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Cob done by using potter clay & sand

Posts: 94
Location: Spain
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I am planning to build a cob oven and I am realizing (through the jar test) that the earth I have has some fine sand and lots of silt, which initially seemed clay (after 30+ minutes, the dirt had almost all settled, and what was left in suspension in water was not much: ie: the water was almost clear). Actually an interesting thing happened which I had never seen before doing jar tests. The amount of dirt swelled initially, occupying almost all the volume of the water, while submerged in water. I marked the level of dirt in the jar after 30 minutes or so, but after a few minutes I saw that the level of dirt was below the mark and after the night it was even lower.

In case I can't find any suitable clay soil, nearby and want to go for the potters clay, how do I know how much Kg of clay to purchase (and sand)?
I know we want a mix that is around 75-85% sand and 15-25% clay (from Kiko Denzer's book), should I just calculate the volume of the external dome and the volume of the internal chamber and subtract the latter from the former to know the overall volume of dirt? Or is it better to have more of each (clay and sand). I know amounts don't need to be precise, but just in order to know approx how much of each may I need (for a ¡n oven of say 90cm (3 feet?) diameter.
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Location: latitude 47 N.W. montana zone 6A
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Hi Antonio;  Welcome to Permies!
I don't have an answer as to how much potters clay you might need, other than to say that having too much is better than not enough.
Have you looked for #50 sacks of dry fire clay? That might be easier to use than potters clay.  I would think that it would be available anyplace there are masons, building fireplaces.  
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