Question 1

I’m looking to build indoor masonry heater type units out of earth for heating and cooking. Any ideas?

Accepted Answer

A: Ianto Evans and Leslie Jackson’s  Rocket Mass Heaters covers rocket stove design for indoor heating and heat storage; Ernie and Erica Wisner’s website offers courses or you can buy their Rocket Mass Heater Guide book. Aprovecho Research Center publishes “Capturing Heat,” volumes1 & 2, that include designs both for efficient cookstoves and ovens. The Masonry Heater Association website has a wealth of resources. The Rocket Stove Experimenter’s corner is a community of tinkerers and rocket scientists. “Answers Questioned.”

Question 2

Is there any effective difference between the Quebec shape and the beehive shape?

Accepted Answer

A: The major feature of the Quebec oven is a longer, egg-like shape. It’s useful if you need a wide door (for pizzas or cookie trays) but don’t want to hugely increase the diameter of your oven, or if you bake in square pans and want a longer shape with corners. The long oven can accommodate a wider door without giving up too much thermal mass. Some say it’s more efficient, but I haven’t seen any experiments that have proved it either way. That said, it’s good to keep the dome 16 inches or less; more volume reduces the concentration of steam which also inhibits crust development — in addition, increasing the distance between loaves and hot mass reduces the intensity of radiant heat.

Question 3

Have you experimented with electric heating elements?

Accepted Answer

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.

Question 4

Is it better to build my oven with or without a chimney?

Accepted Answer

A: When I first wrote the book I thought chimneys made better ovens because they made a faster, hotter fire. I even built a cross-draft oven with a chimney in back which burned great, but used LOTS more wood. It also illustrated a basic fact that I understood, but didn’t quite accept; that is, no matter where you put it, a chimney sucks heat out of the fire. After years of observing slow fires licking the walls of the oven, I now think the traditional oven with no chimney is probably best because heat transfer is a function of “the 3 Ts”: time, temperature, and turbulence. A slower burn gives the oven mass more time to absorb the heat of the fire. However, if smoke is going to cause problems for you, or if you just need to control it, then yes, build a chimney by all means.

Question 5

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?"

Accepted Answer

A: Since the cut of the doorway will tend to angle down, the inside edge of your door may be lower than the outside edge. So be sure to measure door height at the edge furthest inside the oven. Be careful not to add too much fuel at once, as this reduces the space for oxygen to mix and move in the oven, which will makes for a smokey fire. (See p. 100) If your door is the right height, you’ve played with the fire, and the oven still won’t burn, try cutting the door a bit higher, or making a hole through the back of the oven (opposite the door, just above floor level, and at least as big as a quarter. Plug it up with mud or a stick when you bake.) Both provide more oxygen for combustion. And finally, the biggest combustion problems tend to be in the smallest ovens (again, the smaller your oven, the less room for fire. Try using smaller wood, a little at a time, tend it closely, and and let it burn longer.

Question 6

Which is better, the Quebec shape or the beehive shape?"

Accepted Answer

The major feature of the Quebec oven is a longer, egg-like shape. Because it puts extra height towards the rear, it creates a bit of draft which helps draw oxygen to the back of the fire and circulates the heat throughout. It’s also useful if you need a wide door (for pizzas or cookie trays), or if you bake in square pans and want a longer shape with corners. 
 
A longer, more oval or rectangular shape also gives up less thermal mass to the door opening. (For example, a 4’ diameter round oven has about 12 square feet of hearth space, but a 2’ door opening would take out 1/6th of the wall length. A rectangular or oval oven, 6’ long and 2’ wide, has the same hearth area, but the 2’ wide door takes out only about 1/10th of the wall length.)

Question 7

How important is the height of the dome? Can I build an 18” dome? Or a 12” dome?

Accepted Answer

You can, of course, build a taller or shorter dome; just remember that changing the dome height changes the height of the door. Also, in general, the greater the height, the greater the volume, and the lower the concentration of steam, which is what gives you good crust development. In addition, the greater the distance between loaves and hot mass, the lower the intensity of radiant heat and the slower the bake.
 
That said, you don’t want to go too low because you need volume for good combustion, and you don’t want the door:dome height ratio to make your doorway too low (dedicated pizza ovens may benefit from a lower dome). I recommend keeping the dome no higher than 16 inches, as that gives you a 10” high door, which fits most of our pots and big roasts. I’ve built ovens that high as large as 4’ wide, but you have to get the curvature of the dome right to avoid collapse. Big commercial brick ovens often use steel framing to contain a long, low arched roof.

Question 8

One of the blog posts shows a very tall traditional Indian oven? Do you know what the reason is for the different, tall shape?"

Accepted Answer

Good question; I don’t have a definitive answer. As far as I can tell, traditional shapes vary for idiosyncratic reasons. @athenasteen, who has ties to pueblo culture in the SW, says different pueblos named (and teased?) each other according to the shape of their (squat, tall etc.) ovens (see her wonderful video, "Rooted in Clay,"). That said, there are good arguments for keeping domes lower than higher, and for making the door end lower than the rear. See the Q&A above about dome height.

Question 9

Why do you recommend building the sand form with the peak 2/3 of the way back, rather than in the middle. Is it just for smaller ovens, or would a 30 incher also benefit?"

Accepted Answer

This works for all ovens — the theory is that the high point in the dome acts as a chimney, to draws hot gases in and up, thus also improving circulation — and perhaps combustion. The further back the high point, the further the flames and fire have to travel — mixing all the while.

Question 10

Is it better to build my oven with or without a chimney? What are the different characteristics and performances of ovens with and without a vent/flue?

Accepted Answer

This is covered in Build Your Own Earth Oven, but briefly, the most important difference is smoke management — a flue allows you to get rid of it. The increased draft does allow you to play w/limiting the amount of primary air, which can improve the quality of the burn and (possibly — I haven’t measured) increase efficiency. On the other hand, you can also use a firing door to play with w/limiting primary air on a flue-less oven to achieve similar results. I prefer the simplicity of the flue-less design, and also find that it’s possible to get a pretty clean-burning “slow” fire, which seems to me to maximize the amount of time that flame is in contact w/the oven walls — which is a good thing. 
Early on, I thought chimneys were better because they made a faster, hotter fire. I even built a cross-draft oven with a chimney in back which burned great, but used LOTS more wood. It also illustrated a basic fact that I understood, but didn’t quite accept; that is, no matter where you put it, a chimney sucks heat out of the fire. After years of observing slow fires licking the walls of the oven, I now think the traditional oven with no chimney is probably best because heat transfer is a function of “the 3 Ts”: time, temperature, and turbulence. A slower burn gives the oven mass more time to absorb the heat of the fire. However, if smoke is going to cause problems for you, or if you just need to control it, then yes, build a chimney by all means.

Question 11

I’m looking at using flexible pipe for a chimney. Is steel better than aluminum? Should I wrap it with an insulating material?

Accepted Answer

Most aluminum pipe is not made to withstand high temps, so I would strongly advise AGAINST any use of flexible aluminum. In addition, anything but straight up will reduce chimney draft and thus effectiveness.

Question 12

How do I get my chimney thru the roof of my shelter?

Accepted Answer

US building code specifies that wood-fired appliances be vented through a building roof by triple wall insulated pipe that must have a 2” air gap from any combustibles. It prohibits running plain, single wall pipe through a wall or roof structure made of combustible material. That said, you can buy a canvas tent with a metal thimble that allows you to have a camp stove in your tent. So think for yourself and pay attention (see above). Just because you follow what seems like good advice, won’t make your fire any less dangerous if it gets out of hand or you get into a freaky situation.

Question 13

Why not just build a brick oven?

Accepted Answer

First, dirt is  quick — and cheap (brick ovens can easily cost many thousands, especially if you have to hire a mason). I’ve built 12-inch diameter demo ovens in 15 minutes, and full-size ovens in half a day or less; built right on the ground, or on planks and sawhorses, a quick oven can serve for a day or a year. And you don’t need to build a roof: just cover it with a tarp (when the oven has cooled!) or a piece of tin.
 
If you really want a brick oven, mud makes a cheap, quick practice oven that will allow you to make mistakes and learn before you commit a lotta dollars. 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. I’ve built 4-5 foot ovens for restaurants (9 foot on the outside!).
 
Second, Mud is brick. Fire turns the inside of a “mud” oven to brick (quality varies w/soil, clay content, your mix, etc.).
Third, mud grants a lot more creativity than rigid, rectilinear bricks — not that I dislike rectangles, but mud can make a fast-firing, thin-walled pizza oven, a big oven for a home business, a sculpted outdoor kitchen with seating, and more. 
Fourth, and perhaps most important, your kids (and friends!) can do it with you: so many young people know more about computers than they do about the earth that gives them their food. Building an oven might help them understand how it feels to be a plant, with roots in the soil, and leaves that eat sunlight. Perhaps that’s why mud feels so good between the toes (and dough between the fingers)!? And just as the kitchen, or hearth, is the heart of a home, 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.
As you can see in the book and all over the web, 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).

Question 14

Could you please tell me how to make a small wood-burning kiln?

Accepted Answer

Here’s a wonderful example of a low-tech kiln made of mud: If you want to get more involved, there is a lot of other information out there, and many stories on the web. For inspiration that goes way back, try looking up “anagama hill-climbing kilns.” It is possible to fire clay in an oven, a campfire, or a woodstove, but it will only get as hard as a soft brick. If you want to turn clay into glass-hard stoneware or porcelain, however, you have to enclose the fire in a box of some kind, with a big chimney and a lot of draft, in order to achieve high enough heat.

Question 15

In one of the blog posts, you show a brick doorway with door bricks cut at an angle to ease the smoke into the chimney better — but how do you close that door when you want to bake?"

Accepted Answer

The door has a flange that extends to a flat surface beyond the angle-cut brick.

Question 16

How thick should I make the thermal mass under the hearth bricks? I was thinking of adding an 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)."

Accepted Answer

The rule of thumb here is to put the same thickness under the floor bricks as you have in the dome. So if your dome is 3” thick, put 3” of mass under the hearth bricks. As with any rule of thumb, however, there are variables. Redbrick, for example, tends to be less dense and hold less heat than typical firebrick, so a bit more mass might be good. That said, if you don’t fire the oven long enough to saturate the mass with heat, a cooler subfloor can suck heat away, causing floor temps to drop faster than dome temps. So extra mass only helps if you fully fire the oven. If you want quick fires for limited baking/pizza-making, you may be happier with a thinner sub-floor. Insulation is another important factor, because it prevents heat loss from your subfloor. Insulate, insulate, insulate!

Question 17

If I’m building my oven on a wooden base filled with thermal cob surround by perlite, how much insulation do I need between the thermal mass of the floor and the wooden walls of the base?"

Accepted Answer

Fire can easily get out of hand and burn down buildings, houses, and forests. So you have to be careful, and you have to pay attention. I’ve seen freak fires under an oven that could have burnt up both oven shelter and house. Fortunately, the operator was paying attention and caught it in time. So it’s really, really, REALLY important to pay attention. OK. Are you a bit scared? Good. Here’s my answer: You need enough insulation. How much is that? It depends: how long do you fire the oven? How often do you fire it? What kind of wood did you use? What kind of bricks? etc. I’ve built many ovens in wooden boxes, and traditional Canadian ovens were built on wooden platforms. As far as I know, there’s no building code for ovens like these, but in general, you want plenty of distance between the high heat areas and any combustibles. I generally design my ovens so there’s 6-9” of insulation between the front edge of the cooking surface and the piece of wood that makes the front of the box — that’s enough to support the tongue bricks, which often sit on top of the wooden frame. You don’t want horizontal wooden surfaces near the doorway where an errant coal might decide to roll out of the oven and start a fire where you don’t want it.

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