If you can’t find an answer to your question here, in the book, or elsewhere on the site, please feel free to leave a question in the comment section and I’ll get back to you asap (if you scan the comments, you may well find your answer there).
I’m delighted to hear from you and to help; please do write/ask questions! Please be patient, as I have a lot of irons in the fire. Do let me know if you need or want an extended conversation.
Q: How efficient are earthen ovens? A: There are three 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. If you only want to bake a couple of loaves, you can build a rocket oven, like Flip and John’s. Or a Barrel Oven. (Also 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.) Third, there are efficiencies that don’t reduce to numbers so easily. Consider, for example, the efficiency of buying a brand new Prius so you burn less gas on your way to your hi-tech job to earn money to buy organic produce from a farmer far away that you cook in your brand-new, super-efficient electric oven. Is that more efficient than buying an old pickup to haul materials so you can build an oven, grow a big garden, run a home-based business, and have bread to give to your neighbors? Sometimes, learning to burn wood to cook your own food also teaches how to make do with what you’ve got, how to work with other people, how to build other things that will further improve and/or simplify your life, and how to shrink your needs and desires. Which is the “most efficient”? I think that’s a choice worth thinking about — and defending!
Q: I’m looking to build indoor masonry heater type units out of earth for heating and cooking. Any ideas? A: Ianto Evans and Leslie Jackson’s Rocket Mass Heaters covers rocket stove design for indoor heating and heat storage; Ernie and Erica Wisner’s website offers courses or you can buy their Rocket Mass Heater Guide book. Aprovecho Research Center publishes “Capturing Heat,” volumes1 & 2, that include designs both for efficient cookstoves and ovens. The Masonry Heater Association website has a wealth of resources. The Rocket Stove Experimenter’s corner is a community of tinkerers and rocket scientists. “Answers Questioned.”
Q: Is there any effective difference between the Quebec shape and the beehive shape? A: The major feature of the Quebec oven is a longer, egg-like shape. It’s useful if you need a wide door (for pizzas or cookie trays) but don’t want to hugely increase the diameter of your oven, or if you bake in square pans and want a longer shape with corners. The long oven can accommodate a wider door without giving up too much thermal mass. Some say it’s more efficient, but I haven’t seen any experiments that have proved it either way. That said, it’s good to keep the dome 16 inches or less; more volume reduces the concentration of steam which also inhibits crust development — in addition, increasing the distance between loaves and hot mass reduces the intensity of radiant heat.
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 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. I even built a cross-draft oven with a chimney in back which burned great, but used LOTS more wood. It also 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. After years of observing slow fires licking the walls of the oven, I now think the traditional oven with no chimney is probably best because heat transfer is a function of “the 3 Ts”: time, temperature, and turbulence. A slower burn gives the oven mass more time to absorb the heat of the fire. However, if smoke is going to cause problems for you, or if you just need to control it, then yes, build a chimney by all means.
Q: I built a small 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. Be careful not to add too much fuel at once, as this reduces the space for oxygen to mix and move in the oven, which will makes for a smokey fire. (See p. 100) If your door is the right height, you’ve played with the fire, and the oven still won’t burn, try cutting the door a bit higher, or making a hole through the back of the oven (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.) Both provide more oxygen for combustion. And finally, the biggest combustion problems tend to be in the smallest ovens (again, the smaller your oven, the less room for fire. Try using smaller wood, a little at a time, tend it closely, and and let it burn longer.
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 ovens with a metal door support, or a pre-fabricated door and frame, tend to crumble where mud meets metal. Of course, hot metal expands more than hot clay. However, it doesn’t seem to impair overall oven effectiveness. Perhaps the greater risk is thermal shock, which can crack floor bricks and generally weaken oven materials. Reduce thermal stress by avoiding the use of water, either for steaming bread or for cleaning (or cooling) a hot oven floor. 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—silty, sandy, or made w/too little clay. 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. Also, charred wood actually lasts quite a long time, partly because it requires a higher temperature to ignite. So long as it has enough integrity to hold together, it should work.
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.
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 the first layer is soft enough that you can dent it 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, prop up anything that looks sketchy, 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 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 a bit moist. 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? 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 in the book, and Lime in Building.
Q: Should I let the first layer dry before adding the next? A: It all depends. If you have limited time to work, sure, do it all at once. It may take longer to dry, but that’s OK. If you can do it in stages, by all means, let each layer dry out before starting the next. (But do finish the entire layer, because wet material doesn’t stick so well to dry material. And if you don’t let each layer dry out, the rate of drying will be slower—which can be a good thing, as quick drying can cause more cracking, especially if you’re building in the hot sun and one side gets more heat than the other).
Q: Should I fill any cracks before adding the next layer? A: If you’re building with a good sand-clay mix with minimal shrink (see p. 23), you shouldn’t have much, if any cracking. If, on the other hand, 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.
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 the first inch of clay actually gets hot enough to harden (called “bisque” firing, the clay typically turns a lighter, often pinkish shade, and maintains integrity when wet).
Q: What about cracks? Should I worry about them? A: Probably not—cracks happen, but generally they’re not a problem. In general, any material that goes from 50-700°F and up in the course of a few hours is going to relieve the stress of thermal shock by cracking—more or less. The thicker the mass, the more likely the oven can absorb the stress without cracking. It may be that thicker material (and masonry of fired brick?) 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). 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 used to hang the concrete floor slab on rebar and left 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 operators was over-firing the 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. This allows the central wedge of mortar to serve as a “keystone.” If your door ends up with a brick in the center, 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 the 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, mix 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 the 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 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 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).
There are also, of course, numerous other places to see what folks are doing with cob. Here are a few to start with: