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

[Archena]

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

Cat litter has additives that would likely be bad ideas in really hot ovens. Also, have you priced that stuff lately?

I doubt you could work it well enough - cat litter clumps but doesn't hold together well.

[BackyardPermaculture]

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

I know that people say not to buy potters' clay, but I bought a bag for $30 which was enough to do the whole dome - saved a huge amount of time and gave a great result. It was a very fine dry powder in the bag - easy to dry mix with sand and aggregate in a wheelbarrow, much like mixing concrete.

Cheers,
Mick

[CherylACT]

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

Just throwing this out there as an idea on sourcing clay that would work in your cob mix...but have you considered using bentonite in your mix (available at farm supply stores)? How about clay cat litter?

I like the ideas but cat litter wouldn't be good as mentioned becasue they add chemicals to it. Who knows what horrible stuff would be realeased if I heated it to 800 degrees F! Bentonite might be a possibility...

[CherylACT]

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

I know that people say not to buy potters' clay, but I bought a bag for $30 which was enough to do the whole dome - saved a huge amount of time and gave a great result. It was a very fine dry powder in the bag - easy to dry mix with sand and aggregate in a wheelbarrow, much like mixing concrete.



Cheers,
Mick

Okay, this is what I needed to know. What did you buy a 50 lb bag?

[BackyardPermaculture]

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

What did you buy a 50 lb bag?

Yes, I think so (25kg here, maybe 30kg). The mix was 1:1:1 clay:sand:fine aggregate, and then 10% portland cement. You also need fibres - cutting up natural rope works well, about 3 feet of rope cut into inch-long pieces and then separated into the fibres. I think that's better than using straw - either way, you need the fibres to prevent cracks.

Cheers,
Mick

[CherylACT]

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

I know that people say not to buy potters' clay, but I bought a bag for $30 which was enough to do the whole dome - saved a huge amount of time and gave a great result. It was a very fine dry powder in the bag - easy to dry mix with sand and aggregate in a wheelbarrow, much like mixing concrete.

Cheers,
Mick

I just wanted to follow up on this thread in case down the road someone else had the same question. I had ordered a 2 50 lb bags of 'slip' clay from Sheffield Pottery, a clay company that operates less than 100 miles from me, so it wasn't too expensive to ship. They were backordered on that item and when they called to tell me I asked them if that was the best choice for my application. He said it wasn't and suggested 'Hawthorn Fire Clay' which is half the price per 50lb bag so thats nice, and it has a shrinkage of just 10% which will be good for my oven. Thanks for everyone help on this!

[lancepawl]

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

If you have a decent clay source and don't mind spending hours digging, hauling, sorting, sifting, etc., you can go that route. Or buy some bags of RedArt clay from a company called Resco, which will be perfectly powdered and mix it 1 clay to 2.5-3 sand and you'll have an easy time of it. I bought six 50 pound bags at $12.25 a bag, so for about $75 it was worth it and I didn't have to worry about if the clay had too much silt in it or not enough. You can google RedArt and / or Resco to find who carries it around you.

[Sputnikmelon]

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

Yes, I think so (25kg here, maybe 30kg). The mix was 1:1:1 clay:sand:fine aggregate, and then 10% portland cement. You also need fibres - cutting up natural rope works well, about 3 feet of rope cut into inch-long pieces and then separated into the fibres. I think that's better than using straw - either way, you need the fibres to prevent cracks.

Cheers,
Mick

G'day Mick

Being in Brisbane myself I was wondering whether you used the Claypave Fireclay 30kg bags in the above mix? I can't seem to find any other dry fire clay supplies in Brissie. Also what did you use as the fine aggregate?
The natural rope is also a good idea, clearly going to add some tensile strength to the mix without being as painful as straw.
Any other hints on where to get a hold of materials in Brissie?

Cheers,
Leon

[BackyardPermaculture]

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

Hi Leon,

Wow, an old discussion

I haven't been on here in years, moved house a year ago so I should really build another oven.

It's not fire clay at all, it's a regular clay, and the aggregate is just crushed dust from a landscape supplies place.

Cheers,
Mick

[cobblerdave]

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

Gudday Mick
It's great to hear you answer, shame to hear that at present you haven't got on oven. I'm sure you'll sort that out.
It's great to have a clay oven builder answer a question . Davids as always is there to help but its good to have someone like yourself.
I remember your build was one of the first one I read about and it gave me the enthusiasm to think I could build an oven. Thanks... I hope you stick around and help the guys and girls with your particular experience
Regards dave

[david s]

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

Just dig down under the topsoil and you'll get plenty of clay in the soil. Bricklayers clay is way cheaper than a true fireclay and works adequately if you are insistent on buying it.Ant hills are another option. The ants have done all the processing for you.

[Gulf]

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

Ant hills are another option. The ants have done all the processing for you.

Thanks David,
I would never have thought of that. But, it stands to reason, once I think about it..... Where I live, there are only scant inches of true top soil left. A lot of that acreage only has clay below that topsoil. Those %$^....*&^*#! Argentina fire ants build everywhere .

I was taught as a youngin' that all of god's creatures were put here for a purpose. Until now, I did not know why in the hll they were put here .

[Archena]

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

Thanks David,
I would never have thought of that. But, it stands to reason, once I think about it..... Where I live, there are only scant inches of true top soil left. A lot of that acreage only has clay below that topsoil. Those %$^....*&^*#! Argentina fire ants build everywhere .

I was taught as a youngin' that all of god's creatures were put here for a purpose. Until now, I did not know why in the hll they were put here .

Cool idea! <bookmarks thread> As soon as we have our first frost I can start harvesting (Alabama - all the mounds in my yard are fire ant mounds). Thanks!

[sidamo]

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

How many lbs of dry fire clay for a 36" cob oven would you guys estimate using the 3-1 sand / fire clay ratio?

[david s]

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

It depends on how thick you want to make the walls. Use 4/3 x pi x r x r x r to get the volume and divide by 2 because it's a hemisphere. If you are buying powdered clay then buy one bag, make up a small batch of your mix and you'll be able to calculate how much of it you'll need for the required volume you want.