Building with Cob (a bench)

Some notes about building with cob.

Get Ianto Evans' book unless you are specifically making a cob oven in which case get Kiko Denzer's book.

Make sure you are building near a water source. Watering cans get heavy.

Know what clay looks like. Our shake tests weren't nearly as useful as just getting out there and putting our hands in the dirt. The next thing that really helped was practice bricks. Let them dry overnight to see how much cracking you get. Adjust your mix accordingly.

Our soil was nearly all clay so we added an equal amount of sand. Stand on one side of the two piles and blend them with your feet. Don't stand on the clay. If you mush it down it will be harder to integrate the sand properly.

We dug two child sized wheelbarrows full of clay and dumped it on the tarp for each batch. Doing two batches per day we finished the cob bench in 8 working days.

After the clay and sand are blended add water. Add LESS THAN YOU THINK YOU NEED. Fixing a cob mix that has too much water in it is a pain in the neck. A real pain in the neck.

Do the twist on your mud to mix in the water thoroughly. Roll the batch over several times using the tarp. This helps you mix the bottom part you couldn't reach before. Now do your snowball tests. Hold a packed snowball of the mixture up to shoulder height and drop it. Add more "strength" or more "stick-um" as needed. Strength options are sand and straw. Stick-um options are clay and water. Your practice bricks will help you know what consistency you're aiming for. Don't skip making them. The practice bricks can be used later to practice plastering mixtures.

Now sprinkle with a light layer of straw and blend in with your feet. Make sure each strand of straw is completely smeared with clay. For us we used the amount of clay mentioned above, 360 lbs of cheap grade sand, one watering can of water for each batch (and a second watering can is helpful for washing hands and feet) and one bale of straw for the entire project. So not much straw is needed. Now the burrito test. Roll your tarp, then unroll and see if the mixture holds burrito shape instead of cracking and falling apart. If you have a burrito you are ready to build.

Make sure you put down a foundation first! We used cobblestones.

Locate your bench where it will be protected from the rain and ideally where it is shady both morning and afternoon, so that you get the maximum amount of building time. Locate it by your clay soil. Digging for clay takes a lot of time and work. Bringing over a few watering cans, a bale of straw, and a wheelbarrow with a couple of bags of sand in it is pretty easy comparatively. We located our bench at the edge of the playground and the entrance to the nature trail.

Make "loaves" of cob with your hands and begin to build. Keep the project wet between building sessions -- covering it with the tarp and holding the edges of the tarp down with your wheelbarrows works great. Get a thickish stick and poke holes in your top layer before you leave so that the next layer will attach better. Poking and then rotating the stick as you bring it out works best. If you stick the stick in and then pull it right out it may dislodge that loaf of cob.

The first 2 days it will look like you are making no progress at all. Your bench will only be a few inches high. Don't lose heart. All of a sudden it will be huge. We buried a cinder block and some pieces of something or another in our bench. This makes the project go faster both because whatever you bury takes up room and because it gives the cob more surface area which helps it dry, but it actually weakens the structure integrity so don't overdo it. However, this is a great place for your television to go!

Shape the sides as you go. It is easier to shape them when they are squishy than later when you go to plaster. Keep an eye on plumb. Walls can taper a bit from a broad base but overall they should rise evenly. It is easy for the builders to keep building inward a little as they go upward because they want the loaves to stay put as they place them on. A bench should of course be level as well. We found that our straight and thick "poking stick" worked as a level and a plumb bob perfectly.

If you are working with children I recommend the following. Have several digging sites for your clay, as many digging sites as there are wheelbarrows. Each wheelbarrow can have a different color so you have a green team, a yellow team, a red team, etc. Have a digging tool for each child, a trowel usually. While they are doing digging work the adult fills the watering cans, brings the straw out of storage (it MUST stay dry the entire time, no exceptions!) and gets a bag of sand. Kids dump their loads on the tarp. Some children with sensory integration disorders won't want to dance around in mud; these children can be tasked with digging the next batch of clay. The rest of the children mix. Then you need one or two children to keep the bench growing in the right shape (managers) and some builders. The managers control where the builders put their loaves of cob. Take a photo of each step of the process. First the foundation with nothing on it, then the results of each day's work. We are embedding white ceramic tiles, each with a name of a child who worked on the project, along the back wall of our bench.

Prepare to have a lot of people stop and ask you what you're doing. Be ready to explain what cob is over and over. Invite folks to take off their shoes and get dirty! It really is infectious.

A bench is a larger starter project than a low edging around your garden. If you want to give it a try but want to stay small go with the garden border. It will eventually melt back into the earth so what's the harm? And it saves you from having to go to the hardware store to buy those ugly concrete edging things. Kids love this project and you'll have fun too.


Ingredients in cob:
clay
sand
water
straw


The difference between cob and adobe:
Basically, adobe is formed into bricks and dried in the sun. You then build using the bricks. You build with cob while it is still wet, which is what allows you to shape it into curves or any organic form you like.


How much does it cost?
Our bench was free clay, free water, free straw, free foundation stones and free decorative tiles. I paid for 6 bags of 60 pounds of sand each. Total cost $30.00.