In this post, we will be looking at the all the natural materials and mixes you need to build an outdoor pizza oven. We will not be covering building methods in this post. In order to build natural clay outdoor ovens, it is essential to get to know your materials. There are four basic mixes of natural materials that make up a complete earthen oven. They are; thermal cob, natural insulation, earthen plaster, and lime plaster. We will go into the details of each, as well as, a bonus DIY industrial-material at the end. The first and most critical component in most oven builds is identifying and sourcing your clay.
A finished batch of Thermal Cob
Identifying Clay for Building an Outdoor Oven
How to identify clay soil? Get a handful of subsoil and wet it until it is a thick moldable mud. Work it into a ball and squish it against the palm of your hand, open your hand palm-down. If it sticks to your palm it is likely you have a clay-rich subsoil, In this blog we cover further testing to find out if your soil is clay-rich enough to build an earthen oven.
When sourcing clay from the ground it will typically be a mix of pebbles, sand, silt, and clay, this is why we refer to it as a clay-rich subsoil. The subsoil is the layer of soil below the topsoil. Topsoil contains organic materials from decomposing plants, which we don’t want. Subsoil can contain high percentages of clay, or very little. This is why this process is so important. Another reason for doing various tests is that silt can resemble clay.
The best way to source clay-rich subsoil if you don’t know where to start, is to talk with local farmers, potters, or quarries. Once you find a place you have permission to dig and its a place you aren’t disturbing ecosystems, the testing begins. At your dig site remove a portion of the topsoil, digging past any roots. If the subsoil is dry and rock hard, or wet and sticky, it’s a good first indication that you might have a clay-rich subsoil. There is a common misconception that clay is deposited by rivers. Large clay deposits are formed only where there has been very still water over long periods of time. Rivers can only reveal clay deposits or redistribute them.
What is the Difference Between Clay and Silt?
Before we get into more testing, let us take a closer look at clay and silt. What is clay exactly? Clay is a microscopic piece of mineral (stone) that has been worn-down over millennia. It is classified as clay when the particle is 0.002 mm or smaller. Its small particle size and flat plate-like shape gives it its important properties. Because of its flat plate-like shape by working it, you organize the plates to be stacked-up one on top of another. This stacking is what gives clay both its stickiness and its elasticity. What is silt? Silt is also a mineral and its size is larger than clay and smaller than sand, its particle size is from .002 -.05 mm in size. See the image above for an idea of the relative size of sand, silt, and clay. Silt can trick many people into thinking it is clay because it is also soft and moldable and feels a bit like clay. The difference is silt doesn’t have the stickiness or elasticity of clay and it won’t make a durable earthen oven. It is very important to continuing your subsoil tests to be sure you are not collecting silt. I have simplified this considerably, you can learn more about the details of clay mineralogy in this well made video.
What Type of Sand to Use for an Outdoor Pizza Oven?
A quick note on sand. All sand is not created equal. The main variable in sand that is important to build an outdoor pizza oven is the structure of the particle. Beach sand by its nature is very round from the waves rolling it back and forth. Play-sand for sandboxes is usually round as well. This is the type of sand to avoid. You want angular sand that will lock together when packed. Concrete sand works and sometimes you will find angular sand in sand tubes. For a 36” hearth you will need about a cubic yard of sand. This is usually enough to build your sand-dome form as well.
Clay Mason Jar Shake Test
The second test to do is the shake test. Fill a mason jar half full of your soil sample and fill the jar with water leaving an inch of room at the top. Put the lid on your jar and shake it until all the chunks have been dissolved. If it is very dry, you might need to wait a day for the material to absorb the water. After you have dissolved all the chunks and shaken the jar put it on a level surface and wait. After a couple of minutes, you will see the first two layers form. The bottom layer is pebbles, and sand, the second is silt, and the clay is still floating in the water. The clay will take between 15 minutes to 24 hours to settle, depending on the type of clay you have. Now hopefully you can see a stratification of the various components of your subsoil. This can help you determine how much sand to add to your subsoil when making your test bricks. We are aiming for around 75% sand and 25% clay. If your subsoil has high silt content you will want to keep searching in other areas.
If you see that you have a majority of clay this is good. It is also possible to find a subsoil that is 75% sand to 25% clay, this is the perfect building soil. In this case, you would only need to dig, wet, and stomp the material. Another good simple test to do is to wet your subsoil to a typical ceramics clay consistency, work it in your hands, then roll it between your palms into a snake. The longer the snake you can make without it breaking the more rich in clay your subsoil is. On to making your test bricks.
How to Make Cob Test Bricks
Get or make four to five brick molds of the same size. You can use plastic tubs, loaf pans, or wood forms. Make four bricks of different proportions (to your discretion), for example; 1 part soil to 1 part sand, 1 part soil to 2 parts sand, 1 part soil to 3 parts sand. If you think you might have perfect building soil make one brick out of pure soil. Mix your sub-soil and sand thoroughly, compact the mixes into the molds at least an inch thick, and label the bricks or scratch the percentage into the surface of the brick. Put your test bricks into a protected shady place and let them dry. Once your bricks are dry, you can measure them to calculate the shrinkage, and stress test them, to find out which mix is the best. To stress-test the bricks, pinch (don’t bend) a corner between your fingers. If it crumbles it has too much sand. Also, try scratching the surface, if it scratches away with a fingernail, again you have too much sand. If your best brick is shrinking more than 2.5% than you should try adding more sand. If none of the mixes are working it is likely your subsoil is too rich in silt. Your best mix when dry should resemble cement, but not quite as durable. Another simple test worth doing is to build a mini earthen oven with your choice of mix and see how the material performs.
The percentage of subsoil that is best for your mix and the size of your oven, will determine how much soil you will need. For example, If you have a 1 part soil to 3 part sand mix (meaning your subsoil is close to pure clay) and you are building a 36” oven (inside diameter of the hearth), then 8 five-gallon buckets is more than enough. If your soil is dry you will want to soak it for a day. It is much easier to mix your materials when the clay-rich subsoil is wet. You can always dry-out the finished mix by making mud-cake piles and letting it sit in the open air and then re-stomping the mix to reincorporate the drier surfaces of the cakes with the wetter interiors.
Mixing Natural Materials to Build an Outdoor Pizza Oven
Mixing Thermal cob
The first material you will be making to build an outdoor pizza oven, is thermal cob. The thermal cob will make-up the main inner dome of the oven. To make thermal cob you are simply repeating the process you did to make your test bricks (with the winning percentage) on a larger scale. The best method to mix a batch of thermal cob, is to use a tarp. If you are several people stomping, a medium-sized tarp (10’ x 12’) works well. If you are by yourself a small (5’ x 7’) will suffice. First, measure out your sand and put it in the middle of the tarp, next measure your soil, and put it on top of the sand. Now scoop the sand from the sides and cover the clay. Put your bare feet on, have fun, and start to dance! Once the pile is flat you will want to re-pile and mix. It is best done with two people. Grab two corners of the tarp and walk it to the other end of the tarp rolling the mix back onto of itself. Typically you will be rolling the material more than stomping. Once the material is flat, roll it with the tarp and start dancing again, and repeat.
The idea is to completely incorporate the material until each grain of sand is covered by a small layer of clay soil. If you need to add water as you are mixing be very conservative as it can become a sloppy mess real fast, and then you will need to dry it out before you can build with it. As you stomp and flip the mix you will see it starting to come together more and more. Once you get a solid burrito looking object when you roll it, you can consider the mix finished. Some people mix straw into this layer of an oven. The straw will reduce the shrinking of the mix but being on the inside layer of the oven it will burn-out and make this layer more fragile. I have recently considered the addition of grog, which is crushed up pottery. Grog is used in low temperature firing of pottery to stop cracking. I will be experimenting with this in the future and will report back.
* A nice trick in tarp-mixing is to grab the corner of the tarp and spin your hand around, winding the tarp around your hand. This way you aren’t needing to grip the tarp and the wrap holds it for you.
Mixing Clay Slip
Slip is simply a suspended clay solution. For this, you will need as pure of a clay as you can get, a clay-rich subsoil will work fine. Fill a bucket 1/4 of the way with clay and finish by filling the rest of the bucket with water until it is 2/3 full. Mix the slip by using a paint or plaster mixing attachment on a corded drill. By stepping on the side of the bucket you can hold it steady as the mixture turns. Begin by using a low speed on the drill slowly mixing from the bottom and gradually increase the speed and move the attachment around the bottom of the bucket.
Be cautious until you get the feel for it, and watch your shoelaces. Keep mixing it and checking the consistency. It should be like thick chocolate milk that will cover your hand in a clay glove when dipped. You should not see rivulets of water running down and revealing your hand. If you do, add more clay and continue mixing until you get a solid glove, when you dip your hand in. If you don’t have a drill you can work the clay into solution by massaging the chunks of clay together or against the side of the bucket, as seen in the image above on the right. It takes longer, but works fine and is a great meditation.
Making Natural Earthen Oven Insulation
The next material will be your insulation. It is not essential to insulate your oven, but it will make it more efficient. At the least, it is recommended to insulate the hearth (underneath the bricks), to keep the base of the oven from sucking the heat out of your oven. Under the hearth, it is necessary to use some kind of refectory insulation, such as refectory insulation bricks (which are very lightweight and can be shaped with a regular saw), perlite, or vermiculite. Some people have also used empty glass wine bottles, which I can’t recommend as I have never tried it. If you are using a loose material like perlite or vermiculite you will want to make sure that your base fill is well packed so the insulation won’t settle down and be lost inside the base of the oven, you can also mix a small amount of slip with it to help hold it together.
To insulate the dome of your earthen oven you have more options. You can use other natural materials that are porous like straw or plainer shavings (quark-screw shavings of wood), though perlite is the top choice, they will still work. If you use straw, your are making what is known as straw-slip. In making straw-slip there is the additional step of chopping the straw in to 3-4 inch lengths. The easiest way to do this is with a sharp hatchet holding a bundle of straw above a piece of wood and chopping. Grass-clippers or scissors work as well, I have also heard of people using a weed-whacker inside a garbage-can (when large quantities of chopped straw is needed).
Adding a vermiculite insulation layer
The easiest way to mix insulation is in a wheelbarrow. If you are using perlite or vermiculite it is recommended to use a dust mask. Fill the wheelbarrow two thirds full with your insulation material and pour some slip on top of the material. Be very conservative on using the slip. My method is to rub the material back and forth between my hands. The idea is to just coat the insulation material with enough slip to stick it together. Keep mixing and rubbing the insulation mix together until it feels right. If you are using perlite or vermiculite you should be able to form a ball that stays together, but will crumble when pinched. The more slip you add, the less insulative your material will be. Be sure to do a good job packing the insulation material into itself as you build the insulation layer. It is a firm but controlled pressure, too much and it will blow out and you’ll be starting over again.
How to add Sculptural Elements to your Earthen Oven
As you have likely seen many people have turned their ovens into a sculptural piece of art. The layer in which you create the base forms for your sculpture happens during this insulative layer and the finishing details are made with plaster. In my opinion, the best material for adding a sculptural element to your oven is straw-slip. Get creative, have fun, and make your favorite animal or anything you can dream up!
How to Mix Earthen Plaster
The purpose of earthen plaster is to add a layer of protection over the fragile insulating layer of an earthen oven. Earthen plaster is a mix of clay or clay-rich subsoil, sand, and fiber. You can easily transform your thermal cob into earthen plaster, the proportions of clay to sand are the same. The difference between thermal cob and plaster is the addition of fiber and moisture. The best way of mixing plaster is on the tarp with bare feet. Mix it as you did the thermal cob adding a little more water and then add chopped straw on top, and stomp and fold it in. Add a few handfuls at a time, some splashes of water, stomp, and see how your plaster is looking once the fiber is incorporated. You want there to be a high amount of fiber but not too much that it is separating from itself. It should be a thick mud consistency that you will be able to apply with a trowel. There is the additional option to also use manure for your fiber. The digestion of grazing animals processes the grass into a very fine fiber which makes exceptional plaster. I will typically add more sand and less water to a mix if I am using manure. You will need to gauge it for yourself. If your plaster cracks after one coat, adjust your mix and add another layer.
How to mix Lime Plaster
Lime is an extra layer of protection that is more water-resistant than the earthen plaster, though the oven will still need a roof. Lime plaster is added once the oven is completely dry (about 1 month). You will need to score the surface of the plaster layer to give it a texture to grip onto, this is easier to do when the earthen plaster is still fresh. Lime is caustic so be sure to use rubber gloves, a dust masks, and goggles. The lime finish is 3 parts sand to 1 part hydraulic lime. This is easiest to mix with a hoe in a wheelbarrow. Dampen the dry plaster surface of the oven and apply the lime finish the same way you did the earthen plaster.
* Bonus Material - DIY Refectory Cement
Refectory cement is an amazing material, it is more durable and weatherproof than cob and can be used in various parts of an earthen oven. Some might ask why not build the entire oven out of it? You can, but it is not as natural or traditional as cob, it is caustic, so it’s not as fun to work with, and it sets-up fast. I have used this recipe to build cast arches for the oven entry. It is great for this purpose, as then the arch is a single solid piece.
The key is to make the form the correct size for your oven and to make the bottom of the arch flat so it is balanced. The form was built from thin particleboard bent into an arch held in place on a board with removable wooden pegs. Refectory cement can also be used as the mortar to hold a brick arch together, which will be stronger than using cob. This DIY Refractory cement is made of Portland cement, lime, fireclay, sand and water. Fireclay is a heat resistant clay made up of aluminate and silica which you can buy at most home improvement stores and wood-stove shops. For structural integrity it is a good idea to embed some steel wire-mesh in the middle of any castings you make. For casting make the mix the consistency of modeling clay and pack it well. When your finished cover with plastic and let it slow dry for a week or two.
Measure your ingredients by volume (I used a yogurt container) and mix only the amount you will use within an hour or so. Like lime plaster, refectory cement is easiest to mix with a hoe in a wheelbarrow.
• 1 part Portland cement
• 1 part hydraulic lime
• 1 part fireclay
• 3 parts sand
We hope you enjoyed this post on natural earthen materials! Let us know about your own tips and tricks with mixing natural materials for building earthen ovens in our forums. Stay tuned for more informative content! Happy mixing!