Just to be clear, this is a black oven. The fire is burned on the hearth. Before cooking, you have to move the coals to the back of the oven (for pizza) or sweep them completely out.
Mr. Wikipedia says:
Step 1: Foundation
Step 2: Base
To lift the oven to this height, I built a masonry platform. Many people use cement blocks---the type that basements are made from---to create the platform. This is all well and good, but I thought it would be more fun to build a brick platform, with a circular arch.
One source of difficulty was that I didn't really calculate or even try to estimate how many bricks I needed. I bought a pallet of 500 bricks for 0.20 ea and figured that it was really cheap and a lot---probably enough. It was not enough---half, at best. I had to use stones from a farmer's pile for fill and facing on the sides. This added considerably to the difficulty of the project, both in time and effort. The round edges of the stones make them difficult to work with. I adopted a slip-form technique, in which rocks are piled against a form (the boards) and concrete is poured in from behind. The result looks terrible. My wife suggested stucco over the whole mess and I can't disagree.
I did try to calculate the load-bearing capacity of the arch. For my oven, the bricks and mortar are a lot stronger than needed. But for your own oven, you should run the numbers or find someone who will.
The arch was made, step-by-step using a form made from a section of 2x4, 1/4" plywood, cardboard, and shims. First, I put the form in place, raised on extra bricks and shims (so you can get the forms out!). Then, I crinkled up the cardboard and put that over the forms. Then, I mixed up some mortar and put down a few rows of brick. The next day, I pulled the shims, moved the forms, and did another few rows. The brick had holes in them, so I eventually covered the holes with flat rocks and cement. It would be better to have turned the bricks on their sides, but I didn't think of that until long after the mortar was hard.
I filled in the "legs" of the arch with rubble. My neighbor gave me a bunch of rocks and I added in glass bottles and several years of saved brick, cement, and other stones that I had saved for the purpose.
Eventually, a stable flat platform was finished. I kept it covered to keep water from going down the holes in the brick. Water seems to be one of the main causes of masonry problems, especially when you live as near to the north pole as I.
Step 3: Oven
I made the hearth from firebrick. Each brick is 4.5 x 9 x 2" or so. Some would have you lay them bricks so that the oven floor and dome is 4.5" thick, but that seems to take 250-300 bricks. Each brick cost me 2.10 USD, and my budget was too small to accommodate such a design. Instead, I brought 60 bricks and 100 lb of dry refractory cement (HeatStop 50). It was enough, almost. I used a few old dense bricks to finish off the top of the dome. I intended to use firebrick for the hearth and the first row of bricks, but had enough for almost the whole project.
I put down a layer of polystyrene insulation and put a thick layer of concrete mixed with vermiculite. This all served as insulation for the hearth. Unfortunately, I made this base circular. It would have been better to make this entire part of the oven square, so that the firebricks on the edges would not have to be trimmed. In some cases, I had very small pieces, which were an irritation throughout. The 8" layer required almost 4 cubic feet of coarse vermiculite. It was structurally sound within a day but remained soft and spongy for about a week.
Above the insulation, I put a thin layer (1 in) of portland cement, which served to level the hearth. Then, a layer of sand, followed by the firebricks. If you follow this design, skip the sand. It makes the bricks on the edge of the circle unsteady.
After the hearth was finished, I began adding the first course of firebricks that make up the dome. The first course was easy because each brick can be made to fit will (minimize mortar joint) with a single angled cut. Farther up the dome, it is much harder. Dry fit the bricks before you mix up the refractory cement.
I made the forms for the oven dome out of foam insulation. It was harder than I expected. Once the forms were in place, I continued adding bricks until I got to the top.
Step 4: Roof
I cemented several pieces of 2x4 lumber to the base of the oven. These vertical members were reinforced with cross pieces that formed strong triangular sections. On this frame, I added rafters. On the rafters, I placed OSB, roofing felt, and asphalt shingles. The rafters were triangular with 6" of overhand on the sides. Above the dome, there was not enough clearance to allow for adequate insulation, so I made a metal crosspiece and used a reciprocating saw to remove the original wood member. On the front, I made the crosspiece shorter and higher to accommodate the cook's head. If I were to do this again, I would make the pitch of the roof a little higher.
I should have built the chimney first, but I was getting tired of dealing with the plastic cover every night. Instead, I left a hole. In anticipation of the flashing that would go around the chimney, I left some of the shingles loose around the edges of the hole.
The flashing around the roof was the hardest part of the project. It would have been a lot easier if I had known what to do and done it in the right order. I think I spent more time sealing the roof around the chimney than on any other part of the oven.
In frustration, I used a lot of roofing cement, built a chimney cap, and hoped for the best.
The chimney cap was fun to make. I measured the irregular top of the chimney (kids, when you build the chimney, don't make it like this one) and made forms out of the polystyrene insulation. You can see that there is a drip groove that goes around the edges. The base of the cap was custom fit to accommodate the ridiculous jagged top of the chimney.
Step 5: Door
I cast a cement door in-place. That is, I made a form that included the doorway of the oven, the inner arch that is made of firebrick. I used wax paper to prevent any sticking and was careful to avoid permanently cementing the oven closed. I packed my forms full of 3000-psi concrete (2:3 portland to sand, instead of 1:3, which I used for everything else) and patiently waited for it to cure. Full strength, I've read, does not come for almost a month. In the concrete, I included wire mesh for strength and 1/4-20 bolts for attaching the hinge.
I had to make a double hinge because the oven opening is so far back from the front of the oven and because the opening is already too small for a simply hinged door. Instead, it's a 4-bar linkage---door, 2 links, and ground. It's important for the two links to be rigid. I've used 1-1/4 x 1/8" angle from a bed frame (probably mild steel) and 16 ga sheet metal. The length of the two links depends on your oven. The mounting plates on either end of the hinge are 4 x 12 x 1/4" mild steel. I wanted some ability to make minor adjustments, so these plates are mounted with 1/4-20 machine screws instead of being welded. The rest of the connections are welded.
The joints are made with steel round stock, about 0.300" in diameter. I flatted the ends of the shafts slightly with a file and drilled a hole for a cotter pin. The filing makes drilling holes easier because the drill bit doesn't walk around. I think a punch would also do the trick. I didn't have any cotter pins, so I used a small nail, instead.
The whole hinge needs grinding, priming, and careful painting with a good exterior-grade "enamel" paint.
Just the construction took me about 10 hours. Granted, I cut everything with a hack saw and I'm rather green when it comes to welding. Still, it's an undertaking. Many people use simpler doors that may better match your work ethic and are perfectly functional. I like this design because my dome construction was sloppy and casting a door was a lot easier than whittling. I have also been reading about linkage design, so this was a chance to put some theory into practice. My inexperience welding was also a factor because I've been looking for metal-working projects so that I'll have a reason to practice.
In conclusion, I can't tell you what kind of door is best---only that this is the one that I made.
Step 6: Use
Many people have feel that it is terribly important to gradually heat up a new oven. I'm not sure if it's required, but after putting in so much effort, I opted for the more conservative route. So I built several small fires in my oven and then a big one. I'm also told that if an oven is left un-used for a long time (months) it's a good idea to heat it up slowly. Anyway, I like to do a pre-heat fire the day before I need the oven, so it's all the same to me.
I knew it was really hot when the soot burned off the top of the dome!
Now, it's time to figure out how to bake a pizza. You'll need a peel, shovel, metal bucket, wood, friends, dough, and toppings. But that's a whole other instructable.