20 Jun
2001
20 Jun
'01
7:48 a.m.
On Jun 20, 10:56, Chuck McManis wrote:
I had thought it was common knowledge, the reflective
layer in many CDs
is
sputtered aluminum. When exposed to oxygen (as can
happen when oxygen
migrates through the plastic or the plastic is cracked) the Aluminum
oxidizes and turns black. It does look a bit like a fungus but only
because
it tends to follow the grain pattern in the deposited
aluminum.
Aluminum-oxide is black and quite hard actually.
I've seen several examples of this in "real life" and while I have never
seen the process to actually _remove_ Aluminum from the disk it is
conceivable that the Al02 would form a different crystal matrix and thus
change its orientation relative to the original sputtering. That could
leave 'gaps' where the original reflective layer was.
But aluminium oxide is Al203 and it's a white powder. Or a rather
attractive (and, yes, very hard) crystal, known as carborundum, ruby,
emerald, amethyst, etc depending on the impurities :-) The only place
you'd get AlO2 (which is also white/clear, by the way) is as aluminate ions
in solution.
So folks to don't understand chemistry invent the
'fungus' idea and off
it
goes into urban legend-dom.
--
Pete Peter Turnbull
Network Manager
University of York