This summer, using Kiko Denzer and Hannah Field's book, Build Your Own Earth Oven, Tara and I constructed our first cob structure: a pizza oven. We began the project by digging for clay in the woods behind my mother's house. This task was surprisingly easy, and we hit pay dirt in a matter of minutes: just a few feet beneath the topsoil we found a wealth of sticky, reddish clay, perfect for building. Next, we made test bricks with various ratios of builder's sand and clay to determine which mix would yield the sturdiest oven.
Once we settled on the best combination, we got our shovels and went to work digging the foundation for the oven. Optimistically hoping it would be deep enough to reach the frost line, we excavated a 3.5 foot deep pit. We probably should have gone to 5 feet, given how cold it gets in Minnesota, but since the structure is so small, we decided it would be fine. Mostly, we just wanted to practice digging a foundation by hand, as we might try to do that for our house in Vermont. We filled the pit with rocks and pea gravel from a nearby field.
After our pit was filled in and tamped down, we made our best attempt at dry stacking rocks in order to bring the foundation about 3 feet above grade. As it turns out, big roundish stones are really difficult to lay properly. In the photos below, things appear to be going well, but later in the process, when we started to put mud on the foundation, a few rocks shifted. This lead to an utterly disheartening total collapse of our foundation.
Eventually, we regained morale and cleaned up the toppled remains of our first foundation attempt, finding about a foot of it strong and salvagable. So, instead of making our stone layer three feet high, we settled on a foot and a half. This much, at least, we could make sturdy and stable. Building a shorter foundation meant we'd have to operate the oven on our knees, but we didn't care—we just wanted to cut our losses, be grateful for the learning experience, and make pizza already.
Lesson learned: we'll definitely get the help of a stonemason when we do the foundation of our house!