A student at Willard Middle School (Berkeley, CA), with a marshmallow toasted in the cob oven they built for their school garden. (oven and photo by Bernhard Masterson, http://www.bernhardmasterson.com/)
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!
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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.
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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.
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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.
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Mud vs. brick: Nine Arguments for Mud
- 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).
If you get a useful answer to your cob oven question, please consider making a donation. I’m always delighted to hear from folks, and to help when I can, but any website takes time, effort, and love, not to mention answering questions. I’m also delighted at all the good stuff folks are doing and learning as a result of youtube and sharing, but like everyone else, I need enough income to keep the website going. I don’t like to ask for “consultation fees.” I’d much prefer a donation, or a gift (I’ve gotten some wonderful things over the years, from organic pistachios, to hand-made, woolen, fingerless gloves that I wear all winter long…). And of course, you could always buy someone else a gift copy of a book! Thanks for all your good questions!
Jump to: Efficiency, design, etc. | Using the oven | Materials & Construction | Foundations | Mud vs. Brick Efficiency, design, etc.
Hello Kiko,
I heard about your work and book from an earth/clay builder in Eugene. I wonder if you have any insight on this issue I am working with. I built a tandoori oven using clay/mud bricks. It was a lot of fun. But I made the mistake of coating the inside with a mixture of mud and straw. now the wall starts to crumble due to the breakdown of the straw. I want to resurface the inside with a mixture of clay and sand. but whatever I put on peeled off when dried. I tried thick and thin layers and of different hydration ratios. i also tried adding lime. The latest attempt of very liquidy clay mixture applied in very thin layers seemed to work better but it still crumbled and flaked off at many places. it seemed that the new layer just doesn’t bond to the old surface with much strength. Do you have any experience with this scenario — adding new layers to a fired surface? Thank you for your help! (sorry for the unintentional all-cap. It seems that is how this comment box works.)
Hey, Keith,
I’ve heard of tandoori builders coating the interior surface with clay and manure…? But every clay is different. Milk and urine might do things too. You might try painting the clay surface with the milk/urine first, then the clay mix… Since clay has electrical properties (charged ions), different compounds may create different kinds of bonding…
For a more industrial approach, you could also try waterglass (sodium silicate, available at Georgies Ceramic supply). I usually dilute it 50:50 with water. more about that in another blog post, here.
good luck, and let me know if you find a solution!
I am in process building a bread oven barrell design. I saw a Video of someone Laying the clay/sand cob onto a wire mesh as mold on dome. Can you do that? There was a thick layer clay ABOVE MESS and Inside so mesh Ended up betWeen. Will there be expansion issues as metal mesH aLthough thin mAy expand And crack clay. Thanks
In general, yes, metal expands and contracts at a different rate than masonry. That said, if the mesh is thin and well buried, and if the cob contains some fiber (which mayt absorb/allow movement), it might be just fine. I haven’t tried it myself. And if you’re already figured it out, let us know! (Sorry about the slow reply!)
Hi there, i’m thinking of building a cob oven but am worried about how hot the outside of the oven might get as i want to place it close to our aluminium garden folding doors.
So i was wondering how hot the outside of a cob oven gets – is it as hot as the inside?
How far away from walls or doors would you recommend putting the cob oven?
do you think it’s a problem if the cob oven is placed close to or next to metal or wood doors or walls?
thank you!
The temperature of the exterior will depend on what and how you build: thickness of dense masonry, amount of insulation, etc. You might want to look up code and specs for “masonry heaters” for recommended distances between masonry and flammables. (sorry about the slow reply! But better late than never, I hope…!)
Hi KikO great book and now some great food out of my great oven. Im a big advocate of your book and have referred many.
I noticed steam Coming from the door As i COOKEd fish in an open tray with water. THe steam seems to have also moisTIoned a small section of earth insulation above the door forehead. I am wondering if it is not advisable to cook in broth As Steam can compromise THe ovens inner walls or if a vent is required or if the door should Not be in place while cooking such to prevent this issue. I will take a further look inside this a.m if its cool enough. Thanks in advance.
Hey, Raymond, thanks for the note. Was the moisture enough to saturate and soften the material. If so, is there perchance a place where your door leaks? Seems to me that steam is pretty normal, whether you’re cooking fish or bread (or beans or overnight stew or beans). If the moisture laden air stays in the oven, I think under most circumstances, the material will absorb the moisture and it will dry out slowly as the oven cools. But if there’s a spot where the steamy air can escape (and it’s cool enough outside for the moisture to condense) and it persists for long enough, I can see how the moisture might saturate part of the oven material. Let us know what you find out!
Hi. Yes i agree, steam is part of nOrmal cooking process. Looking at it now its cool i can see a minor verticle crack inside above the door. It seems That some of the Steam is Finding its way into this crack which moistens a small area of external Mud where it condenses, It does nOt apPear to be a problem as it has dried out fine and I have attempted (wrongly or rightly?) to seal tHe interior crack with some oven mass material. The oven has been fired for long periods prior and I am happy to hAve seen the soot burn off and the interiors appearance.
Now a RaIn roof is in progress… Thankyou for your response.
GreetinGs,
I am wonderiNg if a Heavy straw cob mix that is in contact with the outside of the oven Stovepipe will combust, as we didn’t think to use fire cob for this part. Thank you!
It all depends. How thick, how hot, how heavy w/straw, how well mixed (or not). If you’ve already done it, I’d suggest a half inch or inch thick layer of plaster over the top, and watch it. The chimney does accumulate heat. If it’s smouldering after you’ve finished firing, it’s a problem…
Hi
I made what i think is a SUCCESSFUL attempt at An oven in thailand with help from your great book.
I am about to cover it with Straw cob and goop. I am a little concerned at the weight the dense oven material will have to bear with 6” of insulation above it. Any comments. (Is it worthy of concern).
Its 26” diameter and 17”” high interior with 2.5” of thermal mass making up the oven.
Hey, Raymond, first, assuming the material is dry, the dome shape is strong (if it’s wet, and you remove the sand too soon, it can easily collapse under it’s own weight alone). But second, the insulation layer should be much lighter than the dense layer. What’s in your “straw cob and goop”? Ideally, insulation should be clay and fiber, no sand.
Hiya, thanks for response. Yes its just straw and clay slip as i have plenty of straw and the earth here has ample clay. I shall search and try to gauge the density of WhaT i should be applying ie lots of straw rather than compacted straw and clay? The dense shell of the oven is COMPLETELY dry and feels sturdy enough, the dome shape Is strong. You suggest sawdust? Would i be Ok/advised to lay a layer of just straw over the oven before going ahead and covering it with mixed slip and straw?
Standard commercial hi-temperature insulation often consists of insulating firebrick, or pumice, both of which look a lot like foam: a solid fireproof material with lots of little holes. Because straw particles are big and bouncy, and because it tends to clump when mixed with clay, I avoid making oven insulation out of straw. I have had pockets of clumped straw burn out, leaving voids and allowing air to circulate. As the heat traveled, remaining straw started to smoke and smoulder, eventually disintegrating into ash and destroying whatever insulative value it had (and to answer your question directly, plain straw over the oven will turn to ash VERY fast — definitely NOT advisable). Sawdust is much easier to mix into a homogenous dough; when the sawdust burns away, you still have enough clay to make foam DIY insulating firebrick! I recommend making a few balls of different mixes, with more and less clay, putting them in a fire until all the sawdust is burnt out, and testing for strength. You should be able to handle the stuff fairly roughly w/out seeing it disintegrate.
Ok thankyou KikO. I will go with Sawdust as you recommend. Just to note, the sub base i did not use straw insulation but rice husks which are dry and fibrous and appear to be of no interest to the bugs/aNts and such here in thailand
Hello Kiko!
First, thank you for all the important information you are putting out into the world! As many have done before me, I too have plunged into the effort of building an earthen oven, using your book as guidance.
Some tech info: My hearth surface is 27 by 36, dense mass is 4” thick and insulation is 4-5 more inches of a perlite/sawdust/straw/mud mix over a layer of spaced out ceramic fiber blankets. Unfortunately, I was unable to find much clay coming out of the ground where I was digging (Minneapolis, MN), so I ended up mixing some fireclay into the dug up soil and added some grog as a substitute for part of the sand. My final thermal mix was approx.: 300lbs screened ground soil (mostly silt I assume), 200lbs fireclay, 300lbs sand, 100lbs grog. The mix was pretty sticky and dried very hard when tested.
So here are my concerns:
After a few firings, there is a crack running down the middle of the dome inside the oven, all the way from door to the back of the hearth. Nothing seems to be falling off or crumbling. The rest of the dome seems pretty solid so far, minus a few hairline cracks that I am less concerned about. I have recently been reading Dan Wing’s The Bread Builders and got to the part about the external combustion due to soot that leaked through the cracks because there was no filler material being used as an insulant. I am wondering if it is something I need to worry about, considering as I mentioned, that I used ceramic blankets for my first layer of insulation. There are also big air pockets because the blankets were pretty spaced apart since I didn’t have enough to cover the entire area of the dome and I didn’t pack the next layer very tight onto the fibers. Initially I thought this soft layer with the air pockets between would make good insulation and provide some room for expansion. But now, it is this air space combined with this slightly bigger crack that actually has me worried. Should I be worried?
Another concern, which also relates to this crack, is whether it may be a sign of week building material. Does having one larger crack, as opposed to having numerous smaller ones, indicate anything about the material’s strength and/or durability?
This concern led me to consider spraying waterglass on the inner surface to add durability to the thermal dome, but I am wondering if that will be as effective after having fired the oven a couple of times, considering that at least some parts of the inner surface are already quite bisque. Are there any potential risks or drawbacks for coating with waterglass?
Thank you!
Omri
Hi, Omri. Sounds like a major project, and well thought out. Which also means, however, that I don’t think there’s a simple yes/no kind of answer. W/that caveat, I’ll pass on a couple of opinions/thoughts: in general, cracking happens. You’re heating a LOT of mass to very high temps. Heat causes expansion, expansion causes stress, stress causes cracks. You didn’t say how big the crack is, but yes, better a few small cracks than one big one, especially if you’re paying attention (as I think you should) to Dan’s warning about soot and long-term fire risk. When you poke around the edges of the crack, does the material crumble easily? If so, yes, you might have a weak mix. I think waterglass is good, after firing or before firing — either way, it will bind stuff together. Assuming the dense mix is OK, you might consider a new/different insulation strategy. What kind of use are you expecting the oven to get? If you want to talk, you can contact me directly at handprint (at) cmug (dot) com…
Hi Kiko, and Thanks for the quick reply,
Poking around the edges doesn’t do much damage to the material except in a few local spots where the crack is a little wider- possibly an 1/8″ or so – but even there it doesn’t seem particularly crumbly. Maybe 2-3 pea size pebbles fell off from the very top and one more from another spot. No big chunks or sandy rubble. Part of this crack, mostly in the back part of the oven, began to appear early on before the insulation was added. In fact, first There was some cracking on the outer side of the mass in the same area, which appeared in the first week as it was drying. I added some clay to fix it before I fired it once or twice with small drying fires. Then I did one stronger firing for pizza, still before insulation was added. That’s when the crack started forming on the inside as well. I added some clay to try and fix it from both sides and let it dry before adding the insulation layers.
Does it make sense to make a soft mix of fireclay/sand and waterglass and try to fill the crack with it? I know that soda ash is sometimes added in pottery to deal with cracking issues. Could that be helpful in an oven application as well?
I baked my first batch of bread last week and it is pretty special. No comparison at all with dutch oven method I’ve been using so far. I am actually in the process of opening a small subscription based bakery and hoping this oven would play a part in that endeavor. If all goes well, I’m hoping to fire it up 2-3 times a week and ideally would like to get three bakes each time. is that realistic? If so, Would that be considered enough usage to warrant re-insulating? if soot does leak out through the crack, is it really possible for it to combust in the thin air pockets between the layers? as a reminder, over the fiber blankets there is roughly 4 more inches of a clay insulation mix.
The entire building process was well documented and I would like to send you some pictures. I’m not seeing an option to do that here so I will send them to the mail you provided but I am happy to keep the conversation here on the chance others may find it helpful.
Thanks for you help,
Omri
Hey, Omri, thanks for pix, and more story — those help. Sounds (and looks) to me like a drying/shrinkage crack. Glad to hear the mix isn’t soft and crumbly — that bodes well. If you start to lose material, or if it falls into your bread, then it might be worth cleaning out the crack and trying a patch. I’d start w/waterglass, however. And given the wide gap between the outside of the oven and your shelter, as well as the additional mix on top of the blanket, if you were to get a fire, it would be pretty hard to miss — also, with the flammables that far from the exterior of the oven, it would take significant heat/flame to ignite the shelter. I think the fire that Dan mentioned was an interior oven, in an enclosed space — I also think they were (over-)firing it daily, which would mean it never cooled. Much less risk when you have a day or two between firings. (That said, we did just have a freak accident here, when an ember got blown into and ignited a cotton oven mitt hanging on the side of the oven. It fell, burning, onto the ground next to some kindling, which was next to the wooden post of the shelter — it could have been ugly, but the baker saw it on their return, and put it all out. Still — good to keep the ground under/near the oven door well clear of flammables! And always tend your fire, even when you take all the precautions!) As for whether or not you have enough insulation, I’d say test it and see. You can always add more. Best wishes from here!
I’ve read the book and began building my earth oven starting with the base filled with gravel. I then used coarse pet bedding for the mortar between the bottles in the insulation layer. I made this insulation mortar with just enough soil to coat the bedding wood chips and for a ball to hold its shape. When I tested burning this ball as suggested in the book, it held its shape but was very fragile and as soon as I applied a little pressure it immediately crumbled. Will this insulation hold weight and the bottles in place? I will have about 2 inches of thermal mass, 1 inch of sand and then the hearth bricks, so not sure how much heat this insulation will actually get as there will be several inches between it and the fire. Thanks in advance for your guidance!
Please add more clay to the mix. Heat makes things move, and many small movements over time will crumble stuff that doesn’t have enough clay. If the matrix crumbles between the bottles, it will make things unstable. It may take years, depending on how often you use your oven, but better to over-build than under-build.
Hi there
I built my oven last year (not quite up to the standards that you have on your site) but it worked out pretty good, with a few exceptions. I have some cracks that were created and it seems no matter what i do, the cracks keep reappearing in the same area.
What is of greater concern is that the fire won’t stay lit. Could I possibly need a higher entrance? It’s about 11 inches high, and the oven itself is around 18 inches or so. The smoke will come out of the chimney that I made, but it also “pools” out from underneath the chimney. The oven creates a great amount of smoke. Initially it burns pretty good but it doesn’t take long before I’m fighting to keep it going. It’s possible that I’m using too big pieces of kindling or i’m just not letting it get big enough before pushing it to the back of the oven. I tried building it far in the back of the oven and lighting it that way and also tried keeping it near the entrance, but the fires continue to snuff out.
I’m not keen on rebuiding it again, i’m hoping that with some modifications i can get it so that there is a nice roaring fire for a good hour or so that will get the oven really hot.
One other thing, should i put another layer of mud on top of it again? It may repair all of the cracks and imperfections.
Thanks for any help!
https://www.dropbox.com/home?preview=IMG_1434.JPG
https://www.dropbox.com/home?preview=IMG_1436.JPG
https://www.dropbox.com/home?preview=IMG_1437.JPG
Hi, thanks for writing. Cracks happen. Heat makes things expand. When they expand, materials have to give. Cracks happen.
I couldn’t open your pix, so don’t know if you sent pix of a fire, but have you seen the firing video, here?
Probably the most common mistake is over-filling the oven w/more fuel than it can manage. Dry fuel and small, lively fires are key. And frequent stoking. Your door to dome height ratio sounds about right. Is it windy where you area? Even if not, a firing door will improve things. Also covered in the video, or the book.
It is always worth double checking door:dome height. Door should be 63% of dome, and the door height must be measured at the inside of the opening. If you need to make the door higher, gently chisel/scrape/abrade until it’s right (make a classic “adobe file” by nailing a piece of expanded metal lath to a stick of the right shape).
Is it insulated? If not, 3-6″ of insulation will help you hold heat. And a layer of cardboard between the insulation and the mass will reduce (or at least hide) any cracking…
good luck! let me know how it goes.
— kiko
Hi! What is the best season to build the oven. I want a commercial one and the monsoon is about to start. Is it safe to build in monsoon? If not then when should I do it?
Thanks
you can build anytime, but you’ll need to keep your materials dry — and work area, so you can control moisture content of the mix. Moisture slows drying time, which isn’t always bad. You can speed things up w/a fire if need be.
Kiko, first I’d just like to say thanks for your guidance. I have built a 4ftX4ft clay oven in my backyard. It was built 6 months ago and is bone dry. However, the floor (firebrick) just doesn’t retain heat. My dome floor is 17”X17” with a 16” dome height. Door is 63% and all specs look good. My floor is as follows (from top down):
Firebrick
layer of sand
Cob mix layer
Glass bottles
3 inch thick patio Tiles
Wood base (homemade pallet)
Cinder blocks pillars
I bake napoletana pizzas. I build a top down fire and heat the oven for 1 hour, until top of dome is hot to the touch. Walls are 900F and floor is 850-900F. I then push all the fire and embers to the side… floor is still a good 850F. Bake the first pizza in under 2 min. 10 minutes later, fire still roaring… but floor temp has now dropped to 650F. My next few pizzas take like 4 minutes to bake which then defeats the purpose of baking in a cob oven. I was thinking maybe I need to fill any gaps in the bricks with fire cement? Any ideas?
Hi, Jeff,
Happy to help. (If it’s useful and you want to say thanks, please consider clicking over to the donations page!) It sounds to me like the solution is going to be a longer fire. To address the cause, I’d need to know how many inches of mass are in the floor and in the dome, but I’d suspect that your floor is way thicker than your dome. If so, the cooler mass beneath the bricks will draw the heat down, away from your pizzas. To hold heat, you need to fully saturate ALL the mass. (an inch per hour is a general rule of thumb, so an hours is pretty short in most ovens — we typically fire ours for 2-3 hours if we want long-lasting heat.) Also, if the outside top of the dome is noticeably hot, I’d suggest covering it with at least 3″ (preferably 6″ or more) of insulation (clay and sawdust is good — see the book for details). In addition, the sand layer under your hearth bricks may be slowing down the rate of heat transfer to the cob below (sand by itself, tho it may be dense, is also full of air pockets, which means it takes longer for the heat to travel from grain to grain).
Thanks for the quick feedback! However, 1 inch per hour?? That’ll be like 7 hours for 7 inches! I’ll definitely give longer firings a go.
The link below is a picture of my oven. From beneath the bricks to the pallet is 7.5”. That consists of sand, vermiculite/clay mix, empty glass bottles, and then four 3” garden tiles. Dome thickness is about 8”, consists of thermal sand/clay layer, insulation straw/clay slip layer, and then outer shell of sand/clay.
Should the bottom of my oven feel hot after a “full” heat saturation? The only part on the bottom I can touch is a wooden pallet… Check out the link of my oven below!
https://www.dropbox.com/s/w81sqp53fdmhdw7/oven%20snapshot.PNG?dl=0
Nice oven! So: 7.5″ minus 3″ of garden tile and 3″ of bottles leaves about an inch and a half of mass under your hearth bricks, correct? That shouldn’t be too much. The “one inch per hour” rule of thumb applies to mass only, not insulation. If I’ve got the measurements and math right, you’ve got 2.5″ of brick plus 1.5″ of sub-hearth, for a total of 4″ of mass. Try a two hour fire and see how it goes.
If your pallet wood is getting hot, you didn’t use enough insulation. Be careful. Heat effects are cumulative, which means that it might not burn the first few times, but repeated heating changes the wood and lowers the temperature at which it will ignite. Make sure that the area around (and under) your oven is free of combustibles and have water to hand.
FYI, commercial ovens typically have about 7″ or so of mass — firing may take 8 or more hours, but they hold heat for a couple of weeks! (For example, Eric Moshier of solid rock masonry (.com) in MN says that: “In our commercial ovens, we use 4” of mineral wool on top of the firebrick and then 8” of rock wool insulation with HD Aluminum foil sandwiched between every 2 layers with a 2” mortar cap on top of the insulation. Our ovens are fired to ~700-800ºF one day, used to bake for 4-5 days or so and are still at 350º 6 days later.”
In a 3″ thick home oven for baking bread and roasting chicken, you don’t need to saturate it at 1,000F — so getting the surface to burn clean (1200F) in 2-3 hours can leave you w/solid temps of 4-500F for several hours. For pizza, however, you do want higher temps — and you have a continuous fire. So pizza ovens typically use less mass. 3″ of mass is merely a pragmatic compromise between baking and pizza that seems to work.
Thanks man, the oven took just a month to build with the help of your book! After an hour of preheating, the pallet definitely doesn’t get hot. It’s still cool to the touch, I highly doubt the heat will get that far since it has to go through bottles of insulation plus 3 inch garden tiles.
Yes I think the sub-hearth might be slightly more than 1.5”. Directly under the bricks is 1” of sand and perhaps 1”~1.5” of a vermiculite/clay layer over the bottles. Then directly under the bottles are the garden tiles. I’ll give a 2 hour preheat a go and see if it retains enough heat to cook some good napoletanas! Much thanks, Kiko!
Hi,
I have your 3rd edition earth oven book however it is not clear to me the step directly after you build your foundation.. I have a dry stacked stone foundation filled with stones, rocks, gravel and sand. My question is: do I cap this foundation with cob first to level it off or do I just put the glass bottle insulation layer directly on my foundation?
Many thanks for your assistance.
you can put the bottle insulation directly on the foundation material, tho depending on how lumpy it is, a leveling layer of some kind might be worthwhile. I also now prefer to place the bottles vertically (open end down) for a deeper insulation layer (9″ beer bottles are good for a home oven; I only use taller wine bottles for commercial ovens.) If you do put the bottles in vertically, it helps to have a couple inches of something loose (sand/perlite) to secure the necks of the bottles in place. see the drawings on pp. 42-43
I am building my first brick barrel oven on a steel trailer. I intend to confine or frame the hearth (two layers of brick on a 1/4″ steel plate table) with a steel frame. I realize that heat from the fire will expand the hearth bricks. I plan to make an expansion joint around the heart perimeter using perlite or vermiculite (advice on which is best or another material would be helpful) between the bricks and the steel containment frame. How wide should the expansion joint be for a 32″x45″ brick hearth? Also for a 49″x 72″ brick hearth. I can send pictures if needed or they can be seen on facebook page. Thanks!
I built my earth oven a couple of years ago and it works beautifully. With a fire on the back end we can run a pizza party for over an hour! Regardless of what I am cooking all the earth oven resources say fire 2-3 hours to ensure the thermal layer is fully saturated. After the pizzas are cooked and the coals are removed the oven will within an hour come down to a 600F range and then slowly lose heat with a partially open door over the next 2-3 days.
I am now experimenting with smoked fish which requires a stable temperature under 200F. The salmon turned out great but I have to wait 2 days after firing for the temps to come down. It’s not conservative to waste wood for a 200F oven or even a higher temp for just baking.
What are your thoughts on a shorter firing time? Doesn’t the thermal layer reach equilibrium through conduction? Why doesn’t a shorter firing time lead to a lower temperature “saturation/equilibrium” level at which point the oven would cool off at the same rate as a longer fired oven does?
Thank you
Hi, Stan, and thanks for the great comment/question. I haven’t tested your assumptions (I don’t do a lot of low-temp drying/cooking), but I share them. The BTUs are the same, whether you fire for one hour or three hours. I would definitely try it. You may still want to fire just long enough to burn off the soot (surface temps of 1200F), as I’d be concerned about flavors from particulates — but then again, that’s what you want for smoking, yes? In which case, maybe you should see how short a fire will work for you?
Sounds like a well-insulated oven! Congratulations.
— Kiko
I recently built a beautiful oven. First layer of clay/sand and second layer of that plus straw. I made the mistake of removing sand and building a small fire same day. Experiencing inner layer cracking, separating from outer layer and some falling. Outer layer still looks good and plan to add another couple inches of insulating layer. Can I add more clay to inside or how can I fix this? My impatience has unfortunately damaged a beautiful project. Suggestions?
Please!!!!
hmmm. Hard to advise from a distance. But. Adding to the inside is always tricky. Sometimes it can work. Sometimes you can just compress what’s left and it will harden it. If the second layer is holding up fine, and if it’s dense enough, it may be all you need. Just add insulation (sawdust & clay, no sand) on top. lettuce know!
Hey Kiko, I finished your book and my oven in 1 month! My oven measures: 27 inch dome diameter, 16 inches high, sitting on concrete base 48 inches X 48 inches which sits on top of 15 cinder blocks. Functions beautifully, gets up to 850+ F.
Here’s the issue — I had to move the oven further away from the house due to smoke issues… it was building up soot on the ceiling of my patio (i know, stupid I put it right under a painted wood ceiling). So I got 4 car jacks, a 1000 lb capacity cart, and after much pivoting and stress on the oven, managed to move it to an open space. The problem is now the hearth bricks of the oven floor has shifted and now there are small gaps… maybe like less than a cm. But I can definitely wiggle the bricks now which is NOT good… they sit on top of a layer of sand, and under that is clay/vermiculite mix + bottles. Any suggestions on how to fix this/ will this actually be a problem?
sorry about the slow reply, but if you get back here, let us know how it worked out. I suspect things are just fine, and that the gaps have simply filled w/ash — or that you took the time to remove and re-place the bricks in a tighter pattern, perhaps leaving the gap around the outside edge, where you could fill it w/clay/sand mix…
I built a 3″ thick slab for the oven subfloor (27″x 27″) above a 3″ insulated layer made of bottles and sawdust with slip. The slab is also surrounded by another 3 ” of insulation. I made the subfloor before properly testing my soil and as it is drying it is beginning to crack, the widest maybe 1/4″ thick. Now that I have made some test bricks and done the water test etc I can see the soil is mostly clay. Will this affect the performance of the oven? Will these be major thermal breaks? Should I take out the slab and redo with a better clay: sand ratio? Or can I leave it in and move on?
I’m lazy, so I’d probably just leave it, or fill the cracks and get on with the rest of it. Fortunately, an oven is a very forgiving thing, and I don’t think such cracks in the subfloor will significantly affect performance. lettuce know!
I built an oven recently but after the first firing the inside firing layer is very crumbly… Touching it with my hand, and it comes away with lots of fine grained particles… Poke it, and it falls apart! I’m pretty sure I have good clay content and not silt in my soil, though I could be wrong… I see above that you mention trying to repair a similar situation with an internal plaster. I’m wondering how to get this to adhere well–make it very wet? Wet the interior walls? Same proportions (though if the first turned to a crumbly mess, should I go with more clay, less sand?)… Your book has been very helpful. Hoping not to have to do a total rebuild… Thanks,
Given your description, it sounds like a secondary fix might be pretty dicey. But experiments are always very good for learning things you might not learn any other way. Yes, you’d need to wet the surface well. Typically, you’d also want to scratch it up/make it textured, but it sounds like it’s not really hard enough to do that w/out possible having it all come off/collapse. Let us know how it goes. I am sorry not to have replied sooner — the website hasn’t been letting me know when comments come in! I hope you managed to resolve your problems. If not, please do get in touch directly at handprint (at) cmug.com.
I’m about to start building my first oven and have 2 questions: first, I may have an easy method for cutting the ends off of glass bottles. I’m thinking of placing the ~2″ bottle-ends all over the first building layer — as my insulation layer. I’d grout them with the kind of insulation mix described in the book and add my finish layer – with the bottles showing, or just covering them up. Separately… I’ve got lots of pine needles here. Could they work in place of straw for insulation? I have a way of chopping them (with my electric blower/mulcher. Thanks for any advice!
Sorry, I forgot to ask the question in the first part .. What are the potential problems with doing this? Will it do the job of insulating the dome well? I remember seeing an oven with exposed bottle ends all over it in a village in Poland many years ago but I never got to talk with the maker.
apologies for late reply; for some reason, I don’t seem to be getting notifications when people comment! Remember that the best insulation is a vacumn — no molecules at all, no density. Failing that (only truly available in outer space), you want dead air space and low density. glass itself is denser than a sawdust (or pine-needle) and clay mix, so if I was going to use bottles, I’d go for the whole bottle, rather than just the (neck?) end, which is more glass and less air.
— Kiko
Kiko, the oven is finished and working great (a year later!?). I made a video documenting the whole process and I figured I’d post here so you could see what I was talking about with the cut bottles. Here it is, cued up to the point where I show them..
https://youtu.be/orsz2nwpn1w?t=10m15s
Hello Kiko,
My family and I used your book to make a cobb pizza oven and LOVE IT! We would like to do the lime plaster to make it look pretty and give it a little more protection from our Phoenix AZ sun and few yearly monsoons (I know from reading above posts a roof structure is still needed). I have the Type S Lime (purchased from Home Depot in building section-it doesn’t say “hydrated”, but it is used for masonry projects) and have purchased a natural earth and ocher pigment to color the lime plaster a “natural sienna color”. Having never done this before, I’m having trouble as I look through blogs and books, determining the right mixture and instructions. Here is what I am thinking is correct. I would so appreciate any feedback or correction.
Plan:
1. Wet down cobb oven to help lime adhere.
2. Apply Lime Primer:
20 parts water
1 part hydrated Lime Type S
3.Apply Lime Plaster while damp (misting as I go along to keep moist)
To make plaster put make lime putty (3 parts) and mix with sand (1 part) and ocher pigment (3-20% weight of dry lime depending on desired color)
Lime Putty
2 parts hydrated lime
1 part water
Some blogs and videos say to make the lime putty at least 3-6 weeks before application (keep in bucket with 1 inch of water to prevent CO2)….Is that necessary?
We are in May in Phoenix and waiting until June or July to apply will mean that the outside temperature will be 100+ F. We also get monsoons in July and August. I will have a roof over it, but wondered if the significant change in humidity is reason to wait until September to apply. I’d prefer to apply asap and mist it three times a day as it cures, before the monsoons come.
Also, most blogs (including yours) indicate 3 thin coats is advisable, but advise about waiting to dry between coats varies so much. For Phoenix conditions, what would you recommend?
Thank you SO much for your help. We made wonderful family memories making mud and have shared our pizza oven with our friends and family. It will be a source of conversation and fun for years to come. It’s even inspired to other families to give it a try!
apologies for late reply; for some reason, I don’t seem to be getting notifications when people comment! For anyone else who might read this, when applying lime to earth, the “lime primer” and wetting should be one step; the “primer” can also be water off the top of the lime putty — makes for “basic,” or low Ph water) — this helps bond the dissimilar materials to each other.
Type S lime is calcium hydroxide, and suitable for lime plaster. Aging is good, but not absolutely necessary (are you really in such a rush?)
letting layers dry is good — it improves suction/bonding for the next layer, and helps make sure that micro-cracks get sealed.
I’m glad to hear about your good mud-memories. And you’re very very welcome. It is SO gratifying and encouraging to continue hearing mud and art stories from folks!
— Kiko
i am building a oven outside my home in beautyfull south MISSISSIPPI out of SAND CLAY and HAY. when dry will this stand up to the heat from the wood burning on the inside of the oven. if so and will withstand the heat how long do these ovens usually last?? thank you for any feed back i can get
apologies for late reply; for some reason, I don’t seem to be getting notifications when people comment! Typically, the interior layer of the oven shouldn’t have organic matter (hay, etc.) in it, or only a little bit (please refer to Build Your Own Earth Oven for more details). That said, if you did use hay, it should be fine. Ovens can easily last decades/centuries, but it all depends on materials, construction, usage, as well as exposure to weather.
best wishes,
— Kiko
Will baking sand in an oven cause any chemical reaction in the oven? Thanks!
what’s “baking sand”?
hi I have built my pizza oven with old red clay bricks inside and out , can I put a thin layer of fire clay over the inside of the roof of pizza oven,
KIND REGARDS F<GREEN.
apologies for late reply; for some reason, I don’t seem to be getting notifications when people comment! To answer, briefly: on the inside, thin layers are very susceptible to spalling, cracking, breaking up due to intense temps and thermal shock. Outside layers should be fine.
— Kiko
I am about to build my commercial pizza oven, and I’m wondering if I can angle the chimney to go through my wall and then go straight up the buildings brick outside wall. Or is it only legal code to go straight up?
Jason,
Where are you building? What style of oven? Would be nice to have more background.
There is little or no code about ovens so there is not much reference there.
My best advice is to work with someone with chimney installation experience and not to skimp on this part of the project. Class A insulated stainless pipe is a good option.
As far as general chimney design….. yes, having two angled bends would be fine. You need to make sure the connection to the masonry is solid so that the bends are putting torque pressure on that connection.
For architectural reasons, one oven I worked on with my mentor had the stove pipe going almost horizontally out after a smooth bend from the throat of the oven to the outside and then straight up. The key is that once the chimney is heated up then it pulls through many configurations.
Hi Kiko,
Thanks for such wonderful resources!
After reading your book (first edition, from the library) and researching earth ovens from many other resources, I designed a 30″ oven and started building a cob base to put it on. With the rainy weather, busy life and the sheer volume of cob I need to mix, it is slow going.
Then I bought the .PDF version of the third edition of your book and I have decided to follow your advice and start small(er). I have designed an 18″ oven and started constructing the base to put it on. This will allow for instant(ish) gratification and I can experiment with a few things. But I have a few question…
1) Oven height
For an 18″ oven, do you still recommend a height of 16″ or would something like 14 be better?
2) Peak of dome 2/3 back
Earlier in this FAQ, you recommended building the sand form with the peak 2/3 of the way back, rather than right in the middle. Why is that? Is that just for smaller ovens, or would a 30 incher benefit from that as well?
My 18 incher is inspired by pictures of a workshop that you taught. In the workshop, two portable 12″ ovens were made over a wooden base insulated with vermiculite and mason jars. I am doing the same (but with perlite) and have a couple of base insulation questions…
1) Perlite v. Perlite & glass
I already read above that clay slip mixed with perlite reduces the R-value, but I am wondering which would better, straight perlite or perlite and mason jars or glass bottles.
2) How deep?
I haven’t bought the lumber to make the sides of my base yet. I can buy 2x8s or 2x10s giving me 6 or 8 inches under my bricks. I gather that 6″ would be deep enough for my perlite or perlite/bottle insulation to protect the wood floor underneath? Would I be better off with 8″?
2) Add thermal mass under floor?
I was thinking of going with the 8″ depth, but adding a extra inch or two of thermal mass under the bricks (I am, by the way going to build this one with red clay bricks (2.25″ thick) to see how they hold up, and because I haven’t been able to find any good firebricks locally). My biggest concern with adding the thermal mass under the floor is that I am afraid it will be too close to the front board of my wooden base. If I build a circular form 18″ in diameter, fill it with thermal cob then surround it with perlite, it will only be about 3″ from the front board. What do you think? Too close? Just stick with the thermal mass of the bricks?
Thanks again,
Ryan
1) Oven height
Q: For an 18″ oven, do you still recommend a height of 16″ or would something like 14 be better?
A: Either one is fine, but remember that lowering the dome height proportionally reduces the height of the door. If that’s not a problem, lower away! (Pure pizza ovens especially benefit from a lower dome.)
2) Peak of dome 2/3 back
Q: Earlier in this FAQ, you recommend building the sand form with the peak 2/3 of the way back, rather than in the middle. Why? Is it just for smaller ovens, or would a 30 incher also benefit?
A: This works for all ovens — the theory is that the high point in the dome draws hot gases in and up, and improves circulation — and perhaps combustion. The further back the high point, the further the flames and fire have to travel — mixing all the while.
1) Perlite v. Perlite & glass
Q: I read above that adding clay slip to perlite reduces its R-value; which would be better, straight perlite or perlite and mason jars or glass bottles?
A: Straight, uncemented perlite can compress and, with repeated heating and cooling, can also shift and cause the oven above to move and crack. The glass jars provide a stable structural connection between oven and base. I suspect (unconfirmed theory) that there would be negligible difference in practical insulative value between these options.
2) How deep?
Q: I haven’t bought the lumber to make the sides of my base yet. I can buy 2x8s or 2x10s giving me 6 or 8 inches under my bricks. I gather that 6″ would be deep enough for my perlite or perlite/bottle insulation to protect the wood floor underneath? Would I be better off with 8″?
More insulation is always better, but I NEVER use wood underneath the hearth floor. The effects of heat are cumulative, and you don’t want someone down the road to reap the (negative) consequences of that decision. Use metal, and stiffen it w/more metal ribs of some kind. Leave ventilation space below, so if it does get warm, the heat will be dispersed. VERY IMPORTANT — FIRE HAZARD!
Q: thickness of thermal mass under floor?
I was thinking of going with the 8″ depth, but adding a extra inch or two of thermal mass under the bricks (I am, by the way going to build this one with red clay bricks (2.25″ thick) to see how they hold up, and because I haven’t been able to find any good firebricks locally). My biggest concern with adding the thermal mass under the floor is that I am afraid it will be too close to the front board of my wooden base. If I build a circular form 18″ in diameter, fill it with thermal cob then surround it with perlite, it will only be about 3″ from the front board. What do you think? Too close? Just stick with the thermal mass of the bricks?
A: This depends on what you’re using the oven for, how much heat you want/need to store, and how well you can insulate. Red brick do tend to be less dense and hold less heat than typical firebrick. In general, the more mass, the more heat you’ll store (if you fire long enough to saturate the mass), and the more insulation you should have. While the book recommends that the mass under the hearth should be as thick as the dome, if you don’t fire the oven long enough, the extra mass under the hearth can cause the floor temps to drop faster than dome temps. If you plan on fully firing the oven for very long baking times, the extra mass will help. If you want quick fires for limited baking/pizza-making, a thinner floor may make you happier.
Fidinng this post solves a problem for me. Thanks!
Hi Kiko…..Namaste from INDIA…
I am planing to build a cob oven inside my farm kitchen with a interior dome of 30 inch. That means the sand dome height will have to 22.5 inch ( 75% of dome diameter) And the door height will have to be 63% of dome height….that is approx 14.2 inch…
My question is…
01. What should be the width of the door? Does it have any proportion?
02. Since its a interior oven, should I build a chimney?
Regards,
Amit Karia.
Mumbai-INDIA
Greetings, Amit! It sounds like you’re working from the old edition — which doesn’t go into much detail on dome height. In general, however, it’s best to keep the dome height low, so your heat source stays closer to the food, and so steam stays more concentrated. I would recommend a 16″ dome and a ten inch door. Door width? About 15″ should be good. And yes, a chimney will be essential indoors. I’d also recommend making the high point of the dome about 2/3 of the way back from the door opening.
Thanks Kiko for the quick response.
When you say 16″ dome….are you suggesting height or dia?
This will be my first oven so just being sure.
Thanks in advance.
Amit.
Sorry……I got it..
16″ dome dia, with 10″ door height and 15″ door width…..Right?
Thanks.
Sorry. 16″ is the dome HEIGHT — whatever the width (I’ve built to the same height even w/a 48″ diam. or larger dome — but you have to be more careful with the shape of the dome — NO flat spots!)
Thanks a lot Kiko…
Hi Kiko,
As you had said that its essential to have a chimney for a indoor oven, I am all set to make one. Can I use 3′ inch dia metal pipe, say 1 ft in length, which will be attached to the oven and then join a 3′ inch dia flexible aluminum flue pipe – 12 ft long, which will be directed outwards.
Amit.
Amit, a chimney for a 30″ diameter interior oven will need to be larger than 3″. I would experiment, if possible. You might be able to get away with a 4″ diameter (round) pipe, but if you can’t test it first, I’d go w/6″ diameter. (Square chimney equivalent would be 6×6.) Try to keep the edges of the oven door rounded or angled to minimize turbulence as the smoke flows up and out. I would strongly advise AGAINST any use of flexible aluminum, for two reasons. The aluminum will not hold up to high temps, and anything but straight up will reduce chimney draft and thus effectiveness. Please be very careful when running pipe through or near wood. The effect of heat is cumulative, and while it may seem to be fine at first (even for years), wood becomes more flammable w/regular exposure to heat. I US building code, hi-temp, insulated, stainless steel pipe requires 2″ of clearance from wood. US code prohibits running plain, single wall pipe through a wooden wall or roof structure. US code is, of course, written for an audience used to pushing buttons and ignoring what keeps them warm and fed, so I’m not saying it applies to you. Only suggesting that you take precautions and maintain an awareness of all risks. Of course, none of the advice you get on the web is a guarantee for what you decide to do! Best,
— Kiko
Hello Kiko,
I appreciate and understand your concern. What if I use a flexible steel pipe instead of aluminum? And also wrap it with a insulating material. Will definitely run the pipe without touching wood or any other flammable material and maintain gaps. Also the oven door is rounded/arched. The only reason to use a flexible pipe is because the distance between the oven and the outdoor opening for the chimney is around 12 feet with L shape and the iron pipe was becoming too heavy and would need lot of other fixtures to hold the pipe. As I mentioned earlier, I am planning to use a 1 feet long metal pipe so that it can take the heat/flame and then join the flexible pipe. I can increase the metal pipe to 2-2.5 feet, if you think the heat/temperature may subside a little before it comes in contact with the flexible pipe. Am I making any sense??
Regards,
Amit.
Good day Kiko,
I’ve read through multiple online blogs and decided on a plaster mix: 3 parts sand to 1 part 50:50 clay manure. A real pleasure to work with. I didnt have much of what i considered quality clay so the first layer worked into the clay slip hay insulate layer was a higher silt content sub soil with about 25% clay i suspect mixed 1:1 with sand.
I left it rough and applied the plaster mix about about a 1/4 inch thick the next day. I had a little mishap with high winds and rain pitching my tarp about but repairs were easy enough. I ended up plastering what i thought would be a finish layer about an 1/8 of an inch thick. Unfortunately, as it began to dry hairline cracks began to show. Do you think it the mix despite what i would think a low clay content? Or is the silty sub soil below drying causing the problem? Id like to think I can apply a final plaster coat to last without cracking at least for a season. Have you a recipe?
Also, I now realize using hay instead of straw may have been a mistake, At the joint of the chimney and the earthen dome an orange fuzzy mold began to spore; inside the oven, at the thermal layer butted up against the fire brick floor as well. The oven hadnt had a burn for over a couple weeks since applying the hay slip insulation.
So, my knee jerk reaction was to fire it for a few hours thinking I need to get that decomposing hay to dry out. Am i on the right path here?
Thanks in advance for your help.
Ken
I just heard baker Jeffrey Hamelman tell a story about an early trip to study w/a master baker in France. After fielding 1,000 questions from Jeffrey the baker finally shouted: “You must feel it w/your fingertips!”
Trouble-shooting at a distance means lobbing out general principles and hoping that the reader will make the appropriate connection between principle, his/her own practice, and his/her own materials. Since I can’t feel it w/my fingertips, I can only encourage you to feel it w/yours! So:
1. in general, it’s best to let previous layers dry completely before adding additional layers, since layers may dry at different rates.
2. in general, cracking is caused by high clay content, not low.
3. in general, an otherwise crack-free mix may show cracks when subjected to high wind, hot sun, or anything else that encourages super fast drying at the surface.
4. in general, drying will stop mold growth, but a moldy surface may need to be cleaned or re-plastered in order to remove or cover mold stains.
5. It all works (usually), especially if you’re not too worried about wrinkles and other unexpected (and often unavoidable) cosmetic phenomenae.
Hi,
Nice anecdote and and apt analogy. My finger tips tell me the final plaster was pretty low in clay content so I’m going with my gut feeling the cause for cracks was mostly due to the base plaster layer which was probably more silt and clay to sand than it should have been. I’m going to let the plaster dry thoroughly before filling cracks and applying another layer.
Thanks again for the quick reply.
Regards,
Ken
Hi,
I bought your book and took to building some months ago. The build is mostly to spec with a 3-4 inch thermal layer topped with a another 3-4 inch cob insulation layer using hay. I added a brick chimney and an arch.
After several small fires across a few days I fired it for ~ 2 hrs resulting in initial temps of north of 1000 F and made some great pizzas. At the time, the top of the dome and midway down the left side wall was hot to the touch but not so that you couldn’t keep your hand there. It was steaming as the cob had obviously not dried completely.
The outside dome temp had me concerned about energy loss. I added a second 3-4 inch insulation layer of hay which had been submerged in a clay slip of heavy cream-like consistency and wrung out like a wet t-shirt. The dome looks like an enormous upside down bird’s nest.
I would like to finish it off with plaster. I am thinking of using manure and or flour paste but I do not know when to apply it. Should I wait until the the hay clay slip is completely dry and wet it up a bit when ready to plaster?
I’ve also read about people applying plaster in 2 steps to straw bales to ensure proper adhesion of the plaster. Should I follow suit? If so should I wait for the 1st plaster layer to dry before applying the second?
Thanks in advance.
Cheers mate,
Ken
It all works; waiting will actually dry things faster, as the plaster slows down the drying of the inner layer(s). wetting first is good. the bale technique, as I understand it, is to put some clay on the straw to improve adhesion for the next layer. As you’ve already dipped your straw in slip, I don’t see it as particularly helpful — but I haven’t seen or felt your materials! Always take the theory w/a grain or three of salt, until the theorist shows up to get muddy…
Many thanks for the quick reply. As its all good, it looks like rain this coming Tuesday means applying plaster on Monday is the best option.
Cheers,
Ken
Hey there,
So i just finished up my foundation for my first earthen oven and am super pumped for this. I made my insulating bottom layer with glass bottles encased in straw and oven mix. I did not think it was supposed to rain last night so i did not cover it. Woke up in the morning (it poured for a couple hours) and its now soupy. starting to become solid with sun on it. Is this ok? i was going to let it sit for a week or so, with a little shelter. Do i need to pull it all out and make a new batch? Also i misread the book and put a bunch of the insulant in the bottles too. Is this ok? Thanks for any advice. Your book is awesome
sorry for slow reply. You’ve probably solved all the problems, but for the record, I might re-do the soupy stuff. Hard to tell w/out seeing and feeling the materials in question, but when in doubt, just do it. Insulant in the bottles shouldn’t hurt anything, but will be (very) slow drying — and of course, until it’s dry, it will mean longer heat up and less heat retention.
I want to start to build a cob oven in a back yard, I have read a lot information about the topic, however I have any doubts.
Is only necessary three layers of mud? What does must be right mix in each layer? Should I wait drying in each layer to follow ? Can I use sawdust instead of straw in the mix? or both things?
I think to build the foundation with a pile of rocks, Can I use clay to join the rocks or is preferable cement?
And finally,
Is essential put glass bottles insulation below fire bricks?
I hope your comments,
Thanks in advance.
these questions are quite thoroughly covered in the book, so I will refer you to that first. Mud and sand make very serviceable mortar for laying up rock. If they are reasonably protected from direct moisture (rain, etc.) they will do the job. Always preferable, I think, to use cheap local materials over expensive ones that must be manufactured (w/LOTS of oil/energy) and transported…
Hello,
I built my oven pretty much “by the book” with regards to size, layers, door height, etc. however I can’t seem to bring it to cooking temperature.
On our last attempt the fire went on for almost four hours (on and off as keeping it on is quite hard) the top of the oven was very hot to touch at the end but not the bottom fire bricks (I did put glass bottles insulation below), and even then, the heat wasn’t enough to bake bread. I have only fired the oven twice, but it has dried for about four weeks.
Am i supposed to keep the door closed whilst firing? at the moment is open until I attempt to cook after removing the coals. Not sure what to do.
Any tips highly welcome. We are feeling very upset and disappointed that our oven doesn’t seem to work well, after all the hard work.
Hey, Marcos, don’t be discouraged! It sounds to me like a case of dampness under your hearth bricks. I’m assuming your glass bottles below the hearth were encased in a sawdust and clay mix? If so, the stuff takes a L-O-N-G- time to dry — regular (daily) small fires will probably work better for drying than a few hi-temp bake days. It can take quite a while before the water finally works it’s way out — but when it has, you’ll know! You DO want to keep the door OPEN while firing. I soak my door in water while I fire the oven — that way it burns less, and adds steam to the baking environment. Keep us posted. And thanks for writing!
— Kiko
We’ve built a clay oven with Maine blue clay in our back yard. It came out beautifully. However, being a perfectionist I felt the minute cracks in the interior needed to be patched. I attempted to do so with a thin layer of clay mix over the cracks. Afterwards during the first very hot firing from what my husband who is a welder says moisture was probably trapped behind the patching and the extreme heat blew the patches off pulling along with them, pieces of the lovely oven wall up to a a sixteenth inch deep and deeper in some spots. some of these areas are 3 to 4 inches wide. the oven still seems to work extremely well even with these discrepancies. It fires up well over 900 degrees. It bakes pizzas in under 2 minutes. I bake bread etc. However, it bugs me to see our oven with blemishes. Is there some way to patch it? Or should I just leave well enough alone?
Patching the inside of an oven is tricky, especially when the damage is largely superficial. I have seen one oven completely coated on the interior w/a thin layer of clay slip, and it seemed to be holding. The trick, I would suspect, is to paint on the slip in thin layers, and to let it dry completely between coats. The other route would be to live with it long enough to temper you perfectionism with gratitude for the amazing properties of a mass oven, which (like a blemished/damaged but still living human body) still works despite it’s blemishes!
We are living in Santa Fe, NM. For the next 5 months. I have been looking at several ovens,much larger than I will build. Can’t wait to get back home and start on mine. Your book is awesome. P.S. My wife is taking a class on cooking using a Horno oven at a Pueblo nearby. Can’t wait to reap the rewards.
Hello Again,
The oven came out fine (for our first one) got it to about 600 degrees and cooked about 10 pizzas but the insulating layer cracked. It looks like some heat is escaping from the cracks otherwise I would leave it. The oven layer is like a rock, no issues there. Can I repair the cracks on the outer layer with more mud, or vermiculite and then cover with the lime/sand finish. Really a great project and the community had a wonderful time gathering materials and of course the final rewards…Let me know what you think
Joe
yes, you can fill the cracks w/more insulating material. It sounds like the cracks were shrinkage cracks rather than expansion cracks from hot material, but to be on the safe side, use a wet mix, and do the fix when the oven is hot (fully expanded).
Q? In regards to construction of clay oven.
We are using quality ex kiln clay and were in the process of building the first layer 75% sand 25% clay when violent weather stopped proceedings midway. Are there any tips bar wetting the rim of the incomplete layer to assist the stitching when we resume the build?
Regards
marty
to get the best bond, you’d want the previous layer to be saturated but not soaked — i.e., you want water all the way through the material, but not too much of it. I’d try laying a wet cloth on top, and keeping it wet (but not dripping) — probably for 1/2 day or more, until the material is close to what it was when you started building. Good luck!
I recently built a mud oven using free clay excavated from a building site (near Walnut Creek, CA). The clay was already 1/3 sand and very sticky so it took 2(+):1, sand:clay mixture very well and made a strong thermal layer. We’ve been using the oven often and having a lot of fun with extremely good heat retention (fully insulated with straw/slip). I’m trying to build another oven with Lincoln Fire Clay which is a pure dry powdered clay from the ceramic store. It doesn’t seem to want to take any more sand than a 2:1 sand:clay mixture. I’d like to get the mixture closer to 25%clay as I’ve been reading is ideal. Is there a way to make clay stickier. Or do you think a 2:1 mixture will perform well enough? I would really like to assemble the sand/clay mixture and bag it up ahead of time to bring to the site. The pure dry clay makes things a lot easier too in my opinion. Its fairly cheap, readily available, no sifting, and would make a replicable recipe. Thanks!
If you want a standardized oven, there are a million places you can get plans, standardized materials/recipes/instructions etc. If you want to learn about what’s under your own two feet, test things out and see how it works.
In any case, material issues are impossible to address w/out seeing and feeling the material. That said, things that can affect “stickiness” include the mix of particle sizes (the greater the range of sizes, the stickier it should be, assuming some variation in particle shape as well), whether or not the mix contains biological compounds (try adding a small amount of compost), as well as the generic ratios of generic sand to generic clay. Of course, there’s no such thing as generic, and I don’t recommend “standard” materials because I think people will learn more from figuring things out than from following standardized recipes. The principle here, is diversity. Life comes out of and generates diversity. Standardization inhibits diversity. CLoning might be efficient for a corporation seeking profit, but I think it’s a dead end if you’re interested in discovering and supporting the kinds of life that will flourish in your particular locale. Tip O’Neill said “all politics is local.” The same goes for materials.
Hello again,
we are looking at the insulating layer and several sites use straw/ hay cob for the outer layer. In your book you recommend using the sawdust… can you use a combination of sawdust/straw mixed with clay and sand for both strength and insulation. Also, your thoughts on a lime plaster finish…
Insulation should just insulate. It does not need to add strength. Sand does NOT insulate, so adding it to the insulation layer is counter-productive. Indeed, you want just enough clay in the insulation mix so that when the sawdust/straw burns out, you’re left with a matrix of closed cells that are just strong enough to support themselves and maintain the integrity of the “foam.” Typically, only about an inch or two of the sawdust/straw will completely burn out. Remaining woody/fibrous material will provide plenty of strength. I prefer the sawdust because its easier to make a homogenous mix. Straw often leaves very large voids which makes weak spots that can cause failures (combustion of un-coated straw, holes (sometimes scary smoke) and, finally, loss of insulation value).
RE: My “thoughts on lime plaster finish” — hmmm. That’s what I’d (politely) call an “open-ended question.” Rather than spend a lot of time trying to guess what you’re thinking/asking, I’ll suggest you re-read the lime-plaster pages in the book, and/or re-frame the question…
Just got your book. Is cement a good conductor of heat? Or would it be better to build base out of cement, then use firebricks for ovens cooking surface. Does cement put off chemicals when heated? Thanks Chris
Portland Cement. Amazing stuff (liquid stone!) But there’s lots bad about it. Here’s a few:
1. environmental cost: huge amounts of energy required for production, which spews greenhouse gases into the environment
2. cash cost: you want to spend money on bagged cement when you can get native clay from underfoot for free?
3. trickier/nastier to work with: once it sets, you’re done. If you got it wrong, you’re still done. And if you work bare-handed or bare-footed, your skin will hurt (crack, blister, redden, itch, burn, bleed…)
4. other health issues: cements sometimes includes toxics from dirty fuels used in production.
5. doesn’t hold up to heat (cement starts to lose strength at 700 F., and will not hold up to typical wood-fired oven temps — in the long run, it will break down and fail). This (and no. 4) make it a very bad choice for a hearth floor. There are specially formulated “refractory cements” that are used for hi-temp applications, but the other drawbacks still apply.
6. indirect effects on human brain: to “cement” something means to make it stiff and immovable. Industrial thinking typically assumes that the stiffer and more rigid something is, the stronger and better it is. This assumption only works in some situations (and, as in procreation, it typically requires an equal portion of an opposite characteristic in order to achieve the desired result). In other situations, it may not work at all. “Rigid thinking,” for example tends to reduce mental function, and can even shorten life.
So. If the question is: in a recipe that calls for clay, should you replace it w/Portland Cement? my short answer is: no.
Our community in Taipei Taiwan has begun to build a “semi” earth oven. I have purchased some refractory clay (castable) and would like to use it in my first layer over the sand dome (oven layer) I want to use a mix of: 1 cement/ 9 sand/ 2 lime/ 2 refractory clay. What are your thoughts on this method? Will it maintain its heat integrity? I made a test brick but have not fired it yet. It appears a bit sandy.
Thanks,
Joe
Joe, the cement is completely unnecessary. Also not beneficial to the mix, as anything over 700 degrees makes it lose strength and sticking power. It (and the lime) will also require you to use gloves or tools for working the mix. Clay alone increases in strength with heat. For general info about “improving” your mix (especially if it seems “sandy”), please read the posts on the rocket-powered kiln and the DIY castable (a/o 2/2014, they are 2d and 3d on the homepage). Otherwise, 9 parts sand with 4 parts of sticky stuff is nearly a 50:50 mix, which under most circumstances would probably shrink and crack a lot. More typical is 3 parts sand to one part sticky stuff. test it out! Make a mini-oven.
Hi Kiko,
This summer with the help of my family, I completed our earth oven. It was a great project, something that really brought us all together. We’ve used it a handful of times, but the weather in upstate New York this year has been pretty wet, most of the time.
I want to continue to use the oven into the fall. I do not have a permanent shelter over the oven. I don’t have a lime plaster either. I have a pop-up shelter up, but I can’t keep it there indefinitely. When I was building the oven, I tarped it between layers. When I took off the tarp there was a lot of condensation, some mold, but I dealt with that and moved on. Now I’m considering tarping it for the winter with a vinyl tarp, but I’m wondering if there will again be condensation? If the oven gets wet from condensation and freezes and thaws, I’m concerned I’ll lose the oven by spring. I thought about building a simple stick frame tight to the foundation with a plywood top and tarp over that whole structure to keep condensation off the dome, but not really sure if that will survive a winter. Any thoughts? Where I built the oven isn’t conducive to having a roof structure over the top, so for next year I’ll have to try some kind of weatherproof coating, but have never worked with lime before.
Thanks!
Brian
Hey, Brian,
You’re right to be worried about condensation — the easiest, quickest solution is just to insure that you arrange your roof/tarp material and leave a gap between it and the oven itself (easy to just lay boards on the oven, vertically, and then lay the tarp over the boards). Properly done, the oven material itself will have room to breath and any condensate will run down the inside surface and drip away from the oven. (This goes for any uninsulated metal roof, as well as outdoor tarping arrangements.)
My favorite simple solution — a very nice, easy metal roof requiring no real wooden structure — is illustrated in the book at the end of the color section, opposite page 37, and captioned “the english muv’s oven” — all it requires is a couple sheets of wide roofing tin, sandwiched together, and screwed along the top seam; then you spread the bottom edges to fit over the oven. Attach the metal along the bottom edges to a long piece of wood or stakes in the ground. There’s a similarly simple hinged roof design on page 30 that a reader sent in. LIME PLASTER ALONE WON’T SOLVE THE ROOFING PROBLEM! You still need to keep the rain and snow off so moisture can’t sink in. Good luck, and let us know what you come up with!
We just finished building the thermal layer of our earth oven here in rural Western Washington yesterday. It was hard work, but a fun family event. I jokingly commented that while most families watch television together, we were having more fun building together. This week we have plans to shear the sheep, which brings me to my question. Can the sheep wool be used as part of our mixture for the insulation layer or even the finishing layer? Also what is your take on using linseed oil as added protection along with a roof? Would the linseed oil make it difficult for the oven to breathe and cause deterioration over time?
Thank you, it because we went to your exhibit at the Mother Earth Fairs three years running, that we finally have begun making an earth oven.
Hey, Kim, good to hear from you. I have not yet tried working w/sheep’s wool. Good Shepherd wool (.com) has info on something called a WEKA panel that combines wool with lime — apparently developed to insulate the interiors of stone buildings in europe. All kinds of interesting properties, possibilities and ideas here, but as I said, I haven’t tried ‘ it. If you do, let us know! I would just be careful about putting the wool too close to the heat source, as it could get stinky if it gets hot enough to burn (tho I haven’t smelled burning wood from the sawdust clay, so maybe I’m wrong). As for linseed oil, I don’t think it would seriously compromise the breathability of the oven surface, unless you were to burnish the clay and really put a gloss finish on it. Good luck!
Our earth oven does not hold heat sufficiently. We’ve burned the wood down to the coals which takes about 3-4 hours. Pulled out the coals and soaked the floor. When we inserted the bread it just isn’t hot enough to bake it in a reasonable amount of time. Do we need to put another layer of insulation on the outside? Why aren’t the bricks holding the heat? Any advice is welcome!
Thank you in advance!
-Tina
How many times have you fired your oven? Unless you’re unusually patient and have allowed your oven to dry thoroughly, the first few firings of a new oven typically cool off quickly due to retained moisture in the deeper layers of the oven (especially under the hearth, and even more especially if you used sawdust-clay under the hearth and didn’t let the layer dry completely before continuing). Otherwise, the only reason a dry oven would lose heat quickly would be poor firing technique (here’s some video tips) and/or an un- or under-insulated floor. Solid mass under the hearth w/no insulation will suck heat out of the hearth floor, so after cleaning the oven, load your loaves quick!
Okay thanks for the tips. We’ll keep firing it up and see how it goes. We did use sawdust clay and because of limited time had to put the insulation layer on two days later. Thanks again!
On 5/18/12 5:49 PM, Greg Varela wrote:
> From: Greg Varela
> Subject: Algae in clay?
>
> Message Body:
> Hi Kiko, I was going to include this in the FAQ, but thought it’s probably not a frequent problem, so I thought I’d direct it to you.
> I pulled about 1,000 lbs of clay rich soil and when I got home, discovered a massive ant colony, billions. I place the soil in drums, blended it all and let it sit for several weeks..perhaps too long? The water has risen and it is green and stinky, as is the clay.
> I’m planning on using it for my oven and to make adobes for a foundation and wondering if it’s an issue. The clay is green/gray in color with no material in it and it bonds as it should.
>
> I’ve shoveled it out and drying it in the sun hoping that when I break it into dust and hydrate it again, the algae will rinse out.
> Over-thinking?
>
> Thanks for your comments,
>
> Greg
oh, this is a yummy one; I’ll have to put it in the FAQ section…
all that living stuff adds life to the clay, makes it stick better, makes it mix and slide easier, makes it feel nicer, etc. etc. That’s why the old Japanese master potters would dig and age clay for their sons/grandsons; what they used themselves had been dug and “aged” (really revived, after long geologic sleep) for them by previous generations. Think fermentation and bread dough (as the label on the hand-made Haitian chocolate that friends just brought back for us said, “la fermentation change tout!” (“fermentation changes everything”).
But if you dry and rehydrate, that will work too… BTW, you don’t need to make it into dust to re-hydrate, just make sure the clumps (related to “clay,” “clabber,” “cleave”) are smaller than a fist and dry dry dry. Then dump them into buckets and let sit for a day or two (no mixing!) If they were really and truly, totally dry, they will slough right down into slip.
— Kiko
Very helpful Q&A.
I’m building my 2nd Pompeii pizza oven on a circular stone/mud foundation. I want the inside to be open for firewood and curious to know the preferred way to uphold the weight of the oven [floor] with minimal or no use of concrete.
My thoughts are either a re-barred slab tied into the inner wall(s) or an iron sub-floor basin, “lipped” into place. Either one seems only slightly complex without having a wood frame in place.
Also, in your book you commented on using perlite/slip for insulation. Any preference between powder or granules? I used powder in my last oven but I’m unsure what the ratio should be to slip. Suggestions welcome. Thank you!
The dome shape concentrates the weight to the sides, so I’ve supported the center/floor weight quite well on scrap/scrounged stuff like old metal fence posts (t-posts or round tubes like those used for chain-link). I use old metal roofing to make a fireproof “floor”, but it needs to be fixed/screwed down securely otherwise it flexes too much. The materials will tell you what they need if you pay attention. I think slabs are generally overkill, myself. Perlite is really, really, REALLY nasty to breathe. Silicosis city. If I have to use it, I get the biggest stuff available, wear a respirator, and try to do it when the air is absolutely still. For subfloor insulation around bottles, I use no clay at all (which would reduce insulation value), but do press it into place around the bottles (avoid over-compaction as that also reduces insulation). For insulating a void over a dome, I pour it in loose, and use a long stick to settle it in place.
I have recently read Rocket Mass Heaters, and wonder if anyone has tried combining aRMH with cob oven? It seems like a RMH burns hotter/more efficiently using less fuel. If this has been tried, any suggestions, hints ideas to be aware of? I am thinking building a second cob oven and wonder if this is an idea worth exploring.
Thanks
first, right here on this site, take a look at Flip and John’s rocket oven, here:
http://www.handprintpress.com/authors/kiko/new-rocket-oven-design-by-flip-and-john/
and Joe Kennedy’s design at the end of his oven story: http://www.handprintpress.com/ovens/an-earthen-oven-odyssey-by-joe-kennedy/
here, too, are a couple of interesting designs/rocket-oven experiments, including Ray Cirino’s design, which Joe mentions:
https://picasaweb.google.com/Jonnygms/RocketOven#slideshow/5598633162314034594 (these guys have a lovely site about their work in Haiti, including a rocket kitchen w/rocket oven! here: http://rechoroket.com/Home.html
http://people.tribe.net/raycirino/photos/ad44c02f-0ce3-41d5-bd43-f9549e6afbc6
Ray Cirino explains some of his design starting at about 9 minutes on this video:
http://www.youtube.com/watch?v=O0GVHuPWp7Y
there is a yahoo group named rocket-stove-pizza for folks with this interest.
here’s a website from a guy working out a design: http://www.woodfiredpizza.org/Rocket
keep us posted on what you find/do yourself!
— Kiko