[Cob] To tiller or not to tiller

[Shannon Dealy]

On Thu, 15 May 2008, Mitch Ventura wrote:

> Thank-you all for your input on the roto-tiller. I was wondering, 
> however, that if the straw is chopped-up (to avoid becoming tangled in 
> the tiller's tines) would that not mean less overall strength? As I

Yes, it will be weaker cob.  The question is how strong do you need it to 
be for your purposes, and what are the characteristics of your fiber?  I 
have worked with cob using straw of all different lengths, running from an 
average of about six inches (even shorter for specialty mixes like floors
and plasters), all the way up to an average of over two feet, and found 
there is a substantial increase in overall strength even between mixes 
averaging 12 to 16 inch straw and mixes averaging 24+ inches (using the 
same straw, the shorter straw was cut down from the longer straw).  Longer 
is better, however, once you get into the 1-1/2 to 2 foot range, it 
becomes significantly harder to mix with your feet, so there are trade 
offs.

Then of course there is the type of straw and it's condition to consider, 
I have used straw made from a variety of crops and harvested in many 
different ways, some of it still somewhat green, in other cases it was
clearly left out in the field to long.  A shorter straw in good condition 
which is not to brittle may be a much better choice than a longer straw 
which is more fragile, particularly if the shorter straw has "good tooth" 
i.e. the surface is rougher so the clay can more readily lock into the 
surface.

FWIW.

Shannon C. Dealy      |               DeaTech Research Inc.
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