Why do arc tubes or fluorescent tubes blacken at the ends?

The cathodes are coated with an emission material; usually thoriated tungsten. Each time lamp is started, the cathodes operate momentarily at a much higher temperature than when the lamp is running. Hence, the higher temperature sputters off this material and it collects on the phosphor coating causing it to "blacken" at the ends. This is why lamp that are operated at a short burning e.g an hour at a time will fail much sooner than a lamp operated 24/7.

The white stuff, I belive, is the emmision material...I've also heard that Barium is used, at least for merc lamps. 

Fluorescent lights do have an emmisive substance on the electrodes this is to reduce the starting voltage to a low enough level that the ballast can ignite it when this substance is worn off ie when you turn it on and off rapidly it wears off the emmisive material which causes the ends to glow orange if it is running on a starter if it is running on a rapid start the lamp will go out if it is a cheap electronic ballast with a very high open circuit voltage which causes the electrodes to run at a much higher temprature than normal which either evaporates the tungsten to the point where it breaks then the ballast is able to set up a field around that electrode lead which causes the lamp to light despite broken/no electrodes the arc melts the electrode supports down to the glass where that cracks and lets air into the tube which blows away the phosphor

Is that what happens when a slimline is EOL? When the ends glow orange and get really hot, it is acually melting the cathode supports down to the glass? And then it goes dead(vacuum loss)? I had F6T5s do this in my lantern.