[Cob] Q re: Clay-to-sand ratios?

[john fordice]

Dear Muddy,
Here is a piece I've written for a cob intro class I teach.  Hope  
this helps.
john fordice - cob research institute.
........................
MIXING THE COB and  BUILDING WITH IT

DESIGNING THE MIX
Scopeing the mix is a bit of an art and requires observation,  
communication with the materials, and some practice.  The goal is to  
get as much sand in the mix as you can & still be able to build with  
it.  The best way to do this is make a series of test bricks from  
your materials and see how they are when they dry out.  After you’ve  
found reliable sources of soil / sand / & straw, you are ready to  
proceed.
Check that the soil contains clay - pour a bit of water into a  
handful of the dry soil - knead / mix it together in you palm till  
you get a handful semi-soft sticky mud - squeeze the mud in your fist  
so the mud extrudes out between your fingers & palm - if this muddy  
handful resists when you open your hand, you have clay bearing soil -  
the stickier it is , the more clay it contains.
To determine your mix it is best to add our components use a small  
measuring scoop - this enables translation of your test results into  
larger construction batches
Mix your test bricks in a shallow mixing container - small cat boxes  
are excellent.
The mix test is progressive and additive - each step is toss tested  
for impact integrity , sticking to your muddy palm, and resistance to  
pulling apart.
A toss test consists of forming a ball of gelatinous consistency mix  
- tossing it up 2 or 3 feet and catching it so it impacts your open  
flat palm - the goal is a mix that has as much sand in it as possible  
and:
1.  does not fracture  on impact
2.  just barely sticks to your muddy palm when
     you invert it
3.  a ball that breaks apart only when you pull on
      it slightly.
Start with one measure of soil - wet it just enough to make a  
gelatinous plastic mix ( be careful to not over wet  - if you do,  
just add a small amount of dry material to adjust back to the  
gelatinous state ) - toss test it.
Do a series of Incremental additions of 1/2 measures of sand - wet &  
mix till gelatinous & toss test each incremental mix.
Keep record of the number of 1/2 measures added.
Continue this process until the tossed ball looses it’s integrity and  
breaks apart easily on impact - this is will happen when you pass the  
limit of how much sand can be added.
Subtract one measure of sand from your recorded count - this should  
be the optimum proportions of soil and sand for your mix.
Make a series of 3 test mixes bracketed plus & minus 1/2 measure of  
sand.
Repeat the same test mixes with straw added - add as much straw as  
you can and still have the mix workable
Form each test mix into a 1 1/2 thick rectangular brick.
Place each brick on a sand covered board to dry - be sure to write  
the mix proportions of each brick into it’s surface.
Allow the test bricks to sit till dry - the brick that is strongest  
and does not crack on drying is your approximate  mix
Congratulations !!!  MIXING THE COB and  BUILDING WITH IT

.........................

On Mar 15, 2008, at 9:14 AM, Ocean Liff-Anderson wrote:

> (Muddy Toenails - Thanks for writing.  I am posting my response to
> the list, because I think others need to comment on this as well.)
>
> Sand-to-clay ratio is highly subjective, dependent on quality of clay
> being used.  I cannot offer any suggestion without being able to see/
> feel/mix your cob.  Kiko Denzer's book "Build Your Own Earth Oven"
> describes how to test samples of cob with various clay/sand ratios,
> as does Cob Cottage's "The Hand Sculpted House".
>
> Although cob construction seems simple and elegant, this example
> underscores the importance of receiving adequate instruction in cob
> mixing to be able to make a strong, consistent mix.  In your case
> failure of the cob is frustrating, but not dangerous.  There are
> examples of cob structures collapsing with fatal consequences.
>
> Ocean Liff-Anderson
> Ahimsa Sanctuary
> www.peacemaking.org
> FireWorks Restaurant
> www.fireworksvenue.com
>
>
> On Mar 15, 2008, at 9:00 AM, Muddy Toenails wrote:
>> Thanks for your posts.
>>
>> So what ratio of sand to clay do you reccomend?
>> With much appreciation,
>> h.
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