Identification by Refractometer

The refractometer is one of the most versatile of all gem testing instruments. It serves not only to determine refractive index, but it can be used to measure birefringence, to ascertain whether a stone is uniaxial or biaxial, and to establish its optic sign. The combined results of these various tests provide conclusive evidence for the identification of many gem species. Like most other testing instruments, however, the effective use of the refractometer requires a basic knowledge of its construction and adequate experience in its use. The stone sets furnished with this course will provide the necessary practice to assist you to become familiar with the instrument you will be using.

HOW A REFRACTOMETER MEASURES R.I.

All refractometers contain a hemisphere, a hemi-cylinder
or high-refractive index glass prism on which the stone to be tested is placed. It was explained in the Colored-Stone Course that each transparent material has a specific critical angle and R.I. For example, the critical angle of the glass hemisphere used in gem refractometers is approximately 31°. This means that of the light passing into the instrument and entering the hemisphere, those rays that impinge on its inner top surface at less than 31° to the normal (perpendicular to the flat surface) are refracted out through the top; on the other hand, those that impinge at greater than 31° are totally reflected back from the hemisphere's top surface and out the opposite side (Figure A).

To fully understand the significance of this in relation to refractive-index determinations, it must be remembered that the standard critical angle listed for any material is always based on the material being in contact with air. Thus, normal refractive indices and critical angles are all related to the change of speed of the light as it passes from air into denser materials, or vice versa. Obviously, if air is replaced by a more dense material, these relationships change and a new critical angle is formed within the hemisphere that also is directly related to the density of the new contact medium. The same condition arises when grease accumulates on the pavilion of a diamond. Because the grease is a denser medium than air, the critical angle of the diamond is enlarged and light that would have normally been totally reflected escapes through the larger critical angle and is lost through refraction from the diamond (Figure B).

Since it is possible to alter the critical angle of a transparent material by changing the medium with which it is in contact, and since the degree of change is always related to the density of the contact medium, the index of an unknown stone can be determined by placing it in contact with a material of predetermined refractive index and measuring the change in its critical angle.

Visually, the critical angle of a hemisphere can be observed from a position behind and below the surface; for example, the position indicated in Figure C. The edge of the critical angle is the boundary of the totally reflected light and from the position indicated, it appears as a shadow edge. The only requirement is that the angle of observation be adjusted so that it is parallel to the critical angle. To permit observing the shadow edge from a normal position above the refractometer, a reflective element is used to reflect the image up to the eye (Figure D). Figure E shows how the shadow edge is superimposed on the R.I. scale, which is located between the hemisphere and the reflective element, and shows the appearance of the scale through the eyepiece of the instrument. A more detailed explanation of the theory involved in these determinations will be presented later in the assignment.

DESIGN AND CONSTRUCTION

A number of refractometers have appeared on the market since the turn of the century, when G.F.Herbert Smith, the British gemologist, designed the first small, inexpensive unit for general gemstone identification work. Earlier instruments usually were quite large and elaborate in construction, being more suited to research work. Following the Smith refractometer came the Rayner and Tully instruments, which are still used today, although the Tully in no longer made.

The use of refractometers was limited to the determination of indices of specimens with flat polished. A method for obtaining indices on curved surfaces was discovered by Lester Benson of GIA.

Duplex Refractometer was developed. This was the first refractometer designed to obtain refractive indices on flat and curved surfaces with equal ease and to combine this highly desirable feature with modern design characteristics. The second refractometer designed for this purpose, the Duplex II.

PLACING THE STONE ON THE HEMISPHERE.

Regardless of the kind of refractometer chosen, the basic requirement for use are the same. First the stone to be tested is placed in optical contact with the hemisphere and the resultant change in critical angle of the hemisphere is viewed through the eyepiece. The scale, which is between the hemisphere and the eyepiece, is calibrated to measure this new critical angle in terms of the R.I. of the stone producing the change.

Since an optical contact between the stone and the hemisphere's surface is required, it is necessary to utilize a contact liquid to eliminate the thin film of air that would otherwise exist between the two media (Figure G). The presence of an air film prevents the stone from altering the critical angle of the hemisphere. The use of the liquid must be higher than that of the stone, and the one that fulfills this requirement most satisfactorily is sulphur-saturated methylene iodide (CH2I2) to which tetraiodoethylene (C2I4) has been added. This is the liquid provided with refractometers.

The application of the contact liquid to the hemisphere requires care. The bottle is equipped with a glass rod with which too much liquid is usually transferred, unless the tip of the rod is touched to the lip of the bottle repeatedly to remove the excess liquid. Only an amount sufficient to spread and cover the contact area between the stone and the hemisphere is required; for most stones, a pinhead-size drop adequate (Figure H). (Note : The contact requirements for testing curved surfaces will be discussed separately under the subject of spot readings.) Because of its high density (approximately 3.20), an excessive amount of liquid may float or tilt small stones of lower S.G. and cause incorrect readings. Its surface tension may hold up tiny stones of greater S.C. and prevent any reading.

If too much liquid is used for a prolonged period of time, the liquid may penetrate the adhesive sealing in the bounder: between the hemisphere and the top plate. This is dangerous, because it is a strong solvent for paint and some cements and hastens the formation of tarnish on optical components. Furthermore, since it is heavily saturated with sulphur, the evaporation of even a tiny drop causes the deposition of hard-to-remove sulphur crystals on the hemisphere and surrounding metal surfaces; therefore, these parts must be cleaned gently with soft, absorbent facial tissue immediately after each use. If dried sulphur appears on or around the hemisphere no attempt should be made to wipe it off, since it is abrasive and difficult to remove. Two or three drops of methylene iodide taken from the 3.32 heavy-liquid bottle will dissolve it readily, and it can then be removed easily with facial tissue.

Since sulphur particles also form on the lip of the bottle, care must be taken to prevent them from being picked up on the glass rod and deposited on the hemisphere, otherwise, pressure of the stone on the particles in drop may abrade the glass. Sometimes, this condition does not become noticeable until the stone is placed on hemisphere. If a gentle movement or rotation of the stone indicates any roughness on the contact surface, remove it immediately and apply a fresh drop of liquid. If stone are being tested that are known to have indices below 1.74, pure methylene iodide may be used in place of the regular contact liquid, thus eliminating the sulphur problem. Sulphur and tetraiodoethylene are added to the liquid to increase the index from 1.74 to approximately 1.81; this provides the highest R.I. liquid that can be used without causing excessive corrosion or etching of the hemisphere.

Of great importance in caring for a refractometer is an understanding of the fragile nature of the hemisphere. In order to obtain the high index required, the composition of the glass includes a high percentage of lead, which reduces the resistance of the glass to abrasion and blows and makes it susceptible to oxidation. With long or constant exposure to an atmosphere strong in sulphurous fumes or waste gasses, such as exist in almost any city, tarnish of the hemisphere is sure to result. A gradual reduction in the clarity of the reading is likely, until none may be seen. Tarnish actually is a metallic-oxide coating. It can be removed by careful polishing with tin-oxide or cerium-oxide powder, but this corrective measure should be entrusted to the skill of the manufacturer. It is feasible, however, to re-polish a scratched hemisphere, since its dimensional characteristics are critical and any change in them will cause difficulty in recalibrating the instrument. For this reason, a scratched hemisphere must be replaced. It is to be emphasized, however, that scratches do not impair the accuracy of a reading, but when a large enough area is scratched, it is impossible to obtain a reading. It is then when replacement is necessary. If a refractometer is used with care, these are minor considerations and it will serve the needs of a jeweler for many years before requiring hemisphere replacement. Abusive treatment, on the other hand, can easily result in excessive damage during the testing of one stone.

Another caution: if a refractometer is continually moved about for any reason, as in traveling, it should be rechecked for correct calibration prior to each use. This can be done by taking the R.I. of a piece of rock crystal, synthetic corundum or other material with a constant index.

INITIAL ORIENTATION OF THE STONE ON THE HEMISPHERE.

To exploit the refractometer fully for determining R.I., optic sign and birefringence, it is necessary to orient the stone correctly on the hemisphere. Although the detailed procedure will not be discussed until the assignment on the advanced use of this instrument, basic R.I. determinations must take into consideration the fact that index readings may change slightly, depending on the orientation of the stone. This is not a problem with singly-refractive materials such as glass, garnet and spinel, since light behaves the same in any direction of travel through them. Therefore, they form only one critical angle within the hemisphere and it is always
the same, regardless of the stone's orientation.

With doubly-refractive materials, however, the situation is different. In one or possibly two directions, light behaves the same as in a singly-refractive stone; in any other direction, it is polarized in two planes at right angles to one another, and the speed of transmission in each plane differs. Obviously, if a specimen placed in contact with the hemisphere transmits a given beam of light at different speeds in two planes of vibration, the equivalent of two distinct mediums in contact with the hemisphere will exist; the result will be the formation of two new critical angles within the hemisphere that are visible as two separate shadow edges. The degree of separation between the two shadows depends on the birefringence and orientation of the specimen.

In line with this, it is important to note that a refractometer always analyzes the optical characteristics of the direction in the stone that is parallel to the surface and length of the hemisphere (Figure I). If this represents a direction of single refraction, only one critical angle and one corresponding shadow edge will result (Figure J). On the other hand, if the direction corresponding to the length and surface of the hemisphere is exactly at right angles to the singly-refractive direction in a uniaxial stone, maximum double refraction, or birefringence, will be obtained (Figure K). Any intermediate position will reveal a separation of proportionate magnitude (Figure L).

At this time, it is sufficient to say that the only requirement for basic determinations is to place the stone table down on the hemisphere and check the corresponding readings for several positions of rotation. If it is singly refractive, a single reading will appear and remain constant in any position. If it is doubly refractive, one reading may be obtained for other positions. However, maximum double refraction will always be encountered on any given facet, and the two extreme readings thus obtained through rotation of the stone will provide the critical, information needed for routine testing.

LIGHT SOURCES

The references to sharp single and double readings applies only when filters or special light sources are used to provide illumination for the refractometer. Usually, either sunlight or simple lamps that emit white light are used for obtaining refractive indices, and the resultant readings are often referred to as white light readings. Instead of a sharp line, white light results in a blue-and-green band that covers approximately two divisions of the scale. All refractometers are calibrated against a sodium light source, which emits a very narrow portion of the spectrum between the yellow and green, at 5890 A.U. The equivalent wavelength of light in a typical white-light reading is at the high-number (greenish) edge of the blue-and-green band (Figure M). Most doubly refractive stones have P birefringence that is less than the width of these normal blue-and-green, white-light readings. Because the two colored readings overlap indistinguishably, such stones usually display one moderately wide reading Figure N(i).

To prove the presence or absence of double refraction, the gemologist utilizes the fact that doubly-refractive readings are caused by light vibrating in two planes at right angles to one another. He holds a Polaroid plate over the eyepiece to view the reading. If double refraction is present, the two readings will be visible alternately as the plate is rotated. The effect will be a single blue-and-green band that appears to move up and down slightly on the scale for every 90° rotation of the polarizer (Figure N(ii)). If no movement is seen in the first position of the stone, it is necessary to turn it to several positions see if any movement is detected when the Polaroid is rotated. If movement is detected, the stone should be examined in several positions, to determine maximum separation of the readings.

It will be noted that all complete refractive-index tables also list birefringence figures. Its determination is one of the more important optical tests. If it is established that a stone has a birefringence of .02 in the 1.62-to-1.64 range, it could only be tourmaline, since an examination of the R.I. tables will show that no other gem material in this index range has this degree of birefringence. Similarly, if a stone gives a reading anywhere between 1.62 and 1.64 but fails to show birefringence approximating .02, tourmaline would be eliminated automatically as a possibility.

On any refractometer scale, anomalous effects may be seen that appear to be true R.I. readings. These effects are caused by faulty lighting or by the fact that the stone's R.I. is above the limits of the instrument. Most determinations are made with the top of the refractometer and the specimen exposed to overhead illumination, even though some models provide covers. Usually, the covers are too small to permit the analysis of stone-set jewelry. A strong overhead light source may be dispersed through the stone and into the hemisphere, creating a spectral image on the scale. This effect can be distinguished readily from a true R.I. reading, since it appears as a complete spectrum, ranging from red through blue, in contrast to the characteristic blue-and-green band of an actual reading.

Shielding the stone with the hand or turning off the overhead light source will eliminate this problem.

A second anomalous effect that may be encountered is interference of light in the thin film of contact liquid. It may be present as one or more bands, but more often it appears as a series of irregular spectral lines. Again, this should present no difficulty, since it produces a complete spectrum.

Although it is not always practical or possible to arrange ideal conditions for testing, the refractometer and other optical instruments perform most effectively in a semi-darkened room when the principal light source is that used with the instrument.

Many of these apparent problems are of concern only to the new student; they will become decreasingly significant as opportunities arise to test various types of stones during practice work.

REFRACTOMETER VS. REFLECTOMETER.

The usual method of refractometer use employs the instrument as a TOTAL REFLECTOMETER; in other words, the spectrum and the lighted area are in the region of total reflection of light entering the hemisphere from below the contact surface. Employed as a refractometer, a check of the usual results is possible. In this case, the light portal at the end of the instrument opposite the eyepiece is blocked off and the light source directed down toward the side of the pavilion of the stone that is opposite the eyepiece. In this situation, the reading is seen as a red fringe separating light from dark areas. In contrast to the first type, the high number end of the scale is dark and the low, light.

TECHNIQUES FOR OBSERVING READING ON DIFFERENT TYPES OF REFRACTOMETERS.

When observing the scale in a refractometer with immovable optical elements, high magnification eliminates the possibility of observing the entire area of contact through the optical system; instead, the image is a highly magnified area at the reading, if any is present. With the simpler optical systems, which are more suitable for all-around use, there is much less magnification, thus making it possible to observe the complete contact between specimen and hemisphere directly through the system. It appears as a dark area superimposed on a light-yellow field; i.e., the yellowish glass ,hemisphere. The size of the contact in relation to the hemisphere depends on the size of the stone. A very small stone will comprise only a small dark area which a shape corresponding to that of the surface in contact with the hemisphere in the center of the yellow field, whereas one larger than the hemisphere will eliminate the yellow field in the area above the reading; i.e., the portion of the scale lower in number than the reading.

Novices sometimes mistake one of the edges of the stone- hemisphere contact area for an R.I. reading. This can be avoided, if it is remembered that contact boundaries appear merely as dark shadow areas with no color fringe (Figure 0). When using any of the instruments that incorporate a removable eyepiece, the standard procedure is to observe the scale after centering the stone on the hemisphere and removing the eyepiece. The eye is moved up and down the scale until the image of the contact is seen at the low number portion of the scale. As the eye is moved up the scale numerically, the contact image appears to move along the scale to the 1.81 end. As this occurs, the image is watched until it is bisected by a blue-and-green band dividing the dark and light portions of the contact area. The R.I. is thus established. If the image of the contact area is moved below the position of the blue-and-green band, the entire image appears light, unless the contact is less than the width of the hemisphere, when the outline of the contact area is visible as a dark rim about a light center.

The contact is now being observed from outside the critical angle formed in the hemisphere, and in this position nothing can be seen except totally reflected light from the source in front of the instrument.

Although the techniques for obtaining R.I., readings with the different refractometers vary slightly, the basic procedure can be mastered by experimenting with only a few stones. It is usually advantageous to scan the entire scale to observe the over-all reaction, and then adjust for the R.I. reading. Having established the reading by obtaining the blue-and-green band, the instrument may then be left in this position and a Polaroid plate held over eyepiece to examine the stone for birefringence. This should be done by alternately rotating the stone and checking the reading with the polarizer, making certain that the stone is kept centered on the hemisphere.