Historical Documents

I wonder at what point the US Constitution became a historical document, therefore earning the privilege of bullet-proof glass.

For how long did the US Senate huddle around the ungainly pieces of original parchment with magnifying glasses, double-checking the rules? Until 1778? 1780?

I just think it’s a funny image, a room full of bewigged gentlemen, licking their thumbs and turning the page on something that is now essentially beyond price.