Frequently Asked Oven Questions

A student at Willard Middle School (Berkeley, CA), with a marshmallow toasted in the oven they built for their school garden. (oven and photo by Bernhard Masterson, http://www.bernhardmasterson.com/)

If you can’t find an answer to your question here, in the book, or elsewhere on the site, please feel free to leave a question in the comment section and we’ll get back to you asap (please do scan the comments first, as you may well find your answer there).

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Jump to: Efficiency, design, etc. | Using the oven | Materials & Construction | Foundations | Mud vs. Brick Efficiency, design, etc.

Q: How efficient are earthen 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: There are two sources I know of for info on earthen, masonry-style heaters/cookers: Cob Cottage Company’s Ianto Evans has recently written a book with Leslie Jackson on how to build a Rocket Stove for indoor heating and heat storage called Rocket Mass Heaters. They also teach workshops (www.cobcottage.com). Aprovecho Research Center publishes “Capturing Heat,” volumes1 & 2, that include designs both for efficient cookstoves and ovens. Max Edleson of Firespeaking is developing a set of plans and instructions for a simple masonry heater that you can build from your own mud brick — we hope to have it available for download early in 2011.

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 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 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. An oven without a chimney burns more slowly, giving the oven mass more time to absorb the heat of the fire.

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Using the oven

Q: I built a small oven that burns OK to start with, but no matter how I play with the fuel, it gets very smokey and goes out. The door is 63% of the interior height of the oven, so what am I doing wrong? A: Since the cut of the doorway will tend to angle down, the inside edge of your door may be lower than the outside edge. So be sure to measure door height at the edge furthest inside the oven. 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 oven, the lower the dome, and the less room for   the fire to burn. A small 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 ovens with a metal door support, or a pre-fabricated door and frame, tend to crumble where mud meets metal. I assume this is because hot metal expands more than hot clay. However, it doesn’t seem to impair overall oven effectiveness. Perhaps the greater risk is thermal shock, which can crack floor bricks and generally weaken oven materials. It comes of using too much water, either for steaming bread or for cleaning (or cooling) a hot oven floor. If you’re being careful with your tools and still having problems with interior oven damage, it may be that your original mix is weak_perhaps too silty, or too sandy. 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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Materials & Construction

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 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?   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_in my experience, cracking is a common phenomenon, but generally not a problem. I don’t claim to be an expert on high-temperature masonry, but I’ll pass on a few things I’ve gleaned: In general, any material that goes from 50-700°F and up in the course of a few hours is going to relieve the stress of thermal shock by cracking — more or less. The thicker the mass, the more likely the oven can absorb the stress without cracking. It may be that thicker material (and masonry of fired brick?) survives the stress of thermal shock by sustaining many micro-cracks, while thinner material sustains fewer, larger cracks. (See David Lyle’s Book of Masonry Stoves). 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 the oven floor. I got vermiculite instead, but it seems too soft and squishy to support much weight_any suggestions? A: Compared to perlite and pumice, vermiculite is soft. If you use it under the floor of your oven, 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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Foundations

Q: What’s the ideal height for a foundation? A: Well, there are several ways to answer that one. For someone who uses the oven a lot (at least once a week, every week — not just summer holidays), I’d say ideal height is to have the floor of the oven at the height of your waist. For someone who uses the oven less frequently, I’d say the ideal height is whatever is easiest to build. If that means you just want to plonk a ring of rocks on the ground, fill them with gravel, and go from there, that’s ideal. Remember, the oven itself is simple. The higher you go with the foundation, the more complicated it gets. Unless you’re making a temporary oven, in which case a temporary foundation can be almost anything that will support the weight, and that can be protected from the heat of the oven fire.

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Mud vs. brick: Nine Arguments for Mud

  1. 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).
  2. Mud is brick. Fire turns the inside of a “mud” oven to brick (quality varies w/soil, clay content, your mix, etc.).
  3. Speed: I’ve built 12-inch diameter demo ovens in 15 minutes, and full-size ovens in half a day or less.
  4. 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.
  5. Sculptural freedom: you’re not limited to rigid, rectilinear brick forms (not that I dislike rectangles).
  6. Temporal freedom: built right on the ground, or on planks and sawhorses, a quick oven can serve for a day or a year.
  7. Carpenterial freedom: roofing not required: use a tarp (when the oven has cooled) or a piece of tin.
  8. 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)!?
  9. Community: Just as the kitchen, or hearth, is the heart of a home, large communal ovens have long been the heart of communities. In the eighteenth century, after the Revolution that brought democracy to modern France, one of the first things the people did was to (re?) assert community control over the big ovens that had been under the regulation of feudal lords. According to Jerome Assire, in his Book of Bread,

In rural Europe, the various privileges [of the feudal system] included rights of banality over both mill and bakehouse, which were both the property of the suzerain. To bake their bread, the peasants were obliged to use these and these alone, and had to pay a duty to do so. People wanted to reclaim what was naturally theirs: the right to make their own food, and the right to join hands so that heavy tasks could be made lighter for all.

From what I’ve seen of ovens built by folks with whom I’ve been in touch since writing this book, ovens and community still go together — whether community is the family, the neighborhood, a co-housing group, or a town with a new restaurant. And it isn’t just the eating that joins us — it’s the joy of shared labor, which can be a novel and pleasant surprise in this industrial culture where people often work alone in offices and facilities far removed from sun, soil, and neighbors (as evidenced on the very last pages).

About Kiko Denzer

I live in western Oregon with my family and run Hand Print Press with help from friends Max and Eva. We are interested in restoring the arts of living to their rightful, traditional, public role, as cultural medium – and think the web is a poor substitute, but until we can fashion something better, we try to make the most of it.

89 Responses to Frequently Asked Oven Questions

  1. Mike says:

    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!

    • Kiko Denzer says:

      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.

  2. Peter says:

    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

    • Kiko Denzer says:

      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

  3. Ankit says:

    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

    • Kiko Denzer says:

      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.

  4. Jeff says:

    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?

    • Kiko Denzer says:

      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).

      • Jeff says:

        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

        • Kiko Denzer says:

          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.

          • Jeff Lee says:

            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!

  5. Erin says:

    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.

    • Kiko Denzer says:

      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

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  7. Bob Wood says:

    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!

  8. Stan Yanke says:

    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

    • Kiko Denzer says:

      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

  9. FirstMUD says:

    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!!!!

    • Kiko Denzer says:

      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!

  10. Jeff says:

    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?

    • Kiko Denzer says:

      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…

  11. Tanya says:

    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?

    • Kiko Denzer says:

      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!

  12. Arnie Sabatelli says:

    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,

    • Kiko Denzer says:

      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.

  13. Rob Duarte says:

    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!

    • Rob Duarte says:

      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.

      • Kiko Denzer says:

        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

  14. Amy M says:

    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!

    • Kiko Denzer says:

      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

  15. CARL LEE says:

    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

    • Kiko Denzer says:

      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

  16. Ember says:

    Will baking sand in an oven cause any chemical reaction in the oven? Thanks!

  17. frank says:

    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.

    • Kiko Denzer says:

      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

  18. jason gellerstedt says:

    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?

    • Max Edleson says:

      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.

  19. Ryan Wood says:

    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

    • Kiko Denzer says:

      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.

    • Kalyn says:

      Fidinng this post solves a problem for me. Thanks!

  20. Amit Karia says:

    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

    • Kiko Denzer says:

      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.

      • Amit Karia says:

        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.

        • Amit Karia says:

          Sorry……I got it..
          16″ dome dia, with 10″ door height and 15″ door width…..Right?

          Thanks.

          • kiko says:

            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!)

          • Amit Karia says:

            Thanks a lot Kiko…

          • Amit Karia says:

            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.

          • kiko says:

            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

          • Amit Karia says:

            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.

  21. Ken in New Zealand says:

    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

    • kiko says:

      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.

      • Ken in New Zealand says:

        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

  22. Ken in New Zealand says:

    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

    • Kiko Denzer says:

      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…

      • Ken in New Zealand says:

        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

  23. Justin says:

    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

    • Kiko Denzer says:

      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.

  24. Baltazar Sequeira says:

    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.

    • Kiko Denzer says:

      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…

  25. Marcos Otero says:

    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.

    • Kiko Denzer says:

      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

  26. Suzette Labbe says:

    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?

    • Kiko Denzer says:

      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!

      • chris joyce says:

        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.

  27. Joe Raymond says:

    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

    • Kiko Denzer says:

      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).

  28. marty says:

    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

    • Kiko Denzer says:

      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!

  29. Roger says:

    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!

    • Kiko Denzer says:

      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.

  30. Joe Raymond says:

    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…

    • kiko denzer says:

      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…

  31. chris joyce says:

    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

    • Kiko Denzer says:

      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.

  32. Joe Raymond says:

    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

    • kiko says:

      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.

  33. Brian says:

    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

    • Kiko Denzer says:

      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!

  34. Kim Conner says:

    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.

    • kiko says:

      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!

  35. Tina Delany says:

    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

    • Kiko Denzer says:

      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!

      • Tina Delany says:

        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!

  36. Kiko Denzer says:

    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

  37. Greg Varela says:

    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!

    • kiko says:

      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.

  38. Dave Smith says:

    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

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