My wife came home awfully excited from the Maine Grain Alliance's Kneading Conference (basically an event where you can be fully immersed in the magic of bread and all of the parts that make it fantastic... a total dough-dork party). She had the opportunity to build a quick demo earthen oven at the conference and had quickly proclaimed that not having one at our house would be unacceptable... especially since she is a baker and the bread, pizza, chicken, pita, yoghurt, and pretty much everything else that comes out of it is amazing. We got a fair amount of our inspiration from her instructor at the conference Stu Silverstein and his book Bread, Earth, and Fire along with a couple of instructables, such as this one from cwolsey that had some elements we liked and planned on using.
Overall the process to build a wood fired oven is straight forward and makes sense as long as you keep in mind a couple of basic rules following along the lines of middle school science :
1) Conduction is both your enemy and your friend - Conduction is the process of energy (heat or electricity) traveling from an area of high concentration to an area of low concentration. Stick your finger in a candle's flame and you will get burned because the energy will travel from the flame to your finger, stick your warm hands on a cold flag pole and you will get cold fingers as the energy travels from your hands to the pole until they reach equilibrium. The same holds true for a 1500F fire clay brick, put some bread dough on it and the energy will transfer from the brick to the bread dough, making sweet, flaky, crunchy bread! So, with this in mind, you need to make sure that energy only moves from the oven to your bread, and that the energy stays in the oven and does not move to the outside. Isolate the energy and insulate the oven correctly and you will be able to cook food in the oven for days!
2) Thermal mass is key - Just because the cooking surface is thermally isolated doesn't mean that you will be able to cook a good pizza or loaf... even if the surface temp reads 1500F. That energy will quickly dissipate as it is transferred into the yeasty goodness you are cooking and if there isn't enough energy available you will end up with a half cooked loaf or floppy pizza. You need to have thermal mass to hold additional energy so that you can go through the entire cooking process. Of course the thermal mass must be isolated from the exterior of the oven, so you must insulate it pretty heavily if you want to be able to capture all of the energy available.
3) An equal balance of form and function - One last rule, but a bit of a segue from the science stuff. If the oven is located in a place you rarely visit, you will rarely use it. If the oven is awkwardly high or low you won't use it. And, if your wood pile and oven are an annoying distance apart, you won't use it. Using a wood fired oven is fun, and everyone is going to want to get involved with the process, so make sure that you have a set up that invites everyone to do just that.
Okay! Let's get on with the build.
Step 1: Materials to Build the Oven - You Are Going to Need a Truck
(30) Firebrick 8-5/8" x 4-1/8" x 2-1/2" - Used for the floor of the oven. We got ours from Camosse Masonry supply in Worcester, MA and they cost about $2.15 each
(20) Red Bricks - Used for the arched doorway. Also got these from Camosse and they were $.80 each
(3) 50# bags of Hawthorne 40 mesh clay - This is for the main thermal mass of the oven's dome. We ordered this through Portland Pottery Supply in Braintree, MA. Came to $50 total.
(6) 6 cubic foot bags of perlite - This volcanic rock has a lot of tiny holes in it making it a fantastic insulator, it was used in conjunction with concrete to make a thermal insulator for the base and top of the oven. It is not cheap, but if you want to cook lot's of loaves of bread or a great many pizzas, don't cheap out. We spent $200 total but you will see that it was worth it!
(1) roll of ceramic insulating material - This was used as a mid-layer insulator to increase the oven's R-value drastically. Once again, not cheap, but worth at about $100. We ordered it from Amazon at this link.
(4) 50# bags of all-purpose sand - Used to form the dome of the oven and as a base to the firebricks and bought at a big box store for $3 per bag.
(18) 80# bags of quick setting concrete - Used for the foundation and also mixed with the perlite for the walls of the oven. These are about $4 per bag.
(2) 80# bags of mortar - Used for building the base of the oven and for the arched doorway bricks. About $6 per bag.
(1) 4x8 sheet of 3/8 plywood - Used to create both the domed arch of the oven's shape and a form for the arched brick doorway. About $20 for the sheet
(5) 1/2" x 10' #4 rebar - Used as support for both the lower foundation and upper foundation. About $5 each.
(1) 8' length of 8" metal flashing - Used as a form for the upper foundation where your firebricks will sit. About $10. You will need some self tapping metal screws to put it together temporarily.
(4) 2x6x8' boards - Used as a form for the initial foundation. About $3 per board. You will need some screws to put them together temporarily
(4) 6x6 Simpson Strong-tie post anchors - We wanted to protect the oven (and us) from the elements so I built a little oven hut over the entire thing. There will be additional details on how to build this later in the instructable. If you plan on putting a structure over the oven (highly recommended) it will be easier to put your anchors in while your concrete is hardening. They are about $12 each.
Step 2: A Solid Foundation for Your 2 Ton Beast
You are going to be mixing a lot of heavy materials. There really is no way of getting around this. You will need to mix approximately a full ton (2000 pounds) of concrete, mortar, and clay. With that said a small cement mixer isn't such a bad idea. We ended up purchasing one on Craigslist for a few projects we knew we would be tackling in the next couple of years so it was a fantastic investment in my opinion, and my back agrees.
The base needs to be set on compacted, level ground. I used a combination of sand and gravel to make a level base for the foundation to rest on. Once it was completely level I built a form for the concrete to be poured into. Our form was 6' square. We mixed up the concrete, poured it in up to the edge of the 2x6's and then leveled it off with another 2x6. We then set the 6x6 anchors into the corners of the foundation so that the concrete could harden up with the anchors in place. I placed a plastic cover over the top of the foundation and gave it a few days to completely solidify. Once it was 100% solid I then removed the cover and the forms and got ready for building the base of the oven.
Step 3: Is Timmy Down the Well?
While your foundation is hardening you can start to figure out how you want to build the base of your oven. We decided to use field stone, which is found throughout the woods here in New England. A good majority of the stone came from the initial tilling of our garden believe it or not. You can use other materials to build your base ranging from pressure treated timbers to cinder blocks. You can use upcycled materials like old bricks, an old galvanized tub, or pretty much anything that will be able to be filled with debris and then topped with concrete. We settled on the field stone since it is both attractive and structurally sound... and was taking up space any how.
Once your concrete foundation is hardened completely you can begin building your base. You will need to mix up some mortar for your joints between the rocks. We just put small quantities of the mortar in a bucket and then carried it around the base while we filled the gaps. I would suggest wearing gloves since the mortar has a tendency to remove moisture from your hands pretty quickly and can really do some harm to them over time. With our field stone base we started with the flattest rocks near the bottom to begin building upwards. As we mortared we added additional layers on top until we got to the desired height. Since my foundation was 5-1/2" high, I made the base go up 29". Once the second foundation is added and firebricks are added the total height will be right around 40", which is perfect for my wife and I to put and pull goods in and out of the oven (she is 5'4" and I am 6'4" and it works great for both of us).
We made the base a 5' circle which was very reminiscent to a well. Once it was completely mortared and had been given some time to set up we began filling it in. The beauty is that you can pretty much fill it in with any solid debris you have around the house. We had recently done a project that required the demo of some concrete and we used the broken up concrete to fill a good chunk of the base. We then used some of the sand and gravel mix we had at the house to fill in the gaps. Leave about 8 - 12" of space empty near the top of the base so that you can add your insulating material.
The first part of this process is pretty darn easy. Open a bag of perlite and pour it on in! Open up the next bag and do the same until you are about 2" from the top edge of the base. You will cap off the top of the base with an insulating cap made of concrete and perlite. To make the insulating cap you will need to mix perlite and concrete together. This is where a mixer comes in handy since it will give you the most even mix with two seemingly unmixable materials. You are looking for a consistency similar to thick oatmeal with about five parts of perlite to one part of concrete mix. We used enough water to bring the mixture to that oatmeal-thick quality and then spread it on top of the loose perlite in the base until it came to the very lip of the base. Let this harden and dry completely before moving on to the next step.
Remember, the entire purpose of putting the perlite in the base is to create a thermally insulated area where the actual oven and all of its thermal mass can rest. The perlite and perlite/concrete cap are both plenty strong enough to hold up an oven sized like this.
Step 4: The Oven Foundation - a Foundation on a Base on a Foundation
Now that you have the base thermally independent of the oven via the perlite and perlite/concrete cap, you can build the foundation for the oven itself. This mass of concrete will sit directly on the thermally isolated perlite/concrete cap you made in the previous step and the firebricks will sit directly on top of it. Through conduction the firebrick will transfer energy to the concrete oven foundation and then that stored energy will be reradiated back into the oven after you fire it, keeping a consistent temperature during the entire cooking cycle.
I used 8" metal flashing that I had hanging around the house to make the circular form and two self tapping screws to attach the ends of the circle together. Use some rocks to help hold the form in place while you shape it and make sure that the form only sits on the insulated perlite/concrete cap, otherwise energy will be lost to the surrounding rocks and base. Once you have your form set up, mix up enough concrete to create a evenly smooth 4" thick circular slab. The concrete should be a little bit looser in consistency to insure that it evenly spreads around the form and can be poured from a bucket. Use a board and level to make sure that everything is level. If it is not you might need to let it set up a bit and move around the mix somewhat. Let it harden overnight and dry completely before moving on to the next step.
Step 5: 3... 2... 1... FIREbrick - Installing the Firebrick Base
First of all, firebrick must be used, don't cheap out and use red brick or some other variant, you'll have a heck of a mess on your hands at some point. Firebrick can go up to very high temperatures and not have any thermal shock issues.
To make sure that the firebrick sits perfectly flat on the oven foundation you will need to put a thin layer of sand at approximately 1/4" thick. Place the firebricks on this layer of sand and carefully tap them with a rubber mallet to level all of the bricks evenly so that there are little to no bumps or ridges. How you set them up is totally up to you, but I feel that the orientation we used worked really well for the size of the oven foundation. Once they are all level you can use a bit of mortar to hold the edges in place, just don't use any mortar between the bricks.
When you build a fire on the firebrick they will heat up, and will transfer energy directly to the concrete foundation, all you need now is some way to trap all that energy and radiate it evenly to the goods sitting on the bricks... enter the oven dome!
Step 6: Do You Want to Build a Sand Castle? - Building the Oven Dome and Archway
The archway: This part is so much fun and really starts to make your oven look like a real oven. Before you can build your dome you will need to make your brick archway for where you will be sending your loaves to the fiery gates of happiness. I ended up making a form for the brick archway by first setting the bricks up on a square piece of plywood that is the length of the front edge of the fire brick. I formed the bricks in the shape I was looking for, keeping the two bottom bricks perfectly parallel with the bottom edge of the plywood. You do not need to cut the bricks at all to do this, just make sure that all of their interior edges are touching each other. Once you get an arch you are happy with and that fits within the parameters of your firebrick platforms size you can use a pencil and mark the interior curve of your archway. From the bottom flat edge of the arch you are going to want to remove 1/2" of height so that you can shim the form into place and can remove it more easily later on. Use a jigsaw and cut the arch out, retrace the arch on a second piece of plywood and then sandwich a couple of pieces of 2x4 between the two pieces of plywood using screws to attach them. This will be the form for your archway. Set the form on the front edge of your firebrick and start by placing your first two bricks flat on either side of the arch. Start working up both sides alternating between each side. Use a small pebble to adjust the angle of the brick so that the interior edges touch. Work your way upwards until you get to the topmost brick... then go back through and make final adjustments. I am not a mason, so this took some trial and error, but after at least 100 firings the oven's archway is still standing strong. After you are done with adjusting everything you need to mix up some mortar and pack it into the joints. The mortar will move a bit after it is dried and you fire the oven, it will even crack somewhat, but don't let that worry you, those bricks should stay put if you tried to keep their interior edges touching, or nearly touching.
Build the dome: Time to play in the sand! You are going to form your oven's dome by building a big dome out of sand first as your form. You will then cover the sand with newspaper and all of the layers of your oven, then remove all of the sand, leaving your dome in place. We originally made a guide out of plywood to help us get the right shaped and height of the oven dome. Basically, I tacked two nails to a wall with a distance apart equal to the width of the firebrick platform's width. I then used a 1/2" rope tied to both nails and let the middle of the rope fall to form a catenary arch. I then adjusted the distance the bottom node of the arch was from the imaginary line between the two nails until it represented the height we wanted our oven's interior to be (18" in our case). I then slipped a piece of plywood behind the catenary arch and traced it on the plywood, took the ole jigsaw out, and then cut out the arch. I used this guide for the initial forming of the dome and piling of the sand and then used it at the end to check my work and make sure I was on target.
To begin making your sand form, pour all of your sand into a wheelbarrow and mix water into the sand until it is similar to what you would want to use to build sand castles on the beach. Take your sand/water mix and pile it up on the firebrick using your catenary arch form to adjust the height and shape. Pack down the sand, smooth it out, and then do a final check with your catenary arch guide.
Step 7: Can You Dig It? - Adding the Fireclay and Digging Out the Sand
Just as the firebrick will hold and radiate and conduct energy into the goods you are cooking, the fireclay dome will act as a thermal mass to radiate energy to the other parts of your food. We made our fireclay layer 4" thick, so plan on using a lot of clay and spending a fair amount of time on this part of the process. Before you dive in, you will need to use some newspaper as a layer between the sand and the fireclay. This will make the process of removing the sand all the easier and prevent sand from getting stuck in the clay and later falling into your pizza.
Building up with fireclay: You are looking to mix the fireclay with enough water to make something with a consistency similar to stiff modeling clay. Wet enough to pack into a ball and when thrown at a solid surface will not shatter, but not so wet that the ball sticks to the solid surface. You are going to take the clay and make 4" thick bricks that you are going to place against the newspaper, stacking from the bottom, working your way up. Keep the fireclay on the thermally isolated cap and around the oven foundation (it doesn't need to be as thick around the oven foundation since it already acts as a thermal mass. You are going to keep tacking the fireclay bricks and smoothing them together right up to the very top. You will then need to smooth over and taper the fireclay over the brick archway. Make sure everything is tight and smooth before moving on to the next step.
Digging out the sand: As long as you got all of your clay nicely layered and tightly butted up against the base you should be able to dig the sand out on the same day you do it. Pull out your archway frame by removing the shims and pulling out the form. Strap on a headlamp and start digging out the sand. Pull the sand into a wheelbarrow and find someplace around the house where you can use it... my kids loved the new sand in the sandbox! Once you have cleared out all of the sand you will have a solid dome with some newspaper that will be burned off during the first firing. Speaking of firing.... here we go!
Step 8: This Little Light of Mine - Curing the Clay Dome With Small Fires
Let's just get this out of the way now. Your nice, smooth, even clay dome is going to crack. There is really no way around it, unless you get lucky. You just made a 4" thick layer of clay and are going to let it all dry at once... there is just no way to avoid cracks, sorry. But don't fret, it will not effect the oven at all and you are going to cover up the cracks with more insulation, whoo hoo!
Small Fires: You want to use small relatively low heat fires with your oven to help dry out the clay completely, but slowly. If you make a big fire in there for the first time you will cause significant damage to the clay and make cracks that actually do matter. Start the fire under the archway. Use kindling cut up and stacked and just keep feeding it for a few hours while pushing it further back into the dome. You can then move up to using larger diameter kindling (4" wide) and continue to feed the fire while pushing it further back into the dome. You are going to generate a fair amount of heat with this process and you will see the clay steaming quite a bit while the newspaper all burns off. After you have fired the oven for the day you can use some of that stored up heat to make some pitas or pizzas (recipes at the end of this instructable). If you are going to cook with the oven at this point you will need to remove all of the ash in the back of the oven unless you are going to do a live fire (more explained at the end of the instructable). We use a copper pipe to blow the ash out of the stove along with a natural bristle brush to remove the ash. We also use a few different peals to move the pitas and pizzas on and off the bricks (details and plans at the end of the instructable too... oh there is so much too look forward to!). At this point you should keep the oven under cover and out of the elements. Use a pop up tent or something that will not sit on the dome like a tarp (melted tarp and pizza never taste good together). You don't want your clay to get wet again since it isn't really 100% a bisque fired clay... just dried and somewhat fired on the interior. You'll be able to really fire the thing once you complete the next step.
Step 9: A Blankie for the Oven - the First Layer of Insulation
We really wanted to make sure that the oven could not only cook a ripping pizza but could cook loaf upon loaf of bread once it was completely fired. Bread requires a fair amount of latent heat to be built up in the thermal mass of the oven. That means that you need to store up lots of energy in the oven foundation, firebrick, and clay dome, but you don't want to lose it to the surrounding environment. Insulation is the key! Our first layer of insulation was a ceramic blanket we purchased off of the giant South American river (Amazon). Using scissors we cut it into strips that fit the oven nicely and doubled up the layers to add twice the insulating potential. We then used chicken wire to go over the blanket and act as a mesh for the insulating concrete layer to adhere to. The chicken wire was just folded and shoved into cracks and crevices to keep it in place. This was the easiest step and a truly satisfying step of the process that payed back in dividends in regards to the ability of the final oven to stay at a constant temperature.
More bricks and rocks: You will probably notice that there are more rocks added to the base along with some additional bricks. The bricks are just plain red bricks that are thermally isolated just as the rocks added are. They are all mortared together and DO NOT touch the firebricks or oven foundation at all. This is so you can easily slide a pizza peal in and out of the oven and have a larger surface area to work with.
Step 10: Some Like It Hot - Adding the Next Layer of Insulation
The final layer of insulation is now ready to be mixed and added to the oven. You will notice that the girth and size of the oven has increased dramatically. You are now going to add a thick 5 - 6" layer of perlite/concrete insulation mixture to the entire dome of the oven. You are mixing ten parts perlite to one part of concrete this time, making a kind of sticky-yet-crumbly mixture that will have little structural integrity but will be one heck of an insulator. You need the stuff to stick to itself and to the chicken wire with no problem, so adjust some parts of water or concrete as needed, but try to get as much perlite in there as possible. Do the same thing as before with making clumps or bricks of the material and stacking them along the dome working your way up to the top. Pack it down lightly (don't go crazy, air is a good insulator) and let it dry overnight. At this point you can still fire the oven with no harm to your insulation. One more layer to go, just make sure your insulating layer is completely dry before moving on.
Step 11: Testing... 1... 2 - Making Sure the Insulation Meets Your Needs
We did a quick test of the oven before we moved on to the final step. Not that we were really planning on adding even more insulation, but it's better to check now before it's too late, right? We got a ripping hot fire going in there and took some measurements using an infrared temperature gun. The interior was 1500 degrees F while the exterior was reading 54F, which actually was colder than the air at that point. Even the red bricks at the front of the oven only reached 90 degrees after three hours of firing. A pizza will cook in 10 seconds at temperatures like this.
Step 12: And to Cap It All - the Final Layer and Some Protection
The final layer for the oven's dome is simply a layer of Portland cement and sand that is smoothed over the entire insulated surface. Mix up the Portland cement with sand (directions on back of package) to be a bit looser so that you can easily trowel it on to the dome and then smooth it over the surface with a float. You only need to make this last layer thick enough to evenly cover the entire oven, about 3/4" thick. This layer will not only make the final product look better but will also act as a decent level of protection for the oven from the weather. Don't rely on this layer only, build something to protect your oven. Speaking of that... let's talk about the structure I built over the oven.
Oven hut: If you remember from the very first step, I made sure to leave space for 6x6 post anchors to be installed at the corners of the concrete foundation. The post anchors were for the main supports of the structure I built over the oven. The design is a modified timber framed design with a dual rafter carriers along the length of the structure sandwiching the 6x6 posts. The rafters are bird-mouthed to fit over the outside rafter carrier and there are purlins attached across the rafters to add rigidity and to give a place for the metal roofing to be screwed down to. The angle braces help hold up the 2' overhang and just look cool too. We have had 65 mph winds recently and the thing does not shudder, it is as solid as the oven itself.
Where's the chimney?: We decided against a chimney and we are happy we did. Chimneys are great but they are a place where heat can be lost and in the case of a wood fired oven, that is not acceptable. Yes, you do get a bit of smoke in the eyes during the first few minutes of firing but once your fire is raging the wood will be burning at a temperature that will quickly dissipate the smoke. The smoke with our oven quickly exits the little hut and often can be seen roiling out of the top ridge vent. There is NO fear about lighting anything on fire between the level of insulation in the oven and the distance everything is apart from each other. My opinion is to avoid the chimney, save the time and money, and more importantly, the heat for the bread.
Step 13: Tools We Use for Our Oven and Live Fires
We use a number of tools to work with the oven and since this oven is markedly hotter than your typical in-house oven, you are going to want to put the oven mitts to the side. You will need tools to start the fire, keep the fire going, and move the fire. You will need tools to shape the dough, cut the dough, roll the dough, put the risen dough into the oven, and then take out the now-cooked dough. Finally, you are going to need tools to clean out the ash if you are not doing a live fire. I am going to break it down in sections for you to make it easier...
Tools to start your fire:
- Axe and maul to split your firewood down into small enough sticks, especially at the start of the firing.
- A log grabber / tongs to move the logs around in the oven's dome and to pull logs out when needed.
- Fireproof heavy leather gloves
- A 5' long copper tube with the end crushed flat but allowing air still to pass through it. This is used (with your lungs) as a bellows to keep a small flame going at the start.
- A pusher of some nature to push back the coals further into the oven's dome. We made ours from an old brownie pan (see the pictures) that we used to make our small peal.
Tools for the dough:
- A large bucket and flat surface for kneading
- A dough or bench knife to cut the dough
- A long pin roller
- A large pizza / bread peal
- A small pizza / bread peal
Tools to clean out the oven:
- A natural bristle brush / broom to sweep out the ashes (once they are cooled of course)
- The same coal pusher to pull out large amounts of ash
- The same copper tube to blow out the remnant ash
Step 14: Recipes
This is an entry in the