Cob: Re: Cobber's Thumb and Knitting cob together

[Shannon C. Dealy]

On Fri, 28 Jul 2000, Frances Grill wrote:

> Greetings: In regards to the bonding of one days cob to another, why not use
> a key-way  to prevent shearing?  If the wall is 10-20 inches thick, a 4x4 or
> similiar sized fence post mudded into the top of a section of wall, then
> removed as the cob is drying would provide much greater strength to the bond
> joint, be easier to fill on the next days cobbing,,,,of course it is no way
> as cool as a cobbers thumb. Obviously, since much cobbing is done in a
> curvecd plane, your key-way might need to be flexible to follow the course
> of the wall. This key-way also works well to anchor a concrete "cap " to
> protect the wall from mechanical erosion from rain, since concrete doesn't
> bond well to eathen structures, but is quite protective.Cheers, peter grill

A keyway still will not provide the degree of bonding that knitting the
cob together will, since knitting actually results in the cob (and it's
straw or other fibers) from the two layers getting mixed together, making
them in essence into a single piece of cob.  A key-way approach (or for
that matter the "lots of holes" approach for dried cob in my previous
posting), both create a mechanical bond between the layers, but not a
merging of the layers, and if you were to find a way to clamp on to just
the new layer once it had dried, and lifted it up, it would separate fairly
easily (aside from the problem of lifting that much dead weight :-)  I am
not saying that this is necessarily a bad approach, as with everything,
it depends on what your needs are, but for maximum strength I would always
recommend knitting the layers together.

Shannon C. Dealy      |               DeaTech Research Inc.
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