Argon gas and some humidity to keep the document from drying out.
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They are kept in the inert gas argon with a slight humidity to keep the documents flexible.
The Declaration is actually kept in argon gas, not helium, with a little humidity to keep the document from becoming brittle. Argon gas is an inert gas and will not react chemically with any other chemicals in the ink or parchment of the document itself to harm it.
I'm guessing it was Mustard Gas. Later Phosgene was used. The Biological warfare in WW1 was relatively primitive, but nonetheless terrifying for all that, especially if all you had was a handkerchief to cover your face.......
Advantages of tear gas would have to be , it splits up large riot groups. Used by police.
Mustard Gas