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Harvesting Clay For Cob?

ryan112ryan McCoy


Joined: Aug 23, 2010
Posts: 45
So I live in North Carolina and wanted to harvest the clay for making a cob oven (I have Kiko's book in the mail).  We have this thick red clay (I don't know what % clay it is) that I wanted to use. 

Has anyone harvested clay before? 

How should I go about it? how do I prevent it from just making huge clods?  how do I dry it (do I even have to dry it)?
Kathleen Sanderson


Joined: Feb 28, 2009
Posts: 977
Location: Near Klamath Falls, Oregon
    
    1
I suspect that your questions will be answered when you get the book, but the process is pretty straight-forward.  You don't have to dry the clay -- you'd just have to add water back into it.  What you do need to do is play with it a bit.  Make test 'snakes' and balls and see if they crack when they dry.  If they do, then you'll need to get some sand (sharp builder's sand) to add to the clay.  I forget the actual percentages (and don't have time to look it up -- I have Ianto Evans' book on cob houses), but the ideal mix seems to usually have more sand than clay in it.  The clay holds things together, the sand prevents cracking.  For a cob house you'd also add chopped straw, but I'm not sure how much straw you'll want in a cob oven.  Maybe some in the outer layer.

Kathleen
Ran Prieur


Joined: Jun 01, 2010
Posts: 66
Location: Spokane and near Diamond Lake, WA
    
    1
First you might want to do a shake test to find out what percent clay it is. You half fill a jar with the soil, fill it to the top with water, shake and stir it until it's all dissolved, and let it settle. Sand will settle out in seconds, silt in minutes or hours, and clay in days or weeks. But just seeing how much sand and silt come out will give you an idea of how much of the rest is clay.

To mix the clay with the sand for cob, the clay will ideally either be a powder, or be completely dissolved in water to form a thick paste. One way or another, you have to get all the lumps out before you make cob. What I do is first dry the clods of clay, then hammer them with a big piece of wood in a five gallon bucket, then dump the small lumps into a 55 gallon drum with water, then stir it into a paste with a shovel. There are other techniques that might work as well or better.
ryan112ryan McCoy


Joined: Aug 23, 2010
Posts: 45
NC red clay is a pretty high clay content.  Today I went out and dug down a bit and got some wet clay, added some sand, a little water and some hay.  I tried a few "recipes" or variations of the ratios and made some test bricks.  using my feet I was able to break up the clay really well, the mix was very uniform. 

Here is what it looks like, I will see how they dry to figure out what is going to work the best.


Kathleen Sanderson


Joined: Feb 28, 2009
Posts: 977
Location: Near Klamath Falls, Oregon
    
    1
Cool!  You've inspired me -- this afternoon I'm going to try that with our 'clay' (it's technically volcanic ash, and light gray).

Kathleen
ryan112ryan McCoy


Joined: Aug 23, 2010
Posts: 45
Went out and made a few more, this time with a mold to see exactly how much they shrink.  Also according to Kiko's book he says the test bricks should be made without hay so I omitted it this round. The other ones from yesturday were rock solid!

Sorry for the bad photo, it was a bit late and it was with my cell phone camera. 

Stan Davis


Joined: Sep 14, 2010
Posts: 26
great tread! i live outside dublin city ireland... i have clay soil and lots of it 

so for the test bricks should i put in sand? or will just use clay and if the test bricks cracks up then i should add sand for the next test batch?
i am going to build my first rocket stove and hope to build a straw bale house down the line

ryan how did your molded clay bricks go? did they shrink and or crack?
Jim Argeropoulos


Joined: Jan 11, 2010
Posts: 96
I have the problem of almost no clay content near by. Even the high clay content soils can't roll a snake. I've decided the only way to get enough clay content is to mix a bunch of soil with water and then drain off the clay rich water to settle in a lower pond and wait for it to dry.
Kathleen Sanderson


Joined: Feb 28, 2009
Posts: 977
Location: Near Klamath Falls, Oregon
    
    1
Silver, I think that's going to be a VERY slow and laborious way to get clay!  What do you want to use the clay for?  Is there anything else local that you can use (indigenous materials/building methods)?

Kathleen
Jim Argeropoulos


Joined: Jan 11, 2010
Posts: 96
I believe you are right. I'll keep hunting, but until then...laborious it must be
My first project will be a cob oven. Next goal is a Rocket Mass heater.
Will Sustane


Joined: Sep 29, 2010
Posts: 10
Silver wrote:
I believe you are right. I'll keep hunting, but until then...laborious it must be
My first project will be a cob oven. Next goal is a Rocket Mass heater.


Like you, Silver, I am out of luck when it comes to local deposits of clay. I would like to try building a cob oven, also a rocket mass heater for a greenhouse someday. What I am wondering is would both these devices function if they were constructed from concrete instead of cob? Or, would they behave in a drastically different or unpredictable way? For example, would a cement oven cool too quickly, or might a concrete rocket mass heater get too hot externally and become a danger? Anyone have any suggestions for us clay deprived individuals?

Will
Kathleen Sanderson


Joined: Feb 28, 2009
Posts: 977
Location: Near Klamath Falls, Oregon
    
    1
I wonder if cement with some additives in it would work?  Something like vermiculite?

Kathleen
Brice Moss


Joined: Jul 28, 2010
Posts: 700
Location: rainier OR
    
    2
for the oven I'd go with castable refractory, expensive stuff but it takes the direct heat of a fire very well,

I would imagine that the advantage of cob is that the inner layers of clay become a porcelain very like terracotta under the high heat, but I haven't had the pleasure of building with cob yet
Jim Argeropoulos


Joined: Jan 11, 2010
Posts: 96
Brice is right. Refractory cement is an option. Kiko does discuss it in his book on ovens. As to the rocket mass heater, I suspect only the high temperature areas would need refractory cement. You could use any number of other mechanisms to work with the bench area.
ronie dee


Joined: Mar 04, 2009
Posts: 588
Location: Cosby MO
    
    2
Kitty Litter is $3. for 25 pounds at Walmart -it's all clay.

Have you seen Paul's portable display RMH? I think it's under portable Rocket mass heater.

You could probably use a wood box with sand and gravel for mass.


Sometimes the answer is not to cross an old bridge, nor to burn it, but to build a better bridge.
Ardilla Esch


Joined: Feb 05, 2010
Posts: 183
Location: Northern New Mexico, Zone 5b
    
    3
Most kitty litter is zeolite not clay.  You would have a hard time getting the litter to bind to itself.  Also, zeolite has water in its crystal structure - if you get it hot enough like in an earth oven, the grains will pop (read: mini explosions).
                            


Joined: Aug 07, 2010
Posts: 271
How is the oven working out?

I just harvested my clay today. Not nearly as nice as in the photos in this thread, but I feel very fortunate to have found what I did.
Jim Argeropoulos


Joined: Jan 11, 2010
Posts: 96
I've started collecting rock for the foundation. I'm balancing time for this project with a mobile phone app I'm working on, cycling, and family.
Troy Rhodes


Joined: Feb 17, 2011
Posts: 247
    
    4
"Most kitty litter is zeolite not clay."

If we add the word "cheap" to kitty litter, then most of them are bentonite clay of one kind or another.  Most of the others are zeolite, a volcanic derived mineral, and unsuitable for this application.

The cheapest wal-mart brand is ground clay, as indicated by the label.  $3 for 25#

I don't know if clay-based kitty litter is good for cob, but it is clay at least.


HTH,

troy
Ran Prieur


Joined: Jun 01, 2010
Posts: 66
Location: Spokane and near Diamond Lake, WA
    
    1
I read somewhere that bentonite clay is not good for cob because it shrinks too much when it dries. But it's supposed to be great for sealing ponds.
                              


Joined: Jun 07, 2011
Posts: 10
Could I use clay from re-digging an old pond? the first few inches on the bottom are swampy gook, but everything beneath that is clay.
Jason Leue


Joined: Jan 27, 2012
Posts: 5
Location: West Oahu
I was wondering if I could use cob to create a pond liner for some garden beds?

I live on the west side of Oahu which doesn't get rain for 10 months a year and would like to conserve water.

What do you think?
Ran Prieur


Joined: Jun 01, 2010
Posts: 66
Location: Spokane and near Diamond Lake, WA
    
    1
Jason Leue wrote:I was wondering if I could use cob to create a pond liner for some garden beds?


Cob dissolves in water, and then you've just got a bunch of wet sand and clay. It's better to use all clay. I've heard the best way to seal a pond is to put pigs in it. Or you could simulate pigs by throwing in manure and doing a lot of wet tamping.
Devon Olsen


Joined: Nov 28, 2011
Posts: 1002
Location: SE Wyoming -zone 4
    
    6
the jar test is a very helpful tip, i have highly SANDY soils, but this may help me to identify if there is even any clay available lol

EDIT:
i did the test, filling about 1/4 with soil
i have about 1/3 jar of sand/small rocks
and about a 1/4 inch of silt thus far
the water is a VERY light grey lol
so my location for my pond definately doesnt have much clay if any, thanks again for posting the idea though, very helpful for giving a more clear idea of clay content in my soil....


Current Cheyenne, WY project
"Do you Hugel?" T-shirts and other products
Jason Leue


Joined: Jan 27, 2012
Posts: 5
Location: West Oahu
Ran Prieur wrote:
Jason Leue wrote:I was wondering if I could use cob to create a pond liner for some garden beds?


Cob dissolves in water, and then you've just got a bunch of wet sand and clay. It's better to use all clay. I've heard the best way to seal a pond is to put pigs in it. Or you could simulate pigs by throwing in manure and doing a lot of wet tamping.


I was wondering about that manure technique. Do you have any more info on this technique ( I think it's called glaze or something)?

Thanks
Ran Prieur


Joined: Jun 01, 2010
Posts: 66
Location: Spokane and near Diamond Lake, WA
    
    1
Jason Leue wrote:
I was wondering about that manure technique. Do you have any more info on this technique ( I think it's called glaze or something)?


It's called gley. Here's a thread about it:
http://www.permies.com/t/3409/ponds/Gley-technique-sealing-ponds-dams
                        


Joined: May 26, 2010
Posts: 278
Location: Iowa, border of regions 5 and 6
Kathleen Sanderson wrote:Make test 'snakes' and balls and see if they crack when they dry.  If they do, then you'll need to get some sand (sharp builder's sand) to add to the clay.  I forget the actual percentages (and don't have time to look it up -- I have Ianto Evans' book on cob houses), but the ideal mix seems to usually have more sand than clay in it.  The clay holds things together, the sand prevents cracking.  For a cob house you'd also add chopped straw, but I'm not sure how much straw you'll want in a cob oven.  Maybe some in the outer layer.


Here's an alternative to straw. On another site, I read a suggestion of use natural rope (they were making a cob pizza oven, 3ft wide at the base, so the recommended length was 3 feet); cut the rope into 1-inch lengths, then separate the strands and individual fibers of the rope before mixing it in to the cob. You'll have to do some back-of-the-envelope calculations to figure out how much extra rope you'll need. YRMV (Your Rope May Vary)
 
 
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