A few weeks ago, the Oakland staff had the unique opportunity to work with Weston Lombard of Solid Ground Farm and he taught us how to build a cob oven.
Cob is nothing more than clay, water, straw, and some sand, mixed together to create a substance, that when dry, can withstand incredibly hot temperatures. Cob is excellent at absorbing and releasing heat, so it’s perfect for creating blistering hot ovens in which you can bake pizza, breads, or anything else you would cook in an oven.
Weston has built several ovens—and a house—from cob, and was a wealth of information regarding supplies and techniques. Below are the basic steps for cob oven building, but for more information, visit his website here and for great break down of steps, check out The Cob Oven Project.
We’re planning on testing it out at a few events at the Columbus store, so keep in touch!
First, your oven needs a sturdy base. We used a pallet (so we could *hopefully* move it around the nursery) and limestone wallstone. The inside was filled with packed clay. 
Weston demonstrated the importance of having clay that you could manipulate with your hands–too wet and it won’t stay together, too dry and it will crack. It also needs the right amount of sand to hold it together…too much sand, it won’t hold.
After laying down several inches of clay (without straw) to create a base, we then built a ring of clay–into which we laid empty glass bottles (soda, beer, or wine bottles work great) to act as insulation. Over that, we added another layer of clay, and leveled the top to be as flat as possible.
After leveling the top, we placed a layer of firebricks (you can buy them at most hardware stores). These are the floor of the oven and do an excellent job of holding heat. 
To shape the oven itself, we piled sand into a dome shaped pile then covered it with wet strips of newspaper. The newspaper acts as a barrier between the clay (next step) and the sand).
After the newspaper, the first layer of clay (still without straw) is applied. This can be several inches thick, and the thicker, the better–thicker walls will retain for heat for longer periods of time.
After applying about 3″ of clay, we added a thick layer of cob: clay with straw mixed into it. Much like re-bar in concrete, straw in the clay helps to strengthen the clay and keeps it from cracking. You don’t want to put a cob layer on the immediate inside layer of the oven as the straw can catch fire!
To finish, we covered the whole oven with one more layer of smooth cob, and “shaped” the whole thing like an upside-down acorn. The door template was kept in place to mark the size and shape of the door. After several days of drying, we cut removed the template, cut the door and removed the sand pile from inside the oven. 
To get the oven going, build a fire inside the oven…the clay walls will absorb the heat and hold it for hours. After the fire has burnt down to coals, the coals are removed, and the bread or pizza can be slid into the oven. Cob ovens can get up to 700 degrees in temperature–perfect for baking pizzas in minutes. As they cool down to the 300-400 degree range, they are great for baking bread.
We look forward to experimenting with the oven and hopefully turning out some delicious pizza for staff and customers!
