Building a Cob Oven

On Saturday, April 2, I went to my friend’s place to help with building a cob oven.  My friends are Mike and Deborah Boehle, of Antiquity Oaks.  She put out a general message to those who might be interested in learning how to do this, and I, along with about a dozen other people, answered!

Cob building has fascinated me for a while.  The fact that you can mix together clay, sand, and water together, and then build stuff – as small as an outdoor oven, to as big as a house – is intriguing.  And I wanted to be in on this project.

Mike and Deborah had already planned this project with Jane, who has built and helped others build such things – so she was the project manager.

IMG_0116Earlier in the week, Mike had poured a concrete base, and when I arrived, he was putting the final layer of the brick base on.  He’d purchased these nifty-shaped bricks, and they were held together with construction adhesive to help prevent heaving and shifting over the years of freezing and thawing.  He also used a ratchet strap to help keep it together while waiting for the whole thing to dry.

Once that was done, the next step was to fill the brick base with empty bottles.  The Boehles, as well as others, had been saving wine bottles, Snapple bottles, beer bottles, applesauce jars – any kind of glass bottle they could.  I thought it looked like way too many, but I underestimated how many bottles it would take to fill that base!  IMG_0118We all worked together to fit the bottles in as tight as possible – biggest bottles on the outside, then the smaller ones.  It’s like a puzzle without a picture to guide you.  The bottles are there for insulation value, and to reduce how much sand is needed.  Once the base was filled, then sand was added to fill in the space between the jars and brought up to about 4″ below the edge of the bricks.IMG_0119

Then began our first cob making.  A tarp was brought over.  Bags of clay and sand were opened.  A hose was made ready.  Sand and clay were dumped on the tarp – about a 5-gal bucket of each – little bits at a time, while water was squirted over them.  Then the mixing began, first by using the tarp – lifting the sides alternately, dumping the mixture back and forth.  Then feet were used.  With boots on.  IMG_0120I’ve not mentioned yet, that it was crazy windy – about 30 mph winds, with stronger gusts – and only around 40 degrees.  Bare feet in wet cob was not going to happen.  After the first batch we got smart and started folding the tarp over the mixture and then stomping on it that way, in order to keep the mess off our shoes or boots.  When the cob was “right” as determined by Project Manager Jane, then it was placed on top of the sand, up to the level of the bricks and leveled.  Then another slightly raised area was made in the center.  This was to be the oven floor.  IMG_0121The dimensions had all been pre-figured by Jane and were based on the size of the base.  Once the oven floor was leveled and smoothed, it was covered by wet newspaper.  The reason for this was to protect it for the next step, as well as to help later.  I’ll get to that in a moment.IMG_0140

IMG_0143After the paper was in place, damp sand was then piled on top of the papered oven floor in a large dome.  (Again, the size of the dome had already been determined by Jane using a cob oven calculator for maximum stability and such.)  The point of the sand dome is to provide support for the cob dome.  When we were almost done with that, some of the group started to make more cob.  Jane had decided that we had enough time left before the coming lunch break to get the first thin layer of cob over the paper-covered sand dome.  With the wind being so bad, things dried out quickly.  By getting this layer on now, the paper would be covered and couldn’t dry out while we were eating lunch and warming up.  So, that was accomplished, we covered it all with a tarp that was secured with logs and cinder blocks – and we went in to eat lunch and thaw out!IMG_0144IMG_0147

(BTW – we had a wonderful lunch!  Deborah made quiche and Tim brought a vegetarian chili.  Oh, and there was an afternoon snack of creme brulee pie and banana bread!)

After lunch, hot tea and coffee and some fun conversations, it was back to work.  The cob making continued.  We took turns making cob and continuing to cover the dome.  The extra bottles that didn’t fit in the base were put around the sides for additional insulation.  IMG_0149After that layer of regular cob was on, it was time for the final layer.  This time sawdust was added to the mixture.  The purpose of that is that the sawdust will slowly burn away, leaving small insulating holes throughout that final layer.  So again, we split into two groups – three or four people would make cob while the rest put it on the dome.  Finally – it was done!IMG_0151

There was going to be a final, thin, decorative layer of regular cob put over it all.  But it was late in the afternoon, and everyone was tired.  The decorative layer was going to have to wait until the next day, and most of the helpers wouldn’t be there.  Jane decided that it would be ok to just make the decorations with the little bit of leftover cob.  After discussing it with the Boehle’s intern, Stephanie, it was decided to just make a recessed design instead of raised.  IMG_0154So Jane got to work making the outline of the sun design, then I worked on finishing it while she worked on the door opening to fit the door Mike had built.IMG_0156

By early evening, it was done as much as it could be for the time being.  It needs to dry out for a week or two.  Then the sand will all be dug out.  The newspaper under the sand dome and over it is the signal of when to stop digging.  Then small fires will be built in it daily for a few days or so to finish drying it out.  Then it will be ready to start baking in.  The Boehles plan to use it for all their summer baking.  I look forward to seeing how that all works out.

By the way – because the oven is made of earth, it needs to be protected from driving rain, which would eventually wear it down and wash it away.  Some people use a tarp, others a permanent shelter, others a box they put around it.  I don’t know what they’re going to do yet at Antiquity Oaks, but if I remember, I’ll let you know when they have it figured out.