Cob is a mixture of clay, sand, and straw similar to adobe. It has been used for building for thousands of years, as it can be readily made from materials available is most parts of the world. It does not require any special tools or equipment, but is very labor-intensive.
Ideal ratio for cob is about 3:1 sand:clay but will vary depending on the exact materials used. You do not need to get the perfect ratio, there is a reasonable broad range over which the cob will work. The larger the structure you are building, the more important it is to get near the ideal properties, so I stronger recommend starting out with a small project before tackling something large. There are some standard tests for cob strength. Cob is labor-intensive, so even a "small" project will take more materials and harder work than you think.
Clay - Clay provides the stickyness that holds the cob together. See Finding and Processing Clay. For cob you can be less picky than for making pottery. A few small rocks and leaves won't hurt the cob. But do be sure it is good sticky clay that will hold together well once dry. Also, for cob the clay can be pretty wet, as the sand and straw will absorb water.
Sand - Sand provides structural support, making the cob much stronger than clay along would be (also greatly reduces shrinkage). Sand grains should be rough and angular, so they lock together rather than sliding past each other. The more sand gets transported, the rounder the grains get. The best sand is collected from right where it weathered out of the rock. River sand is usually okay. Beach sand is usually very rounded and should be avoided.
I found a local roadcut that had some rough sand from weathered granite. I collected a number of buckets of sand, took them home, and screened through 1/4 inch hardware cloth. Screening the sand removes any sticks or larger rocks, and makes sure that any lumps are broken up. The 1/4 inch mesh worked well -- some of the bits were larger than we think of as "sand" but still worked well in the cob. The sand had a little bit of silt/clay mixed in, but I decided it was not enough to be worth removing (clay is okay, but more than a little silt can be bad). I did try washing some of the sand, that removed the silt/clay but I decided it wasn't worth the effort.
Straw - Straw provides some larger-scale support and helps prevent cracking. Also reduces weight a bit.
Putting it all together - Cob was traditionally mixed in a pit dug in the ground, but a tarp is very convenient for mixing small batches. The plastic sheeting used to cover lumber also works (often available free from lumberyards once they're done with it). [Note: I have read that old carpet also works well, but have not tried this myself.] Some small holes in the tarp don't matter but it should be fairly strong. I like to use 2-3 layers of the lumber sheeting. Spread 1-2 buckets of sand in the center of the tarp, and add about a bucket of wet clay. Take off your shoes and start mixing the sand into the clay with your feet. When the mix gets too spread out, lift an edge of the tarp and pull the edge of the cob mix into the center of the tarp. Once the sand and clay are well mixed, pick up a handfull and squeeze it. As you squeeze you should hear and feel the sand grains rubbing against each other, and the lump should hold together well. If it's too soft and the sand grains just slide past each other, add more sand. If it is good and moist, but not holding together, add more clay. If it is started to get dry, add water.
Once the sand and clay ratio seems good, start adding straw. Sprinkle some straw over the cob, work it in with your feet, and use the tarp edges to fold the cob over while you keep working it. Don't add too much straw at once, or it will tend to form a layer that can be difficult to break up. Better to add a thin layer of straw, fold it into the cob, and then add another thin layer. The straw will absorb water, so you will need to add more water periodically to keep the cob workable. As straw is worked in, the cob will start to behave more as a single mass, and becomes more difficult to fold over.
Once the cob is mixed, it is improved by letting it sit overnight (cover to avoid drying out) but for small projects where top strength is not critical, it can be used right away.
Building with cob - Building with cob is both tricky and easy. The basic idea is to tear off chunks of cob from the large mass that you made, and stack them like bricks to make whatever structure you are building. The tricky part is that for maximum strength, the "bricks" should actually be blended into each other a bit, avoiding planes of weakness between them. There is a special tool (a "cobbers thumb") that helps with this, but you can also just use your fingers. The process is a little similar to joining two pieces of clay when making pottery. Again, for small structures getting this just right isn't cricital, but you should get really good at it before you start building a house.
Cob tips: place sand on tarp, add clay, mix well (keeping wet enough to be soft). Add more sand as needed (you have enough when you can hear/feel the sand grains grate against each other when the cob is squeezed). Add straw last, not too much at once or it tends to form layers that are difficult to mix in.
References about cob
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