From Kiko Denzer:
Just a few quick thoughts in response to some of what’s already been posted, and a word or three about insulation, mass, and related principles:Mortar: I dislike spending money on commercially available “refractory mortars,” for a variety of reasons. There are two types, and both require significant industrial and energy inputs for their manufacture, not to mention transport, and neither one can, by itself, prevent failures caused by operator or design errors. I’ve had great success using the old fashioned mix of sand and clay (either storebought fireclay, or local clay-soil from under foot). A ratio of 3 parts sand to 1 part clay is usually good. Since heat makes things move no matter what the mortar, it seems better to me to design for movement, rather than trying to contain the uncontainable (hot molecules will expand).“Standard (red) bricks”: In my experience, most building suppliers stock very dense, brittle, and thermally unstable “facing brick” (some of them are colors other than red). If I’m going to try to make use of old brick, I look out for ones that were used in actual working chimneys (as opposed to modern, lined chimneys) or fireplaces. Generally, such bricks are a bit lighter and softer, and thus more resistant to thermal shock — and fine for ovens and cooking.
In all things relating to fire, insulation improves performance — dramatically. Even though I’ve been doing this for years, I’m always amazed at how much of a difference a layer of insulation can make (even an inch or two will help, but more is better, up to a foot or more!). In an oven, it can mean the difference between having hours or days of useable heat. It also dramatically decreases the time and fuel required for getting a fire up to temperature (and/or surrounding thermal mass). It's also important, in any firebox, to be able to heat up the walls of the firebox as quickly as possible. So the less mass (masonry or earth) there is to suck heat out of the fire, the faster it gets hot. And the hotter the firebox, the better the combustion, the cleaner the fire, the less smoke, etc.
By the by, this is a good place to remind folks that any masonry cooking appliance should be at least as dry as your firewood (or drier, if you live in the Pacific northwest). Wet bricks or masonry of any kind will dramatically slow down and cool off your fire. So I would advise anyone making an outdoor cooking appliance to make sure it’s covered. Even if you only use it in fair weather, if your bricks have been soaked by a rainstorm the week before, they’ll be full of water, which will make your fire cool and lazy.
I’ve had great success making high temperature insulation by using a mix of sawdust and clay-slip. I make the slip by mixing clay-ey soil and water until it’s about the consistency of heavy cream. Then I make a dough of slip and dry sawdust. It’s done when the mix will hold together in a ball. Depending on the application, I will use thicker or thinner slip. Thick slip (more clay) means greater structural strength, but less insulation. Thin slip (less clay), means better insulation but less strength. It’s always worth testing your mix by firing a small ball in a hot fire and seeing how strong the resulting material is.
For insulating oven subfloors, to bear the heavy weight of the oven and the hearth floor, I’ve had good success using upside-down empty glass bottles, arranged with gaps, so no bottle touches any other (to allow for expansion without breakage). I fill the gaps with the sawdust clay (or loose perlite or vermiculite), and compact it to hold the bottles in place. If you use the sawdust-clay mix, it’s worth letting it air-dry before you cover it with masonry.
Finally, it strikes me that all this is a recapitulation of the history of cooking with fire. Equally worthwhile, it seems to me, would be an exploration of simple ways of cooking directly in an open fire, or ways of using the outdoor appliance you already have (oven, grill, barbecue, etc.) to do the cooking you want to do. That might mean a different cooking vessel, a different technique, or adapting the appliance to capture heat for the purpose you want. After all, cooking is just a matter of applying heat to food, whether the heat comes from a grill, open coals or fire, or an oven, most important is to have the heat where you need it, when you need it ... and to be able to balance your need for heat with your available fuel.
Thanks again, and good luck. I’ll look forward to seeing the results.
— Kiko Denzer
Be sure to check out the updated version of Kiko's book Build Your Own Earth Oven.
Are the bottom hearth bricks mortared down? I'm wondering if it might be nice to have the hearth bricks laid down without mortar in case they need to be repplaced. When I built one of Kiko's earth ovens, my hearth bricks were simply tamped down onto a firm sand surface. They worked wonderfully and never had issues in two years of use. The earth dome cracked of course...
Question #2... any ideas on how to use this design for a free stacked version that can be disassembled? To kind of 'try it out' before committing the idea to mortar.
I very well might build this one because it has multiple uses and is a bit cheaper than most of the full sized brick oven options.
"... a freestacked..."
Or maybe even a version that is built on a base that could be moved by forklift?
Nevermind, I see the sand layer in the instructions now. That question is answered. But another has arisen.
On what is the grill resting in the pictures? I can't find anything in the instructions about putting in some kind of shelf on which to rest the grill in the lower position. Of course, you could grill by putting a grate over the top of the firebricks, but one of the pictures shows it at mid-level. Maybe it is just resting on some extra bricks or a metal rack that just sits on the hearth?
This seems to be an interesting idea, hope we can get some
inspiration out of it:
Siemens liftMatic is a revolutionary wall-mounted oven with
a built-in lift. Just press a button to open and close, raise or lower. The
lift allows access to the food from three sides, without doors or handles, so
there is no risk of touching hot surfaces. The ‘liftMatic’ heats more
efficiently than traditional ovens, as rising heat remains trapped within the
oven. Powered by an electric motor, the principle of the ‘LiftMatic’ system is
similar to that of an electric car window. For safety, Its electric motor has
automatic cut out mechanism if the liftMatic recognizes an obstruction. You
choose the best installation position to ensure a clear view into the oven,
& the lift platform can be lowered for maximum comfort.
Energy Efficient: The ‘liftMatic’ heating system is more
energy efficient than traditional ovens. Thanks to the understanding of a very
simple physical phenomenon that the hot air is lighter in weight and so moves
up. For this reason it is energy efficient as the rising heat remains trapped
within the oven so it can’t possibly escape.It will also preserve the heated
air inside while open. If you keep the door of the LiftMatic Oven open for 15
seconds, temperature falls from 200 degrees Celsius to 175, whereas in
conventional oven, it drops by almost a 100 degrees after 15 seconds, so watch
out - don't burn your fingers.
Speed Cooking: A speed cooking function to reduce cooking
time by up to 30% compared to traditional ovens. It takes only 10 minutes to
get completely frozen pizza ready which is almost 5 minutes faster compare to
Online Math Tutor
This Forum page will host a discussion of a design for DIY outdoor cookstove that can be used three ways: baking, grilling or boiling water/cooking. Cheryl Long, Mother Earth News Editor in chief, has used a similar stove that was a rectangular stone box with the top covered with a removable steel plate, utilizing a reclaimed stove door and a stovepipe chimney out the back. The firebox area was lined with firebricks. This stove was built by Carol Mack and John Stuart at their homestead in Newport, Washington.
To bake, you stack firebricks on top of the steel plate and then charge the “oven” with a fire, rake out the coals and put in the bread, pizza, etc. To grill, you remove the steel plate and place a grill on brackets built into the sides of the firebox walls. (The new design may utilize removable brackets that sit on the floor of the firebox.) To cook on top or boil water for canning, plucking chickens, etc., simply remove the top firebricks and build a fire under the steel plate.
Dr. Owen Geiger of the Geiger Research Institute of Sustainable Building and EarthbagBuilding.com will moderate the discussion (firebox dimensions, size of flue, type of stone, fireproof mortars, foundation options — you name it!), and eventually build the stove and write up the plans for publication. We’ve invited John Gulland, wood-heating expert, and Kiko Denzer, author of Build Your Own Earth Oven, to offer advice and contribute to the discussion, too. After we’ve discussed all the issues we’ll put together a comprehensive article with photos and diagrams.
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