Re: [ATM] Does exposed Pyrex lose polish over time?

[Mark Cowan <toolontop@yahoo.com>]

Hi all:

I've heard this "microcrack" theory of subsurface
damage a number of times now - what I'd like to see
from any reputable source is SEM imagery of "cracks"
with a scale <<1 micron resulting from grinding
remaining through polishing...  Especially if the
polishing stage is extended much longer than strictly
obvious.

And it has to be on Pyrex, not the more documented
effects of migration of surface slurries into fluorite
if care isn't exercised to grind through each previous
grit's layer of damage.

If 5 minutes of polishing rendered the mirror
coatable, it seems much more likely to have been the
result of taking off a surface layer of some
well-adhered contaminant that had an extreme molecular
affinity for the glass.  

I've noticed that the only way to get a brilliant
polish that knocks your eye out is by extending it
much beyond the disappearance of all visible defects,
so I /am/ convinced there's micro stuff you can't see
that affects the macro surface performance.

Cheers,
Mark Cowan
Salem, OR

--- Andy Saulietis <iss@pvtnetworks.net> wrote:
> Tom Krajci asks,
> 
> 
> 
> >Microcracks in the polished surface?
> >
> >Does this mean the polish was incomplete?
> 
> Paul Jones (the mirror maker) says these microcracks
> are sub-sub wavelenght in size & result from the
> grinding
> process. After coating I didn't see any visual
> evidence
> of inclomplete polishing (pits, sleeks, or
> scratches) in
> direct sunlight.
> 
> I can't vouch for the accuracy of Paul's theory, but
> we do
> know that 5 min of re-polishing fixed the coating
> corrosion
> proble. Other mirrors I've used that he's made did
> not have
> this problem. They were made from regular sheet
> Pyrex 2 1/8"
> thick.



	
		
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