleanly, you can do it even after finishing your oven.
There are certain ratios between inner oven diameter, inner oven height and oven door height you need to have. The most important here, the oven opening height has to be 63% of the inner dome height. This is essential for a clean burning fire. Further, the inner dome height should be 60-75% of the inner dome diameter. With these ratios and measurements, you can determine the actual measurements for your oven. It's best if you look around for firebricks, before you decide on the size of your oven floor. You should sketch your firebrick layout on a large piece of cardboard 1:1. Then draw the inner and outer shape of your oven on it and cut it out. This will help you in the next step, the form and size of your foundation. You should place the opening of your oven away from the main wind direction. Last but not least you have to plan for a roof of some sort. If you plan to bake in bad weather, make it big enough to shelter you as well as the oven. The oven should be able to breathe, so the moisture can get out.
I would like to strongly suggest for everyone to read the book from Kiko Denzer, "Build your own earth oven". It goes much deeper into the details, he built countless different ovens and shares his knowledge. You will see references to his book throughout this instructable. Don't get me wrong here, you can build a oven with this instructable alone, but maybe you'd like to do it a little different. In this book, you will find different techniques, styles and lots of background information.
Pictures of smaller ovens made at a workshop. One with chimney, made in the sand mound method, the other without chimney was made with the inverted basket method.
Step 2: The Foundation
Since your oven will be pretty heavy, you will need a stable foundation.
In my case, i had a old fireplace that i used as a base for my oven. A friend made a iron reinforced concrete plate for me. It is 130cm/52inches square and 6cm/2.5inches thick weighting around 350kg. This was a heavy lift for 4 persons. I originally wanted it twice as thick and even larger...
If you don't have a fireplace to convert , you need to make your foundation from scratch.
Depending on where you live, you need to make it frost proof. How deep you need to dig for frost proofing, it's best to ask a local builder. I'd dig at least 50cm/20inches in a frost free zone. Then you can build up walls up to the oven floor height. Then fill it up with gravel, and compress it by jumping. Fill the last 10cm/4inches (minimum) with sand. That's where you will put your firebricks as your oven floor.
What bricks you use is up to you. If you have large stones around, use them. Kiko suggests to use "urbanite", that's scrapped concrete you can find at your local dump.
Step 3: Ovenfloor
In my case, the size of the firebricks and the height of my ovendoor determined the remaining measurements. The size of my firebricks is 25cm/10inches square and 6cm/2.5inches thick. This is a pretty thick brick, in the smaller oven seen in step 1, the bricks were only half the thickness. The thicker the brick, the more heat it stores, but then the sand under the bricks will also store the heat.
I layed out the bricks first to see how they arrange. Since i wanted my firebricks to "float" in the sandbed, i had to cut them to shape. I used a disk grinder to cut them to shape, i ground down around 6 disks. You don't have to do it this way, in the smaller ovens we just built the ovenwalls onto the firebricks.
I sketched the layout on my concrete plate, in order to build up my foundation. If you have another foundation, you don't need to do this step, since you already have your sandbed to lay your firebricks on.
I used Perlite as a first layer under the sandbed. This insulates the concrete plate from the hot sandbed. Otherwise the concrete plate could crack from the heat. If you like to do this, just add it between your gravel and the sandbed.
Be sure to compact your sandbed very well and flatten it nicely. My firebricks weren't of the exact same thickness, so i had to adjust every single one to make sure my oven floor gets flat.
Step 4: Building the Sand Mound
To define the interior shape of your oven, you have to build a sand mound. To do this, you also have to insert your oven door. The easiest way to make your door, is sawing it out of wood. It's best to use some fire resistant wood like oak. Before you close your oven for baking, you can soak it in water.
For my oven, i had to mount the oven door somehow. I got it from my neighbor, he has a whole lot of them around in different shapes and sizes. He's a retired blacksmith who still has his shop and is still working, but only on jobs he likes. I wanted to mount it decoupled from the hot oven, so i mounted it to the concrete plate. This was made because of the different expansion under heat. As a next step, i added a ring made of cardboard in order to have the inner walls going straight up. This way, it's easier to remove the ash and i have more usable space for baking. We added some more cardboard to stabilize the sand mound around the oven door. It is important to use "sharp" sand, no beach or round sand. It also needs the right moisture content, otherwise your mound will fall apart. As a further effort to stabilize the mound, we used paper with paste or goo on it. (I'm not sure about paste or goo. I mean the stuff you use on wallpaper.)
Step 5: Building the Oven / Making the Building Material
This is the step, i was most afraid of. Not because how i made it, but because you may want to go on budget and dig your loam out of your backyard. Although this is perfectly possible, it's hard to tell you the right consistency. This is a tactile thing, so it's hard to describe in words.
I used clay powder that i bought in 30kg sacks. With this stuff it's pretty easy. You only have to mix it with the same amount of sand and half the amount of cut straw by volume. Mix it well and add water until the consistency is right. If you add too much water, just add some more clay/sand mix. We used a large tarp to mix and to knead on it. To mix, roll the tarp from one side to the other, so the whole mix will be rolled and well mixed.
Then you need to knead it well. This is very important in order to drive the air out of the mix. It's best to do this by feet, you have much more power in your legs than in your arms. We thought about mechanizing this process. Mixing could be achieved with a concrete mixer, but i don't think it would work for kneading. I think kneading could best be achieved with a bakers kneading machine, but i couldn't get my hands on one up until now.
We then made bricks out of this mixture and used them for building. You should always keep a 90 deg. angle to the sand form. Try to keep the width of the wall constant over the whole oven. The last piece, or the keystone wasn't made as a brick, it was pressed into place.
Now to the diggers among us. I will tell you what Kiko Denzer writes in his book. I can't go that deep into the details as he does. First because of the time i'd need to rephrase it in my own words and second because i don't want any copyright issues coming down on me. Do yourself a favor and get this book, it's worth every buck.
So if you want do dig, you will first have to remove the fertile topsoil which is darker than the subsoil that is rich in clay. You could also inspect construction sites, if you see them digging a cellar or a foundation. You may ask them to dump a truckload at your place, maybe they have some sand around as well.
This subsoil should (mostly does) contain clay, silt sand and small gravel. Your desired building material should contain from 15 to 25% clay. This stuff is hard to dig. It is sticky and heavy around here and it doesn't crumble as a fertile topsoil would. To recognize it, the shovel should leave a shiny cut mark. If you add water, you should be able to roll it into snakes in your hands and bend it with minimal cracking. You should be able to sculpt it. You could do a test by filling a jar half with your soil, the rest with water. Then shake it well, until everything is dissolved. Stand it upright and undisturbed and watch it. In five to ten seconds, sand will settle out. After 30 or so minutes, silt will settle out. Clay takes from days to weeks to settle. So if your mixture clears in a hour or two, you can't use it, because there is no clay.
Now that you don't know the amount of clay in your mixture, you can do a test. Take a handful of your soil, add as much sand as you think is right and work it into a firm ball. This may take some time, since you want it rather dry and compact. Let it fall to the ground from waist height. If it falls apart, you have too much sand. If it goes flat with no cracking, it's too little sand or too much clay. If it almost holds it's shape without or with little cracking, then you've got it. When you make these tests, be sure to take notes on how much sand and soil you used, so you know the mixture for "production scale".
You should make these tests because of this: clay holds water and this water will evaporate. So your building shrinks. To minimize this effect, use as little clay as possible. (15%) The drawback is a reduced "workability". The mixture tends to be crumbly and needs lots of kneading. I used more clay for my mixture, i knew that some cracks will show up anyway, so using a fatter mixture made for easier handling. You have to decide this for yourself.
In the production mixture, you also have to add some straw, cut up to 2.5 to 5cm or 1 to 2 inch pieces. I added half the amount of the sand by volume. (two buckets of sand, one bucket of cut straw) I have to admit, i bought the straw cut up. If you don't want to cut it manually, try a wood chipper.
It will be pretty hard to mix the soil with sand, water and straw. It's best if you invite your buddies over to help you. Your invitation should include some beers and one or two pizza happenings later on. We were only two, my brew-buddy and me. Sometimes it was a bit depressing, when the work didn't seem to have a end. We were asked to help someone build his oven, after he saw mine. He wanted to pay us, but i told him that it's more important to have a couple of helping hands or even better feet.
Step 6: Removing the Sand
If you are building with a rather dry mixture, you can remove the sand form right after you finished building. But if you're not sure, better give it a week or two for drying depending on weather and climate. Our oven wasn't completely dry as you can see, but it felt hard to touch. So we removed the sand to dry it out from the inside as well. To speed up the process i burned large candles inside my oven. After another week, i started my first fire. Start it small, and do it daily for a week or so. After this, my oven was dry and i tried my first bread. Resist your pyromanic urges to load it with too much fuel, as you can see on the third and fourth picture. If there is more fuel, than the oxygen can burn, it starts to soot badly and burns out the oven door. And that's not where you want the heat.
Step 7: Adding Insulation
After baking a couple of batches of bread and pizza, i added a layer of insulation. My oven was almost a 100 deg Celsius 212 deg Fahrenheit on the outside, without insulation.
If you want to do this, think of it from the beginning. That means plan your foundation a little wider. I planned my insulation from the beginning, but i wanted to see how it works without. After applying 8cm/3.5inches of insulation made of Perlite and clay, the outside temperature dropped by half, keeping the heat much longer. You can use different materials for insulation, my choice was a foamed mineral. You could use Vermiculite or pumice, but straw or coarse sawdust from a chainsaw would also work. Maybe you need to make a thicker layer to achieve the same insulating effect. I used a sturdy paint mixer in a electric drill to mix clay, Perlite and water. As you can see, it cracked after drying. We used a silicone cartridge, filled with rather thin clay to fill the cracks. We further made a temporary roof with the tarp we used for mixing and kneading the mud mix. The final roof will be made of fiber concrete(ethernit), but it's up to you what you use. Depending on the distance from your oven opening, it needs to be heat resistant. It worked with the tarp and pine wood without melting or charring, but i will use oak where it gets hot.
We will add a thin layer of clay plaster 1cm/0.5inch when we remove the temporary roof. It's easier to work around the oven without the roof. This is only for the optics and it's not much work.
Step 8: Using the Oven
Now comes the fun part of having such a oven. Maybe the first time you bake, you have not enough or too much heat. In my case, i heat the oven for 2 hours and then take out the remaining ashes. After another hour of "soaking" it's ready to bake. Soaking means letting the temperature even out in the oven. This timing actually matches my normal dough fermenting pattern.
Here you can see a couple pictures of my baking endeavors.
You can see some of my "devices" that i need to bake as well. To remove the ashes, i use a coco-handbrush mounted on a broomstick. I was lucky to get a wooden bread peel for free and one made of aluminum as a gift. I have a infrared thermometer that goes up to 550 Celsius and 1000 something Fahrenheit. You could throw in some flour and see it turn brown. This should happen in 10 to 20 seconds. Then you have the right temp to bake bread.
Since this is my first instructable, forgive me my wrongdoings. Maybe you see some orthographical or grammatical oddities, that's because English isn't my native language.
I will read your comments and correct any errors and omissions, you make me aware of. If there is enough interest, i could make some baking instructables as well. Having a oven isn't the same as using it.
In the meantime, i made a baking instructable for the Challah-like braid. You can find it here:
By the way, the guy you see on the photos is my brew-buddy Willy, someone had to shoot the pictures and that was me. We have a nice homebrewery together and brew on a (more or less) biweekly basis.
Step 9: Pizza Pizza
Today, we just had a little pizza roundup.
I think it speaks for itself, what such a oven can be good for. It was a very nice gathering. The pizzas came out very well, the flammenkuchen's too. First we looked that everyone had one pizza, so the first hunger was stilled. Well, actually, there was a shrimp cocktail before, some little antipasti and some bread as well. Then i started to do flammenkuchen's and cut them up into small slices, so everyone could take as much as they wanted. Later we had some coffee and cake. Still later, some more flammenkuchen and pizza. As you can see, i had a peppering help from a young fella. At some point, i will learn to shape those pizza's and flammenkuchen's nice and round, but in the meantime they just taste great.
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