This post came from several places; Christina Ott and I talked about this on the podcast and I read about it in the Hand Sculpted House.
I think Cob construction has some great benefits to preppers and homesteaders, but before you decide that cob is the material you will build with you really ought to test your soil to see if it is appropriate for your land.
Cob should be a mixture of sand, clay, and fiber – It is my understanding that the best mix is 30-50% clay. Sand gives it form and the fiber gives it tensile strength, but it is the clay sticks it all together. This post shows an easy soil test for cob.
Cob Building Clay Test
- First scrape away the top organic layer.
- Next take a cup or two of soil from various potential house sites and from various depths. (Soil samples can vary a lot even a few feet from each other.)
- Take out any stones or pebbles.
- Put each sample in a quart jar, you should have about 1/3 of the jar full of broken up soil.
- Label each jar
- Next fill the jar 2/3 full with water and either some salt or a couple drops of dish soap.
- Shake well. Very well – make your arms tired, switch off with someone, and then switch back and tire yourself out again.
- Then let it settle. If your soil has sand, silt and clay in it, you’ll get three distinct layers.
- The sand is the heaviest and will sink to the bottom as you watch.
- The silt will settle next, and the clay will stay suspended in the water for a couple days then settle on top of the silt.
Finally, the best soil with have little silt and a lot of clay. As you can always add sand to get the proper consistency.