> but lasting a thousand years isn't necessary for those properties
Yes, it is. Lasting for thousands of years is the same thing as (1) impermeability to microbes (mold / insects / etc...) plus (2) failure to react with local chemicals. Those two things are the things we want, and if you have them both, you last for thousands of years, because there's nothing to stop you from doing that.
Yes, it is. Lasting for thousands of years is the same thing as (1) impermeability to microbes (mold / insects / etc...) plus (2) failure to react with local chemicals. Those two things are the things we want, and if you have them both, you last for thousands of years, because there's nothing to stop you from doing that.