Wednesday, September 17, 2008

Earth Oven Insulation

I know that many earth oven builders have been using a mixture of sawdust with clay slip for insulation, and I have been one of them. A few days ago I made of a mixture of 6 parts sawdust to 1 part Portland cement, added a little water and formed a tight ball. I did this again, but this time I used 6 parts Perlite to 1 part Portland cement.

After both samples had hardened, I subjected them both to oven heat. As you can see by the photos, the one made with Perlite was untouched by the heat, but the one made with sawdust had completely disintegrated. Maybe the sawdust has some insulation value even in a disintegrated state. I don't know. Even if the sawdust had held up, Perlite still has about twice the insulation value.

Sawdust is free, but you have to buy Perlite. It's your decision to make.

For lots more information about bread and earth oven construction:

Here's an email I just from received from Kiko Denzer, who probably knows more than anyone about earth oven construction.

Hand Print Press

Hey, Stu,

Interesting that the perlite held together w/cement. I would have expected both of them to crumble. My understanding is that cement starts to break down over 700F. And with perlite, there’s really no need to use cement at all, unless you’re building an Alan Scott style oven and need the strength. Otherwise, the addition of cement only reduces insulation value.

The practical test for the sawdust clay mix is to get enough clay into it that when the sawdust is burnt out, you’re left w/a foam that’s high enough in clay to maintain it’s form (and when needed, i.e., for under the hearth, to support weight). Obvioustly, it’s not the sawdust that’s providing the insulation — it’s the clay, in which the burning of the sawdust has created voids. It’s a cheap/easy way to manufacture your own insulating firebrick — but it does help to do some tests. Or, to err on the side of too much clay. Or, under the hearth, to use empty glass bottles for structural support.

I have also had success using a mix of clay AND portland with sawdust — that way, you get the quick setting of the cement, and the lasting power of the clay. Tho if you need such a combo of traits, it’s cheaper and nice to use plaster of paris (gypsum plaster).

-- Kiko Denzer

Hand Print Press • POB 576 • Blodgett OR 97326 • 541-438-4300
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