[chris (OP)]

Why are halogen bulbs called halogens? What halogen is used, how and where?

Also, why are you told not to touch the glass of a halogen bulb?

Chris

[lightarrow]

Without looking up in the net (so I can make some mistake) I remember this: they put a halogen gas, usually iodine, in the bulb, instead of nitrogen or an inert gas. This allows the tungsten filament to reach greater temperatures because the metal vapour combines with the halogen gas, forming a gaseous halogenide (halide) which, when comes in contact with the metal filament (molecules bounce back and forth in every directions) decomposes because of the high temperature, releasing the tungsten again onto the filament; the filament's life is then increased a lot: at those temperatures it would vaporise quite quickly, without that expedient.

The bulb is made of pure silica quartz, essentially because this material can sustain greater temperature variations. If you touch it with hands, you release onto its surface a thin layer of hydrolipidic film, which absorbs radiation coming from the inside, even UV radiation because quartz (differently from common glass) allows its passage. This creates higher temperature dishomogeneities in the bulb which could cause it to be damaged.

[FuzzyUK]

The bulb is made of pure silica quartz, essentially because this material can sustain greater temperature variations. If you touch it with hands, you release on its surface a thin layer of hydrolipidic film, which absorbs radiation coming from the inside, even UV radiation because quartz (differently from common glass) allows its passage. This creates higher temperature dishomogeneities in the bulb which could cause its damage.

Sounds about right. Not many sites reveal the real reasons in any detail but there is a similar piece of advice given by Teklight on their FAQ page:
 http://www.teklight.com/faq_s.html

Why shouldn't I touch my halogen bulb with my fingers?

Do not touch the surface of a quartz halogen bulb with bare fingers, as the quartz surface will be exposed to the salts and oils in the hand. This will cause a weak area and lead to premature lamp failure.  If you happen to contact the bulb with your fingers, carefully clean the quartz surface with a gentle solvent such as alcohol.

[Bored chemist]

Part of the problem with fingerprints is that they promote devitrification of the fused quartz. This weakens it and reduces it's resistance to temperature changes.

[Mazurka]

Whoops, so dropping a 35w gu5.3, letting it roll arond on the floor before picking it up and popping it back in might not do a halogen bulb much good?

Oh well best repalce them with 4w LED's as quick as possible...