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Making cob 101 (the basics of basics)  RSS feed

 
pierre-olivier corcos
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Hi,
so I'm interested in cob and started to read on the subject. After reading on blogs I tried to make my own and did a couple of test bricks. Now I also started to read the "cob builders handbook" which is very helpful. I must be retarded because I still don't get it.
So here is how I created my test brick:
1) Make your own clay (as per http://ceramicartsdaily.org/ceramic-supplies/pottery-clay/going-local-how-to-dig-and-process-your-own-clay/)
2) Get sand from old sand box from childhood
3) Get dry straw from a random field
4) Mix all 3 ingredient with different clay-to-sand ratio and make test bricks
5) Let dry in garage for a week
5) As shown in pictures

Then I realized that I did not derstand what cob was at all:

1) Cob is not as hard as concrete (at least not the one I did) since I could easily break my test bricks in 2 by hand. I read somewhere it was as hard...
2) Cob is not at all impermeable to water (as shown in the picture where I wet a test brick underwater for 30sec and broke it in 2 to observe water penetration)
3) In the "cob builders handbook" it is said that plastering exterior can be done, but it is not necessary
4) I really fear that a non sealed/impermeable/plastered cob wall will disintegrate very fast in time (like in the picture i named "What I believe cob does after 1yr")
5) Finally, I guest that I just don't get it, because I know thousand of people built homes in this material, but as I see it (and what my little experiment showed me) is that cob is a pile of dried earth, maybe able to support a roof and all, but surely not resilient enough to withstand rain and wind. I mean if I rubbed my finger on the test bricks is was crumbling a little and I could easily chip it out with my nails.
6) For example, if I build a cob bench to watch my bonfire, will it not dissolve in rain and snow after a year? And if I drop a fire log on it it will obviously chip?

This is my first post on a forum ever, so that you guys if you take the time to answer!
Thanks
P-O
Straw.jpg
[Thumbnail for Straw.jpg]
Straw used
Sand.jpg
[Thumbnail for Sand.jpg]
Sand used
DIY-clay-2.jpg
[Thumbnail for DIY-clay-2.jpg]
Home made clay
 
pierre-olivier corcos
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experiment pictures (continued)
Finger-test-1.jpg
[Thumbnail for Finger-test-1.jpg]
Finger test 3
Finger-test-2.jpg
[Thumbnail for Finger-test-2.jpg]
Finger test 2
FInger-test-3.jpg
[Thumbnail for FInger-test-3.jpg]
Finger test 1
 
pierre-olivier corcos
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experiment pictures (continued)
1clay-0sand.jpg
[Thumbnail for 1clay-0sand.jpg]
test brick 1clay-0sand
2clay-1sand.jpg
[Thumbnail for 2clay-1sand.jpg]
test brick 2clay-1sand
1clay-1sand.jpg
[Thumbnail for 1clay-1sand.jpg]
test brick 1clay-1sand
 
pierre-olivier corcos
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experiment pictures (continued)
1clay-2sand.jpg
[Thumbnail for 1clay-2sand.jpg]
teats brick 1clay-2sand
2clay-3sand.jpg
[Thumbnail for 2clay-3sand.jpg]
test brick 2clay-3sand
1clay-3sand.jpg
[Thumbnail for 1clay-3sand.jpg]
test brick 1lcay-3sand
 
pierre-olivier corcos
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experiment pictures (continued)
Images in order:

what I think cob does after 1yr
test brick after 30sec underwater
All test bricks
All-test-bricks.jpg
[Thumbnail for All-test-bricks.jpg]
All test bricks
Brick-after-30-sec-under-water.jpg
[Thumbnail for Brick-after-30-sec-under-water.jpg]
test brick after 30sec underwater
What-I-believe-cob-does-after-1yr.jpg
[Thumbnail for What-I-believe-cob-does-after-1yr.jpg]
what I think cob does after 1yr
 
Kirk Mobert
Posts: 151
Location: Point Arena, Ca
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pierre-olivier corcos wrote:Hi,
so I'm interested in cob and started to read on the subject. After reading on blogs I tried to make my own and did a couple of test bricks. Now I also started to read the "cob builders handbook" which is very helpful. I must be retarded because I still don't get it.
So here is how I created my test brick:
1) Make your own clay (as per http://ceramicartsdaily.org/ceramic-supplies/pottery-clay/going-local-how-to-dig-and-process-your-own-clay/)
2) Get sand from old sand box from childhood
3) Get dry straw from a random field
4) Mix all 3 ingredient with different clay-to-sand ratio and make test bricks
5) Let dry in garage for a week
5) As shown in pictures


Pierre, I'm on a slow dialup connection here so I only loaded about half of your pictures, it's possible I'm missing something.. I've been at the cob thing for some time now and I teach it, maybe I can help a little.

1) As far as I can see, your soil looks kind of silty to me.. The worm test should look different, it should hold together better. The "clay" doesn't seem to bend very far before it comes apart. The WAY that the dirt is on your hands (in the pictures), the soil seems to dry quickly and dust off the hands. It should cling to you and not rub off easily. It could be that your soil came from the top layer... Try digging deeper.
2) The sand should be angular (ridges, like it was crushed) and have a lot of different sizes of stuff in it. Fine, round or single sized sand isn't as good. Most sand should do fine as a bench test though, I think it's likely your soil sample is the issue.
3) Look for long, hearty straw. If you grab a handful with the straws lined up, take it in both hands and try to rip it in two, it should take a good deal of effort if you can do it at all. If they snap when you bend them in half, they're no good. They should kink but not snap.
4) Try a few mixes without straw first.. Try the best of those with the straw. Do tests with no sand as well.

Then I realized that I did not understand what cob was at all:

1) Cob is not as hard as concrete (at least not the one I did) since I could easily break my test bricks in 2 by hand. I read somewhere it was as hard...
2) Cob is not at all impermeable to water (as shown in the picture where I wet a test brick underwater for 30sec and broke it in 2 to observe water penetration)
3) In the "cob builders handbook" it is said that plastering exterior can be done, but it is not necessary
4) I really fear that a non sealed/impermeable/plastered cob wall will disintegrate very fast in time (like in the picture i named "What I believe cob does after 1yr")
5) Finally, I guest that I just don't get it, because I know thousand of people built homes in this material, but as I see it (and what my little experiment showed me) is that cob is a pile of dried earth, maybe able to support a roof and all, but surely not resilient enough to withstand rain and wind. I mean if I rubbed my finger on the test bricks is was crumbling a little and I could easily chip it out with my nails.
6) For example, if I build a cob bench to watch my bonfire, will it not dissolve in rain and snow after a year? And if I drop a fire log on it it will obviously chip?


1)Nope, it's not. Your cob is clearly not the stuff that we're looking for, but cob ain't concrete.. Oddly enough, it can do better in an earthquake because it's not as hard as concrete. Cob will shake and roll a little before cracking, concrete won't. Good cob WILL be quite hard, though you can still bust chunks off a cob wall with a hammer.
2) Cob ISN'T impermeable to water, nor should it be! A cob building with a high foundation and good eves can handle a surprising amount of sideways rain, but it can NEVER sit in a puddle. If a cob house sits in a puddle, it WILL turn back to mud again. So.... Simple, don't build a cob home in a flood plain and encourage ground water to flow away from your building.
3) Nope, plastering is NOT necessary, though they look MUCH nicer with a good plaster on 'em.
4) NEVER completely seal cob walls, NEVER stucco or cement plaster cob! One of the nice elements of cob walls is that they breathe. Earthen walls regulate the humidity inside a building, moving moisture through to the outside, improving air quality in the home. If you cover cob with an impervious layer, moisture WILL build up between the cob and the layer till there is a failure.. Condensate won't find it's way out and it will build up over time. In England, (where there is an ancient cob tradition) they say that UN-plastered cob (in a well built building) has a "loss of face" of an inch a century. This is in parts of the island where it traditionally rains sideways for a SIGNIFICANT part of the year. If you're concerned about hard sizeways rain, lime plasters are the way to go, they're good with water AND still breathe.
5)Cob IS a pile of re-organized earth! It's a VERY nice, inexpensive and comfortable way to go! If done well and maintained, a little cobby can last a couple hundred years, the oldest one I've heard of is 800 years old. You gotta follow the rules with it or else, but the rules are QUITE simple: High foundations (like knee high) to protect from splash and standing water, Tight roofs with wide eves (good hat), don't build someplace where it floods - ever and guide groundwater away from the building.
6) Cob benches need roofs too! I tell people NOT to build outdoor benches.. Not really an appropriate use for the material. There have been attempts to cover them with various concoctions, but none have really been successful.. Gotta roof over them. If you drop your firewood on a well made cob bench, depending on the thickness of the cob, likely the plasters on the corners will chip. Benches get beat up pretty bad actually.. Like I said, I discourage people from building cob benches.


This is my first post on a forum ever, so that you guys if you take the time to answer!
Thanks
P-O


No prob.
Cheers
 
Jami McBride
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Hi Pierre, and welcome ♥

Cob is not intended to withstand extreme or prolonged exposure to moisture, so your cob bench by the bonfire will need to be covered via a roof of some sort and/or plastered.

With cob the saying good hat and boots is everything. Cob is not a cement and will not substitute for it. It is not weather proof, or non water resistant without treatment - see natural floors. But you get a thick mass of cob with straw, kick wall and other support materials plus plaster and it does stay structurally sound. Look at England's cob houses. Every now and then they do require some exterior touch up, but they are still standing strong. The only way they fail is through neglect, and a standard house will do the same.

As far as I can tell in the pictures your grass is not as thick/stiff as straw (recommended material) to add in cob for strength. I have seen several cob houses/structures here in raining, damp Oregon and they are not pitted like in the picture you show. They do have good hats and boots though.

May I suggest that you buy a bit of Ag clay and straw, then run your tests again, just to establish a baseline of good results, and get some experience in mixing. Try adding a bit of manure in one test too, this gives your cob stickiness, which is helpful when cob is combined with other materials. I believe the brick is to test your mix ratio and not for over all strength, for that you need a bigger piece of cob, so try more of a large bread loaf size.

Just so you know, you can make a cob/cement hybrid but that negates the breathable, natural, non toxic aspect of cob. Better to use Roman cement for your foundation, raising your cob up off the ground (good boots).

I'm so glad you posted your experiences and pictures. If you do continue your experiments I'd love to hear what you find out.



Here is a video that may help you in your quest for cob basics ~







 
pierre-olivier corcos
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@Kirk Mobert:
Woaw, this was spot on! Thanks a lot for your input. I would never of thought about the silt thing by myself, and the straw was kidda obvious so I fell kidda dumb haha (the recipe states straw… not grass).
After reading your reply, I do understand much better what it’s all about, thanks a lot!

@Jami McBride:
Thanks for your reply, I got it now I think: “good hat and boots’’!
I’ll be around, thanks to you all
 
Jami McBride
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Bump


I added a video which may help, all the best!
 
Joy Davis
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How do I find the right kind of clay? I live in Iowa. I have not seen anyone do any workshops in Iowa. My daughter says that she was told that cob houses can be built in Missouri. What about Iowa?
Joy
 
Jesse Matlock
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Location: Central Oklahoma Zone 7A
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Joy- a lot of your questions can be answered in this book
Hand Sculpted House
I've found it hosted on the web if you google it.

In it it describes each element of the cob, why they are important, what to look for, and different types of each element that can be used. It has great information on testing for clay, and also says that anywhere on earth you can find clay that is suitable for cob.

Good luck!
Jesse
 
christina angeles
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hi,
I an new to this cob material and I want to ask some questions regarding about cob material:
1. if I am from the city where can I get the materials needed for the cob material specially soil?
2. What if in my place we can not get soil anywhere because it is prohibited? is there an alternative material for soil in making a cob material?
thanks in advance.
 
Tim Pelton
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Joy,
I lived in Fairfield, Iowa for many years and did a lot of building there. Here is how you find clay. Go out in the backyard with a shovel and dig a hole. The topsoil will be very dark about 6" to 18" thick and easy to dig up. Then you hit a lighter colored, harder to dig layer that can be 50 feet or more deep. Congratulations! You found clay. There is little or no sand in the clay (just as there are no rocks in Iowa, there is no sand that rocks weather down into) so the soil does not percolate worth a damn. And that is why Iowa basement walls are all buckled and reinforced with I-beams. Guys who haul and sell soil in Iowa have two choices - Topsoil and Fill. The Fill is cheap and is mostly, if not all, clay. You will have to buy sand.
- Tim
 
Fianou Oanyi
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Any sort of earth building needs protection from weather. So you need a good roof with large eaves to really protect the render and the exterior from wearing away with water and wind. I have seen a lot of houses suffer a bit for having eaves a little too small. Even a lot of blogs and natural building advocates make the eaves too small. Down the track the buildings will need a lot more maintenance. You want a hat with a nice wide brim to stop the elements from wearing away at your wall.

 
Guerric Kendall
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What are you looking for with those bricks? One that can't be broken by hand? I see a lot on people making bricks to test clay quality, but so far nothing that shows what is a good cob mix after they've dried.
 
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