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Refractory heat insulation - Thermal
Also in wood fired ovens the energy absorbed into dense firing chamber ust be kept in letting it to serve its purpose. It should vibrate only inside the dome and cook for us for a long time. If the dome was not covered with insulation, heat energy from the fire would quickly run away leaving the dome cold.
Dome walls of the oven are made of dense material with high thermal conductivity therefore cooking chamber is capable to soak in heat from the fire and lead it all across in the body. This heat energy also enters right the outside edge very quickly, literally in a high speed. In other words, as well as the dome is hot on the inside, very fast it gets equally hot on the outside. "That means it's loaded with cooking energy :o) 'Oh no no … don't get scared it is very good for you!!! We will just give the oven a nice coat to keep it warm even though it is not winter, and that way the energy is only ours."
Refractory heat insulation types
You can choose from many refractory insulation types. Some are packed in bags and used in a loose form by pouring it dry in around and on top of a boxed in dome of any shape. Then there are blanket form heat insulation (rolled in different lengths or precut into smaller sizes.) You can easy prepare your own mix or buy insulations that are mixed with water before application, these can be cast or built up by hand, after setting the mix forms solid but soft insulating layer.
Dry, loose form heat insulations that can be poured around and on top of boxed in dome:
Ceramic fiber blankets or bats:
Mixed refractory insulation's:
13 : 1 : 1 - Perlite, Portland cement, Lime. Mix all dry ingredients then add small water amount in stages while mixing. This will become solid but stays flexible! Used e.g. for covering of the dome e.g. into barrel or igloo effect on the outside. (It is not insulating concrete or mortar.) Apply in 2.5cm - 1" layers, do 3-4 or more of them allow each layer to set before following one. Last, still insulating layer, can be thinner with double amount of cement. Finally for the weather finish, the oven can be rendered on chicken wire, just like the stucco on old wooden houses.
Thicker insulation application for under the hearth slab
Heat resistant insulation for under the floor slab can be simply 5:1 - mix of Vermiculite : Portland Cement (or often called also GP cement.) This insulating layer has also metal mesh under (the mix is then poured on top of the mesh). Mixing this type requires of approximately 1 part of water and 6 parts of dry matter, which makes it dry stuff to water 6:1 . . . ( in other words: 5 + 1 = 6 parts of dry ingredients which is Vermiculite and cement and water 1 part. All measuring is done fast in volume sense, NOT in weight.) Mix first only the dry Vermiculite and cement together. Then add into it only ~80% of the required water amount and mix. From the beginning it feels rather dry but as you progress with mixing you will realize that adding just a bid of water could make it very runny easily. I place into wheelbarrow 5 cans of Vermiculite, 1 can of cement, mix it with shovel there, then I add 1 full can of water and finish mixing. This makes me full wheelbarrow of insulation. Because insulating ingredients are so much lighter in weight, when compared for instance with any sand, it would be easy to push it out of the wheelbarrow. So take it easy while mixing it. You will see:)
Do not mix Vermiculite based insulation in a cement mixer as the mixer is destroying the Vermiculite granular particles (page with images plus extra reading on vermiculite insulation.)
Thickness of this refractory insulation slab layer should be between 3" to 4" (75mm to 100mm) which is sufficient for the small surface a family sized wood fired oven has. When you make the floor insulating slab; transfer it from your wheelbarrow into place, spread it on the whole surface, and tap the mix on the top to make it more solid. This tapping doesn't have to be done with an insulation applied on the top of the oven because that one just sits there on the dense cooking part. With the insulation that goes under the concrete slab for the oven, lite tapping is required, because the oven is heavy. Till it sets the surface will feel soft like a mulch. But as it dries it also becomes hard.
The same 5:1 makes it light in weight concrete also. Floors are cast out of this mix in high raise buildings to lower the weight of the whole building down. However builders use mostly Perlite instead of vermiculite because as a building material Perlite is brittle. As a insulation in wood fired ovens both Vermiculite and Perlite perform very much the same way.
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