Tuesday, June 30, 2009

Cob building workshop

Last weekend I went to a Sustainable Living Festival at a nearby farm. In addition to about a dozen lectures that I listened to, I also took a cob building class. I've been super interested in cob for years and actually wanted to build our home out of cob ... before we found and bought the one we live in now.

So, now I'm thinking ... cob chicken house.

What's cool about cob is you use what you have. It's a great self-sufficiency skill to acquire. The most common materials are: clay, sand and straw. But again, you use what you have. Bailing twine, sawdust, the earth you dig up from your back yard.

First you lay down a tarp if you don't want the ground to get all icky. Also it helps for rolling logs, as you'll see later. Our instructor, Keiko, laid down sandy soil first. He got this from a river bed.

Then he added clay dug from the ground and dried in the sun.

We all started smooshing it together with our feet. Mine are the red toes.

You add water to get it nice and wet.

Keep stomping and squishing.

Then you do a ball test. Make a ball in your hands and pat it down real good ...

... then you drop it. This batch isn't quite done yet. Needs more sticky stuff. So we added more clay and water.

For this particular demonstration, Keiko was having us make a cob oven. It was a mini one.
First you've got a base (simulated by the milk crate and wood lathe thing), and then you lay down your fire brick.

Keiko interrupted his demonstration to show us some bricks. Instead of free forming with cob, you can also make bricks with a form like this.

This is a cob brick. (It is unfired and used to build in this form.)

You can also fire cob bricks in a kiln (like pottery, 'cuz -- duh -- it is pottery) and they end up looking like this. And they last a really long time.

So back to the oven. Once your cob is ready, you build a sand castle with wet sand. :) This is what you are going to put your cob on.

For this little oven, Keiko had us make the walls of the oven be two fingers thick. It's important to keep your measurements the same all the way up to the top, otherwise you end up with a really thin dome top. Push down on the clay, compressing it but not pushing into the sand.

If a crack happens. Oh well.

Just cut out the broken part and keep working.

Here's all the cob put over the sand form.

Use a piece of wood or a paddle to smack it around a bit to further compress the clay and work out any lumps or weird pooches in the walls.

And then the walls are all smooth.

Cut out a door.

And dig out the sand.

Keiko talked a bit about insulation next. On an oven, you want to keep the heat inside so it doesn't cool off too fast when you pull the fire out to cook in it. (I imagine you'd want insulation on your house, too.) You can actually go up to a foot's width of insulation. At least six inches. Here he's made an insular layer made of sawdust and pottery slip. ('Cuz that's what he had.)

Our next project was a quicker way to make cob that was nice and strong and could be used for walls and such.

Take your cob mixture: sand, clay and water; and add straw. This isn't so much mixed in, as stomped on. Think "squash the grapes." The idea is to keep adding straw and dancing on the whole pile until no mud squishes up between your toes. You don't want a sloppy cob to build with or your walls will smoosh over.

When it was dry enough, we rolled the tarp over and consolidated all the cob together into this log that was strong enough to stand on.

Now you can start building with it. Same as before, compress and integrate the layers together. Don't just lay them on top of each other.

Alternately, you can make coils. (Good for curved walls.)

Now make a giant coil pot!

I got lots of ideas and I'm so glad I went. Next step, dig a hole in my back yard and start drying my clay soil out.

Happy building!

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