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Grinding and polishing glass, revisited

The recent thread prompts me to add my $0.02, here.

How is glass processed by the pros?

"Grinding" uses a loose slurry of hard particles in a coolant liquid to
produce microscopic chips that gradually eat away the surface of the glass. A
hard metal tool is usually used for this stage that won't trap the particles
and produce scratches. Rolling particle action gives a "ground-glass" surface.

"Polishing" or "Lapping" (as distinct from grinding) uses deliberately-trapped
particles to produce microscratches that are so tiny they don't scatter light
effectively. The particles are trapped in a wet pitch, wax or felt-like pad
and prevented from rolling as is common in grinding. The
chemistry/crystal-shape of the polishing material is important for quick, easy
results. Rouge and cerium oxide are common polishing compounds, while diamond
and silicon carbide are used for grinding.

Polishing pitch requires a series of trapping grooves to catch any particles
that come loose from the surface, so they can't roll far and cause much
grinding. Felt or other cloth-like polishing pads do a better trapping job, so
may tend to be faster finishing.

In true polishing, the glass material is moved around but essentially none is
really removed. That's why it is so slow to fix a scratch. The microscratch,
under a strong magnification (electron microscope is needed), looks like a
plowed furrow. It's a groove with raised edges, as material is moved but not
chipped away. The glass looks like it was melted and resolidified, which may
be what happens.

Use coarser grinding material to remove the scratch. 5 or 10 (or even coarser)
micron grit might be appropriate. A cast-iron or other hard, non-malleable
tool is needed for fast results. When the scratch is essentially invisible
under a 10X glass, start reducing particle size (wash off the coarser grit)
and grind for a smoother finish down to maybe 1 micron particles.

Then, polish with a "Pellon" or pitch pad and cerium oxide as a quick process
to reduce the "ground-glass" scatter caused by the tiny chips. [Don't use
rouge, unless your room is basically already a rust red color. :-)]

This all requires some specialized tooling, but it will take hours off the
process described by John. Check out amateur telescope makers for where to get
the materials and tooling.


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