All adobe/mud cookstoves and ovens..

[Donkey]

Agreed that a layer of burlap, after the fact will NOT help.

My process has always been to build a sand form, though I will sometimes use other items under the sand to add bulk and reduce the amount of sand needed. Over the sand, I spread wet newspapers which help prevent drying of the sand while I work AND it makes an easy boundary, an obvious stopping point when digging out the sand.
My oven core material is made of a very sandy cob, so sand that it can NOT be applied in the usual manner. For my soils, the mix that we use for normal building cob is 1 part sand, 1 part clay soil and straw to taste. My oven core mixes are 4 parts sand, 1 part clay soil, zero straw. I also make it as dry as possible and still stick together. This mix can't be massaged into previous layers like builders cob, it's too sandy. It needs to be packed in thin layers, built up from below, pressing only into the oven body (not towards the sand form) while building,. No slapping allowed, only pressing firmly. It's best to arrange something to stand on so that you can always work from above the oven and always have plenty of leverage.
If the oven core layer is packed properly, the sand can be dug out almost immediately. I do like to let it sit for an hour or so, a nice extended lunch break is good here. As with MOST natural building techniques rushing is not advisable, it took me a LONG time to figure out good timing.

[patamos]

Cool

That all makes sense...

So then there is the more laborious step of impregnating the inner finish skim coat of fine sand and clay with strips of fiberglass mesh. General word is that those fibres are good to 700c. Not sure what the long term effect of thermal cycling would be. Maybe some threads in 'materials' offering a closer look at this.
I can understand why some folks would not want the glass mesh inside a black oven, with fibres potentially floating around the chamber. But if it is well buried in the belly of the plaster it tends to stay there...

I'm not sure how well this could hold up a very compromised dome. But at least it is doing the tensile work where one wants it.

Donkey, when you write of 'compressing' are you talking about running a wooden or metal float over the applied layer as it is in a middle stage of drying? Or more what you are doing with your hands/floats as you are laying it up? Or?

[Donkey]

Oh.. Just using hands, push firmly. As I said before, it's best to get up above the oven on something, then you can lean on it with body weight.
I think that if this is done properly, there's no need for the glass fiber. In fact, do further generations a favor and LEAVE OUT the fiber! When someone goes to re-cycle the old oven, there won't be any nasty surprises.
If it's done well, roofed and a good high foundation, it could last for the rest of your life.

[matthewwalker]

While we are on the subject, I have a question for you guys who have built a few all adobe ovens. I've done a few, albeit intentionally short lived ones that lasted through one outdoor cooking season and then I reconfigured the area so they went away.

My question is, what do you guys use to plaster the inside so that accidental contact with a utensil or dish doesn't result in sandy food? That's the main reason I've started doing thin refractory liners first, then building with cob around those. It's partly out of laziness, but also I just had too many sandy pizzas. Of course, my ovens were small so you had to kinda carefully extract the food, which is when the sanding happened, but...just wondering if you do anything special, or just a fine plaster mix?

[Donkey]

I build larger ovens than you do.. With a dome of 16 inches high, all you really have to worry about is striking the edges with the peel or while shovelling out the ash. I like a layer of brick all the way around under the (edge) line of the dome, as well as a brick (or stone) arch opening.
You can see the line of them (at the bottom) in this pic.

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[matthewwalker]

Well, that's an awfully simple solution. So, what you are saying is, stop bumping into the damn oven!

[satamax]

Just an idea for the unskilled!

Couldn't we do a corbel vault (is that the right term?)

Something like this, where each circle of stones or brick is smaller than the one below, reduce the diameter enough, and plonk a refractory slab on top?







Whatdaya thunk?

[patamos]

Ya,
So many of the old (and still standing) bread ovens are brick. Concentric domes...
I've been wondering how best to create a cob mix that can be fired to vitrify. Something fiberless and clay rich, with burlap on the underside to keep it from cracking while drying? The burlap burns off in the first mild firings. Then a skim coat of finish plaster to clean up the surface, then super hot firing?

Just throwing ideas around

[patamos]

Further thoughts in response to Matt's question.

If you make a mix of clay rich finish plaster consisting of 1part clay to 1.5 or 2 parts fine angular sand, and apply it to the inside cobb surface, you might find that it dries quick enough that you can compress it shortly afterwards (within minutes), and thereby clean up any cracks that start to form. This could provide a somewhat more durable surface to bump into.

[wrekinwanderer]

Wow, Donkey what a beautiful cook stove!

A question: if you were to bring the beast inside, is there anything you would do differently?

Mike

[martinpolley]

Hi Donkey. That's a lovely oven. Exactly the kind of thing I'm looking to build. But I have a couple of questions that the photos don't quite answer...

Is the adobe "barrel" the same as in a regular RMH? With the hole at the top allowing part or all of the hot exhaust gas to flow into the oven (which it presumably only does when the door is open)?

If this is how it works, where does the exhaust exit when the door is closed?

When you fire it, do you leave the door open until it has heated up the dome and hearth enough, then you close the door? (Does the dome heat up as much as a regular cob oven?)

What do you use for the floor of the oven?

TIA for helping a newbie out

[Donkey]

martinpolley,
Look carefully at the build pictures, you will see that the heat comes out of the riser, strikes a plug and flows in all directions through several horizontal channels to the outside edges of the dome at the bottom. There is a channel leading down from one side of the oven that takes exhaust (when the door is closed) through a lower hot water heater, then down again to the chimney.
The chimney exit starts quite a ways below the oven but above the level of the fire.

When I fire it, I only leave the door open for about 10 minuets tops. Just long enough for the firebox to come up to heat and go clean. Then, I close the door so that heat will be forced to go down from the oven chamber. The thought is that it will act like a bell, keeping the hottest gasses inside for longer.

I made a "pizza stone" thing with local clay, mixed with wood-ash, sand and a little chopped straw. It didn't quite fire in there and making it just small enough to barely fit through the door in one piece makes it really too small to be a good baking floor. I plan to make a new one in quarters (or something) to fill the whole space.

[martinpolley]

Donkey Thanks for the info! So is the mode of operation for this oven to fire it for a while to get it good and hot, then bake stuff, or to keep the fire going the whole time you're using it?

[Donkey]

The beauty of this oven is that you can use it as it heats, at a range of temperatures; use it as long as you want at high heat while firing (maintaining heat); and let the fire go out and use it while it cools, at a range of temperatures.
It's a thermal mass oven, so it will act like a traditional wood fired AND it can be stoked as it is used for, say, baking pizza for hours on end.

[martinpolley]

Donkey Nice! Sounds like the best of both worlds . How long does it take to reach pizza-baking temps?