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.

cabin cray cray

Three weeks. After that, headed back to live in this tiny, crazy cabin. It's a small cabin on Moon and Stars Farm where our dear friend Holly lives- a farm that's been operating on the homestead scale for about 15 years but is now looking, kind of tentatively, at some expansion. That's where we come in, an extra couple of hands, learning from Holly, helping her out, and seeing what we can create.

I aspire to get it a little organized feeling in cabin, hopefully streamlining all (or most) of our food prep and storage processes to an outdoor kitchen space which we'll be building. You can kind of see the food prep area on the far right of the frame of this photo- the hint is the apple-shaped cutting board designed by a friend of mine for Crate & Barrell. Ooh la la. Also, the tahini bucket with harmonicas on top if it is our "fridge" (bucket on porch in winter.) We will probably want to keep a few little basic food items in the cabin: coffee, tea, breakfast and snacks. Maybe mini-frige level, apples, hummus. I think an ideal use of this small space (about 300 sq feet) that my partner and I share would mean a majority of food processing/elaborate meal prep elsewhere. We have a well spigot outside and the cabin has full electricity. Doing dishes outdoors is the biggest obstacle for meal prep here, but I guess that would be easier in summer. We also have an electric kettle and a hot plate in here, on a blue desk on the right. 

Other clutter decreasers will be getting all the stringed instruments hung on the wall, likely in some sort of artful and spacesaving gallery wall configuration. Maybe we'll transition some of our bucket-storage system (kind of a guy thing, I guess?) to outside while adding some normal indoor-storage in here. I want to build a proper desk with hairpin legs... the desklike structure on the left is an odd end table that my parents pulled out of a dumpster on the night they got engaged to share a bottle of champagne over. Sigh.

I'm thinking of slapping a fresh coat of paint on two bookshelves (not pictured) and that ladder. I do wish I had a photograph of the space before we painted the walls, but the dark-stained wood walls let very little light in and it was really difficult to take a photo. We painted them "desert fortress" starting with a friend's leftover gallon but we had to buy more too. It's actually a pretty dark beige but it looks like white compared to our dark wood. 

As an aside- I have to laugh at any reading I do on the interwebs about restoring buildings, updating/organizing/decorating interior spaces. Those folks are operating at a pretty different level of budget and waste and stuff than I can really fathom. It's a major factor inspiring me to share our processes of figuring things out.

Other exciting prospects are replacing the front door (it's kind of falling apart and really colorfully painted with like a zombie doll and some words, I can't remember) building beds for our little kitchen garden here by the cabin- probably lots of simple medicinals, edible flowers, and salad and things we can eat without much prep. The outdoor kitchen will be inside the larger garden-Holly's garden- and we'll grow more hard-core stuff up there. Also up that way is a garden shed which I plan to unearth, organize, and turn into a more usable space. It's in the inevitable garden shed state of chaos and storage- so it happens when a person is so dedicated to a garden- the shed falls away from the center of focus. But I plan to get the shed whipped into shape- to best serve the garden. In that way, it will also serve the homestead dwellers- perhaps even as a place of quiet solitude for the 2 noisy people sharing the small cabin (us!)

The biggest task of all will be helping Holly and Marc design and build an outdoor kitchen, with a centerpiece cob oven. It's also the most exciting and most badass project....All culminating in September when all this new infrastructure will be dedicated and christened at Holly and Marc's wedding. 

We. Can't. Wait!

bio-char

Been hearing a lot about biochar for a couple years and how it can save the world.

you hear about oxisols transformed into rich, abundant terra pretta over the course of hundreds of years of biochar application. you hear about amazonian paradises.

Biochar is organic matter burned in a low-oxygen setting. It's used as a soil amendment and it's got the added benefit of sequestering carbon back into the earth. Here's what it does for garden soil:

• increase moisture holding capacity and aeration
• increase organic matter and nutrient efficiency
• improve habitat for beneficial soil microbe homies.

Lots of folks talk about making biochar in steel drums to keep out oxygen. We decided to go with digging a pit. Being surrounded by earth on 5 sides keeps oxygen input minimal. As the blaze grows, the wood we add to the top smothers new oxygen from reaching the coals. It's not perfect, but for us, the added benefit of chillin' around a fire is worth it. If you're gonna save the world at least relax while you do you know.

the first iteration of our bio-char fire pit.

after some sawing in the snow, the project's instigator (the chief of char) wised up and dug a new pit next to a big brush pile. conservation of energy. this pit's bigger, so you have to crawl in to light it, which is really fun. 

"I'll just put your camera on the 'girl in pit lighting fire' setting." Credit: Ben Samuelson