High duty fire bricks

[peterbennett]

Re: High duty fire bricks

High Duty, when the alumina content is high it makes the bricks more resistant to very high temperatures and erosion/hot corrosion. The reason is that the other constituent in them is silica. On the net you can find the alumino-silicate phase diagram, essentially at the high alumina end the brick's melting point is extremely high 1828C, whereas once the silica content is above 30% this drop down to a mere 1587C (and stays there unless fluxed lower by other contaminating oxides, e.g. calcia).

So that is the duty side of things clarified, a non-starter for cooking ovens, but what about when in direct contact with the fire, what is the max. temp?


Heat retention is a completely different matter, here we are talking about thermal mass and thermal conductivity. If you want the brick to store a lot of heat you want it to have a high thermal mass (and not lose it by conducting it away to cooler regions), in simple physics terms you want it to have a high specific heat capacity, i.e. the heat/energy required to raise a unit mass, Kg by 1 degree, now here is where densification comes in. If the product is quite dense it will have a higher thermal mass per unit volume than a less dense version. Now replacing alumina by silica or vice versa will [B]not[B] necessarily improve or reduce the thermal mass, it depends on their relative specific heat capacities (& how dense the brick is made when you consider this on a volumetric rather than a weight basis, as noted above). My guess is that their specific heat capacities are probably quite similar, so probably a non-issue?

From tables, an engineering brick has a specific heat capacity of about 1.0 kJ/KgK, whereas a firebrick has a specific heat capacity of about 1.05 kJ/KgK.

So unless you plan to melt steel the high duty firebrick could be a bit of an overkill, or is it?

I have updated another post on this type of topic with info on typical thermal conductivities of firebricks, engineering bricks and a couple of red /buff common bricks (not concrete common bricks). Not an issue if you choose the right brick.

I hope this is helpful and helps clears up the various issues involved (but I don't know the max duty temps.)

[gsp]

Re: High duty fire bricks

Looking for a good source for high duty firebrick in WA. Any leads would help me get my oven built!

[altamont]

Re: High duty fire bricks

Hate to admit it, but I spent almost 15 years with AP Green. As the USA stopped manufacturing this and making that, demand dropped. And fast. The remainders of APG and Harbison-Walker and Kaiser basically ended up consolidating into one. And that one is much smaller now than any one of the originals were. And I personally have concerns about the health aspects too - I crawled in and out of thousands of furnaces over those years.

Back to the original train of thoughts in this thread though:

For our WFO's we all will see just a little difference in the heat retention (and rate of heat-up) between low heat duty, medium heat duty and super duty bricks.

If the information is available, compare the density (weight per unit volume). For our purposes we can assume a greater density will store more heat. On the other hand that means a little more fuel being burned to supply those calories. Then, to get even more rententive, we could start considering the thermal conductivity - a VERY rough generality is the denser material will conduct heat a little bit faster which could imply a slightly higher heat loss.
Concentrate on following the great plans provided here, and INSULATE - bottom, sides and top - any place a firebrick is located: insulate. If in doubt and can afford it, add more insulation.

[Johnny the oven man]

Re: High duty fire bricks

If they "claimed" no asbestos , your widow will be able to sue them.lol

[fornax hominus]

Re: High duty fire bricks

Oh! Asbestos! ... great! I bought and am still using on my kiln a stash of AP Green's EPG50 insulating cement .. grey , fluffy and they claimed no asbestos... they claimed.. sigh!! .. Anybody heard a discouraging word on this?
tim

[dmun]

Re: High duty fire bricks

There was a big consolidation of the refractory industry a few years back, I think partly because of asbestos liabilities. I think AP green was absorbed into the Harbison-Walker refractory conglomerate. Now HW is about the only game in town in the US.

[fornax hominus]

Re: High duty fire bricks

I think Varnon are made by AP Green?

I may be wrong [what? Again?] but I think AP green is no longer with us.
tim

[fornax hominus]

Re: High duty fire bricks

I built my oven with a combination of med and hi-duty bricks .. all salvaged and some vintage from steamship boilers.. I did come across some ''chief'' #1 arch bricks which I used on row 6 , they are the last bricks to lose their soot as they have more mass the further they go into the dome , and I mortared them in . After parging with mortar I put a layer of extreme heavy duty aluminum foil [shiny side in ] over the whole oven , before the ceramic fiber layer. I also have a floor of 3x5x7in. hi dutys and they do hold their heat!!
One aspect that hasn't been mentioned so far is that hi-dutys weigh more! so they have to take longer to heat up , and longer to cool .
I am tossing a handful of snow in these days to add moisture for the bread.. it vapourizes fast!!

[Johnny the oven man]

Re: High duty fire bricks

I think Varnon are made by AP Green?

[gpappas]

Re: High duty fire bricks

Where can I get BF Varnon Slabs

[Neil2]

Re: High duty fire bricks

I suspect, that for pizza and bread ovens, there is no practical difference in heat up time or heat retention between "high duty" and "low duty" firebricks.

[Abear]

Re: High duty fire bricks

I've finally finished our oven with medium duty bricks (40% alumina content) and I can honestly say I'm quite pleased with the bricks. It takes about 1.5-2.0 hours to get about 850-900 degrees on the dome and 650-700 on the hearth. After a round of pizzas, we've done oven-dried tomatoes and brisket overnight with fantastic results! At 12PM the next day, the walls are still reading 250 degrees. I wouldn't be worried about using medium duty bricks - if they're cheap, use them!

[Johnny the oven man]

Re: High duty fire bricks

Low duty refractory bricks are usually in the 18-26% AL203 grade with med. duty about 26-45, and superduty 45% and up. Density usually increases as alumina content increases. Most firebricks are dry pressed, but you can get extruded/ wirecut poor quality product. We sell 40% grade f/bricks to the general public for their WFO's.

[M.J.FULLER]

Re: High duty fire bricks

I purchased several pallets of high duty fire bricks cheap. I have decided to use them on the hearth only. My reasoning is that they will tend to get hot and stay hot- this would seem to be an advantage. How many people have posted on this forum complaining that the hearth is not hot enough to cook their pizza bases? (possibly a technique peoblem).
I also remember seeing in Alan Scotts book a comment that high duty bricks were good for the hearth (I think). I also saw a post on another forum about 5 years ago from Canada. This oven builder owned a foundry and built his entire oven from high duty fire bricks. He said he had no problems.

I am in the middle of the build. I will let you know how I go.

[Abear]

Re: High duty fire bricks

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!