Dried up Ferrofluid?

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

Hi b_limo!
Ferrofluid typically has a very long life span, but some of the older models of speakers are experiencing drying of the ferrofluid because the older formulations do not have the same long term properties of the new generations.  That being said, excess heat, long duty cycles at high voltage and dust and grime from decades of use will cause ferrofluid to gel and solidify.
There is hope!  The fluid can be replaced and it is not really that difficult to do.  Please contact me at [email protected] and I can walk you through the process.

Dumb question, do all tweeters have ferrofluid?

Dumb question, do all tweeters have ferrofluid?

No, you'll have to ask the manufacture if your does, it served two purposes, a way of getting heat away from the voice coil via the ferro fluid to the magnet which acts as a heatsink, also it served to damp"oil can" resonance at hf especially in metal dome tweeters.

Cheers George

Thanks George, I believe Thiel does in fact have ferrofluid per Rob at Coherent Sound .