Why does surface of the incandescent light bulbs become grey over time?

The grey coating on the inner surfaces of incandescent bulbs is due to gradual evaporation of tungsten from the filament while the light is on. This evaporation eventually makes the filament thin and burns out.  Various methods have been developed to prevent this.

Filaments of the first incandescent lamps burnt in vacuum, but it was soon found that introducing inert gas to the bulb reduced the rate of graying. A mixture of nitrogen and argon is used today.

In addition reactive metals such as tantalum and titanium, can be placed near the filament to attract the tungsten so that it is not deposited on the glass. Alternatively, a small amount of abrasive tungsten powder can be placed in the bulb. Shaking the bulb occasionally will remove the gray coating from the surface of the glass.

Graying can be almost eliminated by introducing a small amount of the halogens iodine and bromine. As tungsten evaporates from the filament, it reacts with the halogens which then re deposit the tungsten on the filament. This keeps the bulb wall clean.

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