[Cob] Soil ratio

[Shannon C. Dealy]

On Tue, 2 May 2006, Predrag Cvetkovic wrote:

> Shannon wrote:
>> Your actual clay/sand ratio is meaningless, with some clays it might take
>> 30% clay to make it work, and for others 10% might even be to much.  It
>> depends on how sticky your clay is, and that will vary tremendously from
>> one place to the next or even just from one layer of your soil to the
>> next.  When in doubt make test bricks, that is the only way to be sure
>> what you will end up with.
>
> Just to add: how about testing with some clay in a jar. Shake well and when
> settled, you can see the ratio clay:send in your clay.

The shake test, which I think pretty much everyone used in the early days 
of the cob revival (myself included), I view as being an interesting but 
generally useless test (I think most people I know have stopped using it 
other than for visual interest).  It gives you a ratio of wet clay to wet 
sand which is radically different from the ratio of dry clay to dry sand.
Clay expands greatly when wet, sand basically doesn't expand, some clays 
absorb as much as seven to ten times their volume of water, so that a soil 
which in the shake test might show 50% clay might have 35% clay or less 
than 10%, the only way to be sure is to drain the water off and let it 
dry completely.  Once you know your clay percentage, what does that 
actually tell you about using the soil for cob?  Absolutely nothing! As I 
stated previously, ultimately all that matters is how sticky the clay 
content of the soil is, and more to the point how well it bonds your cob 
together.  There are other tests described in "The Hand Sculpted House"
which are useful tests (I don't have time to give details), but 
ultimately, the final test is how it works in cob, I have made excellent 
cob using soil that is marginal or fails some of the quick tests, this is 
why I recommend that people make test bricks with different mixes and 
once they have dried, check for shrinkage, and breaking strength, 
particularly if you are new to cob (though for any serious project, 
I think everyone should do this before starting).

FWIW.

Shannon C. Dealy      |               DeaTech Research Inc.
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