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I'm building a pizza oven and, to do so, I bought a whole load of engineering bricks (not these but similar). They've got five holes vertically through them, each hole is 2x6x6.5 cm (80 ml) and the walls, both end and dividing, are 2 cm thick by 6.5 cm high.

When the oven is fired up it will reach ~500 °C. If we use some good solid concrete as masonry for the bricks, which will seal the ends of the holes, I'm worried that as the air heats up, the pressure inside will turn them into a sort of ceramic bomb that will make cooking pizza a rather more exciting experience.

Is that likely to happen? How can I calculate the pressure that will be exerted on the bricks by 80 ml of air at 500 °C?

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    $\begingroup$ if there are water pockets in the bricks then the pressure will be much higher. $\endgroup$ – ratchet freak Jun 4 '15 at 12:56
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    $\begingroup$ Bricks are porous, some more than others. So is concrete. You won't be able to put your mouth on them and blow air through, but I'm not sure you can assume the air pockets to be fully contained. The effect of the heat on the brick seems like the main concern, even without holes through the brick. $\endgroup$ – Air Jun 5 '15 at 19:23
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    $\begingroup$ In case anyone else is coming to this now: six months later, the bricks are fine, but the 3 inch thick solid concrete paving slab that I used as the base of the oven has almost cracked through. Moral: gas law and explosions were more fun, but should've listened more closely to the porous / water bits. $\endgroup$ – Toby Abel Dec 7 '15 at 19:12
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The pressure rise is easy enough to calculate from the ideal gas law $$ PV=nRT. $$ In your case $V$, $n$, and $R$ are fixed so you can convert this to a simple ratio $$ P_2=P_1\frac{T_2}{T_1}. $$ Be careful here to use an absolute temperature scale (usually measured in Kelvin). From the numbers you give, the pressure will rise to about 2.6 atm assuming that the trapped air was initially at 1 atm.

In addition we can calculate the energy released by a gas if it were to expand by blowing out the walls without changing temperature. The work done in an isothermal process is given by $$ W_{A\rightarrow B}=-\int_{V_a}^{V_b}p\ dV = -nRT\ln\left(\frac{V_b}{V_a}\right), $$ where the last line assumes that it follows the ideal gas law in the process. If the initial volume is $V_a=80\ \text{mL}$ at 500 °C, then the final volume is given by $V_b=2.6*V_a=208\ \text{mL}$. Putting in the rest of the constants gives an energy release of $\sim 22\ \text{J}$ which is less energy than is released by a typical firecracker.

I make no guarantees of your safety, but I certainly don't think that this will cause your structure to explode. On top of the fact that the pressure isn't particularly high, heating to 500 °C is a relatively slow process and I suspect that the air will find its way out through cracks or holes before it hits the full pressure.

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    $\begingroup$ According to Wolfram Alpha 2.6atm is ~ the pressure exerted by a garden hose, and sealing the brick and sticking a garden hose in it wouldn't give me much cause for terror. Time to build! Thanks Chris! $\endgroup$ – Toby Abel Jun 3 '15 at 19:43
  • $\begingroup$ No problem. I also added a bit about the amount of energy released if the gas were to suddenly expand and it comes out to be significantly less than a typical firecracker. I think your bricks will hold up to it. Have fun building your oven. $\endgroup$ – Chris Mueller Jun 3 '15 at 19:48
  • $\begingroup$ @TobyAbel Due to the process of heat conduction it will also be more likely that the air would only reach temperatures halfway between ambient and 500°C, so the pressure increase will probably be lower. This will depend on how much isolation there will be on the inside and outside of the bricks. $\endgroup$ – fibonatic Jun 6 '15 at 14:40
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I'd be less worried about the shards and more about the structural integrity of the oven. When the bricks crack they lose most of their structural integrity. This may lead to the oven collapsing.

It's much safer to line the inside with firebricks. They are solid and more expensive but you can be sure they won't crack under the heat. They will also protect the wall behind them.

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The bricks themselves should be fine, at least for one cycle, IF they are heated reasonably slowly. Plenty of people have put bricks in barbecues and bonfires. Remember they were already fired in a kiln when they were made, to fuse the clay particles together so that they wouldn't turn into mud as soon as it rained. The only exception is if the bricks are wet. In this case the water inside will turn to steam. Pressure will build up and they will fracture and crumble.

You would be better off with proper firebricks. These, in addition to being designed for high temperature, have a higher air content / lower density which improves their insulating properties.

Firebricks are best stacked loose, or with the proper cement. An individual brick will not suffer much differential thermal expansion, but a large structure almost certainly will, so I'd expect your cement / mortar to crack pretty quickly.

In summary, I agree with ratchetfreak. Engineering bricks for structure, firebricks for high temperatures and to insulate and protect the structure.

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Engineering bricks (solid) are fired at 1200 degrees in a kiln so don't worry about your complicated mathematics , your pizza oven will never get to that temp.if it did or even if it got much much higher the bricks would just melt together.(They call them clinkers) I am a bricklayer with 47 years experience and a lot of that time was building and repairing kilns. When you repair a kiln you only use normal clay for the crowns (internal)as once the kiln is fired up the clay turns hard just like the bricks you are cooking. You only use fire bricks for the fire holes and they are cemented together with texacrete.

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