Bricks and mixers

[Mark A Hoberecht]

At 06:28 PM 11/24/96 -0800, you (Shannon C. Dealy) wrote:
>On Sun, 24 Nov 1996, Eric D. Hart wrote:

>> 2.  What about mixing cob in a cement mixer??  I've mixed stucoo by hand in
>> a wheel barrow and in a cement mixer and the mixer was much easier!!  I know
>> part of the appeal of cob is mixing with you feet but I'm just trying to
>> make it easier.  
>> 
>
>You can mix the earth and water ingredients with a cement mixer, but once
>you start adding straw, I hear that the mix becomes too thick/solid to work
>in a mixer, so you will still need to work the straw in with your feet.


I had heard the same thing about mixing cob by mechanical means, but decided
to try it anyway.  I had also heard that the mixer of choice is a mortar
mixer and not a cement mixer.  I got a great deal on a used mortar mixer
($75), so here's what I discovered.  The mixer does indeed bog down once you
start adding the straw, but ONLY if you're making large batches.  When I
made smaller cob batches, it worked TREMENDOUSLY well.  After lots of
experimentation, here's the recipe I came up with:  add 5 quarts clay
(sifted) and 8 quarts sand (I actually used crusher fines instead of sand
for a more "structural" mix) to a 5-gallon bucket.  I then poured this into
the mortar mixer, and after about 30 seconds everything was well mixed.
Then I slowly added about 2 quarts of water (more or less depending on the
moisture content of the clay and sand).  No need to wait before SLOWLY
sprinkling in straw -- a very large handful.  By adding the straw slowly, it
gets evenly mixed into the cob.  You can start dumping it out about 10
seconds later, then go ahead and pour your next dry mix in.  I usually had 8
buckets lined up and would just mix one batch right after another.  Then I
could turn the mixer off after about 15 minutes and I'd have enough cob to
last me a couple of hours.

I was amazed at how well this worked.  Since I was cobbing by myself most of
the time, I didn't have the luxury of a partner to help with the tarp mixing
method.  Every batch was pretty consistent.  I even was able to put a "too
wet" batch back into the mixer and add a little more dry ingredients to
stiffen it up.  A few words of caution.  If you mix enough cob this way,
eventually you'll get sloppy and the bucket will slip out of your hands and
follow the dry ingredients into the mixer.  LET IT GO!  You can always get
another bucket.  Once you see what happens to that bucket, you'll never
think about reaching into the mixer when it's running.  It's tempting
because the blades are moving relatively slowly (slower than a cement
mixer).  But it's still a 7 or 8 HP engine packing a lot of power.  I also
found that my batches were just a little wetter than the cob typically mixed
in a tarp.  It would tend to slump if I tried to build too high in a day,
but if you're working by yourself and it's a dry day, it's hard to get ahead
of yourself.

If I had to do it again, I would do as Shannon suggested and let the cob sit
overnight.  I found that the cob that sat the longest before putting it on
the wall was the best.  A good sequence might be to mix it the evening
before the day you plan to use it, and just cover it with a tarp overnight.
Your Minnesota weather is pretty similar to ours in Ohio, so I would suspect
that you wouldn't get too much drying overnight.  In fact, you may be able
to rent a mixer for just a day and mix up enough cob to last several days if
you can keep it from drying out.  Good luck!

Mark Hoberecht
scarecrow at lerc.nasa.gov