02 January 2012

Cob Oven Video - Parts One & Two

Here are our first two videos explaining how to build a cob oven.
For more information on cob ovens, I recommend Kiko Denzer's book "Build Your Own Earth Oven"...it's a complete "how to" including how-to sketches, list of tools, inspiring photos, and much more.


Part One shows how to prepare the base and dome mold for building a cob oven. This oven is 36" interior diameter, so the height of the interior is 75% of that, or 27" tall. The thermal mass layer is 4" thick, plus a 6" thick insulating layer over that, so the whole oven is 10" bigger than the fire cavity all the way around. The base is made with stones gathered from the site and held together with a clay + sand (cob) mortar. Also note, the primary reason to use firebrick is that they do not crack under the heat of a fire.


Part Two shows how to mix & install the cob thermal mass layer (no straw) and the insulating layer (lots of straw).  Note that our soil was about 50% clay and 50% sand content.  We want about 25% total clay, so we added an additional 2 parts sand for each 2 parts soil.  Since the soil is already 50-50 sand-to-clay, the total is 1 part clay to 3 parts sand, or 25%.   The thermal mass layer has no straw.  This is the layer that gets hot and holds the heat from your fire.  The second layer has tons of straw...basically as much as you can mix in.  Straw is a good insulator, so the second layer helps hold in your heat.

9 comments:

  1. Let me know what you think of these videos!
    Also, check out our youtube channel: http://www.youtube.com/user/sigikoko

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  2. Dear Sigi,

    enjoyed your video's very much, they're vivid, you ain't afraid to get dirty, love that, thank you for publishing. It seems to me, you're an excellent specialist concerning cob.

    I placed links to your vid's and weblog on our ecological forum in the Netherlands. several readers loved them.

    Keep up the good work, with kind regards, Boy Bakhuys, Holland.

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  3. Thanks so much for the kind feedback. I am so glad to hear that you and some of your readers found them to be interesting. I'll be posting more this year on my youtube channel http://www.youtube.com/user/sigikoko (and will add a notice here when each is posted).
    Enjoy! Sigi

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  4. Sigi, have you any sense of how long a cob oven like this one would last compared to one lined throughout with firebricks?

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    1. Hi Wendy,
      As long as you keep your oven protected & maintained, it will last indefinitely. The nemesis is water, especially water that soaks in and freezes in winter. The most durable protection is to build a roof over your oven. The simplest protection is to cover the oven with a tarp after the last time you use it before winter, and keep it covered until spring.
      Hope this helps!
      Sigi

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    2. Thanks Sigi. The potential for water damage I'm aware of. What I'm interested in is how well the interior of the oven stands up to repeated intense firings. The natural assumption would be that a clay-sand mixture wouldn't last as long as firebricks (even though that's essentially what firebricks are) and I was wondering whether anyone had done any sort of study on this. Clearly, if clay/sand lasts indefinitely, then there seems little point in going to the expense of using firebricks, but if firebricks significantly extend the lifetime of the oven, then they come back into consideration.
      Best, Wendy

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    3. Hi Wendy, Sorry I misunderstood your concern. The inside of the oven actually gets fired, like low-fire pottery, over time. This makes it quite hard & durable. And if for some reason you get any damage inside, you can always re-plaster the inside to make any repairs. Really if you handle the water issue, the cob oven will outlive us both. :-)

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