#21  
Old 05-17-2010, 02:11 AM
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Default Re: Clay Oven on Verandah

Mick,
With the high percentage of unfired clay in your mix you will get a lot of shrinkage which means cracking unless the dome can shrink. You should remove the sand in the middle as soon as the dome is self-supporting , which is probably straight away. Some lime in the mix would probably have helped make a stronger and more refractory brew, but I'm sure what you have done will work, but take a long time to let it dry before you cover it with anything else. The clay in the middle will take ages to dry although the outside may appear dry. Ask any potter who has to fire thick sculptural work. We're talking weeks.
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  #22  
Old 05-17-2010, 05:39 AM
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Default Re: Clay Oven on Verandah

Thanks for the tip David - I'll be especially careful with the drying.

I got the sand out of there the following morning, and put 2 x 200 watt halogens in there for 24 hours - lots of steam was coming off overnight and it's much drier now. But I'm planning on progressing to some heatbead fires.

Also planning to insulate with thick layer of rockwool and then vermicrete, so the dome should have freedom to move.

Cheers,
Mick
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  #23  
Old 05-17-2010, 04:31 PM
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Default Re: Clay Oven on Verandah

If you are going to put anything on the dome other than lime you MUST have an enclosure. I'm pretty sure you were planning on one but it is vital if there is anything on the dome. Anything that can potentially seal moisture in or inhibit evaporation will destroy the cob. Be sure the exterior is dry before you add the insulation.
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Old 05-17-2010, 05:51 PM
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Default Re: Clay Oven on Verandah

Archena,

Thanks for that. I was actually still trying to decide between an igloo with acrylic render and an enclosure. Sounds like enclosure it is!
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Old 05-18-2010, 05:51 AM
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Default First Fire

Just ran my first fire in it tonight.

I ended up getting it pretty hot - dome was feeling quite hot on the outside; definitely pushed a bit more moisture out. No cracks so far, which is encouraging.

Really happy with the design - most of the smoke drawing strongly up the short bit of flue pipe I had on it.

Also really happy with the performance of the AAC layer (Hebel). The blocks cost me $64 at Bunnings and took about 30 minutes to lay on some adhesive. Much easier than pouring a vermicrete layer.
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Old 05-18-2010, 04:52 PM
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Default Re: Clay Oven on Verandah

Great! Congrats!
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Old 05-19-2010, 06:07 PM
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Default Second Fire

I lit another fire in the clay oven last night. Got it a bit too hot one one side - lots of steam came off that side, and one crack appeared when it cooled down.

Easy to fill in the crack with the sand/clay slurry, but it shows how much moisture is still in there and how careful you need to be in getting it out.

The oven was drawing really well; much easier to light the fire this time and very little smoke (which was almost all drawn up the flue). The dome is drying out to an almost-white colour now.
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  #28  
Old 05-20-2010, 02:34 AM
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Default Re: Clay Oven on Verandah

With the kind of thickness you have in the walls the moisture is hard to eliminate from the middle.If the temp gets up to around 300 C expect your wall to start splitting from the middle. You might think that they're dry but they won't be. Don't be in a hurry and let nature do the a lot of the job for you. Try to keep the fire really low but prolonged. You will have added maybe 50L of water. How long do you think it would take to boil a 50 L drum dry? Use sun and wind to your advantage.
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Old 05-20-2010, 04:27 AM
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Default Re: Clay Oven on Verandah

Thanks David - not so easy to keep the fire long but prolonged, but I'll give it a go.

Unfortunately the oven gets no direct sun on it at this time of year, so I will probably have to continue trying to heat it. I think I'll go back to the plan of using heat beads or some other commercial fuel to try to get a consistent small fire.

There's rain coming soon, apparently, so I need to be careful not to let it get drenched too.
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Old 05-20-2010, 05:02 AM
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Default Re: Clay Oven on Verandah

Low and prolonged: build a small fire on two long sticks. As they are consumed move the sticks a little further in. This is how people with limited firewood cook in the Third World. If you need to extend it, add a third long stick before the others are consumed.
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