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Polished Mirror

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Posted by Robert May on August 6, 2006 23:34:27 UTC

The info on how to test a polished mirror is often distorted because of the assumptions of the writer.
If you're unsure of what is right on testing a surface for polish, take a window glass and do the same test on it. The glass should be well polished so the end result should be the same.
I personally don't like the laser test because the light will be reflected by the surface (4% of the light gets reflected) and thus you need to differentiate between the reflection (with it's sparkle due to the laser itself) of the surface and that of a differaction due to a small spot of unpolished out pits.
The only sure method that I've found is to do the Foucault test on the mirror and see if, when the zones are pretty much grayed out, there is any stray light being bounced back by pits. Bright spots when the mirror is almost black in an area indicates that the glass isn't fully polished out yet.
I haven't observed the grinding of the surface so I don't know if you went from the 80 grit to finer grits too fast but this is a possibility if you do indeed have pits left over and these pits will basically take forever to polish out. I always do what I call "chasing pits" as the coarse grits are used. What this is is that you mark where some deep pits are and then grind until those pits go away. then find some new pits and do it again. Then you can go to the next smaller grit and do that all over again with the finer grit.

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