In many HID lamps, the arc tube has to heat up to be able to boil the contents. In metal halide lamps, they have to be hot enough to boil the metal iodides inside, and in some spectral lamps they are just boiling the actual metal itself, like zinc or cadmium or something. My questions are the following:

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1) Quartz Tubes:
What is the highest temperature you can reasonably get with a quartz arc tube? When does it become likely to bulge? Google says that the softening temperature is about 1630°C, but can you safely get up to that temperature with high pressure inside? What methods do they use (other than refractive coating) to increase the temperature?

2) Ceramic Tubes:
How about alumina? Google says it gets melty at around 2000°C, but can that temperature even be reached with tungsten electrodes? I know tungsten and niobium melt way above that temperature, but I would assume the electrodes would be hotter than the actual arc tube.

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Thanks!

The average arc core temperature in a silica bodied halide lamp is in the order of 5700 degrees C , “Lamps and Lighting” 3rd addition 😁
Hope this helps in some way?

@AngryHorse
That definitely does matter, but what I am more looking for is the maximum operating temperature of the walls of the arc tube (the cold spot temperature).

It gives a little detail?

It is unfortunately not possible to get anywhere near the softening temperature.  The actual maximum is limited by chemical corrosion reactions.

For modern quartz mercury lamps with oxide cathodes the absolute maximum is 950C.  Beyond that, the barium component of the emitter coating penetrates the silica matrix leading to devitrification, with a resulting loss of strength and then explosion.

For quartz metal halide the practical upper limit is about 1100C, because there is no emitter coating.  But depending on the filling it may be lower. Eg green lamps containing thallium iodide are very susceptible to corrosion followed by explosion.  Same for thulium-containing lamps.

For ceramic metal halide with saturated vapour, it’s possible to get up to about 1300C before the rare earth halide salt pool eats its way through the entire wall thickness during the rated lifespan, and then causing leakage.  For sone unsaturated lamps it is possible to go slightly higher.

@James
Thanks so much! That answers a lot of questions, I didn't know metal halide electrodes weren't coated. I guess they are jsut thoriated or something.

@James: I didn't know that thallium iodide is very corrosive to the quartz arctube. I hope that the black layer at the bottom part of the arctube of my 70W thallium lamp, isn't a corrosion layer from the thallium iodide.