• ISG Fun with the Refractometer

    29 March 2016

    ISG Fun with the Refractometer

    From last week's ISG Journey Thru Gemology

    For those of you who missed the ISG Journey Thru Gemology last week, I thought we would offer a review today in preparation for tonight's special journey presentation on Maximizing the Refractometer Part 2.

    First, if you have ever wondered what a refractometer looks like with its guts out, just look at the image at left. Here is the inner workings of the ISG Refractometer.

    If you look closely at the lower right side you will see two screws with red coloring. These are your calibration screws and they must be in exactly the right position. If anyone has ever purchased one of those cheap US$90.00 (or less) refractometers on ebay and wondered why your readings were skewed, its because the company did not actually calibrate the refractometer...they just screwed the parts together and sold it on ebay. To get proper readings these calibration points must be carefully positioned and glued tightly.

    The manufacturer of the ISG Refractometer thinks I am clinically OCD about calibration. I personally spot check our refractometers coming into the office to make sure of the calibration. If I find one off...well, you know how I can get about these things. We are very careful about calibrationa, something those cheap ebay selles are not.

    ISG Quick Guide to Proper Use of the Refractometer

    For those who are just getting started, or perhaps need a review of how to properly set up your refractometer for reading, here are seven steps to remember, along with a slide presentation below to help you understand the issues.

    1. Open the top of the refractometer and make sure you have an operational light source.

    2. Very carefully place the refractive index liquid on the stage of the unit, making sure that your dropper or push top (depending on what kind of RI liquid dispenser you have) does not touch the hemicylinder Although the ISG Refractometer has the new cubic zirconia hemicylinder that replaces the very fragile old leaded glass hemicylinder, you still need to use caution in this step.

    3. Make sure you only get a single drop of RI liquid on the stage. Too much liquid will cause your readings to become skewed.

    4. Carefully place the stone on the liquid. Be careful not to set the stone directly on the hemicylinder

    5. Carefully slide the gemstone over the CZ hemicylinder to the position for reading.

    6. Once in place, close the lid to avoid outside light interference, and be sure to use your polarizing filter for all readings.


    7. Remember that refractometers come with focus lenses that allow you to slide the lens up and down to insure the best focus of the reading scale, and more accurate reading.

    OK, so what if you are out in the field and have no electricity and no batteries and need to take a refractive index reading. Well, I learned a little trick from the good folks at the Gem-A that works quite well. Here is the story:
    The Candle Light SodiumD Refractometer Light
    The monochromatic "sodiumD" light wavelength is considered the best, standard light to use when taking a refractive index. This is because refractive indices can vary in light sources, so this is the standard to use. While our candle is not going to make a true sodiumD light source, you may be surprise at just how close you can come to creating one with this method. Here are the steps.



    Step 1: Simply get a refractometer with a gemstone, candle and lighter.



    Step 2: Make sure the refractometer is of the design that allows for an outside light source to be used, as shown at left. (sorry for the brownish ding on the refractometer, this one went through the drunk driver crash into our building)



    Step 3: Light the candle and measure how far up or down you need to move the refractometer to insure the light of the candle is going directly into the light source opening.




    Step 4: Use whatever you have to in order to maximize the candle light into the light source opening.



    Step 5: Get your Morton salt, or whatever salt you have available. Make sure it is NaCl table salt and not some exotic type salt that will blow up when exposed to a flame.


    Step 6: After letting the candle burn down just a bit, fill the melted wax area just below the flame with the table salt. Not too much as it will douse the flame and you will be groping around in the dark looking for your lighter...and that lost shaker of salt that Jimmy Buffett is always looking for in Margueritaville.

    Step 7: If you are really creative and have a diffraction grating spectroscope in your pocket, you can actually see the strong sodiumD transmission line coming out of your candle and table salt light source in the visible spectrum. At left is this light taken with the OPL Teaching Diffraction Grating Spectroscope I got from Gail at the NAJA office (it is amazing). If you want one here is the order link: NAJA OPL Spectroscope.


    Step 8: Take your reading. I fumbled around in the dark but lost my polarizing filter while doing this (yes, bad planning hits the ISG office too) so this image is taken without the polarizing filter. But the 1.554 reading of this citrine is pretty amazing to come from candle light.

    I hope you had fun with our review of the refractometer. Tonight we have an amazing presentation in the ISG Journey Thru Gemology on how to get the optic character and sign using the refractometer. You will not want to miss it.

    If you are not already an ISG RG or RGA Student it is not too late to sign up in time to attend tonight's ISG Journey Thru Gemology. Just click on the link below and join us. This gemology thing...it ain't rocket science you know. But it is fun and rewarding when you work it right.

    At the ISG...we have learned how to work it oooohh...just so right! You do not want to miss tonight, I promise.

    Robert James FGA, GG
    President, International School of Gemology