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Author Topic: Why is it bad to touch the glass on a light bulb?  (Read 5190 times)

Offline Geezer

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While mucking about with some low voltage lamps, the instructions reminded me not to put my great big greasy fingers on the glass envelope of the light bulbs. I suspect there was an overabundance of caution here as these were 4 watt tungsten filament bulbs, but I know it does do a number on halogen bulbs, presumably because they get so hot.

What is the process that banjaxes the glass? I'm assuming it it's a chemical process, but maybe not?


 

Offline SeanB

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Re: Why is it bad to touch the glass on a light bulb?
« Reply #1 on: 01/07/2012 17:50:10 »
The quartz glass will decompose the oils and salts in the fingerprints when they get hot. This then chemically reacts with the glass and weakens it, and the decomposition products absorb heat from the light and heat up further creating further damage until the high pressure inside causes it to burst.
 

Offline Geezer

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Re: Why is it bad to touch the glass on a light bulb?
« Reply #2 on: 01/07/2012 18:16:42 »
The quartz glass will decompose the oils and salts in the fingerprints when they get hot. This then chemically reacts with the glass and weakens it, and the decomposition products absorb heat from the light and heat up further creating further damage until the high pressure inside causes it to burst.

Isn't quartz pretty inert stuff? Does it actually combine chemically with oil?
 

Offline SeanB

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Re: Why is it bad to touch the glass on a light bulb?
« Reply #3 on: 01/07/2012 18:41:53 »
Yes, at high temperatures it reacts with the chlorides in sweat, as it is still an ionic solid, and will react slowly. The oils decompose into carbon and add heat and a reactive substrate to increase the reaction. As well the inside has a high pressure from the gas fill, and when the stress gets high enough.....
 

Offline Geezer

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Re: Why is it bad to touch the glass on a light bulb?
« Reply #4 on: 01/07/2012 19:02:37 »
Yes, at high temperatures it reacts with the chlorides in sweat, as it is still an ionic solid, and will react slowly. The oils decompose into carbon and add heat and a reactive substrate to increase the reaction. As well the inside has a high pressure from the gas fill, and when the stress gets high enough.....

Thanks Sean!
 

Offline RD

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Re: Why is it bad to touch the glass on a light bulb?
« Reply #5 on: 01/07/2012 19:04:39 »
My money would be a physical reason rather than a chemical reason, the residue creates a temperature difference in the glass envelope which causes a local increase in stress : increased chance it will pop.

Another physical theory is the residue reflects heat back into the bulb, causing premature element failure through overheating ...  http://boards.straightdope.com/sdmb/archive/index.php/t-351998.html 
« Last Edit: 01/07/2012 19:14:03 by RD »
 

Offline Geezer

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Re: Why is it bad to touch the glass on a light bulb?
« Reply #6 on: 01/07/2012 19:12:45 »
Uh-oh!

Maybe we need to do a 'speriment. All we need is for somebody to fork over a few quid for some halogen bulbs.

Would they fail if they were contaminated with a drop of highly refined mineral oil?
 

Offline SeanB

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Re: Why is it bad to touch the glass on a light bulb?
« Reply #7 on: 01/07/2012 19:21:03 »
Been there and done that with cheap chinese lamps. All in one batch failed in under 50 hours, probably because they were made with soda glass ( they melted and blew bubbles and went very black inside) while the ones that I had accidentally touched during install ( even after cleaning with a cloth) all had marks on them where they were held. I now use a clean fresh paper towel to hold the lamp taking it out of the package and use it to place it in the holder.
 

Offline Geezer

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Re: Why is it bad to touch the glass on a light bulb?
« Reply #8 on: 01/07/2012 19:27:08 »
All in one batch failed in under 50 hours, probably because they were made with soda glass

Was that using refined mineral oil? If so, would it react with the glass?
 

Offline SeanB

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Re: Why is it bad to touch the glass on a light bulb?
« Reply #9 on: 01/07/2012 19:43:18 »

Skin oils only. MO would smoke way too much. The ones that cremated geckos did tend to explode though.
 

Offline Bored chemist

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Re: Why is it bad to touch the glass on a light bulb?
« Reply #10 on: 02/07/2012 19:46:10 »
The "oils" will evaporate/ burn off (those things run hot).
But the salts in the sweat will kill the bulbs.
The bulbs are made from fused silica and it is always trying to turn back into crystalline silica. It does that very slowly but the salt crystals at high temperatures catalyse that reaction.
The crystalline material has a different density and thermal expansion coefficient so it weakens the bulbs.
 

Offline Geezer

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Re: Why is it bad to touch the glass on a light bulb?
« Reply #11 on: 02/07/2012 20:37:39 »
The "oils" will evaporate/ burn off (those things run hot).
But the salts in the sweat will kill the bulbs.
The bulbs are made from fused silica and it is always trying to turn back into crystalline silica. It does that very slowly but the salt crystals at high temperatures catalyse that reaction.
The crystalline material has a different density and thermal expansion coefficient so it weakens the bulbs.

Ah! Thanks BC.

Do you think I would be able to put a dab of mineral oil on one without causing damage? I might give it a shot.
 

Offline Bored chemist

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Re: Why is it bad to touch the glass on a light bulb?
« Reply #12 on: 02/07/2012 20:51:26 »
What for?
 

Offline Geezer

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Re: Why is it bad to touch the glass on a light bulb?
« Reply #13 on: 02/07/2012 21:34:21 »
What for?

To find out if you are correct.
 

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Re: Why is it bad to touch the glass on a light bulb?
« Reply #13 on: 02/07/2012 21:34:21 »

 

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