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