Making cob 101 (the basics of basics)

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

experiment pictures (continued)

experiment pictures (continued)

experiment pictures (continued)

experiment pictures (continued)
Images in order:

what I think cob does after 1yr
test brick after 30sec underwater
All test bricks

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

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 ~

@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

Bump


I added a video which may help, all the best!

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

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

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.

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

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.

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.

I found some nice videos on YouTube explaining how to make and build with cob:

I am completely new here, and I am new to cob building. I have read a ton of info on the subject, and plan on making some test bricks very soon. My only worry is that I live in Louisiana. But... I don't live in a swamp or in a place that floods. I do have creeks & rivers nearby, but I live on a huge hill that has pretty good drainage. There are a few spots in my yard that tend to puddle up a little but nothing major.
So my question is this, does anyone have any tips for building with cob in Louisiana? I can't seem to find any info anywhere on building with cob in LA. No workshop nearby, etc.

What is meant by the word “lime”?
As used in mortar ? Chemical name? Sources?

Benson Smith wrote:What is meant by the word “lime”?
As used in mortar ? Chemical name? Sources?

They are talking about hydrated lime, AKA slaked lime.  As used in mortar, precisely.

https://en.wikipedia.org/wiki/Calcium_hydroxide

It is also the binder in traditional lime plaster, which is the subject at hand.  Mixed with aggregate (course sand) and fiber (chopped straw or chopped horse hair are traditional), a lime plaster is produced that will make a harder, more durable, and more water resistant, but still vapor permeable surface on top of your cob structure.

Two or three coats of plaster are traditionally recommended, with each coat dry before adding the next.  Your finish coat will be a different mix than your base coat(s).  In my base coats, I used quarry sand and chopped straw - which needs to be finer than the straw used in cob; chop it with a weed whacker in a metal trashcan until the average length is no more than an inch or so.  Apply by hand.

For my finish coat, I used sand-blaster sand - which is manufactured, and therefore a consistently finer size - and no straw.  I attempted to use little tabs of dissolving artificial fibers, as are added to some concrete, but those didn't work out well; couldn't get them to dissolve properly.  The finish coat is applied by hand - or, with the right equipment, you can also spray on your finish coat - and then compressed and burnished with a wood float before it dries completely.

As for sourcing the quick lime, you can buy 50lb bags at most masonry supply stores.  I forget the brand name of the stuff I bought.  You can look at big box stores like LOWES as well, but don't count on that.  They are much more likely to provide only pre-mixed quick mortar and quick concrete products.

Once you have quick lime, you need to mix it thoroughly with water.  I mixed mine in 55 gallon plastic drums using a paddle mixer on a power drill.  There is no precise ratio, just mix until you have a very thick slurry.

Then seal them up and let them sit to hydrate, or "slake," for as long as you can.  Weeks are better than days.  Months are better than weeks.  If you possess the necessary degree of planning and patience, years are better than months.  The longer you can let it hydrate, the more complete will be the chemical reaction that forms hydrated lime.

As it hydrates, the lime will sink and a layer of liquid will form on top, which will protect the lime from reacting with air until ready to use.  At that point, carefully syphon off the top liquid, scoop out the lime you need, and throw it into the mortar mixer to make your plaster - or a wheelbarrow if you have the time and the brawn to mix by hand using a mason's hoe.  Be sure to reclose the drum.  At day's end, poor some water back on top of the remaining hydrated lime and reseal the drum.

In case you are interested, here is the complete lime cycle: 1) mining limestone (calcium carbonate); 2) which is burned in a kiln to produce quick lime (calcium oxide); 3) which is slaked in your water drums to make hydrated lime (calcium hydroxide); 4) from which you make lime plaster; 5) which once on your walls slowly pulls carbon from carbon dioxide in the air to reform limestone (calcium carbonate, once again).

An addendum to my post above: I fear I may have misled by referencing "quick lime" among the steps.  I remember now that the raw material bought at the masons' supply store to begin the process is not packaged as quick lime, bur rather as Type S Hydrated Lime.  Although quick lime represents the corollary step in the lime cycle, true quick lime is far too reactive.  I'm not even sure you could easily find it for purchase if you tried.  So, they partially hydrate it and then dry it out into a powder again before selling it, which makes it more stable.  But make no mistake, you still need to slake it substantially in water before it is ready to use.  I slaked mine for several months.

[quote

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





HI there EXPERTS in cob making and its properties......

OVERARCHING WONDERING:
I have wondered, (apart from anyCodes/local rules that would complicate or restrict such freedoms) could I /anyone use cob as a skin for an uninsulated single story existing home that has hardiplank boards on the exterior of most of the building. I realise that cob needs to breath, but can it become the retrofitted exterior skin to a home?

Q 1) What prep would be needed to KEEP the cob on or aligning with a vertical wall  e.g., put horizontal buts a foot above each other, around the skirt of the building with 'prongs' randomly protruding an inch short of the essential width of the cob cladding? so there is some extra GRIP and HOLDing strength as it climbs the height of the single story to the eaves?  OR what what you recommend?

Q 2) The prongs would/could  be dowel, e.g. old broom handles or what about metal rods or retired curtain rails etc? Does it matter what material the PRONGS are? would PRONGS /spkes be necessary and if yes, they could be different lengths maybe if there was any value in that - depending on what materials are on hand, and rather than waste cut offs, or they could be kindle type cut offs. AS LONG AS THEY ARE fitted into a but that runs around the skirt of the building??

Q 3) when you are doing cob buildings or walls (with boots and hats on)    {as per ....  https://www.youtube.com/watch?v=FZS2ZEN2bTs  } it obviously takes a long time. While you are making it and you haven't put its roof on, I figure you cover it with a tarpaulin or such until it has a roof, if it rains YES?

Q 4) If one's technique with making the fresh cob, and your foundation is complete and you are beginning the cob-construction, how long will it take for a square meter or 3ft x 3ft? with on person hands on, besides the foot work of making the cob. OR what other way can you indicate the time span to set aside to make a cob wall or cladding?

Q 5) the components:  there is clay, straw and  ? sand.   IS the clay something you buy or where do you get clay?
Q 6) the straw, it needs no seeds? or it does not matter?

Thanks for anyone that has the answers and cares to share, I really appreciate that.

cobs to ya!

Lady Curley wrote:I am completely new here, and I am new to cob building. I have read a ton of info on the subject, and plan on making some test bricks very soon. My only worry is that I live in Louisiana. But... I don't live in a swamp or in a place that floods. I do have creeks & rivers nearby, but I live on a huge hill that has pretty good drainage. There are a few spots in my yard that tend to puddle up a little but nothing major.
So my question is this, does anyone have any tips for building with cob in Louisiana? I can't seem to find any info anywhere on building with cob in LA. No workshop nearby, etc.

Why not try and contact the folk / the natural builders group that are highlighted in this video. THEY might be able to help you, or put you onto someone that is in your area or you might be able to have an online Q and A with one of them?

CREDIT goes to someone else in finding and posting this on this thread (not me).

Mud/cob building, education and awareness raising of ancient craft of building sustainably and with manual labour.