I have 'some' cob building experience( helped on a cottage build on Vancouver Island,did a cob oven in interior BC and cob bench in south Alberta)
1/8inch size rocks are fine with the occasional one inch one.
I keep coming back to what the structure is.....clay is the bond for the sand granules and the straw lengths. I used to think that it was the clay that was the main building block when in fact it is the sand. So when I put rocks in that equation,I think can the clay that I squish around everything in the mix hold the rocks as building material.
Yes, those one inch ones hurt the feet a bit and then I would just pick them out but the occasional one will not topple the wall.
Perhaps try making some test blocks with a greater and lesser amount of the stones in your clay and see how they dry and hold together.
Also, it would also depend on the quality of the clay soil..is it pure clay or how much silt/soil is within it?
And, yes, people do put 'things' in the walls to help fill the space and build up faster...this would depend on the width of the wall you're building.
Ianto Evans suggests in his book that 'to reduce the amount of cob needed,you can add hard,non-compostable objects as filler in the middle of the wall.'
Eg: rocks,chunks of concrete,bottles,ect. He writes more about using these on pg.188 of The Hand Sculpted House.
All the best with your build. It sounds like a wonderful project using your colorful clay.
I was reading Ianto's book and they indicate that the random shape of river gravel, sand is the perfect thing for cob. So, I went down to the end of my drive where the creek runs and
gathered some gravel and screened it to 1/4 inch I think and got some of this great orange, almost pure clay along my driveway and made some cob. It turned out really well I think.
It is just a slab about 18"x6"x6" sitting on a large stone. Worked perfect. So, now I'm going to start to test different recipes. This one was about a third clay and I just sprinkled in straw
until it got hard to mix. It actually feels sandier, grittier than I expected. I am going to make a bench also. And, I need to find some cheap or free fire bricks to make a rocket mass heater
test unit. I will update again and hope to hear from any others in NWA area. It would be good to know people to pool resources.
Holly Dozier wrote:I know this is an old thread ... sorry. Im just learning about Cob and I have a question that I can not seem to find the answer too. Everyone is concerned about heating Cob in the winter ... what about cooling it in the summer for us folks that live in NWA. I live near fort smith and the thought of a house that is going to generate heat in the summer is NOT good. Can someone tell me how this holds up to keeping it cool in our hot arkansas summers?
Holly, I wanted to check to see if you got things sorted out with your cob experiment. I live in sallisaw and I will be building a rocket mass heater in the next couple weeks. I'm looking for a source of good clay. I have some in my yard but it's very rocky, too rocky to dig deep. I tried all of the sand and gravel places around here and haven't had any luck finding any. Let me know if you know of a place, it would be appreciated. Thank you!
The clay deposits on my land (upstate New York) are very rocky glacial till, with around 50% stone and gravel and sand, 30% silt and 20% clay more or less. It needs severe processing to make it suitable for stomping, and never with bare feet as there can be sharp bits of gravel, but just removing the rocks bigger than an inch or so and mixing by hoe in a wheelbarrow works fine for small batches and makes a good strong cob by itself. For house-scale building I would use a machine (tractor, bobcat, whatever) to tread and mix it and move it to the building site.