Building a Cob Oven–Part IV

This weekend was the grand dome-raising – we built the sand form for the oven chamber and started covering it in cob for the thermal mass layer. We ran into a few snags along the way so we didn’t get quite done;  but it looks much more like an oven now, rather than a backyard pagan altar.

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Here are the inner baking door (wood) and outer firing door (metal). The handles are just temporary. At least for the outer door we’ll find something more appealing; maybe a nice piece of driftwood.

We needed the doors so we could incorporate them in the form. That should ensure that they’ll fit nicely.

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The form for the baking chamber with the baking door in place. The form is made from wet sand which we’ll scoop out after the cob is dry.

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Here you can see the chimney chamber covered in red clay slip; the baking chamber is already getting a layer of cob. We’re aiming for a thickness of 4” to have a decent amount of thermal mass. That translates to a lot of cob.

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Fortunately, we had a couple of helpers – Nick and Lincoln.

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And here you can see the problem we ran into: the cob slumped and cracked. It was too wet to support its own weight. Apparently, our technique was also flawed: you’re not supposed to overwork the cob, because that just encourages liquefaction.

We decided to trim away a lot of the thickness and apply multiple layers instead. So far it seems to be working better; but we’ll see how things hold up when it starts drying.

We learned some lessons this weekend. This being our first cob build, I guess we shouldn’t be surprised about that. If we ever build another oven, here are a few things to keep in mind:

  • The formwork requires a lot of sand. Buy it in bulk, instead of dragging it home by the 100 lb bag.
  • Take measures to combat slumping: use good technique (see Kiko’s bookfor more details); stack bricks to support the wet cob; or incorporate bricks into the cob wall itself.
  • Have all your materials on hand. It takes a fair bit of time to create the form, apply the release layer, apply slip, and build the first layer of cob. All of this should ideally be done on the same day, so you don’t want to be running out to get more sand or other supplies in the middle of the build.

So not everything went as smoothly as we’d have liked. But cob is pretty forgiving; and there’s no better way to learn than by making mistakes.

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