Making Cob with a Cement Mixer

Steps for making Cob with a cement mixer return.

It’s best to have the clay soak overnight a 5 gallon bucket. This will help soften the lumps of clay and get them to mix better with the sand. It’s also good to use a slightly damp sand to keep down any airborne dust that can be harmful to your health. I start preferably the day before by filling up as many 5 gallon buckets of clay as I think I will need the next day. If I run out, I try to soak the clay at least thirty minutes.

I fill the 5 gallon buckets up to the top with loose clay and then spray and water with the house until the clay is completely covered. When I am making the cob I start by adding nine shovel fulls of loose sand with the cement mixer running. I then add the 5 gallon bucket of wet clay. With very clumpy clay I may break the clumps by hand or with a drill with a steel mixing arm attached to a corded drill.

I let it mix for 2 to 3 minutes, occasionally spraying additional water from the house to the top left of the cement mixer to loosen any sand or clay that is stuck to the drum. The cement mixer should give you audible clues that it is nearing completion of the cob mix by rocking back-and-forth with the cob loafs thumping on the bottom. If it fails to achieve this you should add more water. The mix will be on the wet side, but this is good thing since the two ingredients (clay and sand) will mix better the wetter the mix. The key is to add dry straw at the end to soak up the additional water and give you the consistency that you need.

With the forms as you see in the background, we go up 18” levels at a time. Fewer levels will make for a more monolithic cob wall, as the area where cob layers upon the dryer layer will always be a weak point in the wall. Occasionally, after a few batches I take the shovel and scrape the inside of the mixer a few times to loosen the sticky stuff. I would say one person working with materials already ready, can do about one cubic yard of cob per eight-hour day. You will be quite tired at the end of a full day like this.