How they make insulating firebricks
Here's a quiz. I was talking with the supplier for Super Isol about various "things refractory" and asked him how they make insulating firebricks. Low to medium duty firebricks are made from the same basic refractory materials, but the insulating firebricks are much lighter and much less dense. The air holes stop heat transfer and retention. In fact, I learned that the average insulating firebrick has about 38% alumina. I have always thought that they felt as though they were pumped full of air.
This is the question. How do they put the air holes in the brick? I did not know before, so I thought it might be fun to guess -- or perhaps our engineers already know.
Ok, I'll guess - beads of combustible material, maybe a wood product, like beaded sawdust pellets, along the lines of the pellets used in wood stoves but smaller. Mix with the 'clay' used for a regular firebrick in the desired proportion. Cure the brick and then fire it out.
When I was a kid, we did RAKU pottery in high school art class. We would fire the pottery with lead glazes, and unglazed portions, and pull it out of the kiln with tongs when it was red-hot, and throw it in a barrel of oil-soaked sawdust. The sawdust would burst into flames, and fill the drum with black smoke, which would bring out wierd colors in the glazes (since the usual oxide colors depend on oxygen in firing, depriving it of oxygen in mid firing produced unusual behavior, particularly in copper based colorants). It also produced dramatic crazing in the glaze because of the thermal shock, and the unglazed areas became a dramatic matte black becuase of carbonization.
It also created a giant fireball when the red hot pottery hit the sawdust. We'd clamp the lid on, and huge clouds of smoke would ensue. We lost some eyelashes, and had a great deal of fun, and even produced some decent looking pottery. I don't recall any liability lawyers were involved in the process. Those were different days. I understand that kids in art classes can't even use box knives now.
But to answer the question, I don't think firebricks are fired full of sawdust. I suspect they are aerated with some foamy material, but I don't know.
I think we can date ourselves by how much we got to do as kids. I come from the "many trips to the emergency room" generation.
This all makes the firebrick question sound pretty boring. It is sawdust, which burns out and leaves air holes. The top and bottom of the FB Calore fireplaces also have insulating refractroy top and bottom prices that have lots of tiny little air holes -- I will have to ask how they do it.
I think the Raku difference must be the heat shock of rapid temperature change that makes it somewhat unsafe (fun). But bring the sawdust up to temperature slowly and it should just escape as gas leaking through the brick.
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