[Cob] just cob until it feels right

[John Hall]

Fully agree with Shannon, and what perhaps should be added, is that too high 
a clay content will lead to longer drying times, and long-term shrinkage of 
your walls.
John.


----- Original Message ----- 
From: "Shannon C. Dealy" <dealy at deatech.com>
To: "karl and tabitha o'melay" <karl at omelay.com>
Cc: <coblist at deatech.com>
Sent: Monday, February 28, 2005 6:07 PM
Subject: Re: [Cob] just cob until it feels right


> On Mon, 28 Feb 2005, karl and tabitha o'melay wrote:
>
>> while attending a cob party (me learning to cob) a question arose. if
>> the sand is course do you need to add more or less clay proportionally
>> than fine sand?
>>
>> borrowing a frequent analogy
>> if
>> the clay is mortar
>> the sand is brick
>> the straw is rebar
>>
>> then, logically
>> if the brick is really large, less mortar would be used to assemble a
>> given wall?
>>
>> or does that not hold true in this case?
> [snip]
>
> No.  The sand might be "brick", but it's a bunch of broken pieces of brick
> so depending on "average" shape and distribution of sizes, it could
> require either more or less clay.  To give an extremely oversimplified
> example, imagine a set of wooden building blocks like the ones children
> play with, and how much clay it would take to mortar them together into a
> cube one foot on each side.  Then imagine how much it would take to mortar
> together a bunch of oranges to make the same cube.  Now imagine again
> using a mix of oranges and marbles, less mortar, but still the building
> blocks win.  It is the shapes and size distribution together that make the
> difference, this is part of the reason that round beach sand should
> be avoided if possible (the other being that irregular shapes interlock
> better), as well as why it is recommended that if possible you use sand
> with a good range of grain sizes, the sand grains are stronger than the
> clay, so as your clay content increases beyond what is needed to bind the
> cob, the overall strength of the mix will tend to decrease.  Generally
> this is not going to be critical with a reasonable mix, but you will
> definitely notice the difference if you start playing with really high
> clay mixes for a while.
>
> Shannon C. Dealy      |               DeaTech Research Inc.
> dealy at deatech.com     |          - Custom Software Development -
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