Richard Casady wrote:
On Sat, 26 Jan 2008 17:47:34 -0500, "Roger Long"
wrote:

"Richard Casady" wrote

They do make white greases; vaseline is close to white.
The stuff I used was white but no white grease will be white after that
aluminum oxide gets into i

Aluminum oxide is white. Saphire is aluminum oxide. It is unlikely
that you have ever seen aluminum that was not coated with a
transparent film of the oxide.I think the black stuff has to be fine
particles of aluminum. That stuff can be black when finely divided,
which is true of metals in general. The black in B&W photography is
pure silver, in tiny grains.

Casady

Yes, sapphire is aluminum oxide. However, sapphires come in every
color under the sun from bright red (ruby) to dirty opague black. It may
appear white spread thinly on the aluminum but is like dirt on your car
which appears gray until you wash it and then its black on the rag. That
said, the alo sticks to the al tenaciously and probably wasn't pulled
off by something dragging along it. My guess is the black was plain old
crud!
Gordon