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COB - first try - ask for feedback  RSS feed

Martin Nevim
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Hello, I have discovered cob and I am really fascinated by this material. So I have made a decision to try a small project.

So I have made heated bench. An idea was to make a fire form the side , let smoke through and heat the bench. It seem it works. But at this moment I dont have big experience because of making the bench and let it be dry. And now I can see quite a lot of cracks.

What could be a reason? Wrong ratio of sand and clay? Or is it normall and I need to make next step and patch it? Or fast drying?(but it was covered from direct sun)


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Glenn Herbert
Posts: 2180
Location: Upstate NY, zone 5
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It's hard to tell without being there. It could be too much clay, or the wrong kind of clay (some clays like bentonite swell a lot when they get wet and shrink a lot when they dry.) It could be not enough straw - straw helps keep large cracks from forming, spreading the tension out so there are numerous tiny cracks.

Have you done a jar test, putting some clay and water in a jar, shaking it, and letting it settle? You should be able to see different layers of sand, silt and clay in the jar. This will help you know the proportion of clay in your soil, to figure out the amount of sand to add.

The most useful test might be to just make a series of bricks with different proportions of sand and clay and straw, make marks at 4", 10cm, or 10" spacing, whatever is convenient, and measure the marks when the bricks dry. This will show you what mix has the least shrinkage. Be sure to keep notes so you know which brick was which recipe!
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