Hello! Welcome to our four part series on how to build your own cob oven. This is our very first attempt at building with cob and we by no means know what we’re doing. Please use these posts as a rough guide and learn along with us. If you are interested in building with cob, we strongly suggest finding a “hands-on” workshop to participate in.
Let’s get started with step 3!
Now the long awaited moment. Cob makin’ time! Please remember that if you are using natural clay, your measurements may need to be different. We don’t have much clay in our soil here so we purchased man-made “fire clay” which is a mixture of cement, mortar clay, and sand.
So far this is a dry mixture so add just enough water to combine the two powders and stomp. Since we used the fire clay instead of natural clay, I was not comfortable mixing it with my bare feet. I wore my handy rain boots… with polka dots. The polka dots aren’t necessary, but they help. You know what else would help? More than one person doing the stomp mixing. It gets tiring fast.
Once you feel the mixture is well combined, check to make sure it isn’t too wet. Form some cob into a ball, drop it from waist height, and if it splats flat the mixture is too wet. If it stays somewhat ball formed, you’re good to go. Here is where I made my mistake. I started building my first layer with too wet of cob. Oops! If your mixture is too wet let it sit out and dry up a little. My mix took almost 20 hours to become dry enough to work with properly. Don’t be in a rush like me. You’ll cry.
Start building! Your first layer should be 2 to 3″ inches thick all around. I made little bricks (easier to work with), punched them against the core form, and then smeared the edges of each brick onto the previous one.
At anytime if you should feel like your bottom “cob bricks” that you are building onto are too soft, stop! Stop and wait a few hours for them to stiffen up. If they are too soft, they won’t support the weight of the cob bricks you are adding as you get higher. Trust me. I speak from experience.
Now get your husband
who couldn’t figure out why you were breaking a sweat with the first batch to combine the mixture for the second layer of cob using the stomping method. Combine:
100 lb. of fire clay
75 lb. of sand
1/4 of a bale of hay/straw (a heaping arm-full should do it)
Enough water to combine dry ingredients
I learned my lesson and this time when I felt like the bottom rows of cob in the second layer were too soft to stack on, I wrapped up the exposed first layer of cob in plastic and let the second layer of cob stiffen up overnight.
Now I’ll let it all dry out for about a week before I start removing the sand core a little at a time. You don’t want to take the sand core out all at once because it may cause the whole oven to collapse. Then in about two to three weeks, depending on how well it dries, I will light a small 10-minute drying fire in the hollowed oven for a few days before officially firing up the oven to bake.
Look for “step 4″ of the DIY Cob Oven in a few weeks and see how it all works out.