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When consulting manuals, electricians, online sources, etc., they always instruct you to handle halogen light bulbs with gloves.

The "explanation" that usually accompanies this statement is that oils and salts from a bare hand can "react" with the surrounding glass/quartz, owing to the fact that halogen bulbs are much hotter than ordinary ones. These reactions would cause weak spots in the quartz (or in some versions of the story, the filament), decreasing the lifetime of the bulb.

Is there any merit to this explanation? What are then the precise mechanisms involved in the bulb's degradation when the bulb is touched by a dirty monkey finger?

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It is most likely not a chemical reaction, quartz is quite inert even at higher temperatures but dirt and burned substances can cause thermal gradients and stress in the glass. – Alexander Aug 13 '12 at 13:13
That seems plausible, although I'd love to see some numbers backing this up – Rody Oldenhuis Aug 13 '12 at 13:31
I've never seen them say "react", generally, the explanation is that the extra oil will interfere with the cooling of the lamp, plus it'll burn etc (and might thus affect the lm output of the bulb). – cnst Feb 20 '14 at 3:42

Any oil from your fingers that resides on the outside of the halogen lamp will absorb some of the heat and light from the filament in operation, and cause the fingerprint area to be hotter than other parts of the bulb.

All it takes sometimes is a small differential in temperature for anything made of glass or quartz to develop a weak spot from the additional thermal / mechanical stress.

Wearing gloves or handling the bulb with something that won't leave fingerprints (I usually use a small piece of paper towel) is a very good idea.

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The interesting point in halogen bulbs is that they managge to not, over time, cover the inside of the bulb glass with condensed vapour of the filament (1).
Therefore, the bulb can be smaller, as it's not needed to spread that metal over a large area.

Now, when touching the bulb, a fingerprint is left, which contains various carbon compounds. When switching on the bulb, it carbonizes, leaving slight dark residues (2). Even if they are barely visible, they cover a significant fraction of the surface area, and make that area somewhat less translucent. That causes a reduction of light output over the livetime of the bulb,
adding up to a significant loss in overall efficiency of the halogen bulb.

On the alternative explanations:
The idea that somehow temperature differences are created, and cause a problem by weakening mechanical stability makes no sense - the kind of glass a halogen bulb is made of, quarz glas, is very stable under heat differences, as it changed it's volume only very little, compared to other glass.

(1) Instead, that vapour reacts with the halogen, to a compound that will subsequently split again into metal and halogen on the surface of the filament.

(2) I assume these carbon residues are long-lasting or permanent because the bulb surface is not hot enough to just burn them off.

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So you're saying that it doesn't reduce the lifetime of the bulb, just reduce the net light output? And not by much too, because normally you'd only handle the bulb only once, to put it in... – Rody Oldenhuis Jun 19 '14 at 6:06
Yes, that's what I expect. (The "once" is not relevant, though - it's the amount of fingerprint material when switching on that counts. Before that moment, wiping helps; after, it sticks to the glass pretty strong I think) – Volker Siegel Jun 19 '14 at 6:33

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