Dried up Ferrofluid?

What causes ferrofluid to dry up?  Heat, usage, age?  Is there a usual shelf life, as in, do most 15-20 year old tweeters that originally used ferrofluid, need new ferrofluid?

How is it checked, where is it bought, is the replacement generally the same from tweeter to tweeter?

Thanks ahead of time for any info regarding this!  I figure that there are quite a few good speakers out there that are getting up there in age so this may be something I’ll want to learn to do!