Making Our Cob Oven

The year after moving in, my husband built us a handsome, highly functional mud (or cob) oven in our own backyard. It was quite the task, but one that he undertook with willing hands. He has written up a post describing some of this unique and exciting (though laborious) project. (Note that he keeps saying “we”- really he’s just being sweet, because I was in my last month of pregnancy and didn’t do much more than sift a little dirt and bring him water!) Hope you enjoy. 🙂 

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Being outside. Dirt. Wood. Fire. Bread. Pizza. Food. We love all of these things. We also love making just about anything! Somehow all of this love, moved by some Youtube videos, forum posts, and ultimately Kiko Denzer’s website and book Build Your Own Earth Oven gave birth to the idea of building a cob (earth, mud) oven.

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With the exception of the firebricks and some sand and straw, ours was made exclusively from the clay about a foot beneath our feet and various objects found or gathered from the land around our house. While the building process seemed daunting, it was an enriching experience which we would wish upon anyone. Do try it yourself! If you follow through, you’re sure not to regret it.

Now that we have built one, we hope to adjust the design (to improve efficiency) and build more.

This is not the place for detailed instructions–for those, please get Denzer’s book–some pictures of our building experience follow.

First we chose a site. We wanted it to be conveniently located near the kitchen, but at a safe distance from the house. Our original intent was to build it at the base of the retaining wall outside of our kitchen (you can see the stone-filled hole at the base of the wall in this image), but we finally decided to put it at the top of the retaining wall. This position raised it to a decent height for working and eased the roofing process.

June 2013 139

We dug out the area and outlined the location in bricks (the stake marks the center of the oven floor). While the crushed stone atop the wall provides excellent drainage–much needed to keep moisture from wicking up the cob–we punched holes in a found piece of corrugated drain pipe and used it for drainage around the periphery of the future oven floor. This drains to daylight through the retaining wall.

Summer 2013 008We next built a base out of cob for the sand that would underlay the bricks of the oven floor. So, we had to dig a hole deep enough to get adequate clay…this was the most laborious part.

Summer 2013 021Each shovelful was sifted through a screen or doubled chicken-wire mesh.

Summer 2013 015Summer 2013 017While the screen was finer than necessary, it yielded a good result.

We tried a few simple methods to gauge the clay content (Denzer outlines these in detail).

Summer 2013 020

Summer 2013 019One method involved making sample bricks which we then baked to observe the shrinkage (more shrinkage=more clay).

Summer 2013 022Summer 2013 025The brick on the left was pure clay, the right had some sand mixed in (sand reduces shrinking and cracking). Having found an adequate mixture, we mashed up some cob for the base.

Summer 2013 149

As this was our first time dealing with cob, we were careful to follow instructions… Summer 2013 108

 

 

iPhone pictures and videos 408You must love dirt to do this! We do!

Summer 2013 110Summer 2013 111This base was filled with sand so as to properly lay the firebrick. The method is similar to laying pavers. The floor of the oven is critical–it is the hardest working part–and must be quite even to avoid catching oven tools on the edges of the bricks. Just think of the all the pizzas, breads, pans, and peels that will grace the oven floor in addition to the scraping it will receive from cleaning, etc…it’s important to take your time.

Summer 2013 120We then made a sand-form mold of the inner cavity of the oven. The height of the door is roughly 63% of the peak of the interior cavity–this allows for a sufficient draft when firing.

Summer 2013 140(The door was made of samples of kiln-dried hardwood and a twisted branch.)

We covered the sand form with wet newspaper in order to differentiate it from the next layer: the thermal layer. This is helpful when digging out the sand after the thermal layer has dried enough (you don’t want to dig out the clay!). Summer 2013 144Next we built the thermal layer. This is pure clay because it has a higher thermal mass (it holds more heat) than sand. Summer 2013 152

Summer 2013 166Because I initially overestimated the amount of material I should construct this layer with, ours ended up with a thicker bottom portion (it doesn’t hurt anything).

We then covered this thermal layer with an insulation layer consisting of a thin clay slurry and cedar sawdust. This slows some of the heat loss. We dug out the sand form and gently fired the oven. We generally use wood from pruning or other small pieces as fuel.

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This is the inside of the oven just prior to firing.Summer 2013 2 021The next layer just consisted of cob–a mixture of clay, sand, and straw. Here’s the oven as it originally looked after completion.

Summer 2013 2 080Summer 2013 2 083The flat angled piece in the front is a steel plate we found on the property. It is placed there to help direct the coals and ash into the funnel directly beneath it when cleaning out the oven.

iPhone pictures and videos 572Note the crack that forms over the door when it’s been heavily fired. This is due to a normal and harmless expansion of the clay; it actually helps to keep track of the oven’s temperature. There is a piece of galvanized material above the door to protect the temporary roof (branches and tarps) from igniting.

iPhone pictures and videos 574The oven was used for a year in this form…even through the winter (it is so pleasant to use it while it is snowing):

iPhone pictures and videos 1090Here’s a clip of some bacon corn-muffins going in on a snowy night (they didn’t really need the live fire, but we decided to put them in on a whim after baking earlier that day–they turned out great!).

The temporary roof was finally replaced a year later with a more permanent form (the reclaimed materials were also free to us). We made a table out of part of an industrial wire spool and raised up the ground level in front of the oven to improve the working space. We also resurfaced the oven with a bit of sifted clay (there were a few drips through the temporary roof). Here is the final form as it is today: IMG_0008Expect great pizzas, breads, roasts, soups, and just about anything you can bake in a conventional oven. One of the greatest aspects of cooking in the cob oven is that the heat it absorbs after a firing remains for quite a long time, so you can cook progressively according to the temperature of the oven–pizzas to breads and pies to roasts, all the way on down to drying herbs. We certainly have not used our oven to its full potential but it is a joy to use it. It is deeply satisfying to craft something with your own hands and feet from the most mundane, neglected materials and cook wholesome food with a bright crackling wood fire.   Don’t be afraid to build one yourself. Do what you can with what you have, and don’t be concerned if it turns into something you didn’t quite expect at first–you’ll be sure to learn through every mistake and every problem solved.

 

How we Built a Cob Oven

2 thoughts on “Making Our Cob Oven

  1. Big AL

    came here from Permies.com, (welcome! !!!)and watched the Cob oven job, seems very good, I expect your updates on your blog will tempt me to check out the rest ! Thanks ! big AL

    Reply
    1. Abi Post author

      Thank you for stopping by AL! We appreciate your input and would love to see you back! Permies is a great resource, and a wonderful place to get ideas. 🙂

      Reply

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