CHAPTER I INTRODUCTION AND SCOPE OF THE PRESENT WORK

Size: px

Start display at page:

Download "CHAPTER I INTRODUCTION AND SCOPE OF THE PRESENT WORK"

Stephen May
5 years ago
Views:

1 CHAPTER I INTRODUCTION AND SCOPE OF THE PRESENT WORK

2 Chapter I INTRODUCTION AND SCOPE OF THE WORK 1.1 Introduction 1.2 Gemstone Enhancement Temporary Gem Enhancements Bleaching Coating Dyeing Filling Impregnation Waxing L2.2 Permanent Gem Enhancements Irradiation Laser treatment Heat treatment 1.3 Classification of Heat Treatments Heat Treatments without additives Heat Treatment with additives Recrystallization Beryllium treatment Flux Treatment Surface Diffusion 1.4 Various processes for Heat Treatment Process 1: Development of potential asterism Process2: Removal of Silk or asterism Process3: Development of colour in a stone with a potential for blue Process 4: lightening of blue Sapphire Process 5: Diffusion Mechanism Process 6: Adding Asterism by diffusion Process 7: Adding Colours other than blue by diffusion Process 8: Combination of treatments 1.5 Scope of the present Work 1.6 References

3 Chapter I INTRODUCTION AND SCOPE OF THE WORK 1.1 Introduction Stones such as sparkling diamond, dazzling blood red ruby, green emerald and soothing white pearls are some of the gifts of nature to humanity and are termed as Gemstones. These gemstones because of their colour, clarity and transparency are objects of desire and admiration since ages [I]. The science of gemology has expanded tremendously over the years, as advances in technology have resulted in new gem treatments and synthetic gem materials, as well as in new identification techniques and new developments in gem mining and exploration. These advances in technology have come from a variety of disciplines such as Physics, Chemistry, Geology, and Materials Science so that the science of Gemology has become more dependent on, and established in, several branches of study. Although new occurrences of natural gemstones are found from time to time in various parts of the world, the availability of the best quality gemstones are rare, and there is recovered a much larger amount of lower-quality material (with poorer colour and/or clarity) that has little market value. Researchers strive to develop methods to treat lower-quality gem materials to enhance their appearance and hence their sale ability [2], Historically, many gemstones have been enhanced to improve their appearance. At present, a variety of traditional and advanced enhancements are applied to many natural gemstones such as beryl, corundum, tourmaline, topaz, zircon, zoisite, etc. The treatment of gemstones by various means to improve their appearance extends in some cases back to thousands of years. The past century witnessed growth of gem treatment in terms of new and sophisticated techniques utilized on a widening range of gem materials. The various gem treatment methods currently in use can be grouped Department of Materials Science 2

4 into two general categories, methods to change colour, and methods to change clarity [2], Not all treatment methods are obviously appropriate for every gemstone. The costs of the starting material and the treatment method itself, versus the value of the resulting treated materials, are also significant factors in determining if a particular treatment method will be used. In general, the most widespread treatment methods, especially for colored gemstones, involve a change in colour.two methods in this category, exposure to heat and/or radiation, are similar to processes that can affect natural gemstones in the earth, and as a result, these methods can sometimes produce similar color-change effects as one could encounter in natural gemstones[3]. 1.2 Gemstone Enhancement: The value of a gemstone is determined by the famous 4 big C s: Colour Clarity Cut or shape (or the potential for cutting) Carat (or the carat weight after cutting) All these factors might be influenced and enhanced on the journey from raw material to the cut gem. It is possible to influence these 4 C s with several physical and/or chemical methods. A cut and polished gemstone has a definite carat weight and cut, but its clarity and colour could he further upgraded by subjecting it to treatment techniques [2]. Commonly used treatment techniques are: I.2.1Temporary Gem Enhancements Bleaching: is used to lighten and/or whiten gemstones or pearls. Bleaching is a process that uses agents such as acids or hydrogen peroxide to remove unwanted color from a gem. Only a limited number of materials will respond to such treatment. Department of Materials Science 3

5 Coating: is surface enameling, inking, foiling, lacquering, sputtering, or vapor deposition Dyeing: is used to enhance the color and uniformity of gemstones or pearls Filling: is a process in which surface cavities or fissures are filled with colorless glass, plastic or resin Impregnation: Treating gems with oil, wax, or resin to improve clarity & appearance Waxing: Wax is rubbed on the surface of the stone to improve its luster. Emeralds are oiled with green resins or plastic or special mixture to hide cracks and to deepen the color. To make opals and turquoise more lustrous, oil impregnation is carried out to fill the pores. A desirable brown color or red shade is obtained in agate and chalcedony by spraying an acid solution of iron oxide. Aniline, indigo and synthetic dyes are employed to improve the appearance of quartz, topaz and amethyst. Synthetic growth over quartz has yielded larger and much more valuable emeralds. These practices are carried out in secretive fashion and without the adequate knowledge of reasons for color changes Permanent Gem Enhancements Irradiation: The irradiation sources can be classified into following types: 1. Electromagnetic radiation 2. Atomic particles Electromagnetic radiation commonly utilized to change the colour and clarity of the stones can be categorized as under; Light or ultraviolet light which produces less pronounced coloration X Rays that cause non uniform color and changes are not so significant Department of Materials Science 4

6 ^. 1. Gamma rays that produce uniform coloration accompanied by clarity in some cases. The gamma ray intensity is averaged out around 1.25Mev and can t be varied at will. At this intensity no radioactivity can be induced in gemstones. 2. Atomic particles that can be utilized in coloring gemstones are protons neutrons and alpha particles. Due to their very limited penetration, these are not suitable for commercial use. Neutrons induce uniform color but could cause residual activity depending upon nature of metallic impurities. Often gemstones such as sapphire, ruby and quartz on turning radioactive are stored till they are safe to handle. The gamma rays are the preferred choice since they do not induce radioactivity Laser treatment: The practice of laser drilling diamonds to create an opening through which acid could be introduced to remove a dark inclusion had remained unchanged for many years, until a new version was introduced that took advantage of advances in laser technology. Developed in Israel and referred to as the KM treatment this method does not actually drill a hole into the stones; instead, it uses lasers to create a small fracture from the inclusion to the surface so that the inclusion could be bleached. Most of the 'temporary' enhancements or treatments are basic and self-explanatory, but most of the 'permanent' gem enhancements involve highly sophisticated techniques that can be difficult to detect Heat treatment: is the most popular technique commercially used for the improvement of quality of gemstones. It mainly modifies color, unifies inclusions, increases transmittance and improves luster. Department of Materials Science 5

7 Heat treatment (also known as thermal enhancement) is applied to gemstones such as rubies & sapphires to improve color and /or clarity (maybe some phenomena such as asterism). Thermal enhancement of rubies and sapphires is considered stable and permanent, and it is generally accepted by the international gem and jewellery trade* It s practically impossible to establish a precise dating of the.first gemstones heating experiments. Red agates and camelians revealing evidence of heat treatment were produced in India in 2000 B.C.; many examples were also recovered from Egyptian tombs, including treasures found with Tutankhamen. Over the years there were number of technological studies on the effect of heating gems in various atmospheres and at various temperatures, often with the aim of understanding the causes of colour in the minerals that could be used as gemstones. Since the 1970 s there has been tremendous increase in the use of heat treatments applied to corundum family. The heat treatment used for gemstones can be as simple as placing the stone in a vessel and placing it over a. flame, to heating the stones in sophisticated furnaces having temperatures as well as environmental control. As a result of use of sophisticated instrumentation, made control on heating parameters and helped in optimizing the heat treatment parameters; such as: The maximum temperature reached. The time for which the maximum temperature is sustained. The rate of heating to obtain maximum temperature. The rate of cooling, from maximum temperature and any holding stage. The environment of heating. The nature of material in contact with gemstone. Department of Materials Science 6

All of these techniques are secondary value-addition steps [3]. To carryout them successfully, a wide knowledge of science & technology is essential. 1.

8 All of these techniques are secondary value-addition steps [3]. To carryout them successfully, a wide knowledge of science & technology is essential. 1.3 Classification of Heat Treatments: HEAT TREATMENTS r IWVOASS HLlPiC- I WITHOUT ADOmVB + + BERYLLIUM TREATMENT SYNTHETIC 03RUNOUM HEAT TREATMENT T Heat Treatments without additives Gemstones subject to heat treatment without additives command a higher price than those with any other treatments. In case of heating without additives the main emphasis is laid on the conditions at the time of heating and the temperature. Sapphires are generally heated in reducing environment while rubies in an oxidizing environment. Heat treatment without additives is generally more accepted than any other form of treatments [3]. Figure 1.1 shows natural corundum and Figure 1.2 shows heat treated corundum. Fig 1.1: Natural Corundum Department of Materials Science 7

9 1.3.2 Heal Treatment with additives Lead Filling: During this treatment, rubies are infused with lead crystal glass of high refractive index, as a result nearly opaque stones turn to transparent or translucent. However, this treatment is not permanent and many a times, lead-filled rubies get damaged during setting or mounting in jewelry. Today, lead filling is generally not accepted as a treatment because of the toxicity of lead during the heat treatment process and due to its non-permanent nature Recrystallization In this treatment opaque or very low quality sapphires and rubies are treated in a series of steps to recrystallize them and increase their transparency. Generally the steps involved are very long but at the end the material is completely different from what it was at the beginning Beryllium treatment Beryllium, when added in very small quantities during heat treatment, can change the color of sapphires greatly. As a result of this sapphires of various colours are available in the market which is the result of heat treatment by beryllium process [2], Flux Treatment In this treatment various additives such as borax, sodium carbonate, sodium silicate and many other natural minerals and lab made chemicals are added for various specific improvements in the rubies and sapphire. Flux filled rubies are generally more accepted than lead-filled or recrystallized rubies [2], Surface Diffusion Surface Diffusion treatment is performed on opaque or weak-colored rubies and sapphires. By diffusion, a thin coat of color is coated on the gemstones due to which the gems appear to have good color. However if the stone is recut or even scratched, Department of Materials Science 8

10 the coating goes away revealing its true color. Sometimes rubies and sapphires are coated with titanium dioxide which causes the formation of 6-ray or 12-ray star on the surface when viewed under the sun. These are normally called diffused star rubies / sapphires or diffusion treated star rubies / sapphires [3], Heat treatment of corundum can affect the presence of milkiness and asterism, the colour, and even the internal structure (inclusions) of the material. A number of separate processes have been distinguished, although several may be performed simultaneously [3]. The exact temperature, duration of treatment, and chemicals used for each process will depend on the specific material being heated; considerable variation is expected with corundum from different localities [3]. 1.4: Various processes for Heat Treatment: Process 1: Development of potential asterism Natural star sapphire /star ruby contains a significant amount of titanium oxide. If such corundum cools fairly rapidly from its growth conditions, the material remains clear; the titanium oxide is in "solid solution" in the form of Ti203 (titanium sesquioxide) replacing some of the A1203 (aluminum oxide). If such material, which typically contains only a few tenths of a percent titanium oxide, is held at between 1100 C and 1500 C for some time (say, 1300 C for 24 hours), particularly under mildly oxidizing conditions, the Ti203 converts to Ti02 (titanium dioxide) as follows: 2Ti = 4 Ti02... (I) In most cases, the Ti02 will then form needles of rutile within the corundum and thus produce asterism. This process is used to create synthetic stars in corundum [4] and it is analogous to process of formation of star in corundum in nature. In fact, if a piece of natural corundum containing sufficient titanium oxide was not heated sufficiently in nature to develop a good star, perhaps showing only fine "silk," asterism can be developed by means of such heat treatment [4], Department of Materials Science mtm 9

11 1.4.2: Process2: Removal of Silk or asterism If corundum containing silk or asterism caused by rutile needles is heated to a sufficiently high temperature, typically between 1500 C and 1700 C the rutile will dissolve in the corundum by the reverse of process I: 4Ti02 2Ti (2) When all the silk has disappeared, the corundum is cooled fairly rapidly, so that rutile needles do not re-form as in process I, Asterism may be removed in a similar manner [5], 1.4.3: Process 3: Development of colour in a stone with a potential for blue The color in blue sapphire is explained by a 'charge transfer" process [4, 6].This is widely accepted to originate from the iron-titanium combination. The process involved in these charge transfers is either Fe2" + Ti4+ ^ Fe3+ + xi3+... (3) or Fe3+a + Fe2+b + Ti4+ ~ Fe2+a + Fe3+b + Ti (4) Where a and b refer to different sites in the crystal. In each instance, a single electron is transferred from one atom to another atom and back again. It is important to note that both processes require that some of the iron be in the divalent ferrous, Fe2+, state. Also, sufficient quantities of iron and titanium must be present in the original stone to produce a deep blue. Sapphires containing adequate amount of iron and titanium oxides and if they are highly oxidized may have pale blue, green, yellow, or may be colorless in their original state. Such material may be heated in a strongly reducing environment to o I. convert some Fe to Fe, as follows: Fe203 +H2 ~ 2FeO + H (5) Department of Materials Science 10

12 This change can be achieved by an extended heating of the stone in a hydrogen atmosphere for equation (5) or by packing the material in charcoal, graphite or another substance that produces carbon (mineral oil, sugar, etc), so that combustion with only a small amount of air produces carbon monoxide. The maximum colour possible can be obtained via equation (5) Process 4: Lightening of blue Sapphire If blue sapphire is heated for an extended period in an oxidizing atmosphere (air or pure oxygen), all of the iron may be converted gradually to Fe As a result the removal of one of the essential coloring ingredients, Fe2+, on the left side of equations 3 and 4 takes place, thus lightening the blue color. If the process is continued long enough, a virtually colorless stone may be obtained Diffusion Mechanisms Diffusion is a mechanism in which atoms move from one region to another in solids. The amount of movement increases with both the rise in temperature and the increase in length of the heating process. Atoms of oxygen or hydrogen can move very rapidly in corundum, which explains effects of processes 3 and 4. Formation or removal of rutile needles of silk and asterism in titanium-containing corundum is also diffusion controlled process Adding Asterism by diffusion If the corundum does not contain titanium oxide, enough to form good asterism, it is possible to diffuse some titanium oxide into the gemstone in the form of a thin layer just below the surface. Typically, to produce the desired effect, slurry of aluminum titanate in water is painted onto the stone and then fired at about 1750 C for several days. The stone is cooled and a subsequent heat treatment, as in process /, develops Department of Materials Science 11

13 the asterism. The depth of penetration may be only one tenth of one millimeter. The process appears to be applied primarily to fractured material [8] Adding Colours other than blue by diffusion Just as diffusion of iron and titanium oxide can produce a blue colour on the surface, so can other color-causing impurities be diffused. Diffusion of chromium produces red colour, nickel gives yellow, chromium plus nickel creates the pinkish orange "padparadscha," and so on [4], Combinations of treatments Several of these processes can be combined, for enhancement of the gemstone. Removal of silk and intensification of color can be achieved in a pale sapphire by heating the stone in a reducing atmosphere and then cooling it rapidly, in a combination of processes 2 and 3. The removal of silk will produce an improvement in colour. Some mineral oils and chemicals like sodium bicarbonate are used, on milky white to pale blue "geuda" Sri Lankan sapphires, to prevent crack and possibly to remove some iron [10]. 1.5 Scope of the present Work Gemstones are mined in India in several places of Orissa, Rajasthan, Andhra Pradesh, Karnataka, Kashmir, Chhattisgarh. Kerala and Tamil Nadu but most of the mines yield rough which is not of much commercial value. Corundum is found plentiful in the Precambrian terrain of peninsular India. It occurs in diverse geological environment. A major belt consisting of varieties of corundum and is encountered on either side of close pet granite in central part of South India. It extends for nearly 500 km with a width of about 50 km all along east Karnataka and west Andhra Pradesh from Bellary district to Mysore district and further extends to high grade granulitic terrain of Tamil Naidu. Apart from this belt there are several places in Orissa where Department of Materials Science 12

14 corundum is found, in abundance [1]. Above all Sunjam, Doda district in Jammu and Kashmir yields world s finest sapphires. Gemstone treatment is an evolving art. Every few years, a new technique is developed which treats previously untreatable material, creating new marketable products. In India, scientific institutes have not been involved in any research on gemstones. Upgrading the properties of poor quality gemstones by heat treatment could add value to them by increasing their colour and or clarity. It is difficult to quantify the revenue potential of a heat treatment centre but a successful centre, more than generating profit for itself; will act as a catalyst to the business of gemstones in India. Another major factor is that most of the lower grade ruby mined in India is not suitable for heat treatment by present methods used in Thailand. Specific research is required to develop processes for such Indian rubies. If successful, it will boost mining and the processing of rubies in India. Corundum can be cleaned in acids and proper heat treatment in air can yield desirable results. Due to high porosity of corundum glass filling /flux filling can be carried out. Similarly Quartz is found in many states of India, which can be enhanced to gem quality by heat treatment. The heat treatments were carried out at different temperatures and under different environments. After each heat treatment characterization was carried out. The results revealed that heat treatment in air at 1200 C are the optimization conditions for colour enhancements of Mysore rubies. The silicates were enhanced at 400 C in air. Thus value addition in poor quality gemstones can be obtained at these temperatures. Department of Materials Science 13

15 1.6 References [1] Karanth R.V., Gems and Gem Industry in India, Geological society of India, Banglore, 2000 [2] Themelis T., The heat treatment of ruby and sapphire, Gemlab Inc., USA, 1992 [3] Nassau K., Gemstone Enhancement, Butterworth-Heinemann, Oxford, 1994 [4] Nassau K., Gems Made By Man, Chilton Book Co., 1980 [5] Falkenberg R., The Vemeuil Process. Crystal Growth, Vol. 2, Plenum Press, New York, 1978 [6] Nassau K., The causes of color. Scientific American, Vol. 243, pp ,1980 [7] Nassau K., Heat treating ruby and sapphire: Technical aspects, Gems and Gemology, Vol. 17, No. 3, pp ,1981 [8] Crowningshield R., Nassau K., Journal of Gemology, 1981 [9] Crowningshield R., Developments and highlights at GIA's lab in New York. Gems & Gemology, Vol. 16, pp ,1980 [10] Harder H., Edelsteine durch Brennen von Korunden Fortschritte der Mineralogie, Vol. 58, pp , 1980 [11] Carr R.R., Nisevich S,D., Altering the appearance of corundum crystals. United States Patent 3,950,596, filed December 20,1971, issued April 13,1976 [12] Emmett J.L and Douthit T R, Heat treating the Sapphires of Rock Creek Montana, Gems and Gemology, 1993 [13] Emmett J. L. et al, Beryllium Diffusion of Ruby and Sapphire, Gems & Gemology, summer, 2003 [14] Hughes R.W., Corundum: ruby and sapphire, White Lotus, 1991 [15] Webster R., Gems, Their Sources, Descriptions, and Identification, 4th ed. Rev. by B. W. Anderson, Butterworths, London, 1983 Department of Materials Science 14

Selected Answer: a. an optical phenomenon resulting in 6 or 12 pointed star reflectance, caused by inclusion of rutile needles

Selected Answer: a. an optical phenomenon resulting in 6 or 12 pointed star reflectance, caused by inclusion of rutile needles Question 1 1 out of 1 points Asterism in corundum is: a. an optical phenomenon resulting in 6 or 12 pointed star reflectance, caused by inclusion of rutile needles Question 2 1 out of 1 points The value