If you are thinking of making some cob, for an earthen oven, or for your new home, then you are going to need to start by testing the earth you wish to use. You need to know how much clay and sand are in your earth so you know how much sand or clay to add when you make your cob.
In their book ‘Building with Cob’ by Adam Weismann and Katy Bryce, they say that a good cob should be made of 15-25% clay and 75-85% sand/aggregate.
So if you do your test and your subsoil has this mix already, then give it a go.
So firstly you are going to have to dig below the surface a bit. Generally you are going to have your grass, which is growing from the black topsoil. Get below this and you are in subsoil. The earth here should contain some clay, sand, stones etc. This is what you are testing.
Here are some photos of the road to our site, just about displaying the transition from darker topsoil to clay subsoil. We are living in an area full of clay so we are lucky, and building with cob is very easy.
Next, get yourself a nice glass jar with a lid, and half fill it with the earth you are thinking of using. You can take samples from various places at your site to see how they differ. Remember to mark each one with the area that you took it from.
You may want to remove larger stones before shaking the mix. Add water nearly to the top, shake well and leave in a stable place to sit. Do not touch it and the earth should separate nicely in to its components. The sand and larger stones will fall to the bottom, and clay will sit above.
You can now measure the percentage of sand to clay, and make yourself a lovely cob.
As an example, if your earth is 50/50 sand to clay you would need to use one part sand, one part earth to bring your quantities up to 75% sand, 25% clay.
It is good to play around with different mix combinations till you find one that works for you. You will want to look to see how much your mix cracks as it dries (cracks are normal but large cracks should be avoided and fixed) if it sticks to the wall nicely when you apply it, if in looks grainy or smooth on the wall etc etc
More on making cob in a future post.