(free!) super duty firebrick for the dome?
I'm trying to build my oven this summer on a REALLY tight budget, and since I work at a university with a big ceramics and art program, this summer I can pick the best used firebrick I can find from a kiln rebuild. Now, this is super duty firebrick, 40-44% alumina, 49-53% silica (I'm putting together information on this stuff at my site). My reading indicates the higher alumina may tend to burn bottom in the hearth; but I'm wondering if there would be any problems using it in the dome?
if its the lightweight, almost pumice like, brick then it will not have sufficient mass to retain heat. you use the brick in the dome for heat storage and not as insulation. the correct brick to use should weigh about 7 pounds.
in my opinion, as long as you stay away from the heavy duty lightweight "foamy" firebricks, almost any other firebrick will work just fine.
one other point, if the firebricks have been used in a kiln you might want to get one of those lead testing kits from home depot and run a test on the bricks.
Yes, these are the heavy bricks, close to 7 pounds. They have a dark brown shiny coating; some variety of salt I think. I think I would have to cut them in half with a wet saw, and use the cut face on the interior. The ends and corners may be a bit ragged, but cutting down the middle should give me two nice faces.
These sound like real firebricks, not the insulating firebricks. One thing to think about is balance between the cooking floor and the dome. You can have a pizza oven that cooks between 900F and 1100F, as long as the dome and floor in tune. The top and bottom of the pizzas cook equally fast. In that sense, there really isn't such a thing as an oven that is too hot. You can have a floor or a dome that is too hot. I think that is where the burned pizza bottom concern comes from.
If your goal is to make a good oven from your "found" bricks, I think you are on the right track. An oven with imperfect basic materials, including either hot bricks or regular clay bricks, is vastly superior to no oven.
I think there is an opportunity for discussion around the material you would use for the floor, if you end up using the hot bricks for the dome.
As Jim once said, it isn't rocket science, and the ovens cook great -- warts and all.
As to the floor, the current plan is imperfect hot firebricks in the dome, and purchase low duty firebrick for the floor. If these are 'hotter' brick, maybe I'll just go with the higher dome. My main goal is for the dome bricks not to crack and spall, dropping stuff on my pizzas. If the super duty brick in the dome is most likely going to burn my tops, I'll just save my pennies and go with all low duty firebrick, which I've found locally for $1.20 ea. :-/
While it may be technically possible to later tear down a bad super duty dome, it would take a while to work up the gumption to do so - it looks like building the dome is going to be quite a challenge. I'm the careful sort who normally tries err on the side of caution (except when in conflict with finance).
They shouldn't crack or spall
You seem to have this well under control. I would add that the basic design is key. If you get the oven layout right, with the floor, dome arch and oven opening working in sync, you will be in good shape. I think that is more important than the specific brick.
My rectangular outdoor bread oven continues to frustrate, and I have finally decided to take it out. It is too bad, and something of a waste of time and energy, but summer is coming its time to get it right.
Have fun -- and send lots of photos.
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