Cob Oven -What type of clay should I buy, where can I buy it?

[sidamo]

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?

[david s]

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?

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.

[sublimemick]

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 950-1300?C

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?

[Gef]

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

[di11on]

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?

[Archena]

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

I have plenty of anthills - you're more than welcome to them. The fire ants in them might take exception, however...

[Archena]

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?

Check out this thread for clay alternative: http://www.fornobravo.com/forum/f43/...oven-9380.html

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.

[Gef]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

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

[Greenman]

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.

[Archena]

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!

[rubone47]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

Does anybody know if I could use this product for my cob oven

H.C. Muddox 50 lb. Fire Clay-100011882 at The Home Depot

[fornax hominus]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

Who knew HD had fireclay !. I buy my fireclay at a pottery supply house .. Hawthorne brand , but for oven purposes [lower temps than pottery ] any fireclay should be fine .. find a pottery supply in NJ and compare prices . they may have bigger bags and you will need lots . HD is not always the most economical .. but they are handy

[rubone47]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

Who knew HD had fireclay !. I buy my fireclay at a pottery supply house .. Hawthorne brand , but for oven purposes [lower temps than pottery ] any fireclay should be fine .. find a pottery supply in NJ and compare prices . they may have bigger bags and you will need lots . HD is not always the most economical .. but they are handy

Thank you for your response. How much clay do I need. Thought that I had read somebody was saying a 100 pounds or 2 bags of dry clay was enough.

[rubone47]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

Who knew HD had fireclay !. I buy my fireclay at a pottery supply house .. Hawthorne brand , but for oven purposes [lower temps than pottery ] any fireclay should be fine .. find a pottery supply in NJ and compare prices . they may have bigger bags and you will need lots . HD is not always the most economical .. but they are handy

Those are 50 lbs bags how many bags would I need. Thought I had read 2 bags would be enough

[Brave]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

I think any clay you can get your hands on will be fine. Just dig a hole on your back yard and go down a few feet and you're likely to hit it. You don't need high quality clay that potters use. Although the cob oven will get hot it's not going to get so hot that it will set into a glass.