As a blacksmith, I’ve built several forges -- 4 at last count, and rebuilt a couple of others. I know something about heating and getting high temperatures in an open space.
I also like making pizza and baking bread. Combining these two hobbies leads to the inevitable idea of building a wood-fired oven, but one that goes a bit beyond what I’ve been finding online.
Most wood fired pizza/baking ovens are little more than a heavy construction of brick and stone to hold the heat, a hearth to burn some fuel, and the same hearth to bake the goods on. This means you build a fire, let it burn down, scrape and push it back so you have a hot surface to cook on, then try to make a pizza that doesn’t have a layer of cinders on the bottom. This is why pre-modern-oven bakin would use a thin layer of dough -- called cake, as in the infamous “let them eat cake” proclamation -- as a base for their baking.
Even so, as a fellow pizza enthusiast noted, they don’t really get hot enough (700F or better) to make a good pizza. I know wood, or at least charcoal, can get above 2000F, so that seems a solvable problem.( Read more...Collapse )
After looking at several wood fired oven designs, I decided that the “build a fire on the cooking surface” was a losing proposition, and remembered my historical alchemical furnace designs. The reverberatory furnace, specifically, isolates the heated area from the burning fuel, while allowing the combustion gasses and both direct and indirect heat to interact with the cooking area. Since I generally like smoke flavoring, this seems ideal.
In a reverberatory furnace, the firebox is either behind or below the heating area, with the flow of heat passing over and around the heated object, or in the case of the lower firebox, also heating from beneath. Normally they have a natural-draft airflow; adding a blower and a tuyere (air pipe) to the firebox should increase air flow and raise heat generation to the desired levels. This is how charcoal and coal forges operate, and they both can get to the point of melting steel without much time or effort.
Also, at least for the test bed, I didn’t see the need for a large, permanent structure. A 2’ x 2’ baking surface should do fine.
So, for the first attempt, my design is to take a sturdy stainless steel table, such as used for food service, and build the oven on that. The base layer, the bottom of the firebox, is a layer of firebrick, with 8-10” of vertical space to build the fire in. Sides are also of firebrick. A steel structure holds a double layer of quarry tile or other heat resistant material that forms the roof of the firebox and the floor of the oven proper, or maybe do what the pros use and get a 24” x 30” piece of ½” steel and rest that on the brick sides. While ideally any supports would be of titanium, high-temerperature steel, or other heat-resistant (and therefore expensive) material, cheaper steel protected by a layer of inswool or a protective coating like itc-213 should also do the trick.
On the bottom of the firebox, a perforated pipe accessible from the outside acts as a tuyere, supplying air from a blower to increase the burn rate and heat generation. While in a forge precautions would need to be taken to keep the pipe from melting, in this case the desired temps are well below the melting point of iron, so standard black iron pipe should be fine. If not, a thick-walled copper pipe should do the trick, it’s what is generally used as a tuyere in short-stack smelting operations, with temperatures around 1800F or so.
At the back of the oven, a gap is open between the roof/floor and the upper baking chamber allows the combustion gases and heated air to enter the upper chamber. The upper chamber has either a cast refractory or brick-and-mortar construction, in a classic forward-sloping or half-barrel dome. Construction would involve building a form out of styrofoam, then either applying a thick layer of castable refractory mixed with grog or sand, or building a brick-and-mortar structure around it. Coating the foam with a release agent should allow removal, or it can just be burnt out.
At the front of the dome there will be a chimney with an adjustable damper, both to induce a draft when not using a blower and to control airflow during baking. Both the firebox and oven fronts will have steel doors to help control heat as well.
During operation, a fire is lit in the firebox, and once it is going the doors are closed and the blower turned on. A thermocouple monitors the temperature inside the oven, and when the desired temperature is achieved, the blower is turned off, and any ash that migrated from the firebox to the oven proper is brushed away. The ceramic or steel floor of the oven should hold a high temperature for quite a while, though just having the coals and embers in the firebox should be enough to keep it suitably hot. Wood chips can be added to the firebox for a smoked effect, and an adjustable vent in the firebox door can help modulate temperature and burn rate.
I’ve sketched out a few diagrams, and am looking for the right table. I have firebricks aplenty, and castable refractory and steel are available locally.
I think I have my winter project.
Bill of materials:
stainless steel table $150
Castable Refractory $120
½” x 24” x 30” steel $60 (assumes local pickup, shipping is in excess of $100)
Firebrick x 30 $225 (replacement cost)
Steel, hardware, etc $50
I can go cheaper by making up my own castable refractory or even by using the local clay, but since most outdoor pizza oven kits go for $3000 or more, I figure this is a win.
Chimney is at the front, firebox (with tuyere) on the bottom, there's a hard-to-see gap at the back allowing airflow from the firebox to the baking chamber.