High duty fire bricks

[Abear]

I know this has been discussed in the past regarding high duty vs. low duty but the cost arguement in my case is not an issue. I came across a gentleman who had a warehouse of palletized firebrick which was to be used to build an oven for Owens-Corning. The oven was never built and the bricks have been sitting there for years. He gave me enough bricks/slabs to create an oven - these bricks are KX-99 arch bricks (40% alumina) and BF Varnon slabs (9 x 18 x 4, 9 x 12 x 4.5). Dimensionally, it will be about .5" less than using standard fire bricks but I figure that's not an issue.

So it boils down to the heat issue. Some have mentioned it'll get to hot yet others have said it will keep heat longer for cooking. Rado at traditionaloven.com has said it requires just getting used to how your oven cooks. Am I wrong to assume that the bricks can attain a higher heat than low duty bricks but won't necessarily get that hot if you control your fire? It would be nice to have bricks which could retain heat for a longer period of time and minimize recharging while cooking. I appreciate any comments!

Abear

[Rastys]

Abear
I don't think that they will hold heat longer unless they are hotter and hold more heat to release (if you understand what I'm saying).
The bricks would certainly get hotter but it will be up to you to control your fire. My oven (using conventional low firebricks) heats up in about an hour and I allow it to soak by maintaining only a normal (less intense) fire rather than a fierce one, cooking pizzas within an hour and a half with a dome temp of over 500˚C and a hearth temp of around 430˚C, pizzas ready in around a minute.
If you were to maintain a fierce fire for a couple of hours, then your oven temp could get considerably higher which you would then need to cool which would also soak deeper through the bricks which in turn would hold more heat to release over a longer period.

Rastys

[Abear]

If you were to maintain a fierce fire for a couple of hours, then your oven temp could get considerably higher which you would then need to cool which would also soak deeper through the bricks which in turn would hold more heat to release over a longer period.

I agree with this logic. It stands to reason then if you have a higher duty brick with more alumina, the brick would stay hotter for a longer period of time extending the "peak" cooking time. Would the KX-99's be considered medium duty bricks with a 42% alumina content? I heard high duty had >60% alumina. The KX-99's are a bit more yellow than my firebrick slabs which are more on the tan/yellow side. Haven't been able to find out the composition of the slabs. I'm assuming since they were all together, they must share similar compositions.

[Rastys]

That's the trouble with some firebrick manufacturers. They can't or won't tell you what the composition is because they either don't know or or try to maintain a 'trade secret' attitude.
We have one over here who make 'fire bricks' but their content??? let alonr therir alumina content??? What's that?
They are basically made to be used in fireplaces rather than their conventional clay brick.
Look, you should be fine if you control your fires sensibly and if you can get them at the right price, then go for them.


Rastys

[Abear]

Thanks for the confidence boost! I plan on using the bricks and plan on giving updates of its cooking characteristics.

[jpar]

Abear,
THe KX99 bricks are actually Super Duty bricks that have been high fired. They have less porosity then standard superduty bricks. Varnon brick are Superduty brick. These bricks will be dense(140 - 150 pcf) and will take a little longer to heat up, but will hold heat longer. Make sure you have enough insulation over the dome and under the hearth.

[Abear]

I plan on using the KX-99's on the dome with the Vardon slabs as my hearth and sides. As for insulation, I'm using vermiculite in my 4" hearth slab I plan to pour and will be using 2" cladding and 6" vermiculite on top of dome and 4" vermiculite on sides. Should this be enough insulation? Do you guys know of anyone who has successfully built an oven using super duty bricks? I'm wondering if the porosity of these bricks will allow for enough moisture in the oven or should I plan on soaking the door, say, in water when I plan to bake bread?

[dmun]

Baking moisture in porous bricks is a myth. Once the bricks are up over water boiling temperature they are as dry as can be. Any injected steam in in the air, not the bricks

[berryst]

I used super duties in the floor with light duty on top. This was to much brick so I removed the light duty. Cladding in just not needed in pizza oven IMHO. It's good for bread. I used lots of cladding. Moisture is not an issue...just use a little sprayer to add moisture just before you cook

[Abear]

Great thread so far! If my dome bricks for the barrel oven are already arch shaped, do I need refrac mortar between bricks or can I lay them clean like the floor? I don't have any gaps between the bricks when I dry fit the arch using my template. Could I then just pour loose vermiculite over the bricks with no cladding layer over the dome bricks? I am hoping the super duty bricks would retain more heat thus making the cladding (mass) layer unnecessary -- unless it 's required for some structural purpose.

Can I use this same mentality on the hearth floor? I've read some people have switched the vermcrete layer on top of the supportcrete layer - contrary to the plans I got from Rado - stating it reduces the amount of additional mass which needs to be heated thus quicker heatup times. With these high duty bricks, I feel I've got the higher heat potential using the bricks themselves and wouldn't need the added cladding(mass) under the bricks to prolong heat. Which leads to my question: I've heard the vermcrete is an insulating layer not a support layer which crumbles when scratched. Will this be strong enough to lay FB floor/dome on or will weight compact the layer? The more I read, the more I second guess myself!