[Cob] Dry mixing & Cement Mixers

[Lance Collins]

Hi,

>Dry clay and sand in a mixer will result in clay balls, unless you're 
>using powdered clay from a pottery supply store.

That's not my experience.

If the clay balls then it's not dry enough.   However a mix dry enough to 
have the clay break up will give you lots of dust and the mix will be hard 
to wet when you add the straw.

I have used half bricks to help break lumps. My old mixer barrel has a 
slightly dimpled appearance as a result but that has not caused any mixing 
problems.


>The only luck we've had with a cement mixer (different than a mortar 
>mixer, which has paddles) is to first soak the clay then whip it into a 
>thick runny slip - note, there are no clay balls in the slip.
>
>Then dump the slip into a cement mixer, and gradually add sand to the 
>desired ratio.  The clay/sand mix will be wetter here than you expect with 
>a foot-mixed batch.  Then throw in the straw, which will absorb a lot of 
>the excess water.

I've done that too.   I know it's not politically correct on this list but 
when I was making blocks (instead of cobbing) the wet mix took a week or so 
before I could remove the forms without serious slumping.  I found that 
adding about one percent cement produced an initial set which allowed me to 
remove the forms in about 24 hours.  There was no noticeable difference in 
the blocks with or without the cement.

I've tried a number of earth building techniques and formed the opinion 
that any other method is probably better than the one you are using at the 
moment <g>

Generally though the amount of water you need to have a cement mixer do 
it's thing results in a mix that's a bit sloppy for cob.   If a mix is 
stiff enough for easy cob it will stick to the mixer blades and just go 
round and round without mixing.

Lance





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