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Kiko Denzer on Art

[Cob] cement mixer

john fordice otherfish at
Wed Aug 6 12:31:50 CDT 2008

Thanks for the informative post about your mixing technique.

We used a cement mixer, also with good results, for a project in  
Berkeley.  The technique was all done directly into the mixer:
1. 90 % of the batch water first
2. 1/2 the batch dirt (a clay bearing soil)
3. let mix briefly
4. all the batch sand
5. mix briefly
6. the remainder of the batch dirt
7. mix till forms into one coherent lump -  adding sprayed in water  
or handfulls of dirt & sand as needed to balance the mix as follows
if it will not lump you need more water, or
if it sticks to the mixer sides in a slury you've  used to much water  
& need to dust in more dry material .
this whole process in the mixer takes less than 5 minutes - any  
longer will make it stick in the mixer which is not good
8. dump into a wheelbarrow & transport to a tarp for foot mixing in  
of the straw.

Using big rocks in the mix to add the straw in the mixer is something  
that might work with this technique.
Next time I'll give it a try - that would be cool if it does because  
removing the foot mixing of the straw would really speed things up.
I'm guessing it would work best to add the straw into the water first  
& before any of the dirt & sand.

john fordice
cob research institute

On Aug 5, 2008, at 7:48 PM, phil wrote:

> I have had tremendous luck with a cement mixer and my mix does not  
> come out too wet. To keep it from being too wet a mix, we soak all  
> our clay and then mix it with a paddle mixer on a drill until it is  
> like a thick milkshake. We add this to the mixer in which we have  
> already placed our sand. After letting it rotate for a bit and  
> making any adjustments needed we add two large stones that tumble  
> along with the clay/sand and mash it up while helping to keep it  
> from sticking to the drum. We then add straw. Some straw may need  
> to be added after turning it out into the wheel barrow and moving  
> it to the wall. We also use a good bit of shredded paper in our cob  
> and plasters. It makes it feel stickier and makes a lighter mix. We  
> try to never add just plain water and instead only use clay slip.
> Phil
> -----Original Message-----
> From: "Damon Howell" [dhowell at]
> Date: 08/04/2008 03:23 PM
> To: coblist at
> Subject: Re: [Cob] cement mixer
> I am going to try the cement mixer approach for mixing, since I feel
> like I'll never get finished before winter without some mechanized
> help. I tried the tiller and fourwheeler, but didn't have much luck
> with either. I started mixing by hand in the wheel barrow, which
> seems to take about the same amount of time as tarping, but the
> advantage is you don't have to use your back as much. The mix is a
> tad more wet than tarping, but nothing 30 minutes in the sun won't
> take care of. I saw a video on youtube of someone using a cement
> mixer. It appears to come out of the mixer too wet to build with
> right away, but allowing it to stiffen up before hand seems legit.
> It sure will slash the labor tremendously. Like I said, I will be
> trying it, so afterwards I will let everyone know how it turns out.
> I've heard it's not as good but it seems that if you let it stiffen
> before building, it would yield the same.
> Damon
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