Temperature on Outside of Refractory Bricks

[Leao]

Hi everyone,

I am currently building my first wood fire bread oven. I have lots and lots of experience working with stone but the things I need to learn to take into account for a wood fire bread oven seem overwhelming, e.g. making the structure strong enough to withstand enormous temperatures and also how to maintain high temperatures.

I am making the wood fire bread oven with a beehive appearance (see attached graphic drawings of the front and cross section) although the oven will ne a dome. The exterior structure will be natural rock and normal cement, while the oven itself will be made from these Spanish refractory bricks and refractory cement.

My question is... Is there a danger that the temperature will get so high on the outside of the refractory bricks that it might crack the natural rock and normal cement that the refractory bricks lie immediately next to?

One person I asked said "in normal operations the outside of the bricks will not get hot enough to crack stone under the hearth but other places like near the chimney where all the hot gasses pass over could get too hot." In which case would you advise I build the chimney out of refractory bricks too? I was just going to use a normal metal chimney tube and surround it by a normal cement mix (see images below to get an idea).

Any help would be greatly appreciated!

Leao

[brickie in oz]

Originally Posted by Leao

Is there a danger that the temperature will get so high on the outside of the refractory bricks that it might crack the natural rock and normal cement that the refractory bricks lie immediately next to?

Not if you insulate properly..

[Leao]

Hi, the problem is that I have already built the stand and I haven't taken into account the room needed to fit in a refractory insulation blanket and vermiculite.

I could make the stand bigger but then the entire structure would be an enormous eyesaw. I could also make the oven surface smaller than 80 cm in diameter but then it might not be much use as an oven. The option I am probably going to go for is refractory bricks + refractory cement + a natural rock finish with no insulation.

Am I crazy to build an oven without vermiculite or a refractory insulation blanket?

Leo

[brickie in oz]

Originally Posted by Leao

Am I crazy to build an oven without vermiculite or a refractory insulation blanket?

Yes.
Without insulation you will burn tons of wood for little heat as the heat will just escape through the whole structure.

[Leao]

Originally Posted by brickie in oz

Yes.
Without insulation you will burn tons of wood for little heat as the heat will just escape through the whole structure.

Hi Brickie, if I only use the oven every now and then and when I do use it to bake while the fire is burning... is insulation so important?

I could hopefully squeeze in 2 inches of Perlite or Vermiculite or an insulation blanket but not both, which would be better? I'll ring the local construction firm tomorrow. I'm in Spain and someone I discussed the oven with suggested using salt for insulation. There is also mention of using a clay/wood shaving mix for insulation. Maybe I'll have to look into these alternatives if I can't get my hands on Perlita or Vermiculita.

What I'm really worried about is not that the oven doesn't get to or maintain high temperatures but that the heat will crack the whole structure.

Leao

[brickie in oz]

Ceramic blanket is best if you have little room.

As for cracking the rocks it would depend on the rock type.

[Leao]

Hi, I called my local building suppliers today. They said that they didn't sell ceramic blankets but they do have rolls of fibre glass blankets for $4 per square metre. They also sell big bags of perlite at $6 per bag, but they do not stock vermiculite.

Would you advise buying the a fibre glass blanket, I think I read somewhere on this forum that it burns and should never be used? How many bags of perlite would you recommend to insulate the floor and dome (the dome is 80 cm in diameter on the inside, 102 cm on the outside).

Warm regards

Leao

[dmun]

I called my local building suppliers today. They said that they didn't sell ceramic blankets

They wouldn't. It isn't a building material. You need a refractory dealer.

They also sell big bags of perlite at $6 per bag

That's a good price for a 4 cu. ft. bag. Perlite is as good as vermiculite. If you're limited to two inches that isn't so good, though. The minimum recommended cover for vermiculite/perlite concrete is four inches. If it's just a couple of places near the enclosure you could use the firestopping material that they sell in good builders suppliers.

I read somewhere on this forum that it burns and should never be used

The fiberglass doesn't burn, it's the organic binder. Don't use it.

Am I crazy to build an oven without vermiculite or a refractory insulation blanket?

Don't even think about building an uninsulated oven. If you think an oven is an eyesore, think about having one that doesn't work.

[Leao]

Originally Posted by dmun

If you're limited to two inches that isn't so good, though. The minimum recommended cover for vermiculite/perlite concrete is four inches. If it's just a couple of places near the enclosure you could use the firestopping material that they sell in good builders suppliers.

I have room to do the insulating hearth with 4 inches of perlite. That's no problem. The problem is the sides of the dome. Hopefully I can squeeze in a 1 inch ceramic insulating blanket and 1 inch of perlite around the bottom of the dome and increase the quantity of perlite higher up the dome to the recommended 4 inches at the top.

Does this sound any better?

Thanks so much for your time and patience!

Leao

[Leao]

I managed to get to the building suppliers. The bags of perlite they had were in fact grinded perlite mixed with plaster for rendering walls. They had no vermiculite either, so I bought some arlita (see this post (Insulation material) by Xabia Jim). It seems very common to use arlita in place of vermiculite in Spain. The owner of the store said his dad used it to insulate his oven. They didn't have any ceramic fibre blankets but did sell a rock wool blanket. It looked dusty and nasty, but will this do the trick and not burn like the organic binder in fibreglass?