To put it simply cob is a building material made from water, clay, sand and straw. If only it were that simple. There is actually a bit of science and definitely an art to making cob. The good news however is that we seem to be programed with the natural ability to understand the art of cob making.
OK so what is the science of playing in the mud? Well cob is specially made mud much like adobe. You need to use the clay subsoil. This needs to be free of rocks, silt and topsoil. It's important to not use topsoil because you do not want the roots or seeds mixed into the cob. The last thing that you want is for you oven to sprout a plant and crumble. So why is the clay important? The clay is the binder that will hold everything together. When fired it will become terra cotta and become a permanent layer on the inside of your oven. The outer layers will not get hot enough to be changed into terra cotta. This is important because the outer layers will be susceptible to water damage. Now you have to mix the clay with the right amount of sand and water. This ratio is in about 2:1 sand to clay mix. So let's say you're using a 5 gallon bucket as your measuring tool. You would use 2 buckets of sand and 1 bucket of clay. You want to use just enough water to allow the mix to be stiff. If you form the cob it should want to hold it's shape but, if it's too dry it will not pack and shape well and will be extremely coarse. This is the art part of things. You will quickly learn to feel the differences in the mix and you adjust the mix by adding more water, sand or clay to get your desired consistency.
Now you'll notice that I left out the part about the straw. Here's why, when you are building the base and the thermal layer if you use straw it will burn out if exposed to the extreme heat of the fire. I learned this and that it makes the cob weaker and it can collapse easily. I opted to not use any straw in the base or thermal layer of the oven. Now the insulation layer will have a lot of straw added to the cob to make it insulate well and reflect the heat back to the thermal layer of the oven. The straw in smaller amounts is used in cob to add tensile strength to the cob. This just means that it helps hold it together and that the fibers make the cob more resilient to cracking or splitting.
OK so why add so much sand if you're using straw? Well the sand adds compressive strength to the cob. OK I know more fancy terms but, it is important to understand. Clay will absorb a lot of water but, that means the clay will have to expand to hold all that water. This means that when it dries it will shrink and that means cracking. Here's where the sand comes in. The sand gets coated by the clay and gets bonded to the other pieces of sand but, the sand won't shrink. So the cracks and weaknesses of the pure clay is greatly reduced and the rough sides of the sand gives the clay a good bonding surface. When this mix dries it's almost as hard as cement and that is due to the sand. When dry it can hold a lot of weight.
Now a word about mixing cob. If you ask my children they will tell you all kinds of ideas about cement mixers and other mixing machines that they want to use. The reality is most experienced people all agree that your feet are the best mixers in the world for cob and adobe. I tried using an electric mixer and it didn't work well and we still had to work it with our feet to get it properly mixed. Oh and I also burnt out a motor trying to mix the cob. Your feet really are the best things in the world to use. Cob is also one of those things that gets better the more you work it. If it starts to dry out you just add a little water and work it in. Now I've seen videos on YouTube and other sites where people are stomping and mixing cob with boots on. OK look we've all played in the mud and this is a great excuse to do it as an adult so just embrace it and enjoy. Also clay is widely used around the world to draw toxins from the body. So just think about the fact that you're getting $100.00 or more clay treatment for free by mixing you cob with your bare feet.
Now a word of advice, do not add the straw to the cob mix until the sand, clay and water are well mixed and ready to be used. Why am I saying this? Well if you add the straw in early is will absorb a lot more water than you realize and you cob will take two or three times longer than normal to dry. If you add the straw at the end of the mixing you may have to add a small amount of water, or better yet light clay slip, your drying time will not be affected much.
What was that I said, clay slip? What's that? A clay slip is your best friend when working with cob. It's made by slacking clay then mixing that with water to the consistency that you need. OK I know, what the heck is slacking? More fancy terms but, it has a really simple meaning. Slacking is taking the clay subsoil and soaking it in water usually overnight. It turns the clay into a thick sludge in the bottom of the bucket that when thinned out with water can be rubbed, splashed or even painted on to your oven to be used as glue to bind repairs and additional layers on. This is the slip and it's your best friend if you can't get everything done in one day. A thick slip mixed with some sand is also a perfect filler for the cracks that will happen. The slip can be rubbed or painted on as a binder between the thermal layer and the insulation layer.
One more quick note about the use of straw. The more straw used the greater the insulation value but it reduces the compressive strength of the cob. For anything that you want to insulate then add some more straw but make sure to cover it with another layer of cob with less straw to protect the insulation. The same holds true for using straw in adobe but, we'll get into that next.
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