“The primary construction material is cob. Cob is made of subsoil which is on the campsite. Cob is one of the oldest and toughest building materials, some of its structures date back millennia and are still standing today. It is still a commonly used material due to its accessibility and how easily it can be molded to create curvaceous surfaces. To make cob, we have to dig into the earth for subsoil, and using our hands and feet, mix into it water, sand, and straw so that the clay and straw are consistently woven and held together. That’s something the children will have a lot of fun doing. To mold the cob into the shape of a gourd, we’ll have to build a steel frame in the shape of a gourd and cover it with chicken wire. A layer of burlap drenched in a mixture of subsoil and sand stretches over the structure, then the cob is applied. Once the cob dries in a few days, lime plaster is applied over the entire structure. Two or more layers of lime plaster will help strengthen the structure. The ceramic and plaster reliefs and the instruments will be made in camp classes and put on the gourd. Over time, the structure will look weathered and have lichen growing on it, which will look gracious and settled into the campsite.”
Cob is an ancient building technique that uses the earth itself to make structures. People have been using cob all over the world to make beautiful, organic, one-of-a-kind dwellings and art pieces since prehistoric times. It is fire proof, resistant to earthquakes and cheap! Best of all, regardless of other local resources, almost every place on earth has the makings of cob available. It is a sustainable and accessible building method that has stood the test of time.
Great Video from Cob Cottage in Oregon
What goes into a good Cob mix?
Clay : Clay is found naturally in soil. It’s microscopic plate-like structure makes it lock together easily, helping to hold together the cob mixture. In other words, it’s sticky!
Clay particles under an electron microscope:
Sand : Sand is made of faceted particles that, when mixed with clay, grip onto the sticky clay and lock it together. Sand particles are much larger than clay particles, often visible to the naked eye. There are many types of sand: beach sand, play sand for sandboxes, river sand, gravely sand. When making cob we are looking for a rougher sand, with many faceted edges to grip our mixture.
Sand under an electron microscope:
Straw : When choosing straw, we are looking for the longest pieces we can find. As these strands dry inside the cob mixture they act like rebar, locking together the whole structure with a web of organic fibers.
Water : We will use water to mix it all together!
We Try Making COB!
When we get to Camp the first thing to do is test our soil and make Cob test bricks.
Using the soil in the Shake Test Video we tried making several test bricks using different ratios of Cob ingredients. There’s a bit of a learning curve here, good thing we will get lots of practice mixing! Once these dry we can attempt to break them and see how strong/crumbly they are.
After one week...
RESULTS: The bricks are very close to dry, they perhaps could have sat a little longer (it rained a lot this week, which didn’t help I’m sure). All of the bricks broke in half under pressure, but the ones with less sand held up a LOT better. It is nice to see the straw structure inside the bricks, but perhaps more straw would help it hold its form. You can tell that the clay soil is stronger just based on the clean break in the bricks to the right.
NEXT STEPS: I would like to do another round of bricks, working with more clay soil and more straw. Just for fun, I also would like to try using a mold to make the bricks rectuangular.
This video of a family making and testing cob bricks in Costa Rica was very helpful and informative (they begin the test at the 5:45 minute mark):
We had similar results: too much sand = too crumbly!
Making rectangular bricks using a mold:
COB Brick Test - Round 2!
We used some different methods to be more accurate with this round.
-Measuring cup for accurate ratios.
-Pre-measuring straw so that each brick as equal amount.
-Makeshift brick mold (tupperware) for consistant size and shape.
We have learned that the clay soil-heavy samples work best, so we stuck to that end of the spectrum for this round.
In the video below we test our Cob bricks after drying for 2 weeks.
- All of the test bricks held up very well. This partially had to do with sticking to the Clay-heavy end of the spectrum, sand is crumbly and falls apart easily.
- The 100% clay soil brick was full of cracks, due to clay shrinkage. We want to find a happy medium between crumbly sand and cracking clay.
- Somewhere in the 75% clay soil -> 60% clay soil range seems to be best with this soil.
- Remember! Clay soil has some sand in it (remember our Shake Test?)
- The soil at Camp may have drastically different Shake Test results, so we will need to do tests there as well.
- We know Cob works for building. Now we will focus on the plaster mix that will go on top of our Cob to produce a polished, clean finish. Check out the Lime Plaster page for more info.