Blue T27

[Illuminatio]

I've just changed a popped T27. There's the usual shiny tungsten deposit around the middle of the glass envelope, plus some black flaring. At the ends, however, there is a bright blue deposit on the glass which I've not seen before (25 years).

 

Does anyone know what chemical reaction would cause that? Is the tungsten alloyed with cobalt? Why would it separate out?

 

(Probably shouldn't have been running the rig to dry a freshly painted floor...)

[Just Some Bloke]

(Probably shouldn't have been running the rig to dry a freshly painted floor...)

What paint did you use? Are you worried about the fumes interfering with the lamp?

 

Otherwise, I can't see a problem: people have been turning on the rig to dry a freshly painted floor for time immemorial and I'm sure they will for many years to come! :D

[Illuminatio]

No - the coating is on the inside of the envelope. The paint is just standard Bollom's black as made now by Flints.

 

I simply meant that if one uses the rig to dry paint you have to expect to change a bubble or two the next day!

[Ynot]

Don't know what causes it, but yes I have seen blown lamps with a pale blue coating on the inside of the bubble.

[adam2]

I believe that is is caused by a minute trace of air in the bulb, this reacts with the tungsten filament to form a blue/gray deposit.

 

Any substantial ingress of air will result in instant failure and a yellow/green deposit of tungsten oxide. The blue deposit is AFAIK from minute traces of air.