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Stephen Davey, associate editor for Nature Chemistry, blogged at the Sceptical Chymist about visiting the National Archives and seeing the Declaration of Independence, the Constitution and the Bill of Rights. He was surprised to find that the documents were stored under helium as opposed to argon - and wondered why. That started me wondering as well, particularly since the inert gases are not interchangeable in all circumstances (you can use helium to dilute the air mixture for diving, but not argon, for example.)
Helium is both more expensive (not an issue in this context, the cost of the gas inside the cases has got to be the least expensive piece!) and difficult to work with than argon. It can leak out through materials that seem air and water "tight". That's why those latex balloons that looked so cheery on the day of the party are withered and droopy by the morning. They're waterproof, but not helium proof.
In the 1950s the US National Bureau of Standards (now NIST) was charged with deciding on the best way to preserve the Charters of Freedom (the three founding documents of the United States of America). (You can read the full report here.) Helium was chosen, despite its propensity to leak through many materials, partly because a high purity, local source was readily available but most because of its thermal conductivity.
The designers of the encasements wanted a way to measure the pressure of the helium within the cases without having to open them, or remove a sample. Since the thermal conductivity of helium is very different than that of air, changes in the thermal conductivity (how heat moves between the panes) could be used to detect leaks. Argon's thermal conductivity is similar to air, so if argon leaked out and air in, the change would be hard to detect.
New casements were designed about ten years ago, with argon as the gas of choice this time. Sapphire ports are embedded to allow the atmosphere inside the cases to be monitored spectroscopically - by passing a beam of light through the port. Since the new methods of monitoring don't require the inert atmosphere to have a different thermal conductivity, it allows argon - which can't wiggle its way out the way helium can - to be used.
The photo is from The Science News-Letter, vol. 62 (Dec. 6 1952), p. 359.
Fascinating! I've visited those documents but never thought much about the chemistry of preservation. Why is argon a better choice than helium? Is it just less reactive in general?
ReplyDeleteArgon is probably just as unreactive in this situation as helium, but its size makes it much less likely to leak out. Helium is notorious for leaking through many materials.
ReplyDeleteAh, I was trying to figure out why either would be reactive anywhere. I love it - this will make a great example in a gas unit if I ever get a job.
ReplyDeleteThe high thermal conductivity of helium also makes it a favorable coolant in nuclear reactors.
ReplyDeleteAccording to this press release, it's argon, not helium:
ReplyDeletehttp://www.archives.gov/press/press-kits/charters.html#pressrelease1
Anon...yes, I noted that argon is now used in the last paragraph of this post: "New casements were designed about ten years ago, with argon as the gas of choice this time Sapphire ports are embedded to allow the atmosphere inside the cases to be monitored spectroscopically - by passing a beam of light through the port. Since the new methods of monitoring don't require the inert atmosphere to have a different thermal conductivity, it allows argon - which can't wiggle its way out the way helium can - to be used."
ReplyDelete