Clay ovens are an excellent way to cook without fossil fuels. Construction can be spread over weeks if necessary, and clay subsoil and a shovel are the only things required. The fuel can be anything that burns cleanly – cardboard, wood, even leaves. A small oven can be built quickly and clay and something to serve as a door are the sole building materials required.
This oven is built of clay subsoil, the walls are about 20 cm thick, and the inside is about a meter high. The floor of this oven originally was also clay, but I now use large flat stones. I often use this oven to make charcoal (‘biochar’) for garden use, and repeatedly scraping out the hot coals caused the clay oven floor to become uneven. My first oven was half as high and the walls were half as thick, but it still could bake five loaves of bread at once. That oven took only five hours to build. This larger, second oven took much longer. Both ovens are strong enough for me to stand upon (75 kg).
Building an oven
I have seen many plans for building clay ovens, and decided that they were all too complicated. This is ancient tech! I prefer to build my clay ovens where they won’t get rained on and at a convenient height. This oven is built on a clay-covered pile of rocks to raise it off the ground. A moisture barrier that prevents water from migrating from the surrounding soil to the oven is an asset, but a couple layers of cheap plastic bags didn’t prevent this. I now use a tarp large enough to keep the oven and the surrounding soil dry during wet weather.
Building a dome-shaped oven is simple, but a few tricks make it easier. A dome shape is easier to squeeze together if there is something inside the dome to push against; in my first oven I used a cinderblock with an inverted flowerpot over the top of it, and when the dome was complete I dug out some clay from under the cinderblock to remove it. This oven was built without an inside support. I pre-compacted most of the clay in the bottom of 5 gallon buckets or in a rectangular form made from three boards, but even so, an inside support would have made construction easier.
Optimally, the inside layer of the oven should be of a light, fireproof substance, such as a perlite/clay mixture. This should reduce fuel use, but the quantity of wood needed is still minimal. If sand is added to the clay, this will reduce shrinkage as the clay dries and the dome will crack less. I don’t bother with this step either, as patching cracks is easy and subsoil is dirt-cheap. All the clay came from local subsoil with most of the rocks and roots removed.
The door of the oven should be three fifths the height of the inside of the dome. During construction, the door should be made a little too short and narrow, as enlarging it later is easier than shrinking it. I now use a rusty cookie sheet to seal the oven, so my doorway is built flat on the outside. I have used rocks for a door, and rocks retain heat much better than my cookie sheet, but the local rocks (granite) eventually break and handling heavy hot rocks is no fun. The rocks also rapidly wear down any clay they contact. Some rocks have water trapped inside, and heating these rocks can cause them to shatter, but the local granite rocks don’t do this.
I find that freshly-dug clay has an optimal moisture content for dome construction. Squeezing a handful of clay should not produce any drops of water. If the clay is too wet, the walls will sag – let them dry out a day or two.
Once the dome is complete, it needs to dry out. Using a fire inside to dry it out is OK, but the oven will crack as it dries, and a fire-dried oven continues to crack for several days after drying. Adding more clay to an air-drying clay oven is easier, as it can be done anytime, even before drying is complete. I also use a watery clay slurry to wet the cracked places in the dome before adding more clay – the new clay is more likely to stay put once it dries. Decorating the oven (with clay sculpture/stones/etc) should be done after patching the cracks.
I use cotton rags to handle hot rocks and the hot cookie sheet. Cotton gloves will soon develop holes in the fingertips, and polyester/nylon will melt.
After the oven is more or less dry, and all the cracks are sealed, and the doorway is cut to it’s final size, it is time to build a fire. The first time the oven is used, evaporating water will carry away some of the heat–so it will work better the second and subsequent times it is used. (A rain-soaked oven isn’t ruined, but a dry oven is much easier to use.) A small fire is best, as it is does not waste wood, and flames licking on the doorway will often cause more cracking. However, building a fire inside a cold clay oven is difficult, as air flow is impeded and the clay absorbs the fire’s heat. I build the initial fire just inside the doorway and use lots of kindling or cardboard, and I also use a drinking straw to blow air onto the fire as needed. Once the fire is burning well, it can be pushed to the center of the oven. If the fire gets too large, I use my rusty cookie sheet to block most of the doorway to reduce air flow and thereby the size of the fire. It will take at least an hour for a fire to heat the oven to cooking temperatures.
When the oven is probably at operating temperature, remove the fire. I use a wet stick to push the fire into a bucket of water. An old hoe or shovel is fine for this task, but the heat will spoil the ‘temper’ of the metal, so a shovel used for fire removal will likely break if used for digging afterwards. Once the fire is removed, check the temperature. An oven thermometer is ideal for this task, but a few strips of paper will work. If a small piece of newspaper instantly smokes or burns when tossed into the oven, the temp is over 500 degrees F. If the paper starts to turn brown and black in about ten to fifteen seconds, the temp is about 450 degrees F–perfect for bread. If the oven door does not seal perfectly, wads of aluminum foil can fill any crack. I lean rocks against my cookie sheet to hold it firmly in place during cooking.
The temp of the oven will slowly fall as time passes. This is perfect for bread, as it rises quickly, then ‘sets’ and browns. I prefer to use loaf pans for my bread. Potatoes also do well in such an oven–they can cook in their own skins, and after they are done, they can be wiped clean with a moist towel. Both potatoes and bread are not spoiled if small bits of clay fall onto them while cooking, but cakes and such need to be covered if cooked in a clay oven. After the cooked food is removed, cut wood can be stored in the still-warm oven. This wood will dry out further and burn better, but the oven should be sealed just in case an overlooked hot coal starts the wood smoldering.
A clay oven can also be used to produce horticultural charcoal. My large oven has a bit of aluminum gutter built in, and hot coals are pushed down the gutter into a bucket half-full of water with a waterlogged hooked stick. The oven can get incredibly hot when making charcoal–even larger pieces of wood will burst into flame when put into a really hot oven, and the fire will grow overlarge quickly unless the doorway is mostly blocked. When the oven gets this hot, I find a small opening at the top of the door and a small opening underneath the cookie sheet is enough for charcoal production. Bluish smoke is optimal–when the smoke thins and loses color, the oven should be emptied of most of the hot coals. If the oven is very hot, a smallish load of medium or small pieces of wood will turn into charcoal with the doorway completely sealed.
The charcoal is an excellent absorber of some salts. I sometimes put a bit of fertilizer in a bucket of charcoal soaking in water and leave it overnight before I add the charcoal to the soil. I had to learn the hard way to never leave a bucket of charcoal soaking in water for a long period of time. Mosquito larvae are difficult to see among the charcoal bits, especially if there is only a little bit of water in a bucket mostly full of charcoal. It took me far too long to find where all the %@$&# mosquitoes were coming from.