[Cob] Fire Bricks @ wikipedia

[Jon Kerr]

Hi Gang!

Does everybody know about Wikipedia? Free online encyclopedia,  
http://wikipedia.org.

My personal experience with firebrick is that there are numerous types  
( I bought mine from Smith-Sharpe Firebrick supply in minneapolis, and  
they had several choices/prices depending on how hot a temperature you  
wanted them to withstand).

Here is the wikipedia entry, in its entirety:

Fire brick or refractory brick is a block of ceramic material used in  
lining furnaces and kilns . A refractory brick is built primarily to  
withstand temperature. This does not usually accompany resistance to  
heat flow; in fact, most refractory bricks usually have the highest  
thermal conductivities. This is logical, as thermally-related fractures  
are caused by tensions within the mass of the material. In turn, these  
fractures are caused by different degrees of dilation within the mass;  
these varying degrees are caused by different degrees of heat being  
absorbed by thermally non-conductive materials. Using a thermally  
conductive material negates the whole causality chain presented here by  
allowing the material to absorb heat uniformly, thus dilating uniformly  
and avoiding the internal tensions which lead to fracture. It is  
important for refractory brick to have a high resistance to erosion by  
ash-laden gases and to the fluxing action of molten slag ; it should  
not spall badly under rapid temperature change, and its structural  
strength should hold up well under rapid temperature changes.  
Fire-brick is baked in the kiln until it is partly vitrified , and for  
special purposes may also be glazed. Fire-bricks usually contain 30-40%  
aluminium oxide or alumina and 50% silicon dioxide or silica. For  
bricks of extreme refractory character, the aluminium oxide content can  
be as high as 50-80% (with correspondingly less silica), and silicon  
carbide may also be present. The standard size of fire-brick is 9 x 4.5  
x 2.5 in. (228mm x 115mm x 64mm)

So logically, as they gain and lose heat rapidly, they wouldn't be so  
great for thermal mass per se. I rather more understood their use to be  
to have direct contact with the fire without breaking/exploding etc,  
which is borne out in the above. Regular bricks do okay, but will  
break/wear out sooner. When building a heating stove, you'd another  
material (cob) for thermal mass; the firebricks would pass the heat  
through into the thermal mass. Kiko Denzer's book describes this  
process, which is why the dome of the oven is clay/sand mix, not a  
catenary arch or whatever of fire bricks.

Jon

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