Mozambican Cob

[nicklear]

Today I have started a cob oven build, here in Pemba, Mozambique. I spent a lot of time researching last year whilst staying up to do the night feed for our baby. Unfortunately I am one of those who feel like there's always one more website to read before you can safely start, but thankfully my wife is not one of those and she has encouraged me to get on with it!

My plan is for a 36" oven inner diameter, with 4" thermal layer and 4" insulation layer. Then maybe a finishing plaster.

I don't exactly know where I am getting all my materials from, but I think starting a foundation (hole in ground pictured below) will set the ball rolling. Here they build their homes with a bamboo frame, filled with rocks and then held together with clay-rich soil so I feel that clay can't be too hard to get hold of. I have sourced from our farm some dried grass which I think will work as the straw. I also read on the Uganda thread of termite mounds being useful.

I am actually more concerned right now with what to use for the base as my budget is limited. I will try to post some photos of what is available around here - various rocks, broken concrete blocks, etc. I think I will go for rocks, mortared with cob as put forward on buildnaturally: buildnaturally.blogspot.com/2013/06/build-clay-cob-oven-in-your-yard.html

Some other Q's I have at this stage:

- Some sites recommend a dome height of 75% of dome diameter as opposed to a pompeii brick design which would be more like 50%. Is that right and why? My thoughts were that a lower dome would cook (radiatively) better but that perhaps a cob oven has less strength and a taller dome maybe requires less strength structurally to support itself.

- How much do people appreciate having a chimney? I am inclined to go for a simple chimney-less design because it is easier and I'm not sure what I'd use for the chimney. If I did one, I think I would go with this design: clayoven.wordpress.com/2008/09/08/5-the-oven-entrance-and-chimney/

- Can you waterproof a cob oven (against fairly heavy rains) with e.g. a lime plaster? Or is a roof/tarp the only option?

Any help / encouragement appreciated.

Nick

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[hedgemonkey]

Here's a good simple cob build using reclaimed materials done in Russia. I must confess that when I read Mozambican cob it got me thinking along the dagga lines rather than oven builds.
simplyselfsufficiency.yuku.com/sreply/5183/-20-Outdoor-Cob-Oven#.UFXTFbJmRVU

[limpopomark]

Hi Nick

You may be 1000 miles NW of me but I feel we're practically neighbours, so the least I can do is chuck in my two penneth.

We had a pretty limited budget for our earth oven, though we didn't go with cob - ours is pretty much the same as Simon's from the clayoven blog you reference, but without the chimney, and with woody elephant poo/clay slip as the insulation layer.

I'd say a roof is the only practical answer for rain protection. These things weigh a hell of a lot too so just be sure that the base is super-solid and not likely to shift at all once the ground starts getting wet.

The buildnaturally blog follows - i think - Kiko Denzer's book fairly closely in terms of dimensions and such - so if they say 75%, go with it. I can't remember my figures but it was certainly more conical than a perfect hemisphere. When I was reading around it seemed he'd done a good deal of experimenting to come up with figures like that. The other important thing is that 63% figure for the height of the opening relative to the internal height of the dome - it's what allows the fire to burn properly.

Not sure if i have said anything helpful but this is a great forum and I am sure you will get any help you need here. Good luck with sourcing materials and getting started, and keep the pictures coming!

If you think I can help with anything, holler

Cheers,

mark

[limpopomark]

* i meant northeast... been bugging me all day!

[nicklear]

Thanks guys for the messages. That russian thread was helpful. And Mark, I went through your build the other day - always encouraging to see other ovens that have been built in Africa. I didn't spot the NW mistake - geography not my strong suit. A couple of years ago we went from here to Cape Town via Zimbabwe and back again all on public transport so i definitely have a sense of the distance!

Today we have rocks in the hole in the ground and tomorrow will pour in concrete so I think it will be pretty solid:



I'm still trying to decide how to build up the base. I have quite a few broken concrete blocks like this:




Though at 45p / block, I am tempted to buy enough for the base wall and then use the broken pieces to fill in the centre.

There are rocks around like this, but I don't think there's enough:

[nicklear]

I have a couple of Q's if Mark or anyone else can help:

- After the first cob layer, the thermal mass, do you need to wait a day (or more) before you add the second insulating layer?

- In the strong Mozambican sun and 30 degree heat, do I need to cover the layers (and the concrete base) as they are curing to prevent them from drying too quickly. (www.wikihow.com/Cure-Concrete)

- If I go for a doorway arch, can that use any old brick? And normal cement mortar or thermal resistant clay mortar?

PS Today my friend found some clay. It passes the worm test anyway and I am doing a jar test right now - it seems to be almost pure clay. Here sand costs more than clay so I would have preferred if it had been 50% clay 50% sand.

[nicklear]

This is what the clay soil looks like:



And first test FAILED! We made a pizza stone from 1:1 fine sand and the clay soil (if it worked I want it for my gas oven) and left it in the sun:



I haven't finished the jam jar test yet, but if I am right that the soil is almost all clay, then this test would have been about 50% clay rather than the 20-25% we want - thus the cracking. Or it is the sun. Will do another test with more sand.

[limpopomark]

Hi Nick

On ours, we removed the sand former the morning after we'd made the first layer, and it sat a day or two before we started on the insulating layer, only because we were doing other stuff. It was January here, and hot, so we covered the thing up to slow the drying to reduce any cracking. The general advice to start small fires early may not apply here - based on my experience I would not say it's necessary.

If you make an arch, use clay brick. Those nasty concrete bricks that are everywhere will crack, I am told. I drove past a building site in town and asked the foreman, and he sold me 50 bricks for ZAR50 (a bit less than a fiver at the time, even less now), as i needed some for the arch and the hearth. I used regular mortar (cement + sand), and it's only cracked at the top of the arch but the structure of the arch remains solid. It has been a little temperamental where it joins the oven but I've stuffed gaps with more clay/sand mix. We didn't use any cob anywhere, although our insulating layer is essentially elephant poo cob., i suppose.

Great to have found some clay - our issue was the opposite: sand everywhere and no clay. If you want to swap some clay for some sand...

keep it coming

mark

[hedgemonkey]

My copy of the Kiko Denzer cob book has been pinched by Petethepub but he gives good details of how to conduct tests on different mixes.

From memory you make small bricks of a set length (say 6 inches) with different proportions of sand and clay. You then let them dry and measure what percentage of the length of the brick is lost due to shrinkage. Again from memory I think he suggests that a 10% loss of length of brick is a good mix. I'm sure that someone will leap in and give better details.

[cannyfradock]

Hello Nick

I'm always fascinated following builds in remote area's especially when access to the correct materials is a bit of a problem. Unfortunately I gain more knowledge from builds like the build from LimpopoMark or following the Uganda build from Janine in Uganda on the FB forum.. www.fornobravo.com/forum/f3/hello-uganda-7728.html?highlight=uganda .....than I can give advice on.

Here's my thoughts on your questions so far....

On lots of clay oven builds the dome height has varied from 50% of the diameter to 70%. In all honesty I don't know if a more conical shaped dome make any difference to strength in a clay oven build......I have gone between 55 and 60% on some of my clay oven builds but the opening ..regardless of a hole cut into the clay...or a brick arched entrance has always been 63% of the dome height.

I always build a brick entrance, because 1....I'm a bricklayer and can build an arch in no time and I always have bricks at hand, and 2....it helps draw in the air to allow a fire to ignite and burn sufficiently. The entrance arch is always built slightly larger than the internal arch as this creates a rebate for the door...for bread baking and a hole is also created on the entrance arch top....for the addition of a chimney....which helps draw the spent air out of the oven. A simple aperture/door opening cut out of the clay, after a few days will also work. These ovens can be as simple or as complicated as you want....they will all work. (if you decide to cut a doorway out of the clay. then, make sure that when a door or piece of wood is laid against the entrance....there are no gaps. Do this when you cut the door as you can easily shape the entrance when the clay is still semi-hard.......afterwards you may find this a bit tricky and if baking batches of bread or slow roasts ...a good seal on the door is important.

Your clay looks very dry and a bit of a nightmare to form into cob balls.....if you soak dry clay in water for about a week, it will then become pliable enough to feet stomp the pieces with sand......and extra water if necessary. Normally clay oven builders will say a ratio of 2 to 1 sand and clay will work. If your clay is so pure (as you mention) then go to 3 to 1 ratio. .....this is always in use with sharp sand which has irregular grains. Some other sands are round in grain which don't bind as well as sharp sand sand......any sand will work but you may have to add a little more fine sand than sharp sand to gain the same strength.

I've no idea on the availability of materials where you are, but the cooking hearth will be important. If you can come across decent clay bricks...fired in a kiln, then use them in place of a refractory or fire-brick hearth. Insulation is also important under the hearth so if vermiculite or thermal insulation blocks arn't available, then use a clay/sand cob mixture with hay or woodchips/rough sawdust in the mix.....this will form a honeycomb layer which will add to your hearth massed heat value.

These are just a few thoughts on the points you have raised.....the picture of the blocks you can buy for base walls look pretty naff.....I hope that if you pass on cash, those blocks will be buildworthy...

Please keep us informed with your build.....sorry I couldn't give you more solid advice.

Terry

[nicklear]

Thanks so much guys for the replies and the support. It really helps to not be doing it alone.

I will try to do some test bricks tomorrow - maybe 2:1, 3:1 and 4:1.

Here is my jar test:



I can't see any layering at all, it does look like pure clay. It's not that obvious which is the peanut butter! (Mark - we get trusty Black Cat all the way up here)

I was a bit worried about the clay being dry, but when we made the first test brick it absorbed water pretty quickly and soon was reminding me of school pottery class. But I do think some soaking would be helpful & save us some time.

With the first test - in the sun - it dried really hard within a few hours. I wonder whether it might be better for the insulating cob layer to go straight on to protect the inner layer from the sun and the heat and slow its drying down - what do you think? I have found a tarp now, so we could use that to cover it and also maybe only build in the afternoon to avoid the strong sun.

I have found some clay bricks that are about as good as I am going to get around here and they can definitely take the heat as I used them last year in the exisiting base oven. It didn't have any brick at all in the base, just a slab of concrete, so I just shoved some in and not of them cracked during use. They don't join together perfectly, but the holes filled with ash and it was ok. Maybe better to fill gaps with thermal cob?

I inherited these bricks, but apparently they were expensive. I might buy some cheaper clay bricks for the door arch.

[spinal]

This may not be possible, but in Gambia we had a chap who built a wood fired kiln out of found clay. To get the temperature hot enough to sinter the clay, he attached an old vacuum's exhaust hose to force air in... never really knew how hot it got, but he got it hot enough to make pottery (and bricks for a second kiln).

May be worth asking around if any potters will let you bake a base-stone for your oven...

.

[nicklear]

I have seen clay flowerpots that look like they've been fired so there must be someone who knows how to get a kiln going. It's an interesting idea.

I really want to understand what can be done with the simplest and cheapest of materials here because in our work we are now focusing on microfinance and small business - trying to get people to be self-sustainable. Anything where they can add value is particularly good. I'd like to teach people to make bread ovens (a bread roll is the national breakfast). From what I've seen they are pretty good with clay and bricks, but insulation is often lacking. Just having access to the internet means I have so much to teach.

Anyway, here is the foundation (not the best you will have seen on this forum & with some cat prints - the guy I hired to do it was hired less on his building skills and more on the fact that I know him and he'd just got out of prison and needed some work):



And some dried grass and sand ready for next week:

[spinal]

Looking good!

Another idea for value added products is these:
ukwoodfiredovenforum.proboards.com/index.cgi?action=display&board=smokincuring&thread=587&page=1

They're common amongst the expat community in Gambia and Kenya. Made out of old metal tins (e.g. oil drums) and other recycled bits that would normally be considered garbage. I'm afraid I don't know how they're built, but I am assuming the metal is cut and welded or cut and folded depending on the area.

If you can get your hands on an old arc welder (or find someone local who welds) it could be an interesting side-project.

--- Anyhow, back to your current project. What insulation are you planning to use? As wrong as it sounds, a mixture of dry grass, clay and cow excement/dung works quite well. Coat the oven once it's dry with this, about 4-6" thick. The undigested grass + grass burns off, leaving some gaps in the clay, which then insulates reasonably well.

For the base insulation, I hear using old glass bottles works quite well. Embed them before you build the oven, and the air gaps they keep will prevent some heat loss.

It wont work as well as ceramic board, but it will work better than nothing

M.

[nicklear]

Great to see those ovens - unfortunately I have never seen metal drums here, nor do we have cattle. I am just going with a classic cob mix, though who knows how my clay will perform. Will do some test bricks today.

Re glass bottles - I am trying to get hold of some. Harder than you'd think as most places have to take a case of empties back to get the next crate. If I bought those bottles, that would be one of the most expensive parts of the oven! But luckily I have found a couple of the more western restaurants that source their beer differently and throw away their bottles and they have promised to collect me some.

We are going to build a stone base today, which my friend Clayton from SA is going to do with me. Turned out buying stones was cheaper than concrete blocks too.