Strange car bulb



Removed a bad bulb the other day. I have seen burnt bulbs before but this bulb didn’t seem burnt. Inside the bulb was this strange gray material at the base of the filaments (on top of the glass base where the filaments are attached). I was wondering if any one knew what that gray material was. Can the inside of a bulb get corroded?


It sounds normal. When a standard lamps is on the filament heats up and some of the atoms fly off. They often land on the cooler glass part of the lamp and condense there leaving a thin deposit. The deposit may look gray or silver.


It’s tungsten and some impurities.

Bulbs create light by heating a tungsten filament until it glows in an “inert”" gas, a gas that does not provide the oxygen for the tungsten to oxidize. The heat, however, cause the filament to burn out any impurities amd also outgas some of the tungsten itself, changing the filament structure from fibrous to chrystalline. Everything burned out of the filament deposits on surrounding surfaces. Variations in the actual composition of the tungsten (it isn’t pure) as well as the inert gas, cause variations in the deposition color. Variations in the physical structures as welll as temperature variations cause variations in where the deposition occurs.

The inside of a bulb cannot corrode. It’s basically silicone dioxide, glass, and will not undergo chemical changes. Quartz bulbs, as in “fused quartz” headlights, are only a more pure form of silicone dioxide. Fewer impurities makes the glass more tolerant of heat, allowing the filament to be heated to a higher temperature and put out more light. .