Hi!

im experimenting to find the best mix for making cob with the clay-earth we have here on Swedens westcoast. The testchunks Ive done all have the same mistake on them. When I rub the surface with my fingers, dust comes off. And the corners crumble with pressure from my fingers.
I got the impression that good cob is rockhard when dry and unabel to crumbel except under big force (e.g. sledgehammer)
Is that true?

how crumbleproof should cob be?

Ill be very gratefull for any advice!
I dont know anybody else working with cob, so you guys are my only guidience

Thank you!

my first guess would be that you dont have enough sand in your mixture, of course, it could also be TOO MUCH sand. did you try making test batches and letting them dry to testm for strengthliness?

Sometimes silt looks much like clay. It behaves much differently. If your "clay" contains a lot of silt, it will not bond properly. Clay particles are thin and flat and very small. Silt particles are rounded like pebbles and they crush when dried.Check out the various sedimentation tests to confirm whether you have silt, clay or some mix of the two.

Even if your source is clay, it may contain a large percentage of sand or pebbles. This can mess with your proportions in the final mix if not allowed for in the recipe.

yes - do more tests. make 5 different bricks with different recipes. sounds to me like either you have not enough clay, or poor clay, but the only way to find the right solution is many different mixes.

make sure you do the real clay test as well. sometimes I find people have wet mushy silt and think it's clay. do the palm test and the snake test with pure clay first.

and yes, cob is very hard, but a perfect cob brick will still come apart with a sledgehammer. a 55cm thick wall WILL survive a sledgehammer. you should be able to stand on your cob brick with it on top of two others without it collapsing (think Stonehenge and climb on top).

a bit off topic, but aren't you worried that in a place like Sweden, cob will be too cold much of the time?

good luck,
Tys

I'm guessing that Tys Sniffen and I were typing at the same time and I submitted first. Seems we agree on the possibility of silt.

My climate is warmer than yours and I would never use pure cob due to its poor thermal properties. Cordwood cob would work better. Often small spruce and pine trees are thinned in comercial plantings that cover much of Sweden. You may find a free resource there if you make the right contacts.

Straw clay would be an even better choice.

Thank you for all the replies! they have been very helpfull and informative. Ill post again when ive done some of your recommended tests.

and yes, I have had some doubts about a cob building in this climate.
I even spoke with an arkitekt who has been doing natural buildings since the 70'es. He sad that even cordwood is too badly insulative for this climate.
im gonna meet him tomorrow and discuss more, and show him sampels of my soil.

I really like clay.

Because its technically easy , almost all can do it. my carpentryskills are very limited.
A log cabin is the clasical/logical choise around here. but demands more skilfull labour.

Do any of you have suggestions for other teknices?

strawbales seems to need a timberframe, thus also demanding skills.

Thank you!

N Kofod wrote:

strawbales seems to need a timberframe, thus also demanding skills.

Thank you!

Post and beam framing is suitable for straw bale and it is not difficult to learn. Relatively small logs can be used. Straw bale or straw clay would be much better suited to your climate.

Looking at the traditional architecture of Europe, it can be seen that monolithic earthen wall systems developed in many areas over time. Cob in England, Rammed earth in Gascony(Southern France), Italy etc. The Romans took rammed earth technology with them everywhere they went and it has survived in areas that have a suitable climate. Within France, earthen buildings become less numerous as you head north. Too cold.

Vikings would have seen all of these building methods and they were known to adopt many technologies that made sense for them. They continued with earth sheltered wood. Solid clay walls in a cold climate without suplimental insulation was a bad idea then and it is a bad idea now.

Yes, I continue to agree with Dale. While I live now in coastal, Mediterranean California, I have lived in places where the snow piles high and doesn't disappear until April, and I would NOT build with cob in places like that. Even here there's some challenges with the lack of warmth in the winter to properly heat the mass of our house.

We did a timber frame to hold up our roof and then did cob walls up to it, just to be extra sure here in earthquake country. Timber frame does take more skill than cob, but it's not like finish carpentry or anything. Even if it takes you an extra YEAR to learn and do the work of a timber frame, it's a year well spent compared to the rest of your days in a frosty chilled cob house. A year now will save you firewood chopping forever. And if log cabins are the standard around you, that means that there are trees - logs - and possibly even people around who can teach you or help you on your process.

Better to keep the concepts of cob - small, simple - and apply them to the right sort of insulation structure.

Tys