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!
Bill is excellent and highly recommended. Just rebuilt one of the acoustic couplers from one of my Vandy 2CE Sig’s. Very very reasonable and a great guy.
So, how does one know if their ferro fluid is no longer viable?
As the ferrofluid dries up, the speaker’s high end becomes dull and loses some sparkle. It may not be evident unless one compares it to a speaker with fresh ferrofluid. Typically ferrofluid lasts about 15 to 20 years with normal use. KEF recommends replacing ferrofluid after 15 years. If you go too long with dried up ferrofluid, the tweeter may be permanently damaged. I did the T33 tweeters in my KEF 104/2 speakers a few years ago and it made a significant difference. It was a fairly simple and straightforward process and cost about $15.
So, how does one know if their ferro fluid is no longer viable?
The ferro fluid starts getting thicker after 10-15 years, especially in a hot environments or if driven hard, it becomes non linear (distorts) and yes as ljgerens said  looses it's efficiency "sparkle".

Cheers George