Firebrick composition-what's good and bad?

[ihughes]

I've been looking at firebricks and have found a few options with differing alumina/silica content. The closest I've found so far has 33% alumina and 63% silica (with ferric oxide, titania and accessory oxides all about 1.5%).
Is this too much silica or won't it make much difference?
The suppliers class this as a high duty firebrick - their low duty has about 23% alumina/73% silica. Which is more important to have the higher content??

[maver]

The reason the plans advise against high duty brick is cost and the fact that some high duty bricks are insulating bricks - poor thermal mass. I don't know whether the chemical composition matters as much to this as the method of construction. The weight of the brick helps to assess this - heavier bricks are generally better. Can you obtain the low duty bricks from this supplier? That would probably be ideal.

[james]

It sounds like the bricks you have found are perfect. 30-odd% is just what you are looking for.

We only recommended against high duty firebricks because they are very high in alumina (I don't remember the exact figure, but I think it is something like 60%), and could get too hot when you have a big fire, and then give up heat too fast once the fire was out and you wanted to cook with retained heat. A few % +/- isn't a big deal.
Go for it.
James

[ihughes]

Same supplier can do low/medium/high bricks. Thks for the info. The manual labor side of the project doesn't worry me but some of the technical aspects can get me a bit confused.