ilovecob.com/video/rotocob » [ Quicktime 8.5 MB ]
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I had the opportunity to mix some cob with the rototiller. A neighbor tossed the sand and the clay into place with the Bobcat. Then I went at it with this noisy gasoline drinking machine. It works really well! It made a great cob, the type for building mass walls. It mixes the straw in very well, though it chops it up a little more than I wanted… I just added more long strands for the wall mix.
I guess the straw can get caught up in the tines if its not a little wet.
Great way to mix the mud.

Watch the Making Cob video:

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Cob = clay-rich soil, sand, straw, water + human energy!

People often forget to mention the human energy when speaking of the ingredients of cob! It’s either human energy or some other mechanical method of mixing. Electric mixer cob, tracker cob, tiller cob, cattle trodden etc. I mixed it up on a poly-propylene tarp. What’s nice about using a tarp is you can toss the material around, pulling the edges towards you while you mix. A basic mix might be 70% sand and %30 clay, with straw to ‘taste’. The ratios change from site to site as different sites have different soil content. In this video I was using a 2 clay-rich soil to 1 sand. I probably could have used more sand, but the batch worked fine for me.

Experiment!

If you are looking for a way to experiment; try gathering a bag of sand from a home center and some clay from a local ceramics outlet. If you can, dig a hole and find out what type of soil you have underneath you. You’ll want to go down below a foot to avoid the organic sub soil material. Mix it up. Make a few small test batches… really clay rich, half-n-half, more sand… more straw. You want something pliable, solid, a little chunky and (with straw) tough to separate. There’s nothing like playing in the mud!
What does each batch feel like?
What do they look like after they sit out for a couple of days?

Resources

There are many cob resources on the web. If you are interested in a good, comprehensive book try the book ‘The Hand Sculpted House’, by Cob Cottage members; Ianto Evans, Michael G. Smith and Linda Smiley. Or if you’d like check out part of Becky Bee’s ‘Cob Builder’s Hand Book’ online:
http://www.weblife.org/cob/cob_043.html »