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How to Build an Earthen Oven

Jonathan Townsend

Posted on April 15 2016

How To Build An Earthen Oven

The existence of ovens like this is easily documented for the 18th century. In fact, just about every ancient culture had a very similar oven. There’s one particular wood cut illustration from medieval times depicting an earthen oven built on a wagon.
There are references in 18th century literature and also archaeological evidence that you would find ovens like these in private homes and in fort settings. There are also references to communal ovens where the baker would bake bread for an entire village.

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  • Sand
  • Clay
  • Straw, Dried Grass or Hay
  • paper
  • Bricks
  • Canvas Tarp
  • Water

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Before you build your oven, you have to consider what you’re going to build your oven on. There are historical examples of ovens built on tables, brick or stone plinths, and hearths. You will also need to make sure that your oven is protected from the weather. This is water soluble and it will just wash away with the rain, so if we need this to last a while you’re going to want a little roof over it.

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Once you have decided where you are going to put it, lay out a layer of brick for the floor of the oven and chalk out a design. Ours is about 22 inches across for the inside measurement.

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The walls are going to be about 6 inches think.

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Our door is about 12 inches across so we can get something as big as a pie in without too much trouble.

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There aren’t very many critical things about the shape and the size of your particular oven, but there is one critical thing, and that is the height of the opening tunnel compared to the height of your dome. This has to be a particular ratio or else the air won’t draw through when you’re burning the wood inside. The tunnel needs to be between 60-65%, or about 63%, of the height of the dome.

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Once you have decided on the design of your oven, you should start working on your cob.

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You probably want to make a whole bunch of this cob beforehand. It ages well, so it won’t go bad waiting overnight, and that way, as soon as you’re done with your sand castle core, you can start putting it on right away and you don’t have to worry about the sand drying out and blowing away while you’re making your cob. Make sure to wrap your wet cob in plastic so that it does not dry out before you are ready to use it.

The inner most layer of mud or cob that we’re going to put on our oven is just sand and clay. About 2 parts sand to 1 part clay. You mix those two together so that they’re very well mixed.

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We want to make sure it’s got about the right consistency that we can still work it but it isn’t so wet that it’s sloppy, and you want to make sure to err on the side of a little more sand than too much clay. The more clay you’ve got the more it’s going to shrink and crack. Learning just the right consistency can be tricky.

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You want it to form into a ball, like a snowball and does not deform easily.

Now you want to go ahead and build the core.

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This is going to be like a sand castle, just very wet sand that we’re going to build the oven over the top of. You need to make sure that the sand stays wet until we get the first layer on. Sometimes you’ll see other people doing it with sticks and things like that, but this is going to be much easier and quicker. At the door, you should place a brick wall, to get a nice flat surface to build up against.

Once your inner core is built, you need to put paper on it.

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Wet the paper down and layer it over the sand to give us a layer to separate the sand from the inner surface and make later removal of the sand easier. Once the paper is on you can start putting on the first layer of cob.

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This first layer does not have any straw or hay because that would just burn up anyway. It needs to be about 3 inches thick. Start from the very bottom and work your way up so you can watch as you go to make sure the thickness stays about the same. When you are done with this layer, allow it to set overnight so it will be slightly firmer but don’t allow it to get too dry or else the next layer won’t adhere properly. Scratch up the layer a little bit so the next layer of cob adheres nicely.

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The next layer of cob that we put on is going to have grass or hay or straw in it to give it a lot more strength than the inner layer. We’re going to mix our clay and our sand first. As soon as that’s getting close to the right consistency, that’s when we’ll add our other binding material in.

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We need to mix this up just slightly wetter, since we’re going to add in dry straw, it’s going to dry it up a bit. This will add some amazing strength to it. When it dries up it really binds it together.

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It’s helpful to make this cob up beforehand. It really makes it work better if its couple days old, but you don’t want to let it get too old because as it’s wet for a long time, the grass will start to rot in there, so you don’t want that to happen. If it’s a day or two old, keep it wrapped in plastic so it’s wet and pliable. It’ll really work even better after a day or two.

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So, to make this go faster, I suggest you invite a bunch of friends over. Have a cob party. They can be stomping on this stuff while you’re putting it on your stove. Everyone will have fun.

About five or six big loaves of cob ready to go is a good start. I’m not sure exactly how many it’s going to take to cover this oven, so we’re going to put this on and then see how much more is needed.

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The outside layer should also be about 3 inches thick. Make sure your loaves butt up well with the inner core so there isn’t a big air space between them and just start adding on your cob all the way around.

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You might want to start a little thinner at the bottom than the finish because some of it will sag down into position a little bit as you go.

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When you are done with the main body, remove the brick wall at the opening and add a nice rounded opening to it because it will have more strength than a sharp one.

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Keep in mind that whenever you add two pieces together you really have to work it so that the two pieces adhere to each other and it doesn’t just fall off. Add some sand to the opening to support the new lip around the opening.

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Depending on where you’re at, your environment, the time of year, and what the humidity is, this could take anywhere from 2 to 4 week or longer for it to get dry enough for you to even start thinking about warming it up from the inside. While it’s drying, you don’t want it to get rained on so you’re going to need to protect it from the weather, but don’t cover it with plastic so that it can’t dry. You want to protect it from the rain, but still let it breath.

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After it has started drying out enough to support itself you can start slowly digging the sand out and even pulling off some of the paper. Don’t worry too much about the paper because it will burn out anyway. Every few days dig out a little bit more of the sand so that you can start getting some are into the center of it to dry the inside.

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Once the sand core has been removed and your oven is adequately dry you will be ready to fire it. If your oven is not adequately dry before you fire it, it will cause cracking, or at least more cracking than normal, in the body. Even if you wait like we did, it’s inevitable that some cracking will occur.

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Don’t be alarmed if the cracks are especially big, you can repair them with a little extra sand and clay and let that dry in place. When the oven is close to dry, you can employ a few warming fires to help it along. The walls of this oven are extremely durable.

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It takes a lot to break this material up so if you need to do modifications you really have to chop at it to break it.

Transcript of Video:

In this video, we’re going to show you how to build and earthen oven.

The existence of ovens like this is easily documented for the 18th century. In fact, just about every ancient culture had a very similar oven. There’s one particular wood cut illustration from medieval times depicting an earthen oven built on a wagon. There are references in 18th century literature and also archaeological evidence that you would find ovens like this in private homes and in fort settings. There are also references to communal ovens where the baker would bake bread for an entire village. We’re going to need several things to make our mud oven out of. We’re going to need sand. That’s the major component of our oven. We’re going to need a good bit of clay. This is dried clay you can get at a masonry store or you can get damp clay out of ditch bank. You’re going to need straw or dried grass or maybe hay. You may need some bricks, so some fire bricks, even regular bricks will work, and you’re going to need a canvas tarp to mix your cob together with and you’re definitely going to need a good bit of water.

Before you build your oven, you have to consider what you’re going to build your oven on. There are historical examples of ovens built on tables or on brick or stone plinths, on hearths. On the top of our very sturdy table, we’ve laid out a layer of fire brick. That’s doing to be the floor of our oven. We’ve also chalked out here the design. About 22 inches across to the bottom on the inside. That’s the inside measurement. The walls are going to be about 6 inches thick so we’ve got markings here so we can see about how big it’s going to look on the surface. The door width right here is about 12 inches across that we can get something as big as a pie in without too much trouble.

First thing we’re going to do is we’re going to build the core. It’s going to be like a sand castle, just wet sand that we’re going to build the oven over the top of. Sometimes you’ll see other people doing it with sticks and things like this, but this is going to be much easier and quicker. This is where our door is going to be. I just went ahead and put a couple of bricks in here to be the inner core of the door. They’ll be removed. And right here I placed a brick wall to give us a nice flat surface to build up against.

So we’ve taken about an hour to put this together. We’ve used very wet sand so that it stays into shape. Now we’ve got to make sure that this stays wet until we get our first layer on. There aren’t very many critical things about the shape and the size of your particular oven but there is one critical thing, and that is the height of the opening tunnel here compared to the height of your dome. These need to be a particular ratio or else the air won’t draw through this when you’re burning the wood inside of the thing. So, this is between 65 and 60% or about 63% height here compared to the height there.

The next thing we’re going to do is put paper on this. We’re going to put paper, we’re going to wet it down so that it’ll give us a layer to separate so when we take the sand out it doesn’t stick to the inner surface.

We’ve got the paper covering done on our sand inner core. This will make it much easier to take the core out from underneath it. Now it’s time to make the first layer of cob or mud to put on our oven.

This inner most layer of mud or cob that we’re going to put on our oven is just sand and clay. About 2 parts sand to 1 part clay. You mix those two together so that they’re very well mixed and then we just put it on there. We want to make sure it’s got about the right consistency that we can still work it but it isn’t so wet that it’s sloppy, and you want to make sure to have err on the side of a little more sand than too much clay. The more clay you’ve got the more it’s going to shrink and crack.

So you probably want to make a whole bunch of this cob beforehand. It ages well. It won’t go bad waiting overnight, and that way, as soon as you’re done with your sand castle core, you can start putting it on right away and you don’t have to worry about that drying out and blowing away while you’re making your cob.

So, learning just the right consistency can be a trick. As you see here, I’ve been stomping on this pile for a little while and this is starting to feel really good. It forms up into a ball, like a snowball. It doesn’t deform easily. It’s not sloppy and you can still form it into any shape you want and it’s not too drippy either. That’s what you’re looking for, something that holds together well but still moldable.

So we’re working on putting this first layer on. This is a layer without any straw in it because that would just burn up anyway. It’s about 3 inches thick and we’re starting at the very bottom and we’re going to work our way up, that way we can watch as we go to make sure our thickness stays about the same.

Well, we finished the inner mud layer yesterday afternoon and we let this set overnight and it’s just slightly firmer than it was. We don’t want to let it get too dry or else the next layer won’t adhere to this layer properly. We’ve scratched this layer a little bit so that the next layer of cob we put on here will adhere nicely. This next layer of cob that we put on, it’s going to have grass or hay or straw in it to give it a lot more strength than this inner layer.

We’re going to mix our clay and our sand first. As soon as that’s getting close to the right consistency, that’s when we’ll add our other binding material here.

So, we’ve got this mixed up. I’m going to mix this up just slightly wetter. It’s feeling like a pretty good consistency now under my feet and since we’re going to add in t

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