[Cob] rototiller cob (Washington, DC area)

[Henry Raduazo]

	The weather did not cooperate for our tiller cob party Saturday, but  
we did well anyway, completing a 38 foot strip of cob 8-12 inches  
high in a two hour window when it was not raining. Since we were both  
mining and mixing the clay at the same time we found that it was  
easier to make small batches of about 400 pounds.
	Our mix was estimated to be half sand and half clay. For the sand,  
we used rock dust from recycled concrete. This is a new product which  
has recently appeared at the sand and gravel place and it is ideal  
for cob.
	We started by tilling up a section of the earth and adding water to  
form a wet slop about equivalent to one wheelbarrow full than added a  
wheelbarrow of rock dust and tilled back and forth till the color was  
uniform while trying not to pick up additional dirt in the mix.
	We then dumped in about half a large trash bag of shredded straw,  
(Almost a wheelbarrow of straw.) and tilled only enough to  
incorporate it into the mix.
	We then picked up the mix with a dirt fork and took it to the wall.  
Occasionally we found dry spots or gravely spots in the wheelbarrows,  
which we had to hand mix a little in the wheelbarrow. We kept a  
watering can on hand at the wall to add water when needed.
	The mix was generally quite wet to the point where 12 inches of  
building was the one day limit.
	Anywhere from two to four people is ideal for tiller mixing. Only  
one person means you have to stop work to add water, sand or straw,  
if you have more than four people, you are getting in each other's  
way. Our team consisted of two people building and two people mixing.
	A dirt fork is essential for picking up the wet cob from the ground  
and putting it in wheelbarrows.  I like to use the steel toe on my  
steel toed boots as a fulcrum for prying up the cob (to break the  
vacuum with the earth).
	Having two wheelbarrows is nice too, so you can pick up an entire  
small batch and move it to the wall while another batch is being made.
	It is important to note that we were using a cheap 5 Hp. rear tined  
tiller. The front tined tillers withe power driven wheels do not work  
for small batches, and they are difficult to impossible to use for  
large batches because you can not turn them around or drag them back  
and forth easily.
	Chopped straw is manufactured by laying straw out on the earth and  
running a side discharge lawn mower across it. Note you must wear eye  
protectors and an N-95 or better dust mask when doing this.
	Note: When mixing on a slab you do not have to worry about tilling  
more earth up into the mix so, I preferred to make much larger  
batches of around 2000 pounds. I add water or straw and use a sand  
shovel to turn the mix  while a partner tills.
	I am going to do another weekend session on April 25 for a group of  
no more than 4 people.
	If anyone wants to try tiller cobbing during the week just give me a  
call to schedule a date.
Ed
703-360-2316