[Cob] Roto Tillers and Cob?

[Raduazo at aol.com]

In a message dated 6/21/2006 2:19:54 A.M. Eastern Standard Time,  
soap at whidbey.com writes:

Ok! Some  one please enlighten me on how you use a roto tiller to make COB. I 
need a way  to make cob quickly and efficiently and this sounds like a 
possibility.  

Thanks

David T



David: I am currently working on an article that I plan to submit to The  Cob 
Web and to The Last Straw on tiller cob. The article is pretty much written.  
Now I just need photographs to go with it and I should get them this  
weekend. The article is as follows: 
 
Tiller Cob by Ed Raduazo 
Foot mixing  cob to Build a house is just the sort of thing I might have done 
back when I was  18 or 20 years old and unemployed, but now that I am 65 and 
suffering a few  joint problems getting out a tarp and mixing up a ton of cob 
is not just a hard  day's work, for me it is darned near impossible. It makes 
one ask don't they  make machines for doing that? Well it just so happens that 
they do make machines  for that. They are called rototillers. Tillers are 
designed for turning up dirt  and well that is pretty much what we are doing 
isn't it. But there are a few  adjustments that you have to make in order for this 
to  work. 
The first is using chopped straw.  The tiller has a rotating blades and if 
you attempt to use long straw many of  the strands will wind them selves around 
the tiller shaft and you will spend  half the time cleaning the blades. 
Some of the bailing machines chop  the straw as they bail it, but I have 
never been able to find straw that was  pre-chopped to my liking and have always 
had to make it. Being an old time  gardener of course I have a leaf shredder, 
but the leaf shredder tends to shred  the stuff too fine. The finer stuff would 
probably work and in fact it reminds  me of the straw for "window cob" or 
straw for earth plaster that Ianto had me  manufacturing when I took his class in 
British  Columbia. 
What seems to work better and  faster than a leaf shredder is a cheap side 
discharge lawn mower. Preferably one  with a real sharp blade. I simply lay out 
a series of flakes along side of a  wall, and lowering and raising the handle 
of the mower as I approach each flake  will shred it and throw it against the  
wall. 
Note: Rotary lawn mowers were  not designed to be used as straw shredders. 
Protective eye wear and a P-95 or  N-95 rated dust mask are mandatory 
accessories if you plan to do  this. 
The tiller you use needs to be  one of the cheap tillers with front tines. 
Small light weight is also  a plus, but with plenty of power to drive the tines. 
 I was once given a  Troybuilt rear tine tiller at the National Building 
Colloquium-East and could  not make it work. You need to be able to go forward and 
backward and make really  sharp turns on a small pile of dirt and the rear 
tine tiller would not do that.  The rear tine tillers have a metal plate to 
protect your toes from getting  chopped off and this plate prevents you from 
pulling the tiller back and forth  and prevents sharp turns needed to go back and 
forth across the pile.
      My first tiller was a 5 Hp chain drive  tiller that was fantastic. It 
could power its way through the thickest glop so  that when I was finished 
tilling I was finished. After I wore out the engine on  that one though I 
inherited my Dad's gear drive tiller. Actually it is a belt  drive that goes to a 
drive shaft that goes to the gear box and drives the tines.  This is also 5 Hp, 
but the belt portion of the drive system tends to slip when I  run it into 
really thick cob/sand/straw mix. It still works, but I usually end  up adding a 
little more water than I like or just doing the dry mixing and  finishing on the 
tarp. It still saves you a tremendous amount of  time.
More water makes the cob softer so  you are limited to 6 inches to 1 foot of 
construction in a day and you need two  or more days of drying out before you 
can put on the next layer. This can work  out fine. If you are a small crew on 
a large project, it will take a couple of  days to work your way around the 
wall and get back to the starting point, or if  you are a weekend cobber like 
me you can do couple tons on Saturday and Sunday,  and your wall has five days 
to dry out before the next couple of tons gets piled  on. 
Selecting the mix is pretty much the  same as with foot stomping. More clay 
means a stickier mix that will shrink  more. More sand means less shrinking 
less sticky. I usually go rich on the clay  because that is the free ingredient 
of my mixes, and because if the mix is too  wet, I can do a lot of compensation 
by adding straw. Whereas if the mix is rich  in sand and I try to add straw 
to compensate for a wet batch the stuff will not  stick together. 
My preferred mixing surface is  a concrete slab. (Be careful on asphalt slabs 
particularly cracked asphalt as  the tiller will break up the slab and add it 
to your mix.) The slab surface  gives you complete control of your mix 
ingredients and your mix size. I like to  work with batches of about one ton. This 
is 10 of the deep wheelbarrows filled  to the top. When I did my solar storage 
wall I used 5 wheelbarrows of sand and 5  of clay. In the playhouse job at 
Clarindon we have very sandy clay so the mix is  3 sand and 6 clay. I pour the 
clay and sand out on the slab in alternate layers  to a depth of around 6 inches 
and work around and across the pile then I add  straw and do it again. It is 
good to have an assistant with a shovel who can  pick up clay or sand rich 
material from the edges of the pile and throw it into  the center. 
When the pile looks fairly  uniform, I create a well in the center by letting 
the tiller turn in place for a  while and pulling back to drag the center 
material towards me. Then I fill the  well with water and begin mixing again. I 
find it easiest to turn the center  into a batch of gooey slop and then have my 
assistant throw in shovels full of  dry stuff from the edges and then add 
more water with the hose till I get what I  want. Here is where a good power to 
blade size ratio really helps. With my old  tiller I could mix really thick 
sticky cob that would not slump very much, but  with my new tiller the belt slips 
when the going gets too tough. I was thinking  about removing the outer two 
blades but the tiller is so old that I am not sure  they will come off or that 
I could get them back  on. 
With a tiller it is easy to adjust  the moisture content by throwing in more 
straw or clay or sand if you need to  dry out stuff that is too wet, or by 
spraying water on areas of the pile that  are too dry. 
In one of my jobs I did not have a  concrete slab to work on and I used the 
tiller for both mining clay and mixing  cob. I did this by just throwing sand 
out on the ground and using the tiller to  bring up enough of the compacted 
subsoil to make the mix feel right. The main  difference is that you will use a 
dirt fork instead of a shovel to move the  finished material up out of the pit 
and into the wheelbarrow to go to the  wall. 
Even with tiller mixing cob is a  heavy labor intensive building material. 
Use it wisely. I have been reading  The Solar House (Passive Heating and 
Cooling) by Daniel Chiras, and I believe  that the future of cob is as a structural 
component and heat storage means used  in combination with straw bale and glass 
curtain walls. You can enclose a lot of  space with bales quickly and you 
have a great insulator. With cob and stone you  have a great heat storage means 
and load bearing means. With a post and beam and  glass curtain wall you have a 
great heat collector. They can work  together.
    If you have questions and/or comments or proposed  editorial changes 
please send them to me, not to the coblist. If I like them I  will incorporate 
them into the article before I submit it. 
    I can not send the pictures through  the coblist so if you want pictures 
please send me a separate  E-mail sometime next week and I will try to gather 
up every one who wants a  picture and send them all out at once. (I have dial 
up so you know what that is  like.)  _raduazo at aol.com_ 
(mailto:raduazo at aol.com)