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  #31  
Old 03-11-2014, 02:56 PM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

Thanks! So would it be something like ( 4/3 x 3.14 x 3" x 3" x 2" ) / 2 = volume? Then i would divide that by 3 if using a ratio of 3 sand to 1 clay for example?
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  #32  
Old 03-11-2014, 07:58 PM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

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Originally Posted by sidamo View Post
Thanks! So would it be something like ( 4/3 x 3.14 x 3" x 3" x 2" ) / 2 = volume? Then i would divide that by 3 if using a ratio of 3 sand to 1 clay for example?
No, here is an example. Say you wanted an oven with an internal diam of 100 cm (approx.36") and wall thickness of 10 cm (external wall diameter 120 cm), then you will need to calculate the volume of the walls by subtracting the smaller volume from the larger one.

Volume of a sphere is 4/3 x 3.142 x r x r x r

Volume of sphere (external) 4/3 x 3.142 x 60 x 60 x 60 = 905 litres
Volume of sphere (internal) 4/3 x 3.142 x 50 x 50 x 50 = 524 litres
381 litres

Divide by 2 because it's a hemisphere =190 litres
(Also deduct some for the volume lost from the door. Dry volume is likely to be more than wet plastic volume. The clay will fill the spaces between the sand so resulting wet plastic volume will be close to sand volume only, so test this as I said previously.

Last edited by david s; 03-12-2014 at 01:39 PM. Reason: miscalculation
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  #33  
Old 04-07-2014, 10:52 AM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

Hi folks - I'm planning on building a 27inch diameter oven. I've sourced bagged potter's clay - "earthstone es180 sculpting / pizza body clay" details below

Firing Temp 950-1300C

ES180

A very coarsely grogged clay with a beautiful course texture. Suitable for pizza ovens, raku, large sculptures, saggers and hand built items. This fireclay based body using graded corderite grog has a very low shrinkage, making it highly resistant to warping,cracking and thermal shock. Suitable for oxidisation and reduction firing.

======================

This comes in a 12.kg bag - does anyone know the best clay:sand ratio for this and if aggregate is needed to construct dome?

I see David S' equation above - any one know how to convert this formula from litres to 12.5 kg bags?
Do litres equate to Kilos as per the above - so in David S' example 190kg of material would be required for the dome and insulating layers?

Last edited by sublimemick; 04-12-2014 at 03:26 AM.
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  #34  
Old 05-02-2014, 02:13 AM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

A great project Cheryl, and good to see people coming back to old posts and giving them some oomph.

I love all of this maths - I just worked with what I had, and figured worst case I dig some more up. As it was I had to get rid of some clay at the end of the job.

For people in a similar situation, I would ring the local pool company. In Australia they are always looking for somewhere to offload what comes out of the ground when the pool goes in. And they definitely know what is heavy dense clay and what is light crumbly semi-clay-semi-soil, cos they dug it up in the first place. If you tell them what you are about you should get a sense quickly of whether they want to assist. If not, move onto another company. Someone will want to offload some clay, and will probably be happy to drop it off to you.

Here is a blog about my project: Clay Pizza Oven I apologise for there be no recent photos. It is looking more rustic than ever 3 years on. The cracks are big, but still not venting any significant heat. If I work hard to get good coals I can do 2 kg (flour) worth of pizzas and a chocolate self-saucing pudding after. If I'm a bit slack or caught for time I can do 1 kg worth of pizzas and charred marshmallows after.

I also love the anthill idea too, but the best ones are all on state land, so I might just have to dig around my Telstra cable again when it is time for Mark II.

By the way I did not 'dry' my clay out - it went on less than a week after being quarried. But then again, I have big cracks, so I don't think I have the total solution (yet).

...Geoff
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  #35  
Old 05-06-2014, 04:42 AM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

Hi folks,

This thread is highly informative...

I was originally looking at a cob based igloo type oven - mainly because this is what I first came across online. I also originally wanted my oven to be portable.

With this in mind, I now want to use the material that gives me the best performance for a given volume. I have also sourced red powdered clay from a potter suppliers and I'm told this is suitable. But I can buy refractory cement for much cheaper - so which will result in a better performing oven?

Can you use a cob style igloo design and just replace the clay with refractory cement - or do you need to add additional insulating layers if not going with a clay based approach?

In the end of the day, for a given sized oven, what materials will give me the best performance? Is there any point using clay if it's more expensive and something else will work just as well?
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  #36  
Old 06-04-2014, 08:41 AM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

Quote:
Originally Posted by Gef View Post
A great project Cheryl, and good to see people coming back to old posts and giving them some oomph.

I love all of this maths - I just worked with what I had, and figured worst case I dig some more up. As it was I had to get rid of some clay at the end of the job.

For people in a similar situation, I would ring the local pool company. In Australia they are always looking for somewhere to offload what comes out of the ground when the pool goes in. And they definitely know what is heavy dense clay and what is light crumbly semi-clay-semi-soil, cos they dug it up in the first place. If you tell them what you are about you should get a sense quickly of whether they want to assist. If not, move onto another company. Someone will want to offload some clay, and will probably be happy to drop it off to you.

Here is a blog about my project: Clay Pizza Oven I apologise for there be no recent photos. It is looking more rustic than ever 3 years on. The cracks are big, but still not venting any significant heat. If I work hard to get good coals I can do 2 kg (flour) worth of pizzas and a chocolate self-saucing pudding after. If I'm a bit slack or caught for time I can do 1 kg worth of pizzas and charred marshmallows after.

I also love the anthill idea too, but the best ones are all on state land, so I might just have to dig around my Telstra cable again when it is time for Mark II.

By the way I did not 'dry' my clay out - it went on less than a week after being quarried. But then again, I have big cracks, so I don't think I have the total solution (yet).

...Geoff


I have plenty of anthills - you're more than welcome to them. The fire ants in them might take exception, however...
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  #37  
Old 06-04-2014, 08:44 AM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

Quote:
Originally Posted by di11on View Post
Hi folks,

This thread is highly informative...

I was originally looking at a cob based igloo type oven - mainly because this is what I first came across online. I also originally wanted my oven to be portable.

With this in mind, I now want to use the material that gives me the best performance for a given volume. I have also sourced red powdered clay from a potter suppliers and I'm told this is suitable. But I can buy refractory cement for much cheaper - so which will result in a better performing oven?

Can you use a cob style igloo design and just replace the clay with refractory cement - or do you need to add additional insulating layers if not going with a clay based approach?

In the end of the day, for a given sized oven, what materials will give me the best performance? Is there any point using clay if it's more expensive and something else will work just as well?

Check out this thread for clay alternative: http://www.fornobravo.com/forum/f43/...oven-9380.html (brickless dome on a shoestring oven)

If you're trying to save money buying more expensive materials when less expensive, good alternatives exist is self defeating. I love the idea of a cob oven - but I live where clay is plentiful. The point is a good working oven - don't get hung up on the aesthetics.
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  #38  
Old 06-04-2014, 06:26 PM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

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Originally Posted by Archena View Post
I have plenty of anthills - you're more than welcome to them. The fire ants in them might take exception, however...
That could get interesting. I have a continuing issue with ants (or perhaps they have a continuing issue with me) in that they pack up and leave their home in the base of my oven as the oven warms up. I assume they come back when it cools down again, as they are always there. (Gaps of a month or two between firings may be contributing to this.)

I'm glad the ones leaving are not fire ants.

...Geoff
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  #39  
Old 06-05-2014, 05:52 AM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

Fire ants are rare in this part of the world. The other ants could not be blamed for finding a nice dry house if it stays dry and cool. I have a load of insects here but none have taken up residence in the forno. i use mine every week though.
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  #40  
Old 06-06-2014, 07:22 AM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

Okay, I finally have a reason to be jealous of Australia!
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