cob oven: design research

Heya Folks.

One of the things on heavy rotation in the carousel of my brain lately has been a cob oven that we're going to help Holly and Marc build for Moon and Stars Farm this summer. It's pretty fascinating. I find that there's a lot of plans out there, saying, here's how you do it, and then launching into a design that someone thinks is best. I haven't found a good resource outlining the different design decisions to be made yet. Since I'm still in New Orleans, lounging poolside with gin-and-grapefruits and all that, I'm not working with the client yet (Holly) in Virginia about how she actually wants the oven to be, but I'm doing my best to gain an understanding of different models and options and pro's/con's through research.

I'd like to share some of what I've found, broken down by different factors and design decisions. For most of this, I'm deferring to Kiko Denzer, and his online resources, without having actually tracked down his book though I plan to borrow it from a friend soon.

size
K. Here's where you need to start thinking about what you want from your oven. What volume you'll be baking at, frequency, etc. The smaller the space, the quicker and more efficiently it will heat. The larger, the easier it will be to make lots of loaves or pizzas at once. Burning lots of wood to heat a large space to cook a single loaf won't be an efficient use of your resources.  If you are say, feeding a hundred people in your community each day, you'll want a bigger oven for ease. Basic size of an oven is measured in diameter.  Kiko Denzer says that a 21" diameter floor area (inside the oven) "will bake several loaves, smalls pizzas, chickens, etc. and needs less wood and firing time. Our 31 by 23" home oven bakes a dozen 1.5lbs loaves, and 3-4 hours of fire will make two batches of bread, as well as casseroles, turkey, vegetables, soup, beans, etc." His biggest mud oven is at a restaurant, 4x4 ft inside, 9 ft diameter outside.

Though there are plenty of taller ovens out there, they say it's best to cap interior oven height at 16", regardless of diameter. This is because the lower ceiling concentrates steam in the baking area for consistently crusty and lovely loaves.

Another good rule of thumb for body mechanics comfort is to place the oven's floor at about waist high, if you plan to use the oven on a weekly-or-so basis. If you plan to use the oven just once in a while, it may be best for you to place your oven lower so you don't waste energy building up a foundation without receiving regular benefit from your efforts.

shape
The two main shapes I've encountered are the dome/beehive/horno and the Quebec, an ovoid shape. The Quebec is best if you need space for rectangular cookie sheets, or if you want a wide door without having to make a large diameter oven (like for pizza, foccaccia, or cookie trays.) The long shape allows for a wider door without sacrificing a lot of thermal mass.

Another shape point is that it's recommended to have a door height that's 63% of the total dome height. So if you go with the 16", you're looking at a 10.08" door.

chimney vs. no chimney?
Okay, the million-dollar-question. So here's what I've found: there are a couple of schools of thought here. The chimneyless camp builds simple domes and allows the smoke to escape through the door opening. This is the model for the traditional horno ovens in the American southwest. This will absolutely stain the front of the oven with soot. It's also absolutely not appropriate for use indoors. The thinking is as follows: 1. an efficiently burning fire won't make all that much smoke (remember that smoke is unburned fuel escaping. If we build top-down fires we can minimize smoke) 2. A chimney will hemorrage heat during the baking portion, even if plugged. 3. it's simpler to build a chimneyless structure. 4. Most importantly, a chimneyless fire will burn slowly, giving the thermal mass more time to heat per wood used ratio.

traditional horno ovens, NM and AZ

The pro's of designs with chimneys are: 1. Quicker burning fire cuts back on firing time. 2. ease of use (no chance of smoke in face while tending- even if say, you can't bend over to get under the smoke) Of course, if you're indoors, you absolutely owe it to your health to go ahead and build the chimney.

Kiko Denzer, the expert to whom I find myself deferring, puts it like this:

When I first wrote the book I thought chimneys made better ovens because they made a faster, hotter fire. Now I think the traditional oven with no chimney is probably best_if the smoke won’t cause problems. If you need to control smoke, then yes, a chimney is best. I changed my mind after experimenting with a cross-draft oven, which burned well, but used lots of wood. It illustrated a basic fact that I understood, but didn’t quite accept; that is, no matter where you put it, a chimney sucks heat out of the fire. An oven without a chimney burns more slowly, giving the oven mass more time to absorb the heat of the fire.

Other snazzy design details I've seen: an arch built into the foundation to store dry wood (and cut back on stones needed to build up the height of oven floor.) I've also seen some funky models with chimney outlet holes that double as a burner. I think that might be for the advanced cob oven builder. I will also say the cob oven building project is lauded as a really great intro to natural building and can help us gain confidence for bigger cob projects (yeah yeah yeah!) I've bookmarked a bunch of good further reading on a pinterest board (also the sources of the featured images) if you wanna check it out.

figurine of wood fired oven, 500 BCE

I'll close with a really beautiful thought from Kiko Denzer about the role of community ovens in the time immediately following the French revolution. Before the revolution, the French were only baking bread in big ovens under the regulation and ownership of feudal lords. With this new democracy, one of the first things folks did was reassert community control of these big ovens. These ovens represented self-sufficiency, mutual aid, and autonomy at the human level.

"Yes brothers of the plow, the great world must be fed, and heaven gives the power to the hand that holds the bread." - from a song by George F. Root, 1874.