[Cob] just cob until it feels right

[Shannon C. Dealy]

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.
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