I had to change a brake light in my vehicle yesterday.  The filament had burned out.  A apparent carbon stain was on the inside of the glass bulb which for me begged the question: Where did the carbon come from?  The filament is made of elemental tungsten...no carbon compound in it to release carbon when it burns out.  There is usually some inert gas inside the bulb also...usually elemental argon, I believe.  Other than that there is just the glass stalks that hold the filiment in the void of the bulb.  Therefore, just where does the carbon come from that stains the inside of the glass when the filament burns out?

The only thing I can postulate is that if the filament were not pure tungsten, but something like tungsten carbide, that would be a source for the carbon stain.

So, what is the source for that apparent carbon stain?

I would assume the coating on the glass is the vaporised metal from the burnt out filament.

As Strange has implied...
What makes you think the 'stain' is carbon ?

( I recall doing vacuum deposition of thin metal films on glass substrates in 2nd year Uni )

3 hours ago, MigL said:

As Strange has implied...
What makes you think the 'stain' is carbon ?

( I recall doing vacuum deposition of thin metal films on glass substrates in 2nd year Uni )

I was assuming it was carbon from the fact that it was Black.  So, maybe not carbon at all, but vaporized tungsten.

15 hours ago, Bushranger said:

So, maybe not carbon at all, but vaporized tungsten.

It's a pretty continual vaporization process over the life of the bulb. If there was a LOT of tungsten residue, the bulb could have been an older style that used a vacuum inside the bulb instead of inert argon. Argon greatly reduces the loss of tungsten atoms to heat, but there's always a bit of evaporation going on. 

Quote

Wikipedia

An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated to such a high temperature that it glows with visible light (incandescence). The filament is protected from oxidation with a glass or fused quartz bulb that is filled with inert gas or a vacuum. In a halogen lamp, filament evaporation is slowed by a chemical process that redeposits metal onto the filament, thereby extending its life.

https://en.wikipedia.org/wiki/Incandescent_light_bulb

 

Note the halogen scavengers against filament evaporation.

Additionally in the bulb is a little bit iodine in, which reacts at the high temperature of the filament to WI₂ . This deposit on the filamnet again and get decompsed again, it enhance the life time of the filament. But some of the vapour will deposit also on the glass it self. 

Edited September 7, 20196 yr by chenbeier

7 hours ago, chenbeier said:

Additionally in the bulb is a little bit iodine in, which reacts at the high temperature of the filament to WI₂ . 

You have got that the wrong way round.

https://en.wikipedia.org/wiki/Halogen_lamp#Halogen_cycle

Yes you are right.