The crossed filters technique, not to be confused with the polarized crossed filters technique, is based on the emission of white light that is filtered by a blue medium. This filtered light, which is now blue, illuminates a gemstone that is observed through a red filter. This technique is conducted in darkness.
With this technique it is possible to determine the presence of chromium in the gemstone, namely in the ruby, red spinel, emerald, pink topaz, and alexandrite; however, if the gemstone has iron content, the effect can appear very decreased or completely eliminated. The observation of a ruby through this technique makes the gemstone look like a piece of burning coal. Take note that a synthetic ruby exhibits this effect intensely because it does not have iron content.
The pioneer of this technique was Sir G.G. Stokes (1819-1903), not for gemology, rather for the determination of weak fluorescence samples. In 1959, Sir B.W. Anderson applied a similar technique, based on the Stokes observations, for use in gemology. It fell back upon a 500cc glass lab balloon, filled with a saturated solution of Copper(II) sulphate pentahydrate (CuSO4 ·5 H2O) in distilled water. This balloon was illuminated by a strong incandescent lamp (500 W), and then placed in an isolated and cooled box. The filtered light (blue after passing the solution) hits the gemstones, which were observed through a red gelatine photographic filter. The gem should be over a black support. The general idea can be seen in Figure 1.
In 1993, Dr. D.B. Hoover and A.F. Theisen published their results showing that the predominant chrome excitation occurs in a blue band centered near 440 nm. The interested gemologist could find several applications for this technique in the IGS Reference Library. For us, in this article, the main objective is to turn the cumbersome device pictured in Figure 1 into a portable and reliable system.
The new LED technologies are very simple and quite inexpensive primary sources of light, so you should utilize them whenever possible in your gemology lab. For a pocket version of the crossed filters device, buy an inexpensive penlight — the type that uses two AAA or AA alkaline batteries — and a strong blue LED at any electronic parts store. Replace the bulb, or white LED, with the blue one. See Figure 2 to make sure you connect the blue LED correctly.
It is important to point out that the magnitude of the results can be different when we use a LED or copper sulphate; however, it is still an effective portable crossed filters device.
Now, you will need a red filter. You could use a sheet of red plastic, a red photo filter, or you can choose this unique option: the barcode readers used in shops and supermarkets have a red plastic filter for the incoming/outcoming red laser. This device, when broken, usually goes to the trash, rather than being repaired. Try to find one, remove the plastic cover and use it. But take note: this plastic is a special acrylic glass. Do not try to cut, drill, or flame it becuase it will crack. You should use it as is. In Figure 3 you can see our adaptation. From left to right, we have the red filter with a handle, the adapted penlight, and a special box. This box, should you choose to add it, contains five blue LED strips, a circular overture with the red filter underneath, and a circular lateral overture (not shown) to introduce a gemstone or a small piece of jewelry. The interior is painted black with spray paint.
Finally, you may be wondering about our previous comments about some diverse magnitude results between LED or copper sulphate usage. Well, if you want the best of two worlds, find two transparent gemstone boxes, one small and one big. Prepare the copper sulphate saturated solution — do it warm. When it cools, filter it well, and add one drop of sulfuric acid to stabilize it. Glue the bottom and the cover of the boxes using some epoxy glue, making sure that no air or liquid will escape. Next, flame a nail and make a hole in the lateral side of the box. With a syringe, inject the liquid inside the box. Do it slowly, letting the bubbles come up as the box fills. When done, close the hole using a generous layer of the same epoxy glue. Also, at any lateral face of the box, glue a rod or L-shaped support that allows the box to be fixed to some support. Cover the rear and the front of the box, and paint the lateral parts in black.
Now you have a good system of copper sulphate crossed filters, and you can use it in spectroscopy observations like we have done in Figures 4 and 5.
This technique is quite useful, and, as an example, you can look to Figure 6 and guess how to separate a natural red spinel from a synthetic one.