Why the fluorescent lamp turns to black in colour at each ends when it is fused?

The tube has wire filament heaters at each end like flashlight bulbs.The heaters glow when the light is switched on and ionise the gas inside.The ionised gas is subjected to a high voltage and a plasma ( like a lightning bolt) flows from end to end. The excited plasma causes the white powders on the inside of the glass to glow. When the filaments burn out they blacken the ends with metal oxide and it is then almost impossible to get the plasma started again without hot filaments glowing. 1-2000 volts would then be needed