Soil analysis help

HI,

I am attempting to build a cob oven solo and i have never cobbed before.. I am using soil from the bottom of my now defunct dam and hoping it is ok. the ratio i am going with is 1 soil to 3 sand because to me the soil test looks more clayish...but i basically am guessing and would love anyones input who has some more experience then i do..
I am at the point where i have just made a subfloor pad out of the 1:3 cob mix and as it it drying i can see some cracking... so do i add more sand and worry about the sand being too much and the oven slaking/falling apart.. or just go with the mix. I made some test bricks but i may have over tested them because basically in the end all of them smashed apart (maybe i dropped them 10 times...is that too much?) is cob gonna break eventually?

Because i am doing it solo and just going from cob oven blog to blog and i have kiko's book, i am anxious about stability long term...
thanks for your help!!!
oh and the soil test has been sitting undisturbed for about 1.5weeks.

Get some fiber in there to give it tensile strength while it is drying. My recipe is to use a blenderful of shredded newspaper with water for every 3-4 gallons of cob. A little trial and error and you will soon know how much fiber it takes to prevent cracking.

Hi,
will the sub hearth floor be fine as it is? bricks will go right on top. should i amend the floor with fibre- but that would mean pulling it off and reprocessing.. My concern is the thermal layer of the dome and what i can do to prevent cracking there- i don't see people adding fibre to that layer?

Erin Cannings wrote:Hi,
will the sub hearth floor be fine as it is? bricks will go right on top. should i amend the floor with fibre- but that would mean pulling it off and reprocessing.. My concern is the thermal layer of the dome and what i can do to prevent cracking there- i don't see people adding fibre to that layer?

It should be. As long as your bricks for the hearth are mortared well, it shouldn't be a problem. I have some small cracks in the mortar in my RMH, but they have negligible effect on how it draws air.

Maybe you can get cob to dry slow enough to not crack if you make it thick enough. Like in rammed earth construction. All that weight helps fill in the cracks in the drying process. But I was impatient. I wanted to cob over the top of my RMH and fire it up the same week. So I put in fiber and fired it two days later. It threw off lots of water, but because of the fiber, there was minimal cracking.

it was probably the lack of fatness of the subfloor slab that made it crack a bit- its probably only 2 inches thick- thanks for that information.