Halogen lamps blackening?

Factual Questions
1

I know halogen bulbs don’t like being dimmed and can blacken if they do, but one of my halogen torchiere lamps the bulbs keep blackening, and I wind up replacing them before they’re burned out because they get so dim. There’s two dimmers in sequence, a (code legal) dimmer controlling the wall outlet, and a dimmer that came with the lamp, which is left in the on position. Probably 95% of the time the wall dimmer is on full, and the dimmer on the lamp is always on.

I’m wondering if halogen bulbs just blacken with use no matter what you do. (This lamp is on probably from 8:00 in the morning when I start work till 2:00 in the morning when my sister finally goes to bed), of if maybe I have a problem. The dimmer on the lamp doesn’t click in the full on setting, so I’m wondering if it’s still engaged and needs to be bypassed.

2

I would guess that the lamp is being run at 85-90% of full power, which will result in much lower filament temperatures, and the resulting envelope blackening. Try bypassing the dimmer(s) and see how much brighter it is. The Halogen Cycle was specifically designed to prevent envelope blackening, so there is clearly something wrong.

3

By code, a dimmer can’t control a receptacle. If you check the voltage, I would bet the voltage at the receptacle is below 120, causing the blackening of the bulb. Try replacing the dimmer with a standard switch or using a different outlet to see how the lamp holds up.