Polishing glass to achieve optical transparency

polishing optical glass sculpture siren

After a glass sculpture is fired in the kiln, the first result is an opaque surface. The piece needs to undergo several stages of grinding and polishing to achieve optical transparency.

Cold work is a time consuming process. I use a vibrating lap machine, which works with an abrasive called silicon carbide mixed with water to obtain a slurry paste. Polishing a 5-sided piece can take up to a total of 125 hours (around 25 hours per side).

The process starts with 80 grit (or 120 depending on how much glass you want to remove) and it goes up to 220, 400, 600 and 1,000 grit. Each stage requires between 3 and 5 hours of grinding. The final step in the polishing process is cerium oxide mixed with distilled water on a felt disc, which is used to achieve optical transparency.

For a faster glass grinding and polishing process you can use a lapping machine with diamond discs.

Optical glass is extremely demanding. If you’re not careful cleaning up the disc every time between phases you are likely to have an unwelcome guest: scratches.

Glass scratches are likely to happen when you haven’t cleaned the disc properly and there are micro particles from the previous grit hiding on the disc surface.

polishing process glass
polishing optical glass sculpture
Polishing glass
This video shows the process of cleaning the disc to make sure there is no previous grit left on the surface. It is a pretty tedious process — you have to take the disc outside every time and use a high water pressure gun and a brush.

The motorized lift for the disc is pretty convenient as the plate is about 40 kg!

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