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.