From: Mark Holm 
To: atm{at}shore.net
Reply-To: Mark Holm 


Recently I did a BVC mirror with cerium oxide then switching to zirconium oxide.
  I noticed that the material removal rate seemed slower with the zirconium
oxide  (judged by rate of figure change using similar strokes) and the
surface seemed to look smoother as well.  I seemed to get a fairly visible
"printing" of
the square lap facet pattern onto the mirror surface with cerium oxide, but
not with zirconium oxide.

I haven't had the experience of switching from cerium oxide to rouge, so can not
comment directly on that, but my experience suggests it may be worthwhile to try
for something slower than cerium oxide for final polishing and figuring.

Cerium oxide has a well known chemical action in polishing that is a major
contributor to its speed.  When trying to final polish or figure using
typical amateur methods, this may be more of a hindrance than help.  The
pros and advanced ATM's may have figured out how to tame it, but for the
rest of us, going with something milder may be easiest.

Mel Bartels says zirconium oxide also has a chemical action, but my limited
experience says it is slower and finishes smoother than cerium oxide.
Zirconium
oxide is more expensive than rouge, but doesn't have anywhere near the same
messiness factor.

Some observations I made when using cerium oxide made me think it might be going
after the glass even when I was not polishing.  I tended to leave the mirror and
lap together for rather long cold pressing spells.  It seemed that the lap
pattern "printing" was more pronounced when i did this.  This
makes me wonder if
the cerium oxide was etching into the glass even when there was no
mechanical action.  This is certainly still at the hypothesis stage with
me.  Perhaps others have observations that would support or refute it.

Mark Holm
mdholm{at}telerama.com

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