How to make cob - was Cob and strawbale combo

[Shannon Dealy]

On Sun, 15 Sep 1996, patrick newberry wrote:

[SNIP]
> 
> I  plan on adding a cob fireplace as well. 
> My question was how thick should the cob fireplace be when it comes through
> the roof. I mean with metal chimminies they use the 3 layers thick.  My
> wooden beams will pass by the fireplace/stove.

I am not sure about this, but if you can find a specification for brick 
chimneys, the cob thickness should be comparable.  If you are doing a 
conventional style fireplace, you will probably want to line the fire box
and probably the chimney with metal or fire-brick to make it last a while.

[snip]
> speaking of clay....I started to think a lot more about when I begin the cob
> wall. On not so  easy to do thing was to get the clay into a powder enough
> substance to mix with the sand. Any tricks to getting the clay in a state to
> mix with the sand?
> 
I wouldn't worry to much about this, just break it up as best you can.  I
think your best bet is to use the blade of a shovel to chop it up into pieces
a few inches across, then mix with the sand.  Finally, mix in some water
and then use your feet to work the sand and clay together.  You will probably
have to do more of the foot-work mixing than normal, but it shouldn't be
a problem (easy for me to say, I'm not the one doing the extra mixing :-)
You might try just letting it sit for a while after adding the water, 
this will give the clay time to absorb the water making it easier to work.


Quick description of process of making cob:

1 - Mix your local soil with sand or clay to get the right mix, 10%-30% 
    clay, the rest will be sand(minimum 50% of total mix) and silt.
    Try to keep the mix relatively free of twigs, thorns & rocks (don't
    worry if it has a few small (1-1/2") rocks unless they are sharp.
    A small amount of these items won't hurt the mix, but they will hurt 
    your feet later.  Mixing can be done with a shovel, small cement 
    mixer, or by placing the ingredients on a tarp & using the tarp 
    to "roll" the ingredients back and forth (preferably with a 
    partner).  A good size initial mix is about 10 - 15 gallons (we used 
    5 gallon buckets for measuring), after some practice, you may wish 
    to increase the batch size to about 25 gallons.

2 - Add water, this will vary a bit depending on your clay content.  This
    one still gives me trouble (it's always either to much or to little).
    My best guess is that a large (25 gallon) batch will require 2 - 3 
    gallons of water (definately less than 5 gallons).  Start off on the
    low side of the water amount, it's easier to add more than to remove
    excess.  Mix again using the same technique as was used in step one
    (adjusting the water amount if necessary.  The final result should
    have a consistency similar to wet cement.  Don't over work yourself
    mixing, it should only take a couple minutes to perform the mixing
    once you've had a little practice.  At this point you are only trying
    to distribute the ingredients evenly through the mix.

3 - Now the serious mixing begins.  Using your feet (either bare foot or
    with a disposable pair of old shoes) run in place (stomp, jump, walk,
    etc.) on the mixture to break up the clay and force the sand & clay to
    bond together.  Once the mix has spread out a bit, push it back 
    together and repeat the process.  If you are using a tarp, this can 
    be done by pulling an edge or corner of the tarp over the mixture to 
    pull it back together & repeat with one or more of the other corners.
    If you are doing this on bare ground, shovel it back together.

4 - Once you have broken up the large clumps of clay and the mix is fairly
    consistent, begin sprinkling straw lightly over the mix as you work the
    mix with your feet.  Periodically fold the mix over on it self, continue
    adding straw and working the mix with your feet.  When the mix is done,
    it will be very stiff and when folded over on itself, you should be able
    to stand on it without sinking in more than about 1/4".  It is important
    that the straw be thoroughly mixed in, there should not be any clumps.
    Steps 3 & 4 should take 5 - 10 minutes for a 25 gallon batch once you
    have some experience.

    You now have Cob, but what do you do with it?  Will Dudley Doright 
    save Nell from the runaway train?  Tune in next time for the exciting 
    conclusion when the Red red Robin comes Cob cob cobbin along ...  :-)
    (would that be considered corn cob? :-)

    Sorry, I get a little wierd after 3AM.


Shannon Dealy
dealy at deatech.com