Posts tagged ‘Earthen Clay Oven’

CREATING A COB OVEN – PART 1

Using clay, sand, straw and sawdust from our land, plus a few fire bricks and some discarded wine bottles, we are building a cob oven a few steps from Underhill House — and  we will soon be baking wood-fired crusty breads, sizzling pizza and soft-centered, crunchy veggies for ourselves and the building crew.

Brad Selz, one of the  crew, has a lot of experience with the ancient straw-clay mixture called cob.  He suggested we build a cob oven together, and for the past few months we’ve been working with him when time permits.

Brad’s bible for this kind of work is Build Your Own Earth Oven by Kiko Denzer and Hannah Field.  I recommend reading it before you start your own oven.

We are in the home stretch now, racing to finish before things get too cold.

Brad put together a base with stones left over from the slip-form stone wall we built as a foundation level in Underhill House.  The stones came from a quarry about 10 miles from our land.  (See my posts on building a rock wall using slip form Part 1 and Part 2)

We leveled up the top edge and filled the gaps below with a cob of clay and sand.

Then to create an insulating layer, we filled the center of the rock base with wine bottles embedded in a mixture of clay and sawdust.  The sawdust will burn away leaving a porous clay material.  We capped the bottles with more clay-sand mixture.

Doug and Brad sifted a bed of sand for the firebricks.

The local, somewhat rocky sand had to be sifted because the firebricks need to sit very evenly.

If the edges of any bricks stick up, they will snag as food is slid in and out of the oven later on, so Brad spent some time gently tapping them level.

Using a pencil and string, we drew a circle with a diameter of 28″, then put a 16″ high stick in the center and began to build the sand dome to the top of the stick.

The sand dome is used to form the interior space of the oven, and shaping it re-awakened the  joy of building sand castles along the shore as a child.  The gritty crunch of damp sand is so satisfying.

Suspense builds along with the dome.

In a sand structure of these proportions, avalanches are unavoidable.  After a certain point, the sand can no longer be patted in place.  It must be ever-s0-gently eased into position for the slightest shock wave can have disastrous results.  Fortunately our cave-ins were few and easily repaired.

Ultimately the sand dome is polished to a satiny finish by lightly rolling scraps of smooth wood back and forth around the surface.  A smooth interior surface will let the heat circulate more fluidly.

The sand is separated from the cob by a layer of newspaper strips coated in clay slip.  We mixed up this slip combining clay and water in a 5-gallong bucket and and stirring them together with a paddle on a power drill till it was the consistency of a chocolate milkshake.  It looked so good I had to keep reminding myself not to lick my fingers.

Thin strips of newspaper conform well to the shape of the sand dome.

The cob for this layer was about 50:50 sand and clay with just a little extra clay.  It was a crisp fall day Sunday, but not too cold to take my shoes off.  I was wearing rubber garden gloves so I could keep my hands clean enough to use the camera, and so I was eager to give my feet some real contact with the cob.  The clay and sand are mixed by pulling the edged of the tarp inward and tossing the two together a number of times.  Then water is added very carefully.  You want a mix that will ball up in your hands but not too sloppy.

Now comes another fun part.  The mud-pie process of building a fist-wide layer of cob over the sand dome.  It is important to make sure the pressure is always downward.  If you press inward against the sand at this point, you can put a dent in your dome.

This process meant revisiting another truly joyful childhood state — making mud-pies.I haven’t had so much fun since I covered the brick bar-B-Q in our  back yard with finely-formed mud-pies to bake in the sun. My parents didn’t appreciate my handiwork, but I imagined that as earlier people built the structures that made their world work, their children were encouraged to play around the edges, hands in the mud, familiarizing themselves with the feel of the materials and perfecting skills that would serve them well all their lives.

Putting your hands into mud is putting your hands into prehistory -

  the place where most humans have existed

and the time when we didn’t have to worry about crashing the entire ecosystem with our clever, modern comforts.

The weight of the cob above causes the lowest layers to thicken as you build up.  You can fix that by using a saw to slice off the excess and then add it back at the top.

At first I could not get my cob layer as smooth as Brad’s but the more you work with it, the more comfortable you get with this stuff.  We used every scrap of the last batch and completed the dome just as the sun was sinking.  Working with these elemental materials on a gorgeous, crisp, sunny autumn day is about as good as it gets.

I’ll be posting again very soon on the exciting conclusions when we add another fist-width layer, this next batch containing sawdust to heighten the insulation factor, and then a final, thin finish coat.

The whole oven will have to have a protective wooden lean-to shed to protect it from rain.  We will be making that from scraps of wood from Underhill House.

I can almost taste the pizza.

September 25, 2012 at 2:10 pm Leave a comment


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