COLOR ENHANCEMENT OF FANCY SAPPHIRES BY HEAT TREATMENT
Color enhancement of fancy sapphires by heat treatment
E. Akhmetshin, T. Bgasheva
D. Mendeleyev University of Chemical Technology of
The main mines of colored sapphires are situated in South-East Asia,
The article deals with a certain variety of
The present level of development of science and technology offers great opportunities in the gem enhancement. There are many different ways to change the properties of stones but not all of these changes can be determined by existing methods. Changing the natural properties of stones is made with different purposes included improvement, removal or changes of stone color imperfections hiding, hardening of the material, improving its appearance. It is estimated that the proportion of enhanced stones on the jewelry market by the end of the 20th century had reached 60-70%. Corundum gem varieties (ruby, sapphire and color sapphires) occupy a considerable part of the enhanced gemstones and almost all of corundums figuring in commerce nowadays are treated with one or another way of enhancement. The most common method of enhancement is a heat treatment. Heat treatment enhancement effects on the whole stone, is stable in time and irreversible under normal conditions.
Color is the most visually prominent feature of precious stones and it considerably determines their value. The most valuable varieties of transparent corundum are ruby (red), sapphire (blue) and sapphire padparadscha (pink-orange). Other color varieties of corundum (fancy sapphires) have less cost than the above mentioned ones. By the intensity of color bright saturated stones are considered as the most expensive. The cost of the stone also decreases because of the not uniform coloration. Although there are some objective criteria appraisal of the jewelry corundum and quality of its tint is a complex issue and mostly it is subjective. In contrast to diamond these stones are valued not so much by cleanliness as by color: the original tint combined with a small defect is preferable to the more ordinary color.
Chromofore impurities in corundum
Corundum α - Al2O3 is crystallized in the trigonal crystal system. The crystal structure of corundum is the closest hexagonal packing of anions, O2-, in which two thirds of the octahedral voids are occupied with the aluminum ions Al3+. Coloration of corundum is mainly due to ions Al3+ that are replaced with 3d - ions (ions of such elements as Sc, Ti, V, Mn, Fe, Ni, Cu, Zn, elements that have unpaired electrons in external orbitals) during the crystallization process. This is possible because the size of the 3d - transition element ions is close to the size of the ions Al3+. The available data on the parameters of crystal field of 3d - transition elements substituting Al3+ ions in the lattice of corundum (Al2O3) suggest that the first few excited states of all elements lie in the visible region. All 3d - elements, if they are present in corundum, will participate in coloration mechanisms. Mechanisms may cause the absorption band from weak to strong, electronic transitions in which lead to a variety of sapphire colors ranging from purple to red.
The color is a superposition of light waves that are not absorbed by mineral when light passes through a mineral or reflects from the its surface, and therefore one or another optically active centers in the mineral which selectively absorb light of certain wavelengths will be responsible for the coloration. In corundum transition d-elements present in mineral as an isomorphic impurities are these centers. These elements are often referred to as chromophore.
Chromophore ions can be both isolated and bounded and can occupy different structural positions relative to each other. Chromophore effect of the presence of transition element ions strongly depends on the structural component. Thus, impurity ions can occupy either separated from each other octahedral positions in the lattice or occupy the neighboring octahedrals. In this case intensive electron exchange interactions between the ions of transition metals can arise accompanied by the formation of additional energy levels of these ions and by the appearance of additional absorption bands in the spectra. In the mineralogical literature these bands are called in different ways: the absorption band of charge transfer metal à metal or absorption bands of exchange-coupled pairs. These bands are tenfold, hundredfold intensive than absorption bands of the spin-allowed d-d absorption bands as they corresponds to the d-d transitions with a considerable touch of electronic states. Not infrequently isolated impurities themselves do not contribute considerably to the overall color of the mineral while when if they are simultaneously present in neighboring positions of the lattice it causes visible changes in mineral coloration.
The influence of the color of colored sapphires chromophoric centers
The main chromophore impurities causing coloring of sapphires concerned are: chromium Cr (III), iron Fe (II and III) and titanium Ti (IV). Chromium (III) and iron (II and III) are in the state of ions, titanium (IV) can be both as ions and solid phase (rutile needles TiO2). Depending on the concentration and quantity ratios between chromophore impurities the color of corundum can has different intensity. Thus, Cr3+ - red color, Fe3+(Fe3+-Fe3+) - yellow color, brownish and yellowish tints (undesirable in red stones).
There are intense broad absorption bands in the visible region of optical spectra due to spin allowed electronic transitions in the chromium ions that give the red color.
The intensive broad absorption bands conditioned of spin-allowed electronic transitions in chromium ions present in the visible region of optical spectra and give the red color.
The yellow color of iron-containing corundum is due to the combined influence of intensive short-wave edge of absorption band associated with the charge transfer band and absorption bands in the visible spectrum, most of which are caused by d-d transitions in Fe3+ ions. The absorption bands of Fe3+ - Fe3+ appear in the same spectral range as isolated ions Fe3+ but the former have a lot more intensity and clearly defined structure conditioned of the splitting of levels due to exchange interaction.
The absorption bands associated with Fe2+ and Fe2+ - Fe3+ appear beyond the visible range of optical spectra and have a weak influence on color (greenish, bluish tones).
If the sapphire has only a small amount of Fe2+ ions or Ti4+ only, it can remain colorless. However, when Fe2+ and Ti4+ are present simultaneously and in the neighboring positions in the lattice an intense blue color is a result. This type of absorption is caused by intervalence charge transfer Fe2+ - Ti4+. Titanium and the iron ions are combined in pairs forming a “biparticles” (Fe, Ti)6+. The absorption band in the visible spectra is caused by the charge transfer Fe2+ à Ti4+ .
Except for the above mentioned chromophore impurities some quantities of impurities V, Ti, Mn and other elements are responsible for tints in sapphire colors. Since these impurities are present the investigated colored sapphires only in trace amounts their detailed consideration is not carried out.
Experiments and results
For heat treatment of fancy sapphires was used the high-temperature electric furnace with an induction heating element. The experimental setup is shown in fig.1*. Depending on the gem material and the targeted color of sapphires after treatment the different temperature regimes are specified, additionally (if required) a special gas environment is created by the use of selected reagents possessing different redox strength.
Heat treatment mechanism*
The main variables available for control during the heat treatment of fancy sapphires are: the redox strength of the heat treatment atmosphere, the working temperature and holding time at this temperature. The type of the atmosphere in which heat treatment is carried out is a factor determining the direction of the process. For example, for initial sapphires containing impurities of chromium (III), iron (II and III) and titanium (IV) under oxidizing heat treatment the formation of additional amounts of Fe (III) will take place, and that will cause the intensification of the yellow components in the resulting color. Under reducing atmosphere the amount of Fe (III) falls and a decreasing in intensity of yellow tints occurs. By reducing of iron new chromophore centers Fe2+ - Ti4+ are produced which can give violet, cherry and blue tints to the treated sapphires. The concentration of Cr (III) does not change during neither oxidizing nor reducing heat treatment. The resultant color of the samples will consist of induced during heat treatment and that which is caused by the chromophore centers do not undergoing changes during treatment namely Cr3+.
*Heat treatment mechanism is confirmed by microprobe analysis data and optical spectroscopy (in this article these data are not given).ListenRead phonetically
A few series of heat treatment experiments of fancy sapphires were carried out in reducing and weak-reducing atmospheres upon the various conditions (redox strength of atmosphere, working temperature and holding time at working temperature were varied) within the temperature range 1100-1800°С. The color of all samples was fixed by means of the GIA GemSet. The results of experimental work are shown in Fig. 2-5.
Fig.3. Heat treatment of colord sapphires at 1400-1700 ° C in reducing atmosphere.
Fig.4. Samples of color sapphires before and after heat treatment (D = 2,5-
1 – samples before heat treatment, 2 - samples after heat treatment.
Generalized data from experiments shows that:
Reducing heat treatment at 1100-1200 ° C: in the orange-red sapphires (group oR, RO / OR, rO, O) the red component in their color becomes more pronounced. The color of the yellow and orange sapphires (yO, oY) depending on the atmosphere redox strength and the composition of the sample can be shifted both into the red tones and into the yellow ones. Yellow sapphires (group Y) lose color intensity and shift into the light Y colors and low saturation. Yellow-green sapphires (group styG) shift into green colors (yG, slyG).
Oxidizing heat treatment at 1100-1200 ° C: sapphire groups oR, RO / OR, rO, O, yO, oY are not undergo changes in color. Sapphires of Y group does not change color or may become more saturated depending on the atmosphere oxidizing strength and the composition of the sample.
Reducing heat treatment at 1400-1700° C: red-orange sapphires (group rO) depending on the atmosphere redox strength can either stay within their color group slightly changing the tone and saturation or get “new” color in the range from orange-red (RO / OR , oR) through red (R) up to cherry (stpR) and violet (rP) tones. Heat treatment at 1800°С leads to full loss of jeweler properties of treated sapphires.
Composition and structural features of presence of the chromophore impurities determines the resulting color of sapphires after heat treatment.
Compliance with certain parameters of heat treatment (the redox strength of heat treatment atmosphere, the working temperature and holding time at working temperature) for a given color group allows to get one or another possible color of the sapphires after heat treatment.
Positive results attainable during heat treatment increase the cost characteristics of sapphires. Heat treatment allows to obtain merchantable color from the original fancy sapphires of unpopular colors and does not require relatively high pecuniary costs.
At this stage of work authors believe that the reducing heat treatment of orange-red sapphires is the most optimal treatment on the basis of the developed method:
- in the range of 1100-1200 ° C (groups oR, RO / OR, rO) color gets red tones,
- in the range of 1400-1700 ° C (group rO) color shifts into the red and purple tones.
This method can be used for other colored sapphires: the authors are working on developing technology for the dark blue sapphires for the purpose of enlightenment and enhancing of quality characteristics of such gem material, as well as for sapphires of other color groups for color improving (at this stage principles of heat treatment for yellow and green sapphires are established). Application of the given kind of enhancement is perspective for the corundums containing chromophore impurities with variable valencies, with an aim of improving their qualitative characteristics.
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