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The refractometer hemicylinder or hemisphere can oxidize over time and interfere with refractive index readings. Fortunately, removing the tarnish is not difficult. “The Path of Light Inside The Refractometer” by Donald Clark.


I purchased a used GIA Duplex II refractometer. The hemicylinder was scratched. I had a Graduate Gemologist take a look at it for me. The report was that it’s usable but a little fuzzy. When I look into my lens, I see a black bar that rises and lowers as I move my head in an up/down motion. But I never see a green line, faded/fuzzy or not.

Here’s my dilemma: do I have the instrument looked at by another person, find somewhere to have the refractometer hemicylinder repaired, or purchase another refractometer in better shape?

Any advice would be most appreciated.


Removing Tarnish From A Refractometer Hemicylinder

I had a similar problem after buying a used GIA Duplex II refractometer for my business. The problem you’re experiencing is common and easily remedied. Because of the high lead content of the glass and subsequent softness, the surface of the refractometer hemicylinder will actually tarnish.

To remedy this, you’ll need to re-polish the hemicylinder. This is easier than it sounds. You’ll need some common supplies for lapidaries and about 15 minutes.

  • Mix a small amount of cerium oxide with distilled water to make a thick paste. (The trick is to keep the liquid level low. Too much water and you’ll have a whole new set of problems).
  • Take a piece of cotton, like an old T-shirt, tab it in the paste, and transfer some of the mixture to the refractometer hemicylinder. (Try to avoid the edges).
  • Begin by rubbing the surface of the hemicylinder in a circular motion. This will remove the tarnish or oxidation very slowly. (Rubbing in a circular motion avoids directional scratches).
  • Continue polishing for about 5 to 6 minutes.
  • Wipe the refractometer hemicylinder clean with a clean piece of cloth.

Since obviously it’s been a while since the last polish, you might have to re-polish the surface of your hemicylinder three or four times before you remove all the tarnish and get a crystal-clear reading. Basically, you’re polishing the glass much like you would a gemstone, so be patient and thorough.

I recommend re-polishing the refractometer hemicylinder every six months to keep your readings accurate. Gemologists do this on a regular basis to get a sharp, clear dilation line for accurate measurements.


Douglas LeGrand, GG

A Sign of Possible Oxidation On Your Refractometer Cylinder

One indication of possible oxidation on the table of your refractometer cylinder is if you can see any rainbow banding on its surface. This is not a guarantee that it is oxidized, however.

Roy Auerbach

The green line or green shade cutoff appears between the light and dark part of the scale. The green line marks the refractive index reading. “Duplex II display.” Donald Clark.

The green line or green shade cutoff appears between the light and dark part of the scale. The green line marks the refractive index reading. “Duplex II Display” by Donald Clark.