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belfryboy 06-08-2009 04:24 AM

Rammed earth base.
I am about to embark on a clay WFO. For the base I intend to make a rammed earth plinth, then build into this the insulation layer and hearth. Has anyone any experience of a build like it.

My main reason for a build like this is the amout of clay I have available. A nearby cave is being excavated and alredy we have a pile of clay about 6 cubic metres! This clay powders down to a very fine dust with very little inclusions, and when mixed 2:1 (sand:clay) sets rock solid. I fired this mix and it ends up with a nice terracotta finish.

jpmort 07-30-2009 09:57 AM

Re: Rammed earth base.
And... what was the result? am thinking of doing something similar, but I unfortunately do not have a good supply of clay. Maybe I can ask a farmer?


Nev Grady 07-30-2009 11:47 AM

Re: Rammed earth base.
That's for your hearth. What about your dome? The old way of building ovens was the wattle and daub method. A dome shape of interlaced willow branches was made and then plastered with clay. This was built up to the required thickness then allowed to dry. The oven was then fired up in stages, much like the recommended curing method for the Pompeii Oven, then fired up to a very high temperature. The clay set like brick and the willows burnt away. Hey Presto a Clay Oven! Worth a try just for the fun of it if you have that much clay available!

Wiley 07-30-2009 11:56 AM

Re: Rammed earth base.
My next neighbor is really into cob construction and he gets alot of his clay from the local redi-mix batch plant. To translate that into English english the "redimix batch plant" is where one can order out concrete by the truck load. Here they wash the material as it comes from the pit. Straight from the pit is called "pit run" (duh?) after washing and sorting they end up with piles of clean sand and washed gravel sorted into several sizes. The wash water goes to settling ponds; where after it clears, it is recycled to wash more pit run. After time the settling ponds become filled with sediment and they switch to another pond allowing the filled one to dry (somewhat). They then empty it with front end loaders. This is the material he uses with his cob construction remixing it with clean washed sand. He gets his clay for the price of hauling if he shovels it and for a nominal charge if they load it. It is almost all clay with some very fine sand and virtually no organics.

Perhaps they have a similar process where you live.

Hope this helps,

dmun 07-30-2009 12:42 PM

Re: Rammed earth base.

The clay set like brick and the willows burnt away.
It's unlikely that with a wood fire, unless you use serious blowers and forests of wood, you are going to vitrify (render brick-like and insoluble) your clay oven. It can be done, of course: they made bricks long before modern furnaces, but the clay has to fired to bright red temperatures to accomplish this feat.

Nev Grady 07-30-2009 01:07 PM

Re: Rammed earth base.
"Setting like brick" doesn't necessarily mean vitrification! The term was meant to be "becomes hard as brick" but then again I'm not a purist nor pedantic!

jpmort 07-31-2009 07:54 AM

Re: Rammed earth base.
well, very interesting really. I have a slope and I want to make a clay/ earth oven for pizzas in it, by flattening part of the slope. And I am gathering enough knowledge to overcome my fear of starting and making a mess of the whole thing. I have though about cob and I wonder if it will be strong enough for a low dome. I was thinking of clab as I assumed it would be harder and more resilient over time. I heard that cob can flake off over time.

I then plan to put insultation over it, and then some NHL lime mortar in an attempt to keep it as weatherproof as possible. I cannot build a roof over it as it will sit right in the middle of my garden, and it needs to be as low as possible.

What I am thinking of is to flatten the earth, then shape it and ram it down hard. Then cover it with a layer of the NHL lime mortar, then a layer of insulation, then some terracotta tiles. Thats the base and cooking surface. Then I construct the dome...

As soon as the weather this summer improves I want to start...

Have you got any pics?


Wiley 07-31-2009 11:02 AM

Re: Rammed earth base.
1 Attachment(s)
JP, I'm guessing your request for a photo is addressed to me. If that is correct here is a photo of my nearest neighbor's cob WFO.

I'm going to make a suggestion that you might consider or not as you know your abilities. My WFO is lined with a steel dome, a hemisphere which was one end of a spherical propane tank. I have also seen propane tanks with ends that are elliptical and so used for the dome would make a low dome that would eliminate issues of possible roof collapse. Not saying to build the whole of steel like I did but have the dome of steel covered with clay (or other refractory heat sink) with a more traditional entrance and chimney of brick and chimney tile.

And speaking of a chimney, most cob WFOs I am familiar with do not have a chimney.

Hope this helps,

jpmort 08-01-2009 02:38 AM

Re: Rammed earth base.
Hi Wiley,
Sorry for not making the photo request more clear, but you but yes it was. And thanks for the pic. Great looking oven. Thats the kind of inspiration I need to get off my behind and get going! Do you know how weatherproof it is?

It is a good idea about the metal dome, but I think I will opt for the traditional approach, and see how that goes. Especially as having clay/ earth will allow the oven to dry out.

As for the batch plant that is near you... I will have a look around, but I think what you are saying is to look for a place that may have clay as a by-product to what they do. Good idea.

I think I need to use a form of sand on something similar to shape the oven, as mine is going tobe quite low. If it was coned shaped or semicircular than I think I could have used branches. This is all talk right now, when I get going it will become obvious what I can and cannot do.

Thanks again,


Wiley 08-01-2009 09:34 AM

Re: Rammed earth base.

Originally Posted by jpmort (Post 60902)
Do you know how weatherproof it is?


The short answer is "not very" so one needs to provide some sort of protection even if it is just a tarp for when it rains. And it has been known to rain in the Pacific Northwest, although not so much where I live.

We are in the rainshadow of the Olympic Mtns and so while Seattle (and even towns just twenty miles to the south of us) get on average 40 inches of rain per year we get 18 to 20 inches.


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