Up To Date

So if you've been reading this blog from the beginning then you know I started writing it after we began  building the oven. You can say we are building with cob, earth, clay or adobe and they're all true. We began with the idea of just cob but, after looking at my design and getting some hands on experience I found that mixing the different types of construction has great benefits and will be mixing them as this project progresses.

So this project has taken a long time so far. We began collecting rocks and urbanite in July of 2010 and it's now January 2011. Unfortunately with my injuries I can't do to much at one time and my kids have school and some are in other states so we really only work on the weekends and vacations. During the summer here in Arizona we could only work outside for an hour or so at a time and during the winter we've had a few weeks of freezing temperatures.

So what I've learned so far about working with weather is important. In extremely hot weather you have to keep your work damp or it dries to fast and cracks but, damp is not wet and too wet means your work falls apart. With cob you can only do a layer or two at a time and you have to let the cob set before adding more. In the hot weather the cob can set in 5 to 10 minutes but in the cold weather it can take 30 to 45 minutes. You can work in the cold weather but, not if it's going to freeze. The cob doesn't set right, it will freeze and expand then be very week and crack. So don't do any real work if it will freeze before it can dry. So I don't recommend building in the winter.

I have also learned that the rain is your enemy. If you know or even suspect that it might rain before you get your project done and sealed then get a good tarp is your best friend.

You can see in these pictures where the rain washed some of the cob off and smoothed out a lot of the rough sides. I covered the dome and chimney of the oven but, my tarp wasn't big enough to cover the base as well and this is what happened. So believe me when I say rain is the enemy.

So this is where we're at right now with our project. Hopefully by the end of this month the freezing temperatures at night will be over and we can begin the next steps. We need to add the insulative layer, outer layer, repair the base and begin making all of our adobe bricks. Can't wait to get started again.

The Outer Door Story, The Closure

Well once I got the door fabricated, aligned, the hinges attached and hung I had to figure out how to keep the door closed. I settled on a simple latch system. Well I thought is was going to be simple, little did I know.

OK so the first thing I did was to attach the upright portion of the latch to the frame I built on the oven. I had left a stub a stabilized stub on the right side of the oven. I had to add a short horizontal piece so that the vertical latch was moved past the bricks so the 3/8 inch rebar had clearance room so it didn't scrape the brick every time it was opened or closed. This was the easy part.

Now here is where it got interesting. I had to figure out how to offset the handle and attach it to the door. So I came up with the idea of using a bolt and welding the handle part of the latch to the top. This way I could drill a hole in the door and put a nut and washer on the backside and all would be good, yeah right! It turns out that the handle needed support. Without the support the handle flopped all over bouncing off the brick and it didn't hold the door in place. So my solution was to use 3 nuts that were over sized just enough for the bolt to pass through them. The bolt that I used had a smooth shank with the end threaded so it wouldn't catch on anything. So I measured it out and three nuts were just long enough for the offset that I needed. So I welded the nuts together and them lined them up over the hole that I had drilled in the door and welded them to the the door. This worked great to support and firm up the door handle.

You can see here in this picture that I ended up using 2 nuts on the backside of the door. 1 nut was just too loose. Even when I tightened it down just by using the handle it would loosen up and it would pull the handle off angle. So to solve the problem I used a flat washer as a backer and set one nut to the right position then used a second nut screwed up tight against the first. This keeps both nuts from moving and has worked very well.

With the handle being attached like this it's not solid with the door so the handle itself stays cool enough to touch even when the oven has been in use for hours and the door is too hot to touch.

Overall I've been very pleased with how the door came out as well as how it functions. Now I just can't wait until we get the outer layers on so the frame will be hidden.

The Outer Door Story, The Hanging

Alright so we discussed how I came up with the air gap in the door as well as how I built the door. This was the easy part. Now I had to figure out how to hang the door and since this outer door came about as an after thought this presented some problems. I found that the people that did design their ovens to have an outer door usually had a metal arch insert that the door hung on. Well my problem here was that to use the insert I would have had to build it in as a permanent part of the oven and I didn't. So time to be creative.

I had to figure out how to distribute the weight of the door fairly evenly around the dome of the oven and how to mount the door to the oven. Well I looked at what I had and figured that I could use the rebar and wrap a couple of rings around the oven and support them. Then I figured I could encase the rebar inside the oven between the thermal and insulative layers. Well that was my plan any way.

So The following pictures is what I came up with as my solution.

So this is is my initial frame work. I used the 3/8 inch rebar and wrapped one strand around the lower end of the oven and a second strand about mid level. I used an arch of the rebar just behind the chimney and one piece from the chimney over the back of the oven. I welded all the connection points so the frame was rigid enough to support and distribute all of the shifting weight of the door and keep the lock steady.

OK I guess that I should tell you how I setup the the hinges. This was one hell of an experience. The first thing I did was to grab my propane torch, a vise and a small sledge hammer. I figured that I could heat up the rebar clamped in the vise with the torch and hammer it into shape. Yeah right, that worked about as well as a hole in the bottom of a glass. After heating with the torch and about a dozen hammer blows the end that I was trying to bend snapped off and my shoulder was killing me.

A new plan was needed. I had to figure out how to heat the rebar enough to soften it so I could bend it without aggravating my injuries any more. OK here was where the light bulb went off, I have an oven that gets really hot. So I built a good hot fire and used the oven as a forge. Talk about a three ring circus. So here I have two 20 feet long pieces of rebar that I have to get into the fire to heat up. Well I have my sons Jason and Charles holding the piece of rebar at one end and the middle. That way they can hold it and move it while I guide it to where I need it to be.

So we get the first piece in and start heating it. When I saw that it had gone past red hot and started to go orange I figured now is the time. So I had the boys pull out the rebar and swing the end around so that I can clear the corner of the house and get to the vise that is sitting on the counter area. After about four times of trying to line things up, because teenage boys never stop moving, we got it in the vise. After about four soft hits with the hammer I had about a 90 degree bend but, it started to cool down to much to continue so we did the dance to get it back into the fire. After letting it heat back up we pulled it out and danced around again to get it lined up. This time I was able to use the vise it's self crimp back the end the rest of the way making a nice loop on the end of the rebar. We set the first piece aside to cool and repeated the entire process a second time. It really was a good show and by heating the rebar in the oven it saved my shoulder immensely.

So this is a picture of how my hinge loops turned out. Also just a quick note on their alignment. I used a piece of 1/2 inch rebar that I checked with a level for plumb, that's straight up and down not the fruit, before I welded all of the frame together in order to make sure that the door would hand level and even. OK so it was a good and ready to hang the door. Now I had to figure out the rest of my hinges.

I used the 1/2 inch rebar I had left for the hinges. As you can see in the picture above I offset the hinge so the door would sit into the the arch about 1/2 inch. I used the 1/2 inch rebar for the hinge so it would be stable and not flex when the door was open.

As you can see in this picture I made the vertical ends of the hinge in two pieces. I did this so that in the future I could remove the door.

In this picture you can see that I used three flat washers in order to align the door. The first washer I welded on to the vertical part of the hinge so there was a flat smooth surface to rotate on. The other two washers I welded on the top and bottom of the hinge loop, but before I welded them I used four wedges to align the door and setup all of my spacing. I used the the flat washers on the hinge loop to lock my alignment in place and provide a bottom flat smooth surface for the hinge.

One quick note to add here was that after I hung the door I had to stiffen and support the hinge side a little bit more. You can see the extra rebar that I added in the picture before the last. The added stiffness stopped the twisting and the support that goes to the ground stopped the bouncing when I would open the door. All of the supports will be encased inside the rest of the outer layers of the oven and base so none of it will be seen when it's done.

The Outer Door Story

Alright I said that in this post I would go into all of the fabrication details, the math and my mounting system for the outer oven door. The first thing that I had to do was to figure out the math that would tell me how large of an air gap would be needed. I based my calculations on what I could find from the rocket stoves. I figured out that the optimal air gap was between 20% - 23% of the height of the door. So my 17 inch tall door needed a 3.4 to 3.91 inch air gap. Armed with this knowledge I settled on 3.5 inches for the air gap and then set off to find the materials to build the door.

I knew that wood wasn't going to work well so I had to look at using metal. I dug all through my stash and found 5 mild steel plates, 1 mild steel strip and some 1/2 inch rebar. I also bought 3 20 foot pieces of 3/8 inch rebar for about $11.00. I measured it all out and laid out the pieces and found that I had just enough steel for the door. I split one steel plate lengthwise for the top of the door that I trimmed to fit the curve of the arch.

In the picture you can see how I laid out all of the steel pieces. I cut the steel strip and used the leftover pieces for the bottom legs. I used an angle grinder to grind down the edges of all the steel plates so there was space for the weld beads. I then welded all of the pieces together and ground down all of the weld beads on the front and back so I had a nice fairly contiguous surface on both sides. Now any of you that have done any welding know that when you take multiple pieces and weld them together they like to twist and buckle. So as you can see in the picture above I added a frame of 3/8 inch rebar and a cross brace to help remove some twisting.

As you can see from the picture of the back of the door I also added a cross brace going the opposite direction of the one on the front side of the door. This was to help remove the rest of the twist from welding the pieces together.

OK so why am I showing this picture? Well this is for several reasons. The first is that I've seen a lot of people try to weld metal pieces by pressing the edges together and then running a bead of weld on top. However when they grind down the weld bead the pieces fall apart easily. By grinding the edges down it allows the weld bead to become part of the metal plate so when you grind down the top to make it smooth the joint stays strong and solid. This makes for a much stronger and longer lasting door.

My second reason is that by beveling the edges of the plates and then running your welding bead in the groove more weld is in contact with the metal plates so that when you grind the bead smooth you will remove a lot less metal. So you waste a lot less which also makes the process cheaper because welding wire is not cheap. 

The third reason for this is there is much less welding needed to fill any gaps on the back side of the panel after the front side side welded up. Which again saves on welding wire and means less material has to be removed to smooth out the surface of the door.

Now as you can see once put together you can hardly notice all of the individual pieces of steel. OK now let me be honest here, it took me several days to do this and when I was done at the end of each day I was hurting bad and I was exhausted but, it was worth it in the end. I just want to warn you that if you go this route that the little hand grinders are deceiving. The amount of torque they produce in conjunction with the angles you have to use them at really wears you down. So I know how long it took me to do this and how much it hurt to do but, since part of the goal of this project was to spend as little money as possible I used what I had.

One more point about the picture above. You'll notice that I did not weld the rebar frame to the metal plate the same way. Due to the heating and cooling of the door I wanted to allow the door to expand and contract without breaking the welds holding the frame on to the steel plate. By spacing out the welds attaching the frame to the steel plate it does just that. This makes for a much longer life span of the door with fewer problems or repairs down the road.

Alright yes, I know that I had said I wanted the door to look like the old cast iron doors. Well it will as you use the oven. I looked up how the cast iron doors were treated to prevent rusting. The simple answer was oil. The very same way cast iron pots and pans are seasoned. Oil is used to create a carbon layer on the metal giving it that distinct black color. Over the years everything from lard to vegetable oils has been used for this. I settled on olive oil since that's what I had. As I've used the oven the door has darkened considerably. I coated the front and back of the door as well as all of the exposed rebar with the olive oil right before I fired the oven for four times. After that it will get darker as time goes on and should only need to be coated about once a year.

As you can see from the the pictures above the door does begin to get darker and take on that traditional look of the cast iron in a fairly short amount of time. OK I should point out also that on the backside of the door I did have to add one more piece of rebar to the top section of the door. This was to stop the top of the door from curving backward. The problem didn't  show up until after the first firing.

I'll describe how I hung the door and keep it closed in the next post so this one won't be to long.

One Or Two Doors, Both Please

Let's talk doors. The reality here is that you will need at least one door to close the door opening that is cut into the thermal layer. The purpose of this door it to close up the oven and allow all the heat and steam to be kept inside the oven to cook your food and not allowed to flow out the door opening. I made this door out of some 2x3 lumber that I had on hand and some screws.

This is really simple construction. I measured the door opening width all the way up and cut then stacked the lumber pieces inside the opening. Just to be safe I numbered each piece starting with #1 so I didn't mix up the order since my door opening is slightly bowed. The top of the door I just used the arch template and marked the curve. Then I used skill saw and cut down to my curve line by making angled shaving cuts.

I did screw the backside braces on first before I cut the arch. This worked OK and if I make another oven door I plan on building it a little differently. First off I would use angled cross braces to make the door stiffer and less prone to shift. I would also have two handles not one and I would change the handle orientation from horizontal to vertical.

This is by far from the only design you can make. I found oven doors made from all kinds of materials and in different designs but, I used what I had.

Now after cooking in our oven for a while I learned that trying to cook with live fire in the oven wasn't quite as efficient as I wanted. I started thinking about the fuel efficient stoves that I was planning on building as part of the kitchen and wondered how I could use that idea to make the oven more efficient. Well after all my research I came up with a design for an outer door. Similar to the cast iron doors you would see on on the old brick bread and pizza ovens.

So my question was how can I make and hang this door as well as make it work to make the oven more efficient? What I found out was that by adding the outside door I could reflect and recirculate the heat in the oven while at the same time reducing the turbulence in the front of the oven. As you can see in the picture above without the door there is a lot of turbulence. This is easy to see when there is smoke in the oven as the turbulence shows immediately.

As you can see in the picture above the door helps to direct the air flow and reflect some heat back into the oven. It also makes the fire burn hotter as the oxygen mix in the fire is better controlled. It reduces the flames but, increases the embers which increases the heat and reduces the smoke making the oven more efficient.



Here's what the door looked like when I first made it. Since this door was an after thought I had to build it based on the size available inside the brick surround and I had to find a way to hang the door. Since this outer door has worked so well I felt that it should be something to point out and suggest that you think about if you plan on building your own oven. I will go into the fabrication of the door, the required math and how I hung the door in the next post.

The Thermal Layer, The Cracks

Once the thermal layer is fired and put into use you will inevitably see cracks. There are two types of cracks I found that happen. One type of crack is the expansion crack from the initial firing. When the oven cools these cracks don't close. The nice thing about cob is that to repair these cracks you just wet the cracks with some clay slip and work in more cob to fill the crack. Then smooth them out with a damp finger and let them dry for a day or two before you fire the oven again. This technique works on the inside and the outside of the thermal layer.

The second type of cracks is the expansion that appears while firing. These cracks close back up when the oven cools. This I have learned is normal in clay ovens. If the cracks close on the outside of the thermal layer then you don't have to try and fill them and if you put the insulation layer over thermal layer it helps hold the thermal layer together. You will need to keep an eye on the inside of the oven. If a crack opens with heat and then closes as it cools then you're fine. However I found that early on some cracks on the inside will start that way and then suddenly they stay open and never close. Those you need to go and fill.

As long as regular maintenance is done your oven should last. That was another statement that I read over and over but, I had no idea what it meant at that time. Well now I can tell you that regular maintenance means checking and filling cracks.

Since my first pizzas I have roasted chicken, vegetables, herb potatoes, hams and baked bread. During all of this cooking, it was so good, I had to begin checking and repairing cracks. Since then the cracks seem to have stopped showing up. I could see how in the future with temperature and humidity changes cracks might occur so we'll just have to keep a close watch and take care of them.

The Thermal Layer, Firing Your Oven

This was a serious source of frustration for me. Everything I read and watched said something different. Some sites said fire your oven right away while others said to wait for weeks and then build small fires slowly building in intensity over many different time frames. Still other information said not to fire the oven's thermal layer separately but, to build the entire oven and let it dry for weeks and then fire it. Still others said just build the dome of the thermal layer and just start using it. Well not all of these sites had longevity in mind and some of them were based on the basic principles of fire makes things hot and were not really designed to be very efficient.

So I made a couple of phone calls to get some information from The University of Arizona. I told them what I was doing and what I had read and asked for their opinion. They were interested in what I was doing and asked me how I was building the oven as well as what I had based my oven design on. I explained that I had settled on the ancient ovens of Pompeii to use as a model for my oven. I also explained that I had added a chimney and discussed the math that I had used to arrive at my conclusions. They responded very positively about my project and offered some suggestions about firing the oven.

Their suggestion that I settled on was to allow the thermal layer to cure for a week then begin building progressively increasing fires once a day for several days. The firing took about a week. The first fire was a small paper and tinder fire. The next fire was slightly larger with paper, a little cardboard, tinder and some kindling. The next fire was again larger made with with paper, cardboard, tinder, kindling, and small branches. The last curing fire was again larger made with with paper, cardboard, tinder, kindling, small branches and some small logs. The duration of each fire also grew longer as the fire intensity grew hotter.

Now as it was explained to me the final firing had to be at an extreme heat for a several hour duration. This would help cure the thermal layer, force any expansion required and begin to change the inside layer of the oven to terra cotta. So I built a good  hot fire for the last curing fire. I started building the fire at about 11:00 AM and continued to build it up until about 9:00 PM.

As you can see I had a seriously hot fire going. You can see how well the chimney works to draw up it. Some nice flames and heat here. We were sitting about 12 feet away from the fire and it was hot.

I took this picture at about 2:00 AM. You can see just how hot the coals are and heat was radiating off of the dome. It was pretty amazing at this point to see what a bunch of dirt and sand can do. The next morning was a Friday, there were still coals burning and the dome was still hot the the touch.

Since it was Friday I decided that I would wait to make dinner until my wife got home. I built a fire in the oven and had my wife stop at the store and pick up fixings for pizza. She came home with fixings for 3 18 inch pizzas with the works and hot wings. So that Friday night was the first meal I cooked in the oven. I moved the coals to the back of the oven and swabbed the bricks with a wet rag to clean off the ash. I put the hot wings in a shallow pan and slid them into the left side side of the oven. Then I used the the right side and the center to cook the pizza. I only put two in then when I pulled them out I put in the third. I wasn't sure how much room I would need. Next time I'll just put all three in together there was plenty of room. All I can say is that pizza really has to be the first thing to cook in your oven. Wow there is just nothing like pizza from a wood fire oven. Hungry yet?

The Thermal Layer, Adding The Chimney

After I removed the arch frame I began working on the chimney. I used three bricks on each side of the arch to span from the arch to the thermal layer of the oven. I then used one brick on the front and back to span the gap at the top. This allowed me to have about a four inch flue that runs the width of and behind the arch that goes up into and 8 inch by 8 inch chimney.

You can see in the picture above how I positioned the bricks to span from the arch to the thermal layer. Any gaps I filled with cob and smoothed out. I also used some clay slip to help smooth and seal everything inside the flue.

In this picture you can see how I used the bricks to span the gaps on top. This gave me a square base to work off of. I just filled the gaps with cob the began building the chimney keeping the same 8 inch by 8 inch size inside. I worked the cob up a couple layers at a time. My chimney is about 4 to 5 inches thick all the way around on top of the bricks. I worked the chimney up to 6 feet tall.

Then I went back and started adding cob against the side of the brick risers. From the front tapering it out to the thermal layer. My reasons for adding this was to provide good support for the weight of the chimney as well as to reinforce the risers for the arch. I also added cob around the sides of the chimney to make it very stable so there would be no concern in high winds.

You can see in the the picture above where we began adding the cob on the sides of the chimney and on the sides of the risers. Since you can only put down a couple layers at a time we were taking a break waiting for the cob to set. Charles and Jason thought it would be fun to attempt to convince me they could sleep in the oven again.

This is one of those design elements that you need to decide on. You don't have to have a chimney but, I wanted to be able to cook with some live fire at times as well as I wanted the smoke to be drawn off so it wouldn't be in our faces. So in order to do those things I needed a chimney. I am really glad that I built the chimney as it helps a lot. I can honestly say that I won't build another oven for myself with one but, for you the choice is yours.

The Thermal Layer, Finishing The Door And Arch

While we left the thermal layer to cure I decided to finish cutting the door before the cob got any harder. I had made a frame for the arch in order to support the bricks while I was laying them. I made sure that the outer arch was about two inches taller than the height of the oven door. I measured out centered and marked the final height of the door on the oven. The I used the arch template to mark my curve of the oven door. Then I decided I wanted a one inch reveal of the oven door. OK I know more fancy words. The reveal is the part of the door the you see that sticks out past the bricks. It looks like a frame that is visible inside the frame of the bricks.

I built the frame with a 2x4 base and 1x2 risers and arch. In order to get a nice smooth curve I made kerf cuts 1/4 inch apart 1/2 inch deep with a table saw. Then I nailed the arch to the center riser and then the end risers. I also ran the outer risers along the blade of the table saw to cut the ends of the arch smooth and in line with the outer risers. This technique created the template, curve and corners beautifully. However I ran into a small problem when I used the form to make the arch. This form needs more support than only the center riser. As you can see in the picture below the weight of the bricks flattened out the sides of the curve. So I would recommend adding one or even two more risers on each side.



Yes that is another one of my kids. Jason was trying to convince me that the oven was big enough to be his new room. You can also see what I was talking about where the curve flattened out on each side. You can also see the inner door and it's reveal that I was talking about.

OK now on to the cutting of the door. I wanted to find something to make a clean smooth cut for the door through the cob. I realized that I had a MultiMax vibrating tool that had blades designed for cutting grout and concrete. I figured this would be a good tool to try. The MultiMax worked like a champ. The only issues I had was that I had a tendency to cut at a downward angle because of the bolt on the bottom of the tool and holding and working that much vibration killed my wrist. I found that if I ran a shallow cut or two into the thickness of the side walls of the door then went back from the front and cut out the opening in layers it worked very well. If you wanted to use a hacksaw at this point instead of the vibratory tool you could, but that MultiMax cut the time down considerably.

To build the arch I started from the outside and worked in to the center. Leave the very center brick for last as this is the keystone. This is the brick that will hold and lock the arch together. I used about 1/2 inch of cob mortar between the bricks and the pressed the keystone into place. Also remember to fill all around the top of the risers to provide a solid bed of cob to lock the arch in place. It sounds harder than it is. Leave the arch support in place for a day or so the mortar can harden. There you have the first brick arch I ever built but, it won't be my last.

The Thermal Layer, The Next Step

After the thermal layer sets you have to cut open a door. You need to do this for two major reasons. The first reason is that the thermal layer will shrink slightly as it dries and if you leave the sand form in place then the thermal layer will crack and fail because the sand form won't give and allow the required space for the shrinkage. The second reason to open up the door is so that the inside of the thermal layer can dry at an even rate with the outside of the thermal layer.

Now hopefully for your oven you've already done the math and you know how tall the center point of your door needs to be. Remember that for optimum performance the top center of the door should be 63% of the internal height of the oven. The measured height of the sand form is a safe measurement to use. The width of the door should be about 1/2 of the diameter of the internal oven space. For us that meant a 24 inch wide and 17 inch tall door. At this point that is important to know because you need to make the door smaller than your finished sized. This is a somewhat controversial technique to use if you have watched the YouTube videos and read other blogs and websites. However if you start smaller and then adjust your finished size you get a cleaner and sharper door opening. I'll touch on this again later.

Now on to cutting the first door opening. Well if you read all the websites and blogs as well as watch all the YouTube videos they all tell you to use a knife or a hacksaw blade. What a freaking joke! First of all our clay soil literally gets rock hard. Then you add all of the sand and this stuff rivals concrete in a matter of a few hours. I tried a knife and it did nothing for me at all. So I tried a hacksaw blade and that was worse. So now I to figure out what was going to work. Then I had a bright idea to use a hammer and chisel. Wow, this stuff is so hard that it dulled my chisel in a matter of minutes but, the chisel did the job. My shoulder and back was killing me afterward but, the chisel did the trick.

I cut the door opening to roughly 14 inches tall by 17 inches wide. This exposed the newspaper covering the sand form. The cool thing about removing a chunk of cob like this is that it can be added and worked into the next batch of cob. So we just tossed the cob chunks into a bucket and set them aside for the next batch. Now I just cut away the newspaper and we were ready to start removing the sand form.

Just start pulling out the sand and putting it in buckets, on a tarp, in a wheel barrow or whatever you want but, keep it separate from the clay soil so you can reuse the sand in future batches of cob. Because we built such a large oven we had to really reach in to get the sand out. Below is a picture of our son Charles working on removing the sand form.

In this next picture you can see how Charles has been removing the sand to work his way into the oven to remove the sand in the far back or the oven.

No Charles didn't do all of the work alone. I worked on the beginning then when I needed a break he took over. The he needed a break and I took over again. Charles then took over and finished getting the rest out. Expect that when you do this your finger tips will be rubbed smooth and sore. I know that when we finished removing the sand I had to take two days to recover and be able to move again.

Be ready with enough containers for that sand. You'll notice the cart in the first picture. We filled that twice over plus some when removing the sand form. The wet sand packs very dense and when you take it out it seems to at least double in size so be prepared.

This last picture is of Charles sweeping out the last of the sand from the back of the oven. As you can see it's a big oven since he can fit completely inside it.

After all of the sand was swept out we pulled the loose layers of newspaper off of the inside of the oven. The newspapers that were stuck we just left on because after the oven is fired they would burn off. So now at this point we had to let the oven dry and cure. When you do this please don't get impatient as the cob must cure with walls this thick before any firing. If you fire it too soon it will generate steam and any steam pockets inside the wall can and will explode. The steam explosions can blow out anything from small holes to large sections. To make sure this didn't happen we waited a week to cure the dome.

Building The Thermal Layer

The thermal layer is the layer that makes your oven work. I read that same line over and over again as I was doing my research. I kept trying to figure out exactly what this meant. To put it simply it means this layer is the oven. The insulation layer helps hold the heat and the outer layer seals and waterproofs everything but, the thermal layer is actually the working part of the oven. This layer is central to the entire oven project. It will be the layer that holds and radiates the heat of the oven. This layer will also be a monolithic dome that will be the structural support for everything else on the oven.

Due to the exposure of fire and extreme heat I chose to build the thermal layer with no straw. This is essentially the same mix as a clay brick or terra cotta. Remember the mix is 2:1, two parts sand and one part clay. I settled on this mix with no straw because when I made a test ball with some straw and fired it in the barbecue it was weak and crumbled but, the one with no straw was hard as a rock. I wanted the strength in the dome.

OK so why am I making such a big deal about this one layer? Well there are several reasons for it. One is that this layer needs to be as even and constant in thickness as possible so that the heat is always even inside the oven. Another reason is that it is the most difficult layer to make due to the multiple steps required for this type of construction. With that said the truth is if you pay attention to the details it's not that difficult it's just a lot of steps.

Step one was to make lots of cob balls. Now we made a bunch of the balls close to the same size so two rows of cob balls made the wall between 6 to 7 inches thick. I would highly recommend not using the balls but making cob logs. I ran into some trouble with the cob at times due to the double layer of cob balls that would have been alleviated if we had made the the logs 6 to 7 inches long at the start.

Step two is to begin placing the cob. Your sand form is not going to move so don't be timid or too light handed when placing the cob. You want to place the cob with a little slap / force always angled in and slightly down towards the sand form. You also want to pat down to make sure of a good bond but, remember that a patting a little goes a long way. If you pat too much your hand will become sticky from the clay and will pull the cob up and open gaps. I learned to use a motion that was like a flat handed karate chop. The hard part in this process is to continue adding the cob so that the top of the cob stays perpendicular to the sand form and consistent in thickness. This gives a good bond and makes for a solid dome.

I know all this sounds strange but, after you put down the first couple of rows you'll get the hang of it. A technique that I used was to place the cob with my left hand while pressing and shaping at the same time with my right hand. This technique also allowed me to keep the thickness consistent. I had to measure the width of my left hand to determine where to hold my right hand. This seems a little awkward at first but, I got used to it very fast.

So when the sand form is all completely covered and you are happy with it then you need to give it some time to set. This was tough to judge since the cob seems to set and solidify in no time at all and because it took us two days to build the thermal layer since we ran out of sand. Once the thermal layer is set then on to the next step.

One side note here is that when we began building the thermal layer I very quickly realized that the brick risers that I had planned to build as part of the arch would have to be put in as the thermal layer went up. I didn't have enough room on the base where I was putting the door. So I had to include the first six layers of the risers. My risers were mortared together with the same cob mix as the thermal layer. This also embedded the risers into the thermal layer which made them very solid.

The Oven Form

At this point we were starting to get excited thinking how we were so close to being done. Wow we had no idea how much work this was going to be but, it was worth it. Well the first thing you need to do is to find the center of your oven floor again. I found that a string lined up with the center of the door bricks and the first coarse worked well. Then I used a combination square to run the second string to verify I had good right angles. I knew my planned my internal oven size was 48 inches in diameter. So after finding the center of the oven floor I had a starting point. I marked an X so I knew where the exact center was. Then I took a short straight piece of 12 gauge copper wire, a pencil and some string. I tied a small loop on one end and slipped it around the wire then measured out to the end and the other tied loop at 24 inches and slipped it over the pencil. This gave me a way to mark a 24 inch radius to give me a 48 inch diameter. I just held the wire in the middle and stretched the string taunt and marked the circle on the bricks.



OK so why am I doing this? This was to define the internal oven space. This has to be done because in order to create the void that will become the internal oven space we made a sand form. A word of warning here, this will take more sand than you expect. You will need to wet the sand so it sticks together. I started by building a small pile and slowly adding to it. I worked the mound of sand out to the line that I drew. I also made a mark on a 2x4 at 26 inches. I used this as a guide for the top of the sand mound. The reason I chose 26 inches is that the most efficient flow of air through the door is when the ratio between the height of the door is 63% of the height of the internal oven space. So with that to get a 16 inch door I needed an oven height of 26 inches. That takes into consideration a slight shrinkage as the dome cures.



I found that by using a trowel I got a nice, smooth and even finish on the sand form. This is important because the smoother the sand form is then the smoother the inside of the oven will be. Once we had the sand form filled out to the line we marked and to the correct height we then began covering the sand form with wet newspaper. The newspaper acts as a separation layer between the sand and the cob. The wet newspaper will also help keep the sand damp so it will be easier to remove than if it was dry. Once that was all done we were ready to start building the thermal layer of the oven.