cob building - when clay isn't clay

I have a question about the clay used in cob building.

A geologist told me there isn't truly any clay in the majority of the Seattle area. Though I know people use clay-like soil to build cob here.

I visited some acreage in Northern Idaho where the owner had the soil tested and was told the clay-ish patches were sand and silt, definitely not clay. We played with this clay-like soil and seemed it would make decent cob.

Does cob require the strictly geologically defined clay, or is "clay-like" good enough?

Jocelyn Campbell wrote:
I have a question about the clay used in cob building.

A geologist told me there isn't truly any clay in the majority of the Seattle area. Though I know people use clay-like soil to build cob here.
...
Does cob require the strictly geologically defined clay, or is "clay-like" good enough?

I think you just answered your own question.  If people build cob with it, you can too, aye?

Try it.

Seattle is mostly built on a filled tidal wetland, so you might need to look for sources in the nearby hills.  Check out the cob popcorn stand in Vancouver, or any local cob projects you can find, and ask them what they used.

If after you try it, you still want a stickier mix, you can often find "recycled" clay at ceramics studios and community colleges.  They toss unfired scraps into a pail or bin of water, and the teacher is supposed to go back in there and turn it into nice clay again after all the students go home.  It's a lot of work to recycle it into ceramic-grade clay again, but as a cob amendment it's superior quality.  They are often willing to donate it to a worthy project, and give themselves a break.

-Erica Wisner
www.ErnieAndErica.info

We've got heavy clay soil here, but I was told that it isn't clay, either.  It's very fine volcanic ash (we have three near-by volcanoes).  However, it looks like clay, it acts like clay, so as far as I'm concerned, it's clay!!

Kathleen

So if we go out somewhere and find something that is "clay like" and then we build a house out of it, we aren't going to have our house fall apart a few months later and all of the cob experts say "well, duh, THAT's not clay, it only looks like clay - they should have sent a sample off to a lab to be sure!"

??

LOL - the idea of a "cob expert" telling you, "They should have sent a sample off to a lab to be sure."

They might, however, recommend building something small as a test case.
Make a few bricks.  Make an earthen oven, garden wall, or chicken-house.  Adjust the mix to suit local soils.  Let it dry out, thump it a bit, and see how it does. 
If it doesn't seem sturdy enough, change your technique, or add something else to the mix to make it work. 

Cob is like baking bread: the secret to a consistent loaf is knowing when to adjust the recipe.  Or, if all you can get is buckwheat, make pancakes.

There's a saying, " Build your barn first."  I used to think that meant you had to take care of business before you take care of yourself.
  But now I understand it differently - I've seen other peoples' "first houses."
If you build your "dream" project first, you're going to get all caught up in the design details, you'll inevitably start doing something and then see a better way to do it, and now you have to decide whether to keep doing it the bad way so it matches, or switch to the better way in mid-wall.  The fear of failure itself will keep you from doing your best work.  If you build a cob house with soil you don't trust, you'll make the walls 3 feet thick and kill an ox mixing it all.
   Build the barn first: build something quick, useful, but not perfect.  Work out the kinks in your supplies and technique.  Your cows can stare at the wobbly corner all night, and you don't have to.

Or better yet, build someone else's barn, or two or three, then build your own, and then build your house.  Practice makes perfect - or at least, it makes for realistic expectations of your own capacity, more accurate labor estimates, and confidence with local materials.

p.s. I've seen heavy silty loam with less than 14% clay used in straw-"clay" infill, and it worked fine, even though it wouldn't meet any geologists definition of a clay soil.  It's not the clay that makes a cob house stand up: it's the bits of sand and gravel.  Clay just keeps them from washing away like a sandcastle. 
  So the feel tests are the ones that matter: is it sticky?  Can it be rolled into a "snake" and bent around your finger?  Will it stick to the bottom of your hand?  Does it swell when wet, and crack when dry?

Let me know if you ever find a lab that will test local soil and tell you it's good for building cob houses, aye?

This is interesting. The region I'm moving to has a lot of towns named things like "Red Banks" and. North Mississippi is all rock hard red clay. No good at all for growing anything but cotton and soybeans, so I sure am hoping it works for a building material!

Krummholtz wrote:
This is interesting. The region I'm moving to has a lot of towns named things like "Red Banks" and. North Mississippi is all rock hard red clay. No good at all for growing anything but cotton and soybeans, so I sure am hoping it works for a building material!

That's great news for me since I'm building a tiny little cob cottage in North MS. And the "red clay" means I should be able to find a nice deep red clay for the adobe floor!  Thanks for the info Krummholtz!

Clay is present in most soils, and clay can be separated out fairly easily. On a small scale, fill a mason jar 1/3 with soil, 2/3 with water. Put the lid on, shake it up, and set in down.  The sand and rocks will settle very quickly. Next, the silt settles. Clay will be on the top, and may take a day to fully settle.

Why pan for gold when you can pan for clay??

You can check out a cob building book, or search on the Web for the 'test' of good cob material.

I will see if I have it written down anywhere, but it goes something like you scoop up so much of your soil in question, put it in a jar with water, shake it and let it sit over night.  How it separates tells you if you need to buy sand, or clay to add to your soil.  If it is close then you make a ball and drop it from 3' I think, if it stays together.... you've got cob!

This is just the gist of what I remember, but you get the idea.  There is no place to send off soil samples for cob testing - LOL but the tests are simple and you can do them yourself.

Here is the recipe and test for cob - http://weblife.org/cob/cob_043.html

If this doesn't answer your questions, check out a cob book from the library ♥ they cover how to achieve cob.

Blessings,

~Jami

geologist said its not clay.

haha! my geology Prof was one Dr. Burt Woodland. We called him Burnt Woodlands cause when we did our washington geology trip he took us to clear cuts and burned forests so we could see the geology around us.

he referred to himself as a professor of golly-gee, and made it clear that of the five or six common languages in which people around the world speak of soils- whether USDA, UNFAO, USGS or whatever, that NEARLY ALL SOILS have some clay in them.

its been estimated that clay is available on over 60% of the earth's surface without need for digging beyond a spades thrust.

it might be 1% clay, but its there.

as for Seattle, much of it is built on clay soils. these were both scours from glaciers where bedrock was not carved out, and geologic relocations from volcanic events. because of this Seattle soils tend to be gravelly, loaded with ash, and so on. but think about it: clay is decomposed feldspar. feldspar, inturn, is primarily aluminum and silica.these re common ingredients in rock, and its a more a question of granularity (clay is generally considered to be that portion of fine-grained soil that is finer than 0.002 millimeters. so its anywhere  silica and aluminum containing rocks - most of them-have had time to decompose)

thats enough science. we can talk electrochemical properties and so on later.

Erica is right on. its tactile. intuitive. once you play with it a few days, you will almost instantly know if any soil you are playing with is buildable. shaker tests and worm rolling and drop tests and so on help clarify exactly what to mix with the soil to optimize building potential. but the "is it possible' is a intuitive skill developed through direct contact. clay is everywhere. it just needs to you touch it...

the biggest problem Ive had with cob has not been finding clay, but having the TIME to work it. its labor intensive. and in humid climates, since it doesn't dry so well, it isn't really worth a dang- I tried building cob in tropical Australia nd it just say there wet for weeks, despite not getting rained on. would not dry out- 90-95% humidity is not the place to build with cobclay...though other applications might work if you can find a way to get the material dry, like kilning bricks or entire homes such as calearth has done...

Deston Lee wrote:other applications might work if you can find a way to get the material dry, like kilning bricks or entire homes such as calearth has done...

I wonder if a solar-thermal driven ventilation system would be enough: an air intake that runs between carcoal and glass, perhaps, with well-designed airflow in the interior and a nice tall chimney to exhaust the structure?

you might get that to work, consideirng that ventilation is basically a fluid (air) motion caused by heat transfer (temperature) and pressure (a coefficient of elevation, temp and humidity, I believe...?). something like mr.hobbit's plenum (http://www.sunnyjohn.com/indexpages/shcs.htm)  could drive a solar powered passive heat exchange with very low investment and operational overhead. where humidity and sunlight occur, I imagine a solid thinker could adopt his notions to a passive system that used the pressure differential to drive a condenser, and recirculate dry- cooler air back into the room...where it would be heated by the sun, pick up more moisture from the bricks and then.... Ive tangented from jocelyn's question, but as an aside its worth noting.

I recommended this book - 'Building Green' - on the alternatives roof topic -- http://www.larkbooks.com/catalog?isbn=9781579905323
A Very complete manual.  They described building a house step by step with great photos, too.  One of the walls was cob and they discussed how they determined the percentage of clay in their soil.

Jami McBride wrote:Here is the recipe and test for cob - http://weblife.org/cob/cob_043.html

Thanks for posting this. Wasn't really sure what cob was... but pretty much covers it .... and very pleasant reading.

I have some really sticky stuff under the river sand on the banks of the river. I could make up bricks of different mixes .... as suggested .... and see if it is worth making something with. I think cob-building a whole house looks to be extremely hard-work. I used some of this heavy stuff trying to build up my bank against river rising and flooding and it was very heavy and hard to work... exhausting. With a crowd it could be really fun though. Start with a rabbit outdoor rest place.... or dog kennel... or something...very cool in summer.

I think thatching as a roof would look excellent on the soft forms of a cob house.

Chelle

Just wondering.... Both cob and adobe are building with mud. What is the difference?

Chelle

I may be off base here, but I think the difference between cob and adobe is the clay/sand ratio and the use of straw.  From what I recall, adobe doesn't stand up to water as well as cob does.  Of course, in the desert, not much of an issue.

Hi Tamo,

Thanks. From what I read in that book that Jami posted, cob has quite a wide range in ratio clay to sand.

I went looking and found this:

Adobe is a natural building material made from sand, clay, and water, with some kind of fibrous or organic material (sticks, straw, dung), which is shaped into bricks using frames and dried in the sun.

Maybe it is cob when applied freehand and adobe when baked as bricks in the sun first..... Or cob is sand clay and only straw...  not just any fibrous material...  ?

I have realised that this is a bit OT. I'll re-post as a new thread. Sorry.

Chelle

If I recall Becky Bee's book correctly, it covers the clay test but if not extremely wet asnd you can roll a handful of it into an elongating snake between the palms of your hands then it will probably work for cob.

Most earth building soil mixes will work well at 30% clay and 70% sand but as Becky Bee mentioned if short on sand add more straw.  The 70% sand and aggregate must include any aggregate in your clay also or you will be sjhort on clay and it will not stick together well.  My clay has enough claystone and rock in it that a 50/50 mix of natural clay w/it's aggregate and sand is very good.

Cob is continuously built upon until the project is done - meaning not bricks- you can stop a bit and poke holes for good adhesion on the next start.  New cob does not like to stick to old without some dampening and working it a bit or also a layer of slip clay (milkshake consistancey) painted over the joining section.  Adobe is mostly made into bricks, however cob is also known as "coursed adobe" in some places meaning it is continuously layed in courses..

Joel Hollingsworth wrote:
I wonder if a solar-thermal driven ventilation system would be enough: an air intake that runs between carcoal and glass, perhaps, with well-designed airflow in the interior and a nice tall chimney to exhaust the structure?

I have read that a poor mans wood kiln can easily be made from black plastic and a dehumidifier. I'm sure it would not be too difficult to eliminate the dehumidifier. Not hard to get water to condense on clear plastic and then follow the plastic down and out of the system.

Problem is, once it is dry, will it stay that way or be an eternal fight?

put your roof on. let it dry here in the PNW its air flow dries things out all heat does is make it faster. this is why a passive solar house is a good thing.

diffrence between cob and adobi is that adobi is topsoil and cob is subsoil.
dont know about you but i dont want my top soil in my walls i want it in the garden.

Ernie,
Laughing, cause sometimes I am not sure I want the topsoil in my garden. Problem with old properties, you don't know what the heck is in the soil. Asbestos, lead, ddt... then again, who wants that part of their house. Know of any simple toxin filters .

look at Paul stamits mycelium running (misspelled i'm sure)
or call fungi perfecti and ask. IIRC they are doing research on just that detox subject.

it is interesting to think of toxins in soil being sequestered in walls. I might not build my house out of it but i think a handsome garden wall might do the trick. best thing would be to degrade it till its none toxic but sometimes that might be a bit difficult.

great, now you send me off on some other area of interest i never knew i had. looks like there's a wealth of info there for me to learn. lost several oak trees in the last 2 years (one was red oak) and it might have had something to do with fungi and/or bugs. added to my list to learn

if you get to talk to Paul you will like him........ he's a fun guy.

Paul is a fungi?

Cool. 

As a ceramicist I'd like to point out that clay particles are flat, and silt particles, while the same size, are round.  This is important!  You can press your thumb into clay, and the detail of your fingerprint will be visible because the flat particles have slid up against one another in a new arrangement, with an interlocking structure. 

Silt can act very much like clay when it's wet, but it doesn't have the same "sticky" or "slick" feeling, and that's because it's little balls rolling against each other.  Silt does NOT have the same structural strength as clay.  I'd say it's very important for a person to know what their dirt is made of before building a house out of it. 

The "jar test" can be extremely misleading for silty soils.  The fine silt particles will settle on top for sure, but that doesn't turn them into clay! 

The best test for questionable clay/silt is to make a snake, or whatever you want, and let it dry completely.  Clay will be hard, difficult to break, but silt will crumble easily. 

Here in northern cali we have volcanic silt as well.  I used very silty dirt in a light straw clay mix as the wall infill of my cabin, and it worked well enough for this purpose.  The straw is the structural component of this wall system, though.  I used the same mud for my rough coat of plaster (not because I didn't know it was silt, but out of desperation to get my house liveable before it snowed), and that's when the silt really became apparent and problematic.  The mud layer crumbles so easily we had to cover the walls entirely with cloth or wooden boards.  (if I wanted to, I could probably get a higher clay mix and continue mud plastering the walls, but we decided this cabin isn't worth the effort as we hope to live in here for less than five years)

I think using silty soil is no substitute for clay, especially for high-clay content mixes like cob.  Telling yourself "it might work" could lead to a nightmare of a building, and possibly dangerous structural compromises. 

I'd take Erica's advice and mine some clay studios for leftovers. 

Or, (actually maybe after you've gone to clay studios and really get a feel for what clay is) you can hunt around for natural deposits.  They exist, usually in a big layer that gets exposed when things get water-logged and that part of the hillside literally slips downhill.  Clays on the west coast seem to be white or grey, thankfully making them easy to spot.  The red dirt is the iron rich volcanic ash and crushed up lava, not at all like the "red clay" back on the east coast.  We have way newer geology on this side of the continent.   

marina,

perhaps there is some clay-like silt that is very fine that would be poor for ceramics, but acceptable for cob?

Lots of good information here on the jar tests and such.  I am a novice but researched and tested a fair amount. 
The difference between Adobe and Cob?  Adobe is made into bricks, dried and stacked dried.  Cob is Adobe used wet.  Basically both are clay, sand, straw.

As for humid climates, and clay vs silt etc.  I had lots of these concerns so began constructing several walls, thought maybe I'd make a porch or something.  One was cob and cord wood, one more of a wattle and daub (cob over sticks, more hollow in the middle)  They got to about 3 ft high and other stuff took priority, never even closed the tops. 
The soil test via jar, snake etc said the soil wasn't really clay (although lots of folks seem to call it that) The open, unfinished walls withstood several major tropical storms, lots of plain ole downpours, and 1 hurricane!  4 years later they were still almost 2ft tall, so in all that rain wind and weather not at all what a finished structure would have to endue my silty, straw, sand cob held up way better than I would have expected in the conditions.  I live in the "Texas Tropics" (think  miami) in what is basically an old alluvial plain. 

Straw works to bind the particles together by sucking tiny bits of moisture and particles into the core, paper does this also So into one of my cob mixes I used shredded low grade paper that was not especially recyclable (very limited here) And it worked wonderfully, used about 10-20% replacement for the straw.  Test Test Test!

Next I am going to try a bottle wall with cob.

The thing no one I've talked to has a a clue on is how well Cob holds up against termites, they do nest in the ground, and we have the flying kind that attack roofs as well as the burrowers. They are unreal on my land, I think because of the farms around me who do spray.  Anything paper or wood on the ground is consumed in a couple of weeks!  Anyone?

Serenity, would chickens (or other poultry) possibly help with the termite problem? 

Kathleen

Krummholtz McCoy wrote:This is interesting. The region I'm moving to has a lot of towns named things like "Red Banks" and. North Mississippi is all rock hard red clay. No good at all for growing anything but cotton and soybeans, so I sure am hoping it works for a building material!

You might want to watch the Back to Eden Film if you thing the only thing that can grow there is cotton and soybeans. You can find the film here: http://backtoedenfilm.com/ It's definitely worth the watch.

I love permies.com. Just had to say that! That said, I am about to pull my hair out if someone can't tell me what is up with my soil. My soil acts like clay looks like clay and passes every single test like clay except the shake test, water and soil in jar. When I put it in the water, all of it settles to the bottom in 15 minutes or less. but I've done the worm test, the drop test, and every test I could think of and it always passes. The other problem that I have is my soil is loaded with slate. I mean loaded. If I dig more than a 1 1/2' I hit a solid slab of slate, and all my soil is loaded with little chips of this junk. I am really interested in making cob for many different projects but am unsure as to whether or not my soil can be used. What can I do?

I have big doubts about getting a response in what seems to be a dead topic, but here is my question anyway.

We are almost ready to start infilling our workshop walls with LSC and possibly trying a wall with woodchips/coarse planer dust with slip.
We live in an area where most of our soils are either a microscopic brown sand and grey glacier silt (both with clay but in negligible amounts),
However,  there is a few local areas where there's a higher clay content soil (still with extremely fine silt in it).
I was really confident about using this soil as it is really sticky and it does a nice thick slip.
But.... I was reading recently about expansive clays (the type we have, it does tend to crumble when dry) and some say that it's a definite ''No go''...
Though I had read that it had been done succesfuly even with a heavy silt soil.
So I was wondering about adding cement powder, we do have several bags left from our foundation building, to aid as a binder in the slip mix.... My concerns would be breathability, maybe it could or has been done? (Or maybe type S Lime? although I would have to buy/import bags since the closest source is at a good 2 hour drive from our place...)
Maybe the solution here is to have a permanent form work like wood lath or some kind of wattle on both sides of the walls. But even then, I would fear that the straw would get ''washed'' on the exterior until it dries.
As for finishing, we were planning on doing a single lime plaster and putting rough cut, live edge pine boards as an exterior sheathing (as soon as everything is dry of course).
Thank you in advance!

When you comment on a topic, it bounces to the top of the forum so it might have a better chance than you think.

As for adding cement, I don't know much about it but cement is added to compressed earth blocks and rammed earth sometimes to stabilize them. You could also try looking for earthcrete. Hopefully someone will have experience that can give you better advice.

Even if it is not clay in a strict sense, it should become vitreous if you heat it up enough. Porcelain isn't clay either. And lots of good glazes are made from local fine sands. Bricks might be your thing, and then covered with lime plaster. Done right, you also get a good bit of biochar out of the arrangement. If you make a test bowl and fill it with water, if the water leaks out, you should add a flux like ashes, neph sy, or minspar to your "clay" body. This will prevent spalling in freezing weather. Coarse sand will prevent cracking during the firing. If you nee more plasticity, add bentonite (2-5% will be plenty).

Erica Wisner wrote:

Jocelyn Campbell wrote:
I have a question about the clay used in cob building.

A geologist told me there isn't truly any clay in the majority of the Seattle area. Though I know people use clay-like soil to build cob here.
...
Does cob require the strictly geologically defined clay, or is "clay-like" good enough?

I think you just answered your own question.  If people build cob with it, you can too, aye?

Try it.

Seattle is mostly built on a filled tidal wetland, so you might need to look for sources in the nearby hills.  Check out the cob popcorn stand in Vancouver, or any local cob projects you can find, and ask them what they used.

If after you try it, you still want a stickier mix, you can often find "recycled" clay at ceramics studios and community colleges.  They toss unfired scraps into a pail or bin of water, and the teacher is supposed to go back in there and turn it into nice clay again after all the students go home.  It's a lot of work to recycle it into ceramic-grade clay again, but as a cob amendment it's superior quality.  They are often willing to donate it to a worthy project, and give themselves a break.

-Erica Wisner
www.ErnieAndErica.info

hey erica, or anyone out there, Today I got a whole load of clay from the ceramics department, can someone help me as I am a Noob and dont know the mixture if I use pure clay and sand.
please let me know. I want make cob for rocket mass heater if thats important


Fraser

fraser stewart wrote:

hey erica, or anyone out there, Today I got a whole load of clay from the ceramics department, can someone help me as I am a Noob and dont know the mixture if I use pure clay and sand.
please let me know. I want make cob for rocket mass heater if thats important

According to this link the proper ratio is 1 part clay, 4 parts sand, and 1 part straw.  I don't know how accurate that is but there is a squish test, a drop test, and a brick test.  Basically, you need to experiment with mixtures until you get the right elasticity.