I can't find a good answer to this on the internet no matter where I look. One I am trying to find out most acutely is the Treaty of Fontainebleu 1814, but any treaties of that era will do if no-one knows the answer for that.

For the sake of discussion, some follow up questions you may want to answer:

How well do treaties of that material last?

How important is the preservation of the original document in international law?

and

Is the ink an important consideration when considering material and conservation?