Tuesday, May 1, 2012

Brick Oven Specs #4

The most distinctive functional difference between cob/clay ovens and modern incarnations of ancient brick oven designs is the insulation.
In ancient times and as recently as early 20th century brick ovens were heated and kept heated with the acknowledgment that heat would both continue to be supplemented with additional fire and fuel and that the heat would continue to move through the walls of the masonry and dissipate.
Neither of those conditions need to exist anymore.
Refractory insulation in a number of forms can prevent most of the heat loss and prolong the effective time that an oven can bake product.

In order of most economical to most expensive, there is:

Cast vermiculite/Portland Cement mix.  Agricultural vermiculite and Portland Cement mixed to an 'oatmeal' consistency is formed in a wood frame over whatever solid base will support the oven.
Pros: cost per oven is low, materials easily found.  Cons: Smoothing the top requires patience and skill and the whole mass takes a long time to dry.

Pre-bought refractory insulation: Mt. Savage Refractories Company makes a bagged version of the above. The cost is about 5x and the mix is rated to 2000 F.
Pros: Solid, reliable.  Cons: a bit of money.

Foamglass: Four inch thick panels are perfectly flat. Oven floor goes on top.
Pros: A lot of insulating value, can be cut with an old handsaw, and very light.  Cons: Fragile until installed and very expensive.

The first two materials can be used on top of the oven.  Refractory blanket (expensive too) can be laid over the finished dome. If building an exposed dome there needs to be an expansion space between the cast insulating layer and the shell or the heating of the interior bricks will crack the outside of the dome.

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