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Step 5Building 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.
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.
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You can also build this oven making a mold with fine and wet sand as you want inside. Cover with clay, as you did. When dry, open the door and take the sand out for next building. You may use soil cement in a ratio of 18:1 with bamboo slivers either to build the oven or to cover it. Ok, it's a little bit more expensive.
How much water to use with soil cement? Get in hand some clay and squeeze. Falling just a drop of water is perfect.
To avoid cracks mix sugar with clay. Warning: the mass is slightly softer with sugar. Wait to dry completely and fire strongly. Using cement, don't forget to wet for 3 weeks, to keep cement cold and just use after 30 days. You can use sugar with cement too. Very good!
Then, good pizzas, good breads and much more!
I'm guessing the temperature of the oven turns the clay into ceramic? But I wonder if the straw burns without the lime to protect it?
The clay in the cob is only fired in a small layer on the inside, if at all. This fired portion will be waterproof, but that's not much help on the inside.
The straw will burn in this layer, leaving holes in the cob as added insulation. Since it's not that much straw in the mix, it's of no concern.
I have to admit, i can't remember having read about quicklime in cob. But a roof also helps to keep me dry, if it starts to rain... and it's easy to build.
Doesn't quicklime reduce the moisture permeability of the cob? This could be a problem with trapped moisture in the thermal layer. The expanding vapor could crack the thermal layer if this happens.
But it could well be, that by the fire in the oven, the slaked lime would be fired back to quicklime and get hygroscopic again.
For general building, it makes a lot of sense of course.