The magic part of how an outdoor clay oven is built, is how the hollow form is made. We call this “forming the void”, or jokingly “the step of build nothing”. There are two basic ways you can form the void, the sand form method, or the basket method. Each method has its benefits and disadvantages, in this post we will cover the ins and outs of each.
It takes a lot more sand than you would imagine to build a sand form. Typically I will use something that can take up space in the middle of the oven; bricks, cement blocks, cordwood, tiles, etc. Just be sure whatever you use is small enough that it will fit through the door once the oven is built. You should already know the height of your oven door and the inside height of your oven. The door height should be 63% of the inner dome height. This ratio is very important for having a good draft in your oven. Typically I don’t make a dome height over 18″ which makes the door height about 11.5″. There are cases where you would want a larger dome height depending on the ovens’ purpose and size.
The Sand Form Method
The sand form method of forming the void, is by far the most used for forming the void of an oven because it is quick, relatively easy, and makes a nice smooth interior. It’s like building a sandcastle in the shape of a pear cut-in-half, laying on its side. There are a couple of factors that will contribute to the difficulty level of this method. First, the type of sand you use. Though any sand will do, larger course sand (like that used to make cob) will not hold together as well as a fine and round sand. Second, the amount of water added to the sand, too wet of a sand mix and it will sag, too dry and it won’t hold its desired shape.
Once your hearth bricks are laid and you are ready to build the void of the oven, you will need to prepare a few things. The opening of the oven needs to be blocked in such a way that you can remove it to dig out the sand after the oven is finished. Along with blocking the entry comes the actual forming of your oven’s entry.
Brick Arch Door Form
If you are building a brick archway you will want to build a wooden form in the shape of your archway, but a little shorter than your desired door height. This allows you to put in two wedges under the door form. Once your oven is built the wedges can be pulled out and the door form is easily removed. Cut two shapes of the archway form out of plywood and sandwich several pieces of 2×4 or 4×4 between the two pieces of plywood. Be sure the two pieces of plywood are lined-up level on the bottom. Add a screw to the front of the form to act as a handle for removing the form (as seen in the drawing below). Having the depth bewteen the two pieces of plywood allows you to stably rest the bricks against it. I recommend using a true arch shape where the bricks form a half-circle. This form is self-supporting and very strong. If you are using straight sides (as in the images) I recommend using a refractory cement mortar instead of cob between the bricks. Also, the corners need to have wedges of brick seen in the first image above.
Cob Arch Door Form
The benefits of a cob archway is you don’t have a joint of two different materials (brick and cob). Issues can arise from the difference of expansion and contraction of the two materials making it crack. Though Cob entryways might crack, they are easier to repair. To block the opening for cob archways you have many options. I have used buckets, and a large rubber ball in the past. If you are using a round object such as these, you just need to complete the bottom of the arch with sand. Be sure to measure to keep your 63% height ratio. A bucket works well because of its tapered sides.
This is another benefit of cob entryways, you can bevel the sides of the entry much easier. Using a plug door with beveled entryway sides gives your door a great seal for baking bread. The other method is to build the entryway form directly with the sand. The challenge in this is blocking the front. To do this, you can screw one board to the bottom of another the width of your door opening. Place a stone or brick on the bottom board and build up against the front board (as seen in the middle image below). If there is enough room in front of where your entry stops you can simply hold up the board with some bricks.
I do not recommend “the cutaway method” where you build a sand form dome and cover it all the way around with cob and cut the door out after. In my opinion, this method lacks a good strong entryway. Also if you are making an insulation layer the cutaway method poses even more challenges. Typically I taper the insulation away from the entryway, as it is a more fragile layer, especially when using straw or wood shavings.
Building the Sand Form Void
The first step in forming the void is to mark the bricks where the inside edge of the oven will be. This is easily done (if your oven is round) by first finding the center of the hearth, then using a string tied to a pencil, mark the circle using it as a large compass. If you have an oval oven mark it by eye. Make your pile of bricks in the center of the oven leaving enough room around the edges to build your sand form. Measure and break a stick for your dome height and place it standing upright in the center of the oven (as seen in the picture above). This gives you a reference to aim for as you build up the sides. Once your form gets close to reaching the top of the stick, you can remove it and finish the void form.
The actual building method of forming the void with the sand form method is a bit counter-intuitive. Most people would think to pile the sand in the middle and work towards the edges. Though this can work, a better method is actually to build up the sides using your hand as the outside form and creating a concave bowl, meaning the outside edge of the void form is one level above the inner part as you build up (see center image above). The first 6 inches of the void form (just above the hearth bricks) should be straight vertical. If the form curves in from the hearth, you will be reducing some of the usable space of your hearth.
Having the right amount of water mixed with your sand is the key. I usually put the sand in a wheelbarrow and add a little water at a time, mix it up and test its ability to build, and tweak it as needed. It is very hard to add sand (especially if it is the angular concrete sand used to make cob) to the outside of the form, you will find that it shears off. That is why we build from the outside in. The building of the sand dome takes gentle but firm pressure, pushing too hard or slapping it can collapse the form very easily.
With practice, you will get there. Once the void is formed you can cover it with wet newspaper. Start with a large open sheet on the top of the oven and hang the others off of it. cut the overhanging pages on the bottom (or aligning them) so they won’t get sandwiched between the bricks/foundation and cob. Covering it with a newspaper is not a necessary step but it will give you a clear visual of the edge of the void when you’re digging it out.Though the oven can be dug out directly after the oven is completed, I recommend waiting two weeks. This allows some of the moisture to leave the oven and the cob to firm-up. After you have dug it out use small drying fires to finish the drying process before a baking fire.
The Basket Form Method
The basket method of forming the void is a traditional and ancient way to build your oven. I use this method when I don’t have much sand. As I am a traveler, I have built many home ovens at places I have rented. So I have opted to build these ovens at near zero cost. Its worth saying that there is also something very satisfying about weaving boughs of trees into the form of your oven. Many things about oven building is a mimicry of Nature and her elements.
This ancient method was used by the Quebecois of Canada in their famous ovens. Starting on page 49 in this great PDF The Bread Ovens of Quebec by Lise Boily and Jean-Francois Blanchette, you can see images of the steps of weaving a basket form, which we cover below.
Weaving an Oven Basket Form
First, you need to collect a good amount of thin straight green saplings or branches and de-branch them. Ideally, you find lengths that are very flexible and don’t taper too much. Start with your thicker sticks pushing the ends down as deep as you can into the sand and gravel (in the base of your oven). Try to get them as close to the sides of the hearth as you can. Begin in the center and measure your oven dome height and adjust the stick accordingly. Next make one hoop for the oven door, again measuring your door height. Make sure it is 63% of the dome height. Now span the distance between these two hoops making a gradual slope.
Finish getting the hoops spanning the width of the oven as solid as possible by packing the sand around where they enter the sand in the oven’s base. Take some thinner more flexible sticks starting with the thin end and weave it from the front of the oven door to the back. Bend the thin end down, and plant it into the sand as close to the hearth as possible. Do the same for the rest of the sticks spanning the length. You can leave the sticks long, sticking out of the oven door hoop. Tie all the branch crossings together with natural twine. Now you can safely cut any branch ends sticking out of the oven door hoop. Measure your oven dome height and the door height to make sure they didn’t move, and adjust if needed.
One of the hardest parts of this is getting the ends to stay well anchored, so they don’t slip, or get pulled out by branches weaving through them. Now that your basket is finished you need to cover it with a natural cloth that will burn out when you start the drying process, I usually use burlap. The extra burlap at the bottom can be pushed inside the oven. Forming your entryway can be done in the same ways as I describe in the Sand Form sections above. In the oven above I used two pieces of wide metal flashing and supported it with sticks. Maybe not the best choice, but it is what I had on hand, and it worked pretty well. This was a zero-cost oven I built in 2010. The bricks were all donated by a ceramics studio that had extra firebricks. My wife and I were baking 50 loaves once a week for the farmers’ market in this oven! We had a blast!
This oven was built before I knew about not adding straw to the inner cob layer, you can see some of the straw had sprouted inside the oven. Even though the oven did crack some, it was still structurally sound. We baked in it every week for three years until we had to move.
After you have finished building your cob oven, I would wait two weeks before starting some small drying fires. Once you feel the majority of moisture is out of the cob walls, you can fire it hot and burn out all of the basket form. Usually, pieces will remain even after a hot fire, so you can simply remove them by hand if they are free. Once all the basket structure is removed, fill in the holes that the branches left around the edge of the hearth with cob, and clean up the inside edges. Having a clean smooth inside edge where the wall meets the hearth makes cleaning out the ash and coals much easier.
I hope you got something beneficial out of this post! I look forward to hearing about your experiences and questions on forming the void of an earthen oven. If you have a response of some length please post it in our forum. Click the bell and be informed of any new posts from Ovens DIY! Get busy building nothing and form that void!