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Dry cob texture/durability?  RSS feed

 
Cassie Smith
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I have read and watched so many videos and nowhere have I seen a description of what DRIED cob should feel like or how it strong it should be.
Will it flake?
Will it crumble when crushed?
Should it be gritty or smooth? (I'm guessing if its smooth there's too much clay?)
should it be hard like an actual brick?
Will it scratch?
Should I expect some flaking if I fire it?

I've made several "bricks" and have tested several different clays.
(Luckily, I live in Ga and my hubby works at a kaolin plant.) I think at this point I may be over thinking and getting overwhelmed by the different clays we have here.
I'm not 100% confident in my mix to start on a project just yet but maybe getting answers to some of those questions may help.

Any and all help is appreciated.
 
Glenn Herbert
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Location: Upstate NY, zone 5
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If your husband works at a kaolin plant, you should have easy access to the highest quality clays. Mix kaolin with about three times as much sand, and straw to taste for tensile strength, and you should have excellent cob. It will likely have a gritty surface, unless you trowel it smooth while still just slightly damp, but should not be crumbly. There are earthen or lime plasters you can use as the finish coat for more durability; for those, you want to leave the cob surface quite rough.

It won't be hard like a brick, but with good clay it will take some effort to scratch it with a fingernail. More sand up to a point will make it harder to scratch.
 
Cassie Smith
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Glenn Herbert wrote:If your husband works at a kaolin plant, you should have easy access to the highest quality clays. Mix kaolin with about three times as much sand, and straw to taste for tensile strength, and you should have excellent cob. It will likely have a gritty surface, unless you trowel it smooth while still just slightly damp, but should not be crumbly. There are earthen or lime plasters you can use as the finish coat for more durability; for those, you want to leave the cob surface quite rough.

It won't be hard like a brick, but with good clay it will take some effort to scratch it with a fingernail. More sand up to a point will make it harder to scratch.


There are a bunch of different kinds of kaolin found around here. I think that is part of what's throwing me off. Too many types. Each one has completely different uses/properties.

I really want to use red clay. To me its so much prettier and easier to use. The only thing is it's brittle and crumbles when dry. Its about 85% pure...maybe that's the problem? I don't have enough sand or coarse enough sand?

The kinds of clay I've tried are used in concrete. I read not to use concrete? Its also extremely hard to break the clay up. It doesn't absorb water to break down and is a PAIN to bust. Im thinking it may keep the walls from "breathing" properly?

The cob tests I've made with the red clay hold up pretty well to fire and hold their shape.
Once its dried it can crumble with little effort. I've seen several cob structures that look like red clay is being used so I'm confused as to why I'm having issues.

Thanks for your input!
 
Glenn Herbert
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Location: Upstate NY, zone 5
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If it's high-purity clay, it will be quite solid when dry. A high proportion of silt will make it crumbly when dry. Fine silt grades into clay, and I have heard of people who used a silty mix with good results. It would really require looking at the actual material and testing it to be sure what is going on.

The bulk material you use is not related to the finish you can get; I would use whatever you can get that works for building, and find attractive materials for the finished surface.
 
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