Q: what’s the difference between a cob oven and an earth oven?
A: Usually, they mean the same thing, but sometimes when people say earth oven, they mean a simple pit oven, like a Fijian lovo or Samoan ‘umu.
Q: How efficient are cob ovens? A: There are two things to consider here. First, since you have to heat up the whole mass of a retained heat oven in order to bake, clearly, cooking just a few loaves of bread won’t make efficient use of all your fuel. However, the more you cook, the more of the stored heat you use up, and the better your efficiency. Insulating an oven increases efficiency even more, as does using it again before it cools off (daily use, of course, is best). Alan Scott told me that one baker he knew cooked 630 pounds of dough with the heat from 130 pounds of wood (the oven was hot from the previous day). That was sixteen and a half bakes from a single firing! Better than 250 two pound loaves! That’s almost 5 pounds of dough baked per pound of wood burnt — pretty good! Second, smoke is unburnt fuel, so a smoky fire is not efficient. If your oven is already hot, it will burn cleaner because the masonry won’t be sucking heat out of the fire so fast. (see the upside down fire.) There are, also, many designs for wood-fired ovens. Some burn much cleaner and more efficiently than a cold earthen or masonry oven. However, you can make your mud-oven burn cleaner by using the driest wood; starting with a small fire, building it up slowly, and insulating. Or, if you only want to bake a couple of loaves, you can build a super-efficient oven that heats just air. (See the next question. If you want a super efficient, clean fire and a retained heat oven, (and you live in a cold climate) you may want to build a traditional masonry heater, such as those described in The Book of Masonry Stoves. (See Resources, and the chapter on fire, p. 93.)
Q: I’m looking to build indoor masonry heater type units out of earth for heating and cooking. Any ideas? A: Cob Cottage Company’s Ianto Evans wrote a book with Leslie Jackson on how to build a Rocket Stove for indoor heating and heat storage called Rocket Mass Heaters. Aprovecho Research Center publishes “Capturing Heat,” volumes1 & 2, that include designs both for efficient cookstoves and ovens. There’s a lot of folks experimenting with stoves and sharing their questions and answers on a public bulletin board, here.
Q: Is there any effective difference between the Quebec shape and the beehive shape? A: Interesting question. If you need a wide door (for pizzas or cookie trays) but don’t want to hugely increase the diameter of your cob oven, or if you bake in square pans and want a longer shape with corners, a long oven can have a wider door without giving up so much of its thermal mass. That longer, egg-like shape is the major feature of the Quebec oven. If that suits your needs, then I’d say it’s a better design for you. Some say it is more efficient, but I haven’t seen or made any tests. Whichever design you use, try to make your dome no more than 16 inches high, since a big volume reduces the concentration of steam during baking, and that means your loaves won’t get so nice and crusty.
Q: Have you experimented with electric heating elements? A: One reader wrote that he had tried a 3000 watt range element in an oven made of refractory cement. He said it never got up to temperature, even after 8 hours. A nichrome kiln element might provide more heat. And, like a kiln, I imagine it would be good to place the element in a channel, to increase surface area and contact between the element and the oven material. But that’s going to get complicated. Also consider that (in all cases except hydro- or solar power) electricity requires burning fuel to generate electricity to generate heat again_wasting large amounts of energy at every step_a very inefficient way to bake. I met a commercial baker who switched to propane when he got tired of splitting wood for his brick oven. But he said wood gave him a “deeper heat”_longer lasting and more effective.
Q: Is it better to build my cob oven with or without a chimney? A: 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. A cob oven without a chimney burns more slowly, giving the oven mass more time to absorb the heat of the fire.
Q: Is it better to insulate the hearth with horizontal bottles or vertical bottles? A: Depends on how much insulation you need. For an oven that needs to be able to hold heat for as long as possible, or for a commercial oven that’s fired multiple times a week, like this one at Gathering Together Farm, vertical bottles will provide more insulation. But they also require more material overall. You get to choose!
Q: I built a small cob oven that burns OK to start with, but no matter how I play with the fuel, it gets very smokey and goes out. The door is 63% of the interior height of the oven, so what am I doing wrong? A: Since the cut of the doorway will tend to angle down, the inside edge of your door may be lower than the outside edge. So be sure to measure door height at the edge furthest inside the oven. If your door is the right height, you’ve played with the fire (see p. 100), and the oven still won’t burn, try cutting the door a bit higher, or making a hole through the back of the oven. Both provide more oxygen for combustion. The hole should be opposite the door, just above floor level, and at least as big as a quarter. (Plug it up with mud or a stick when you bake.) And finally, the biggest combustion problems tend to be in the smallest ovens_which makes sense, since the smaller your cob oven, the lower the dome, and the less room for the fire to burn. A small cob oven should work, but might need smaller wood, closer tending, and longer burning.
Q: Do you find that cleaning the oven causes damage? A: I’m pretty careful to keep the hard edges of my scuffle and peel away from the oven walls. It is harder, sometimes, to be so careful when tossing wood into a raging fire_but not impossible. I have noticed that cob ovens with metal door supports, or pre-fabricated doors and frames, tend to crumble where mud meets metal. This is because hot metal expands more than hot clay. Overall, however, such an oven will still be effective. Perhaps the greater risk is thermal shock, which comes of putting water on or in a hot oven, either for steaming bread or for cleaning (or cooling) the oven floor. The shock of extreme temperature differences causes extreme shrinkage, which can crack floor bricks and generally weaken the oven. It If you’re being careful with your tools and still having problems with interior oven damage, it may be that your original mix is weak; perhaps too silty, or too sandy. In this case, rebuilding the oven may be your best solution.
Q: The snap-swivel on my scuffle keeps breaking — help! A: I had the same problem, so I made a stronger swivel by wrapping a turn of stiff wire loosely around the end of a cotter pin or the end of a nail. The other end of the wire holds my rag, and the cotter pin or nail is secured thru a hole in the end of my wooden handle. It works great.
Q: My nice wooden baking door is getting terribly charred. Do I just have to keep making new ones? A: I soak my door in a bucket of water while I’m firing the oven. That way it chars less, and adds a bit of steam.
Q: I built an all-clay oven but it’s crumbly, and falling apart. It was definitely clay, not silt. What did I do wrong, and can I fix it? A: The mix may have been too dry, or not tamped hard enough, or both. Either would prevent the clay from cementing into a single, solid mass. Remember that it’s water that makes the clay stick together. If you dug your clay in the summer time, and it was dry, it can take a long time for it to fully hydrate. In fact, you might have to soak the clay for a couple of weeks, then let the wet clay dry out a bit before you build. You might be able to save it with an internal plaster (if you can reach_see safety note below), or by just brushing out the worst of the loose material_it still ought to work, if the floor bricks stay secure and the walls are thick enough (you can always add more thickness if needed).
Q: How do I know when it’s time to pull out the sand form? A: I find it easiest to build with a dry-ish mix (see p. 33-35). Then you can pull the form as soon as the first layer is done. However, if you can make a dent in the first layer with your finger, you should wait. Test the material again in a few days (or weeks, depending on weather and your mix). To let air circulate and aid drying, dig a narrow tunnel into the base of the form (if the sand collapses, stop!) When it seems ready, dig a shallow hole into the sand form to expose a fresh bit of the first layer. If it’s still soft enough to dent when poked, wait! If, when you do pull the sand form, part of the oven does collapse, stop and let things dry out. You may be able to patch it up with a sticky plaster (use lots of straw and clay). You may also need to poke sticks or nails into the hole to give the plaster something to hold onto. If it was a minor collapse, and your oven is pretty thick, it might not make any difference and you can just leave it.
SAFETY NOTE: If it’s a big oven, and you do apply an internal plaster, don’t put your arms and head in the oven without another person there to help in case of collapse. I’ve never heard of it happening, but I don’t want to.
Q: What’s a good plaster if humidity is a problem? A: If, by “humidity,” you mean moisture in the air, there’s nothing to worry about; neither lime nor mud plasters tend to soak up (unprecipitated) atmospheric moisture. Lime plaster is not necessarily “better” than plain mud, and both kinds of plaster still need a roof.
Q: Once the cob oven is complete, is it best to wait until after it’s been used for a bit before the finish plaster is applied? A: Earthen plasters don’t go through any chemical changes, so speed of drying has minimal effect_apply them when it suits you. If they don’t stick, spray the oven with water, or make a wetter mix. A good lime plaster, on the other hand, is best applied when the oven is still moist all the way through. Lime plasters require water and time to effect the chemical reaction that makes them durable. Therefore, they are best kept damp (even covered) for a week or more. If you use lime plaster on a dry oven, soak it before you start. Best to use lime water (i.e., water that has had a bit of lime soaking in it to make it alkaline) to improve the bond. Lime is caustic_see cautionary note below.
Q: Is there a difference between mason’s lime and agricultural lime? Â Where can I get mason’s lime? A: Agricultural lime will NOT substitute for mason’s lime. Ag lime is powdered limestone, or calcium carbonate, (CaCO3). Farmers use it to make soil less acid. Mason’s, or “hydrated” lime, is limestone that has been cooked at very high heat (over 1500° F.), which drives off a carbon dioxide molecule (CO 2 ), to create CaO, or quicklime. With the addition of water, CaO turns to calcium hydroxide Ca(OH)2 . So lime plaster, when exposed to air, goes through a chemical reaction by which the soft calcium hydroxide exchanges a molecule of water (H2O) for one of carbon dioxide (CO2), and reverts to limestone. Any masonry supplier should have mason’s lime. Get it as fresh as possible, since, like cement, it “goes off,” or starts turning back to limestone if left to sit too long.
SAFETY NOTE: Mason’s lime is caustic_read the bag, and use gloves, goggles, and respirator as suggested. The best lime plaster is made by slaking quicklime into lime putty which, as long as it is wet, won’t turn back into limestone. But quicklime is hard to find, and slaking gives off immense heat; please do your homework, and be careful. See the Resources section and Lime in Building.
Q: Should I let the first layer dry before adding the next, so I can fill any cracks before adding the next layer? A: If you’re building an all clay oven, the answer is a definite yes. Pure clay shrinks a lot_I’ve had cracks as big as a half an inch. If you let it dry completely, it’s easy to fill cracks from the outside before adding another layer. If, on the other hand, you’re building with a good sand-clay mix with minimal shrink (see p. 23), the advantages of letting the first layer dry are negligible.
Q: Would it be good to fire the first layer to harden it? A: Firing the first layer won’t make any difference to the second layer. If you take apart an old oven, you’ll see that only about an inch of clay bisques, or gets hot enough to harden (it goes lighter and is no longer sticky when wet).
Q: What about cracks? Should I worry about them? A: Probably not—cracks happen, but generally don’t cause significant problems. A few things I’ve gleaned about high-temperature masonry: In general, any material that, in the course of a few hours, goes from 50-700°F or more is going to relieve the stress of thermal shock by cracking — more or less. The thicker the mass, the more likely your cob oven can absorb the stress without cracking. It may be that thicker material (fired brick masonry) survives the stress of thermal shock by sustaining many micro-cracks, while thinner material sustains fewer, larger cracks. (See David Lyle’s Book of Masonry Stoves). I imagine some cracking is simply due to the fact that materials expand with heat. I have seen at least one large masonry oven where the front hearth bricks had been pushed out a full half inch farther in front than on the sides! Some masons address that problem by building expansion joints into the masonry surrounding their fireboxes — i.e., they leave a piece of cardboard between firebox bricks and the rest of the masonry — when it burns out, it leaves a gap. Alan Scott hangs the concrete floor slab on rebar and leaves a ¾ inch gap all around it — partly to keep heat from being conducted away from the slab, but also partly as an expansion joint. Cracks can be useful. Some bakers recognize baking temperatures by the width of their oven cracks. But cracking can cause problems too. They allow flammable soot and heat to escape from the oven, and the soot, or nearby flammables, can ignite. The Bread Builders talks about two bakers who barely missed severe fires, both partly due to oven cracks (p. 140). However, one of the operations was over-firing their oven, trying to get more out of it than it was designed for. And of course, fire is only a risk if flammables are too close to a hot oven. Some commercial ovens insulate with loose, non-flammable material, like perlite or vermiculite, on the assumption that it will fill cracks, and prevent the escape of heat and soot.
Q: I made a brick arch doorway and the center brick came loose — what did I do wrong? A: The bricks hold best if the inside corners are actually touching and if there’s a joint at the top center, rather than a brick (apologies for the drawing). This allows the central wedge of mortar to serve as a “keystone.” If your door ends up with a brick in the center, try to make sure that the narrow end of the gap is a bit smaller than the brick. Then knock off just enough of the corners of the brick so that it will wedge into place, and make a keystone. If the gap is too wide, try turning the brick on edge, or orienting it longwise, and carving it into a keystone.
Q: I couldn’t find any pumice to insulate under my cob oven floor. I got vermiculite instead, but it seems too soft and squishy to support much weight_any suggestions? A: Compared to perlite and pumice, vermiculite is soft. If you use it under the floor of your oven, mixing it with some clay slip (see p. 86) and let it set up hard before you build on it. As for pumice, another source might be a concrete block and/or brick manufacturer.
Q: What’s the ideal height for a foundation? A: Well, there are several ways to answer that one. For someone who uses their cob oven a lot (at least once a week, every week — not just summer holidays), I’d say ideal height is to have the floor of the oven at the height of your waist. For someone who uses the oven less frequently, I’d say the ideal height is whatever is easiest to build. If that means you just want to plonk a ring of rocks on the ground, fill them with gravel, and go from there, that’s ideal. Remember, the oven itself is simple. The higher you go with the foundation, the more complicated it gets. Unless you’re making a temporary cob oven, in which case a temporary foundation can be almost anything that will support the weight, and that can be protected from the heat of the oven fire.
- Dirt is cheap. And if you thought you had to have a brick oven, mud is a good way to practice. You can make mistakes and learn before you spend hundreds (or thousands) on bricks and a mason. Start small: a 21″ diameter floor area will bake several loaves, small pizzas, chickens, etc. and needs less wood and firing time. Our 31 by 23 inch home oven bakes a dozen 1.5 lbs loaves, and 3-4 hours of fire will bake two batches of bread, as well as casseroles, turkey, vegetables, soup, beans, etc. My biggest mud oven is at a restaurant: 4×4 foot on the inside; 9 foot diameter outside (see back pages for photo and story).
- Mud is brick. Fire turns the inside of a “mud” oven to brick (quality varies w/soil, clay content, your mix, etc.).
- Speed: I’ve built 12-inch diameter demo ovens in 15 minutes, and full-size ovens in half a day or less.
- Custom design; you can make a fast-firing, thin-walled pizza oven, a big oven for a home business, a sculpted outdoor kitchen with seating, and more.
- Sculptural freedom: you’re not limited to rigid, rectilinear brick forms (not that I dislike rectangles).
- Temporal freedom: built right on the ground, or on planks and sawhorses, a quick oven can serve for a day or a year.
- Carpenterial freedom: roofing not required: use a tarp (when the oven has cooled) or a piece of tin.
- Most important, your kids can do it with you _ especially good for young people who know more about computers than they do about the earth that gives them their food. Find out for yourselves how it feels to be a plant, with roots that can follow water down into the fertile soil, and leaves that eat sunlight. Perhaps that’s why mud feels so good between the toes (and dough between the fingers)!?
- Community: Just as the kitchen, or hearth, is the heart of a home, large communal ovens have long been the heart of communities. In the eighteenth century, after the Revolution that brought democracy to modern France, one of the first things the people did was to (re?) assert community control over the big ovens that had been under the regulation of feudal lords. According to Jerome Assire, in his Book of Bread,
In rural Europe, the various privileges [of the feudal system] included rights of banality over both mill and bakehouse, which were both the property of the suzerain. To bake their bread, the peasants were obliged to use these and these alone, and had to pay a duty to do so. People wanted to reclaim what was naturally theirs: the right to make their own food, and the right to join hands so that heavy tasks could be made lighter for all.
From what I’ve seen of cob ovens built by folks with whom I’ve been in touch since writing this book, ovens and community still go together — whether community is the family, the neighborhood, a co-housing group, or a town with a new restaurant. And it isn’t just the eating that joins us — it’s the joy of shared labor, which can be a novel and pleasant surprise in this industrial culture where people often work alone in offices and facilities far removed from sun, soil, and neighbors (as evidenced on the very last pages).
If you can’t find an answer to your cob oven question here, in the book, or elsewhere on the site, please feel free to leave a question in the comment section and we’ll get back to you asap (please do scan the comments first, as you may well find your answer there).
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