You will need a good amount of sand, 2 or 3 times more sand than clay, depending on how fine your clay is. You can find suitable free sand near most creeks or rivers, the only place the sand is too fine is at the ocean. Ask the county road crew or a local contractor if there are any sand bluffs in your area.
If there are any dams or canal systems, or any water ways that are managed in your area you might try talking to them and see where they take the accumulated sands, In my area the canal that runs through town fills up with sand and needs to be emptied by back hoe and they are always looking for a place to put it.
It's true that cob works best with a lot more sand than clay, but you first need to find out how much sand is in the clay you have. What is the source of your clay, and have you examined or analyzed it to find its composition?
Location: Western Washington State
posted 2 years ago
Not much clay in the soil I have, so was just going to use dry fire clay.
Stacie; An option for you might be to enclose your mass with regular clay bricks. This allows you to easily mix any clay / dirt / and sand into a cob like mud . Use as many large stones or concrete chunks as you can and fill in the spaces between with your mud/cob mix. The stones are superior to cob at holding heat and your mud is simply filling in the air gaps. Sandy clay mud can often be found on a forest service road or next to a running creek . Contractors excavating a foundation routinely run into a clay layer. They would happily give you as much as you wanted! If you can find a substitute mud , your fireclay/ sand can be used to glue your bricks and around your core , instead of in your mass.
Not all who wander are lost... J.R.R. Tolkien
Companion Planting Guide by World Permaculture Association