Protecting paper documents from fire is quite straightforward with, for example, a Sentry fire chest. This product has been tested hold the inside temperature under 177°C (350F) for 1 hour. This is fine for documents, since they will not combust or deform. However, I'm looking for an inexpensive solution for items that would take considerable damage at these temperatures.

Sure, I could house the fire chest inside another larger safe. Or build myself some kind of drywall, brick and mortar box to house the fire chest in... but I'm looking for something smaller.

My idea is, if I stored my valuables inside a stainless steel Thermos, could this offer sufficient additional insulation if put inside the fire chest? The plastics of the Thermos might deform at some point, but could I get enough insulation? A Thermos works both ways, but how well...?

  • 2
    $\begingroup$ Given that 176°C for 1 hour is not adequate, what is your criteria? $\endgroup$ Nov 14, 2018 at 12:34
  • $\begingroup$ When it comes to combustion remember three thing, 1- Time, 2-Temperature, 3- Oxygen, clearly you can control oxygen, if the temperature rises above 233°C or 451 F (pun not intended), the papers will decompose to its components in absence of oxygen. $\endgroup$
    – user14407
    Nov 14, 2018 at 12:51
  • $\begingroup$ @ratchetfreak The fire chest is listed at a "Class 350 Rating", I would like to be close to the "Class 125 Rating", under 125°F/52°C for the hour $\endgroup$
    – Figaro
    Nov 14, 2018 at 12:55
  • $\begingroup$ Would this need to be portable, or presentable? Otherwise a small stone chest built from refractory material as used in ovens would be sufficient, if coupled with water on the inside. $\endgroup$
    – bukwyrm
    Nov 14, 2018 at 13:15
  • $\begingroup$ @bukwyrm Doesn't have to be presentable just reasonably small $\endgroup$
    – Figaro
    Nov 14, 2018 at 13:27

1 Answer 1


Radiative heat transfer will be quite high in a house fire, so not every construction made for 100°C temperature differences will be applicable here- look for constructions that have glowing-metal temperatures to contain (ceramics ovens...), and reverse the design to work for heat applied from the outside. Also put something with high heat capacity (water is ideal) into the container to soak up the power that will inevitably make it through - you might even try a two-shell design (small thermos in fire chest?) that has the water after the first shell, so you can gain from the fluid-to-gas energy consumption of water at boiling point (remember to leave opening for steam, otherwise you built a bomb) while at the same time nailing the maximum temperature around the thermos to 100°C for as long as there is fluid water, thereby bringing the amx temperature down to what the thermos was designed for.

If you add material beside the valuables that has a melting point slightly lower than your maximum of 52°C you can soak up more power before the temperature rises further - phase shifts usually take a lot of energy, so it might be worth a shot. Phasechange to gas would be even better, but the materials with boiling points below 52°C are probably not safe to release in a fire, and retained the pressure would rise, raising the boiling point and producing structural issues.

Also remember that a house fires can trigger other violent events like falling debris, so the container should be quite sturdy to begin with, and the door-seals need thought.

  • $\begingroup$ Falling debris is a valid point, this is why I would house the Thermos inside the commercial fire chest. It has been drop tested from 9 meters after it has suffered a fire. The fire chest is made from a drywall/Gypsum, which contains water that evaporates to protect the contents of the chest. But yeah, how well can a Thermos isolate when the inside of the chest is heated up? Nothing evaporating about a Thermos... Also, since the fire chest if water-proof (and I want to keep it that way in a fire), whouldn't putting something evaporating inside it compromise this seal due to building pressure? $\endgroup$
    – Figaro
    Nov 14, 2018 at 13:06

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