Cob help and advice please

My soil is made up of 2.3% gravel, 16.5% sand, 68.4% silt and 12.8% clay. I know I have the bare minimum requirement of clay but am worried about all the silt. I made a test brick and it baked in the sun well.....although did wash away in the end as there was no render on it. I am about the start cobbing this spring and want to know if my soil will hold up. If not, is there any additive I can use rather than shipping in new soil?

That is quite a bit of silt. Best idea is to make something more substantial than a brick and see how it does. Seems there is a general fear of not having the right soil for cob, but the best test is to build something small and see how it performs.

If it turns out that it does need more clay, then just add more clay. Even if you add another 20% clay, you still have 80% of your material on your doorstep.

Thanks to you both for your comments. I was considering making a bread oven to test it out but my stone wall footings are already in and I haven't got any room left. where on earth do I get neat clay from......it's not something I can order from b and q! LOL. In America I notice that they can buy sacks of dry clay powder that you could add as an additive But I have not found anything in the uk like that.

DO NOT GO AND BUY CLAY IF YOUR SOIL ALREADY HAS CLAY IN IT!

Sorry for yelling, but... if your soil already has clay, there is no need to go and purchase clay, is all you have to do is separate the clay that is already in your soil, away from the other materials. Which is really easy to do!

It sounds like you've already done a shake test on your soil, so you're half way there to separating the clay out of your soil. When you do the shake test all the soil gets agitated, when you stop shaking the heaviest particles start to settle first, the sand and silt settle in the first few minutes after you stop shaking because they're heavier than the clay. But the clay stays suspended in the water for a while and takes about 30 minutes to settle. So while the clay stays suspended in the water just pour it off into a separate container, just keep an eye as to when most clay has been poured off as the silt will start to come in behind it. Voila now you have 100% clay.

So if you take 2 equal volumes of your soil and do the above with 1 volume and added it to the other raw volume soil, you've almost doubled the amount of clay in your soil!