Preprints
https://doi.org/10.5194/egusphere-2024-1529
https://doi.org/10.5194/egusphere-2024-1529
04 Sep 2024
 | 04 Sep 2024

Luminescence and a New Approach for Detecting Heat Treatment of Sapphire

Teerarat Pluthametwisute, Lutz Nasdala, Chutimun Chanmuang N., Manfred Wildner, Eugen Libowitzky, Gerald Giester, Gamini Zoysa, Chanenkant Jakkawanvibul, Waratchanok Suwanmanee, Tasnara Sripoonjan, Thanyaporn Tengchaisri, Bhuwadol Wanthanachaisaeng, and Chakkaphan Sutthirat

Abstract. For decades, unravelling heat treatment of sapphire has been a challenging issue. The present study offers new aspects that support the detection of heat treatment of sapphire. Natural unheated sapphire is distinguishable from heated sapphire by its orange to red luminescence under long-wave ultraviolet (LWUV, 365 nm) light, whereas blue luminescence under short-wave ultraviolet (SWUV, 254 nm) light indicates their heated counterparts. UV-excited photoluminescence shows a linkage between a broad emission spectrum within the orange to red region and orange to red luminescence of natural unheated sapphire under LWUV illumination, as well as an emission spectrum around the green region and blue luminescence of heated sapphire under SWUV illumination. Furthermore, the presence of melt inclusions within dissolved silks may be used as an indicator of heat treatment of sapphire. It seems that Fourier-transform infrared (FTIR) spectroscopy alone is inadequate for distinguishing unheated and heated sapphire. The application of orange to red, and blue luminescence together with melt inclusions offer a novel and practicable procedure for more precise differentiation of unheated versus heated sapphire.

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Teerarat Pluthametwisute, Lutz Nasdala, Chutimun Chanmuang N., Manfred Wildner, Eugen Libowitzky, Gerald Giester, Gamini Zoysa, Chanenkant Jakkawanvibul, Waratchanok Suwanmanee, Tasnara Sripoonjan, Thanyaporn Tengchaisri, Bhuwadol Wanthanachaisaeng, and Chakkaphan Sutthirat

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-1529', Anonymous Referee #1, 22 Sep 2024
    • AC2: 'Reply on RC1', Teerarat Pluthametwisute, 15 Oct 2024
    • The number of studied samples is unclear. Chemical data of 15 samples are presented, so it is assumed that 15 samples were studied. It is really needful though presenting a list of the studied samples with their weight, dimensions, photos before and after heat treatment etc.
      • Answer: The sample sizes are already stated in Figure 1, Line 162: 'Sizes range between 4 and 12 mm.' (The incorrect '0.4 mm' will be corrected.)
    • The author used a specific way to heat the samples. However, corundum might be heated by using different ways. The reasons selecting these specific parameters for heat treatment should be explained in the manuscript.
      • Answer: To completely break down needles and silk inclusions, we heated geuda sapphire at 1650°C over a prolonged time. Lines 398–400 detailed the used condition.
    • Methods used should be better described. For instance, the authors are mentioning in lines 135 to 137 “Luminescence phenomena were observed and photo-captured both before and after heat treatment. The images were obtained under LWUV (365 nm) illumination using a commercial UV lamp, and under SWUV (254 nm) illumination by means of a DiamondView device”. Information about the power of the lamps as well as the observation distance should be mentioned. The authors are mentioning that SWUV (254 nm) was done by a DiamondView! In gemmology SWUV luminescence is usually observed by using a 3 to 6 watt SWUV lamp (similar to LWUV) and putting the sample around 10 cm from the lamp. Noteworthy that DiamondView is not emitting at 254 nm but at around 225 nm; it is important to check that again. Also, the conditions used to acquire photos need to be explained.
      • Answer: We will revise and clarify accordingly.
    • EMPA results need to be revisited. Gallium concentrations measured are extremely high for metamorphic sapphires and presence of manganese, potassium as well as calcium are most likely erroneous.  
      • Answer: Detection limits (50 ppm) may be reported and stated that these elements may be close to detection limits and are negligible under this study.
    • In lines 284 to 289 the authors are mentioning “For example, the presence of the 3309 cm-1 FTIR absorption peak was used as an indicator of heated corundum (Hughes and Perkins, 2019; Saeseaw et al., 2020; 286 Soonthorntantikul et al. , 2021). However, there were some inconsistencies presented recently: in some cases the 3309 cm-1 peak was also found in unheated sapphire, thus it is not a reliable indicator of heat treatment (Hughes, 1997, 2017; Hughes and Perkins, 2019; Saeseaw et al., 2020; Soonthorntantikul et al., 2021).” However, the presence/absence of the FTIR band at 3309 cm-1 is not used for the identification of heated sapphires. It is the presence of the band at 3232 cm-1 which indicates that the sapphire is heated. For that please check again the article Saeseaw et al., 2020 where it is mentioned in the abstract “This study also showed that Fourier-transform infrared (FTIR) spectroscopy, specifically the peak at 3232 cm–1, is a useful technique to detect low-temperature heat treatment in pink sapphires from Madagascar”. Presence of this band indicates heat treatment to all coloured sapphires of metamorphic origin. However, this criterion cannot be applied for the identification of heat treatment of basalt related corundum. It is strongly recommended to revise the text and better interpret the FTIR spectra.
      • Answer: We will revise the text and remove the citation of Saeseaw et al. (2020). Our experiment reveals limitations, as we did not observe the 3232 cm⁻¹ and 3185 cm⁻¹ peaks in any samples, suggesting a need for luminescence’s approach.
    • The parts dealing with FTIR spectroscopy and band interpretations need to be thoroughly revised. For example, is it mentioned to lined 272 to 275 “It should also be mentioned that the presence of CO2 peaks at 2339 and 2360 cm-1, as well as the C-H stretching related peaks at 2856 and 2925 cm-1 of all samples remained the same after heating. In contrast, their intensities vary dramatically (see Figs. 6 and 7).” The bands at around 2900 cm-1 are spurious; not linked to the sample but rather linked to finger fat/oil traces or oil contamination of the sample. Also, the bands linked to CO2 might be spurious, linked to the sample or both. For instance, in Figure 7b the CO2 related FTIR bands are not linked to the sample (e.g. negative bands). In case the authors desire to better study these bands, it might be worth to clean thoroughly the samples before measuring (so the oil linked bands are disappearing or decreasing) and purge the FTIR instrument to decrease the spurious bands.
      • Answer: The CO2 and C-H related bands are artefacts that were shown only because they are bracketed by two spectral regions of interest, O-H stretching and, in one case, diaspore bands, as seen in Fig. 7.
    • Some FTIR bands are presenting polarisation phenomena, did the authors took in account that? If yes, please add more information in the text.
      • Answer: In this case, we did not make any polarized measurements.
    • The authors presented spectra before and after heat treatment. The samples might be zoned. Did the authors try to acquire the spectra at the exact same position? If yes, please explain it in detail in the text or repeat the experiments.
      • Answer: Yes, we did acquire the spectra at the exact same position.
    • The absorption spectra presented are of average quality; e.g., Fig 5b for the heated sample G18 the band at 580 nm is saturated as well as 5c for the heated samples G11 and G12 the spectra present fringes. Absorption spectra in the UV (from 250 to 400 nm) are important for sapphires. It is suggested also presenting this part of the spectra.
      • Answer: Absorption spectra in the UV range for sample G18 (Fig. 5b, red line) are excluded due to noisy spectral envelopes and misleading artifacts. The poor quality and fringes result from the geuda samples' characteristics but do not significantly bias the spectra.
    • In sapphires luminescence is usually strongly zoned; how the authors tackled that? Did they acquire spectra in different areas?
      • Answer: PL measurements were consistently taken from the same area.
    • In the section 4.2 where the possible origin of the band at 525 nm is not clear to me. It is also important to notice that excitation by using laser or by lamp can give different phenomena. To obtain a clearer image, it is strongly recommended acquiring emission and excitation spectra on the samples before and after heat treatment. Also mapping of the luminescence might also help to better understand the various bands. It is also very important including unheated samples presenting the same reaction under SWUV lamp as well as some synthetic samples with similar phenomena.
      • Answer: We agree. The origin of the 525 nm band is currently unclear. However, I believe our findings will encourage gemological specialists to further study sapphire luminescence before and after heating. Future research should consider this.
    • What exactly the authors are proposing as approach in this manuscript to identify heat treatment is unclear. Did all samples present the same PL spectra with 325 nm laser excitation after heating? Or they propose a combination of several methods? If yes, a kind of flowchart might be better to illustrate that to the readers.
      • Answer: The flowchart is presented below:

      Overall, the manuscript needs to be reorganised, and some experiments/measurements need to be repeated. 

        • Answer: We will simplify the manuscript for better readability.
    Citation: https://doi.org/10.5194/egusphere-2024-1529-AC2
  • RC2: 'Comment on egusphere-2024-1529', Anonymous Referee #2, 03 Oct 2024
    • AC1: 'Reply on RC2', Teerarat Pluthametwisute, 15 Oct 2024
  • Status: closed

    Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
    • RC1: 'Comment on egusphere-2024-1529', Anonymous Referee #1, 22 Sep 2024
      • AC2: 'Reply on RC1', Teerarat Pluthametwisute, 15 Oct 2024
      • The number of studied samples is unclear. Chemical data of 15 samples are presented, so it is assumed that 15 samples were studied. It is really needful though presenting a list of the studied samples with their weight, dimensions, photos before and after heat treatment etc.
        • Answer: The sample sizes are already stated in Figure 1, Line 162: 'Sizes range between 4 and 12 mm.' (The incorrect '0.4 mm' will be corrected.)
      • The author used a specific way to heat the samples. However, corundum might be heated by using different ways. The reasons selecting these specific parameters for heat treatment should be explained in the manuscript.
        • Answer: To completely break down needles and silk inclusions, we heated geuda sapphire at 1650°C over a prolonged time. Lines 398–400 detailed the used condition.
      • Methods used should be better described. For instance, the authors are mentioning in lines 135 to 137 “Luminescence phenomena were observed and photo-captured both before and after heat treatment. The images were obtained under LWUV (365 nm) illumination using a commercial UV lamp, and under SWUV (254 nm) illumination by means of a DiamondView device”. Information about the power of the lamps as well as the observation distance should be mentioned. The authors are mentioning that SWUV (254 nm) was done by a DiamondView! In gemmology SWUV luminescence is usually observed by using a 3 to 6 watt SWUV lamp (similar to LWUV) and putting the sample around 10 cm from the lamp. Noteworthy that DiamondView is not emitting at 254 nm but at around 225 nm; it is important to check that again. Also, the conditions used to acquire photos need to be explained.
        • Answer: We will revise and clarify accordingly.
      • EMPA results need to be revisited. Gallium concentrations measured are extremely high for metamorphic sapphires and presence of manganese, potassium as well as calcium are most likely erroneous.  
        • Answer: Detection limits (50 ppm) may be reported and stated that these elements may be close to detection limits and are negligible under this study.
      • In lines 284 to 289 the authors are mentioning “For example, the presence of the 3309 cm-1 FTIR absorption peak was used as an indicator of heated corundum (Hughes and Perkins, 2019; Saeseaw et al., 2020; 286 Soonthorntantikul et al. , 2021). However, there were some inconsistencies presented recently: in some cases the 3309 cm-1 peak was also found in unheated sapphire, thus it is not a reliable indicator of heat treatment (Hughes, 1997, 2017; Hughes and Perkins, 2019; Saeseaw et al., 2020; Soonthorntantikul et al., 2021).” However, the presence/absence of the FTIR band at 3309 cm-1 is not used for the identification of heated sapphires. It is the presence of the band at 3232 cm-1 which indicates that the sapphire is heated. For that please check again the article Saeseaw et al., 2020 where it is mentioned in the abstract “This study also showed that Fourier-transform infrared (FTIR) spectroscopy, specifically the peak at 3232 cm–1, is a useful technique to detect low-temperature heat treatment in pink sapphires from Madagascar”. Presence of this band indicates heat treatment to all coloured sapphires of metamorphic origin. However, this criterion cannot be applied for the identification of heat treatment of basalt related corundum. It is strongly recommended to revise the text and better interpret the FTIR spectra.
        • Answer: We will revise the text and remove the citation of Saeseaw et al. (2020). Our experiment reveals limitations, as we did not observe the 3232 cm⁻¹ and 3185 cm⁻¹ peaks in any samples, suggesting a need for luminescence’s approach.
      • The parts dealing with FTIR spectroscopy and band interpretations need to be thoroughly revised. For example, is it mentioned to lined 272 to 275 “It should also be mentioned that the presence of CO2 peaks at 2339 and 2360 cm-1, as well as the C-H stretching related peaks at 2856 and 2925 cm-1 of all samples remained the same after heating. In contrast, their intensities vary dramatically (see Figs. 6 and 7).” The bands at around 2900 cm-1 are spurious; not linked to the sample but rather linked to finger fat/oil traces or oil contamination of the sample. Also, the bands linked to CO2 might be spurious, linked to the sample or both. For instance, in Figure 7b the CO2 related FTIR bands are not linked to the sample (e.g. negative bands). In case the authors desire to better study these bands, it might be worth to clean thoroughly the samples before measuring (so the oil linked bands are disappearing or decreasing) and purge the FTIR instrument to decrease the spurious bands.
        • Answer: The CO2 and C-H related bands are artefacts that were shown only because they are bracketed by two spectral regions of interest, O-H stretching and, in one case, diaspore bands, as seen in Fig. 7.
      • Some FTIR bands are presenting polarisation phenomena, did the authors took in account that? If yes, please add more information in the text.
        • Answer: In this case, we did not make any polarized measurements.
      • The authors presented spectra before and after heat treatment. The samples might be zoned. Did the authors try to acquire the spectra at the exact same position? If yes, please explain it in detail in the text or repeat the experiments.
        • Answer: Yes, we did acquire the spectra at the exact same position.
      • The absorption spectra presented are of average quality; e.g., Fig 5b for the heated sample G18 the band at 580 nm is saturated as well as 5c for the heated samples G11 and G12 the spectra present fringes. Absorption spectra in the UV (from 250 to 400 nm) are important for sapphires. It is suggested also presenting this part of the spectra.
        • Answer: Absorption spectra in the UV range for sample G18 (Fig. 5b, red line) are excluded due to noisy spectral envelopes and misleading artifacts. The poor quality and fringes result from the geuda samples' characteristics but do not significantly bias the spectra.
      • In sapphires luminescence is usually strongly zoned; how the authors tackled that? Did they acquire spectra in different areas?
        • Answer: PL measurements were consistently taken from the same area.
      • In the section 4.2 where the possible origin of the band at 525 nm is not clear to me. It is also important to notice that excitation by using laser or by lamp can give different phenomena. To obtain a clearer image, it is strongly recommended acquiring emission and excitation spectra on the samples before and after heat treatment. Also mapping of the luminescence might also help to better understand the various bands. It is also very important including unheated samples presenting the same reaction under SWUV lamp as well as some synthetic samples with similar phenomena.
        • Answer: We agree. The origin of the 525 nm band is currently unclear. However, I believe our findings will encourage gemological specialists to further study sapphire luminescence before and after heating. Future research should consider this.
      • What exactly the authors are proposing as approach in this manuscript to identify heat treatment is unclear. Did all samples present the same PL spectra with 325 nm laser excitation after heating? Or they propose a combination of several methods? If yes, a kind of flowchart might be better to illustrate that to the readers.
        • Answer: The flowchart is presented below:

        Overall, the manuscript needs to be reorganised, and some experiments/measurements need to be repeated. 

          • Answer: We will simplify the manuscript for better readability.
      Citation: https://doi.org/10.5194/egusphere-2024-1529-AC2
    • RC2: 'Comment on egusphere-2024-1529', Anonymous Referee #2, 03 Oct 2024
      • AC1: 'Reply on RC2', Teerarat Pluthametwisute, 15 Oct 2024
    • Teerarat Pluthametwisute, Lutz Nasdala, Chutimun Chanmuang N., Manfred Wildner, Eugen Libowitzky, Gerald Giester, Gamini Zoysa, Chanenkant Jakkawanvibul, Waratchanok Suwanmanee, Tasnara Sripoonjan, Thanyaporn Tengchaisri, Bhuwadol Wanthanachaisaeng, and Chakkaphan Sutthirat
      Teerarat Pluthametwisute, Lutz Nasdala, Chutimun Chanmuang N., Manfred Wildner, Eugen Libowitzky, Gerald Giester, Gamini Zoysa, Chanenkant Jakkawanvibul, Waratchanok Suwanmanee, Tasnara Sripoonjan, Thanyaporn Tengchaisri, Bhuwadol Wanthanachaisaeng, and Chakkaphan Sutthirat

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      Short summary
      For years, mankind has heated sapphire to increase its blue color and value. Gemologists have struggled for decades to detect heat-treated sapphire. Each test costs money. High-tech instruments like FTIR are costly. As a result, luminescence under SWUV and LWUV light provides a cheaper and more practical technique for identifying heat-treated sapphire. This work highlights that blue luminescence under SWUV light could indicate heated sapphire, whereas purplish red may also be helpful.