After long periods of use, a grey spot develops on the inside of a bulb. This is as

dust inside the bulb condenses on the top.At high operating temperatures, tungsten evaporates from the filament & gradually forms a gray film on the inside of the bulb. Eventually, however, a thin spot develops in the filament. The inert gas atoms collide with the evaporating tungsten, causing some of the tungsten atoms to return to the filament. When this happens, the filament will break & the bulb fails.