Sunday, October 10, 2010

Cob Oven at Kev & Kimberly's

On 10/10/10, a dozen permaculture enthusiasts from the area helped us start a cob oven.

Anika, Weston, Connie and Kevin work on the base of the oven. Weston built his own cob house this summer.

Prior to the work day, we got some gravel and sand from a quarry 2 miles away. We dug a foundation pit and saved the clay for building material. We dug it about 20 inches deep. 30 would have been better, both to protect against frost heave in winter, and because we eventually needed more clay.

Cob is a British word for a gob of clay, sand and straw. You mix these materials with a little water using bare feet. Anika, Jason and Eliza mix while Mollie looks on.

Dan works on the foundation.

We made the walls of cob during the work day. Later on, to gain experience with different techniques, we built a cantilevered arch out of corbels, or cobs with long, parallel straws added for strength. This took over a week, because each layer had to dry before we added the next one.

The completed base with corbel-arch firewood storage.

We lay down sand, then firebrick to form the fire and cooking surface.

The sand form for the oven cavity is about 70% as high as it is wide.

Kimberly starts to build the inner dome. This sand-rich cob will hold the heat from the oven fire. There's no straw in this mix. The cob blocks are pressed into each other, not into the form.

Kevin finishes the inner dome.

The outer dome uses vermiculite rather than sand. This provides insulation. Straw provides structural strength and further insulation. We knit the cobs into each other using a wooden "cobber's thumb."

With the dome nearly complete, we cut into the wall and scoop out the sand. This hole became the door of the oven. The oven is 11 inches tall inside, and the door is 7 inches. 7/11 is a good ratio for efficient fires. This photo was taken November 22.

The dome finished, but still drying. We need to keep it under a tarp much of the time so the rain doesn't wash it away. We lit a small fire once to speed the drying. But before we start cooking with it, we plan to plaster and whitewash it.

This project gave us some experience with earthen materials before we design our cob cottage this winter. A cob oven can supposedly crank out pizzas every 3 minutes for hours from just one firing. It may even remain hot enough to bake cookies the next day. When the weather warms up again, we'll fire it up and see how it works!

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