 Refractometers come in all sizes! According to Edward Boehm, the above unit was designed by Robert Shipley Jr. and made by GEM Instruments in 1949. It is a fully functional version of its much larger cousins and was considered by his esteemed grandfather to be one of the most durable portable refractometers ever made (Boehm, 2002). This one, as well as the two just below, came my way via John Koivula and though the functionality doesn't escape me one whit, as a collector of eclectic miniatures I was quite smitten with this tiny, very fine instrument. I have put it to great use! | |
 Two amazing refractometers On the left is a Russian-made prototype whose precision is comparable to that of the Shimazdu and Eickhorst refractometers which I have also used. Housed in a hefty all-metal body, its prism is much harder than the lead glass used in other models. Shown using the fiberoptic light from the nearby spectroscope as a light source, its phenomenal optics produce a razor sharp image. It has a removable analyzing lens and a built-in filter for sodium equivalent light. A nice feature is that the cover is very easily removed making it simple to make "brightline" readings. See: Hoover, D.B.and Williams, C. (2007) Better refractometer results with bright line technique. Journal of Gemmology, Vol. 30, 5/6, pp. 287-2961 (see also the follow-up letter to the editor in vol. 7/8 from Alan Hodgkinson in which he gives examples). On the right is a classic Rayner Dialdex. Instead of reading an internal scale, the user rotates the dial on the outside to align an internal straight-edge with the shadow edge. The RI is then read off the calibrated scale on the dial. I had a hard time imagining this procedure before actually trying it while helping with a workshop at Gem-A in London; I now find it a pleasure to use. That was a lesson in why it is important to be familiar with all kinds of models; I was very lucky to be able to master it quickly in the few minutes before students walked in! (see Darko Sturman's The use of the polarizing filter on the refractometer (pdf)). This refractometer comes with a polarizing filter and a yellow filter which nest together on the ocular. Shown alongside is a battery operated yellow sodium equivalent LED light which can be used as a light source, also from Gem-A Instruments. This is a very handy portable light source for travel as well as for use in a lab. In the background is a very special and rather rare GIA GEM Instruments sodium vapor lamp which provides monochromatic light of exactly 589.3 nm for sharp, very accurate refractometer readings (similar looking standard utility lights produce white light which then passes through a sodium equivalent filter; this is adequate in the majority of situations). Here the light aperture is flush with the back of the Rayner refractometer. | |
 (top to bottom) Duplex II, GemPro, Kruss knockoff, Duplex III Of the four here, my favorite is the Duplex III, especially for travel. It is very small, light and has a huge viewfinder with an easy-to-read scale; its one drawback is a lack of a protective cover. It is very easy to do cabochon spot readings with this model, as well as with it's larger desktop cousin, the Duplex II; both are made by GIA GEM Instruments. The GemPro is from Jeff Wildman's Gemological Products and is very similar to the Duplex II, though of smaller size and seemingly much sturdier build. It has a removable sodium equivalent filter and analyzer. The remaining refractometer is a Chinese model obtained in Chanthaburi, Thailand. This one gives very sharp readings and is calibrated exactly (not all are, so buyer beware). All the models above can be illuminated with a Maglite or the little LED light pictured above when away from electricity. At the lab bench, one may use a fiberoptic light, the light from a polariscope unit or a standard gemological utility lamp such as the GIA GEM Instruments model. With all but the Duplex and GemPro models which are read from a distance, the use of a refractometer stand is recommended to bring the unit up to a comfortable working height. Gem-A makes a nice metal stand which adjusts to hold any model and a light source as well. These are used by students in their gemology classrooms and by people working long hours in a laboratory, but is also appropriate for any user who doesn't wish to develop a permanent neck crick. Alternately, any sturdy box will do, such as the one that is used to store the refractometer or a simply a wooden block.  Stand with Rayner refractometer and LED light source. | |
There is an abundance of misinformation about RI liquid toxicity, especially on the Internet and in outdated literature. This substance requires training and careful handling both by new students and entrenched old-hands. Take a tip from a master: rather than using your finger, manipulate small stones on the stage with a pencil eraser to avoid any possible skin contact with RI liquid (Alan Hodgkinson, 2008). Every human has different susceptibility to triggers of illness; when handling a substance with known toxicity, it is prudent to teach and to follow recommended safe handling procedures. Avoid skin contact, inhalation of vapors, and ingestion - don't chew on that eraser! One must also keep in mind that recommendations change over time with new information; so please keep current. The readings on a refractometer are limited by the RI of the hemicylinder and the RI of the liquid itself. The higher the RI limit of the hemicylinder, the higher the RI of the liquid must be to make use of that limit - unfortunately the toxicity goes up exponentially requiring at least gloves and ventilation, while at its higher limits a lab hood is also mandatory (arsenic salts are used in some formulae). Standard TIR refractometers in use outside of labs usually have a hemicylinder of lead glass with an upper limit RI of 1.90; the highest liquid easily available is 1.81 which thus becomes the effective upper limit. RI liquid is made from methylene iodide (di-iodomethane) saturated with sulphur giving it an RI of 1.78-1.79. The sulphur alone can cause a bad skin or respiratory reaction in some individuals. Increasing the RI to 1.81 requires the addition of tetraiodoethylene - which is carcinogenic. Alternatively, tin iodide (tin tetraiodide or stannic iodide), a reddish salt, may be dissolved in methylene iodide to increase refractive index above 1.780 in a fashion similar to using sulfur without the addition of tetraiodoethylene (methylene iodide is combined with sulfur up to 1.780 and sulfur and tin iodide up to 1.800). Different manufacturers use different formulae so it is important to know what a particular brand contains and thus what the specific safe handling procedures are. These products, as well as heavy liquids used for SG and immersion techniques, must be used with great caution. It is now recommended by all current literature, gemological schools and laboratories to avoid any skin contact and inhalation of vapors. Some schools have changed to 1.79 RI liquid in light of its lower toxicity. A note on crystal formation and effects of evaporation: when the methylene iodide is allowed to evaporate, the concentration of sulfur or tin iodide is increased thus increasing the refractive index of the solution until a saturated solution forms. More evaporation results in the deposition of crystals of the solids. A tightly closed bottle will not deposit crystals, but you may get some crystals from ones formed when a wetted cap exposed to the air lets the methylene iodide evaporate. These liquids are very dense causing any debris such as crystals to float noticeably on the liquids' surface. Color stability: in sun the fluid will become very dark after one day. When stored in the dark the color is stabilized for up to 5 years by the small piece of copper normally included in the liquid. | |
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