From ase–(at)–icron.net Sun Jan 21 19:55:15 CST 1996
Article: 5482 of rec.audio.tubes
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From: ase–(at)–icron.net
Newsgroups: rec.audio.tubes
Subject: Re: Fingerprints :the real story
Date: 21 Jan 1996 18:27:42 GMT
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> Andrew McWhirter
> Some guys wrote:
> >>>>>Also – Don’t
> >>>>>touch the tubes with your fingers, a primary form of tube failure is
> >>>>>people’s fingerprints, which migrate from the outer portion of the
> >>>>bottle,
> >>>>>to the inside ( over several years ) and turn the tube gassey
>
> >>> If you’ve seen old tubes – that people touched, just try to wipe
> >>> those fingerprints off. You can’t, they are already inside the
> >>> glass (the glass here being a LIQUID, but a very, very viscus >>>liquid).
>
>[text deleted]
> Once the fingerprints are on the outside again it is a simple matter of
> wiping them away with a lint free cloth.
>
> For the above reasons, police are now dusting for prints on the outside
> of windows at all crime scenes where there may be free electrons on the
> outside of the building. If that fails, they just take all the glass
> (just in case the prints are still trapped inside).
>
>
> Seriously guys, the fingerprint just gets etched into the glass by the
> action of the acid, accelerated by heat. You can’t wipe it off after
> this happens, but you can still get any remaining chemicals from the
> print off by cleaning (for whatever good that does). If stuff can
> migrate through glass, shouldn’t Louis Pasteur’s soup be a bit dried out
> by now?
>
>
>>>>
Great post! I loved it!
The real, REAL, _REAL_ story with finger prints on tubes is that they
promote devitrification of the glass. Glass is called “vitreous” because
it has no crystalline structure. However, the major component
elements of glass are silicon and oxygen, and these elements would
just love to make a crystal called quartz. Fingerprints contain
impurities which give the silicon and oxygen a starting point to grow
quartz crystals. The heat makes the process go faster, so this problem
is more severe on power tubes. It is a problem because the quartz
crystals have intrinsic strength but are not attached firmly to the glass,
and do not contribute to the strength of the envelope. However, they
are made from the envelope and consume it as they grow. The glass
will eventually become thin enough that the envelope leaks or breaks.
This is a significant problem with the pure quartz tubes used for very
high temperature (in excess of 1000 degrees Centigrade) processing
of semiconductor devices, and goes much faster on vacuum tubes
because the glass envelope is quartz melted down with metallic
oxides to reduce the melting point and make it easier to work with.
