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I saw many kiln designs that lack an opening for hot\pressurized air to come out , or any pressure valve.

But when air is heated to such a high temperature (1300C) inside a closed chamber (the kiln), won't the pressure inside increase enough to explode the kiln ? (These kilns usually operate for several hours under this high temperature)

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    $\begingroup$ How's that a chemistry question? Sounds like physics to me. Now to the point: maybe it would, maybe it wouldn't. Also, consider the possibility that these kilns might not be hermetically sealed. $\endgroup$ – Ivan Neretin May 13 '18 at 13:11
  • $\begingroup$ @IvanNeretin I wanted to make sure i am not missing any chemical reactions inside the air when it reaches high temperatures, this is why i thought it would be good to ask it here, i can remove it if it's not relevant. and about the hermetically sealed, i didn't think tiny gaps between the kiln door or lid would be sufficient to release pressure from the kiln, and these doors are usually heavy $\endgroup$ – soundslikefiziks May 13 '18 at 13:36
  • $\begingroup$ The tiny gaps might pretty well be sufficient. After all, a kiln does not get hot instantly. As for the reactions, why, there surely may be some, but why would that be important? $\endgroup$ – Ivan Neretin May 13 '18 at 13:49
  • $\begingroup$ @IvanNeretin I don't know , i thought about bulling a tiny kiln for ceramics, and i didn't want to explode in the process ;-) , So you are saying the reactions would be negligible ? $\endgroup$ – soundslikefiziks May 13 '18 at 14:29
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I think in general the physics would make it unlikely that the air pressure would build up faster than it leaks out, attempting to seal a furnace is unlikely to happen by accident.

A bomb calorimeter or pressure vessel made to specifically seal the inside from outside does not have a loose fitting lid over a cast refractory edge, it is made to much finer tolerances.

Chemical reactions are not likely unless you have a significant mount of vaporisable fuel in the charge and this is contained until at some point it lifts the lid and lets vapour out and air in with glowing elements to ignite the mixture, a flame may result, explosion still unlikely as volume small and not contained any longer.

In practice most kilns have a hole for temperature sensor that is not airtight and would let pressure equalise as they are heated.

Unless a specific atmosphere (inert, reducing, oxidising, carburising) is required in a kiln there is no real need to make them air tight and they rarely are, even then it is often achieved within a protective wrapping of the load and supporting chemistry inside the wrapping like charcoal, cyanides etc).

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