kevin_hulmes@uk.ibm.com on mon 23 dec 96
*** Resending note of 23/12/96 13:50
Jeff Lawrence (with obvious tongue-in-cheek) asked about the colour of
Aluminium Oxide. Bill Aycock replied, pointing out that it was
colourless although seen as white when in powder form. Bill also said
that Aluminium corrodes rapidly in the air. All of this is true, but
there was a misunderstanding over the colour of the metal itself.
When pure Aluminium metal is exposed to air, it oxidises immediately,
giving a very thin transparent layer of oxide which then prevents
further oxidation. This layer is relatively inert both physically and
chemically at room or cooking temperature, and if you scratch it away,
the exposed metal immediately oxidises again. Because the oxide layer is
transparent and colourless, you see the natural colour of the metal
through it. Thus the smooth side of cooking foil is shiny and silvery,
and the rougher side is less so. When you roughen the surface, it gets
darker and the metallic shine disappears. As a very fine powder, it has
a dark grey colour - almost black, especially when wet.
Aluminium is rather soft, so when you clean your tools or wheelhead in
the presence of an abrasive material (like clay), you create a fine, wet
dust which is "black", even when exposed to air. Each grain of this dust
consists of Aluminium metal coated in an extremely thin layer of oxide.
The same thing happens with steel tools, but the metal is harder, and the
oxide layer is permeable and doesn't prevent further oxidation, so less
metal powder is produced, and it becomes rust over a few hours if not
protected by oil or similar. Stainless Steel powder doesn't rust, which
is why the black marks cutlery puts on some glazes stay black!
Incidentally, Aluminium Oxide crystals with small levels of impurity are
found occurring naturally as ruby, emerald, topaz, sapphire and amethyst
in addition to the corundum mentioned earlier. So, Jeff, the ball's back
in your court. What colour is it?
Bill, really sorry to hear about the post-operative infection. I hope it
clears up quickly, and you're back with us all "on Persimmon Hill" very
soon!
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