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https://doi.org/10.5194/egusphere-2024-1529
© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/egusphere-2024-1529
© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
04 Sep 2024
| 04 Sep 2024
Status: this preprint is open for discussion.
Luminescence and a New Approach for Detecting Heat Treatment of Sapphire
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.
How to cite. Pluthametwisute, T., Nasdala, L., Chanmuang N., C., Wildner, M., Libowitzky, E., Giester, G., Zoysa, G., Jakkawanvibul, C., Suwanmanee, W., Sripoonjan, T., Tengchaisri, T., Wanthanachaisaeng, B., and Sutthirat, C.: Luminescence and a New Approach for Detecting Heat Treatment of Sapphire, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-1529, 2024.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
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RC1: 'Comment on egusphere-2024-1529', Anonymous Referee #1, 22 Sep 2024
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I went through the manuscript entitled "Luminescence and a New Approach for Detecting Heat Treatment of Sapphire” by Pluthametwisute et al. with great interest. The manuscript deals with an important issue for the gemmological and scientific community and the idea of better understanding/studying the luminescence of the heat-treated sapphires is good. However, the manuscript is difficult to follow, and it lacks in several aspects. Please find below a list of suggestions:
- The main issue of the manuscript is that the authors are claiming that luminescence under SWUV is observed solely for heated sapphires. However, similar luminescence reaction might be present under SWUV in unheated sapphires (check figures 10 and 11 at https://lotusgemology.com/resources/articles/156-heat-seeker-uv-fluorescence-as-a-gemological-tool) It is unclear whether such unheated samples could be characterized/identified as unheated or not by using to their approach. Before proposing a new approach, it is strongly recommended to study such samples and check whether these can be identified or not.
- Similar reaction under SWUV might also be observed in synthetic corundum. It is also recommended studying synthetic samples with similar luminescence under SWUV and compare them.
- Samples’ reliability is unclear. The main problem is that the gems might be heated directly around the mining area. Samples confidence level (see for example those used by GIA; https://www.gia.edu/gems-gemology/winter-2019-building-research-collection-understanding-development-gem-deposits) should be mentioned.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Some FTIR bands are presenting polarisation phenomena, did the authors took in account that? If yes, please add more information in the text.
- The authors presented spectra before and after heat treatment. The samples might me 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.
- 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.
- In sapphires luminescence is usually strongly zoned; how the authors tackled that? Did they acquire spectra in different areas?
- 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.
- 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.
Overall, the manuscript needs to be reorganised, and some experiments/measurements need to be repeated.
Citation: https://doi.org/10.5194/egusphere-2024-1529-RC1
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Teerarat Pluthametwisute
Department of Geology, Faculty of Science, Chulalongkorn University, 10330 Bangkok, Thailand
Institut für Mineralogie und Kristallographie, Universität Wien, 1090 Wien, Austria
Lutz Nasdala
Institut für Mineralogie und Kristallographie, Universität Wien, 1090 Wien, Austria
Chutimun Chanmuang N.
Institut für Mineralogie und Kristallographie, Universität Wien, 1090 Wien, Austria
Manfred Wildner
Institut für Mineralogie und Kristallographie, Universität Wien, 1090 Wien, Austria
Eugen Libowitzky
Institut für Mineralogie und Kristallographie, Universität Wien, 1090 Wien, Austria
Gerald Giester
Institut für Mineralogie und Kristallographie, Universität Wien, 1090 Wien, Austria
Gamini Zoysa
Mincraft Co., 10370 Mount Lavinia, Sri Lanka
Chanenkant Jakkawanvibul
The Gem and Jewelry Institute of Thailand (Public Organization), 10500 Bangkok, Thailand
Waratchanok Suwanmanee
The Gem and Jewelry Institute of Thailand (Public Organization), 10500 Bangkok, Thailand
Tasnara Sripoonjan
G-ID Laboratories, Bangkok, 10120, Thailand
Thanyaporn Tengchaisri
Science and Technology Park (STeP), Chiang Mai University, 50200 Chiang Mai, Thailand
Bhuwadol Wanthanachaisaeng
The Gem and Jewelry Institute of Thailand (Public Organization), 10500 Bangkok, Thailand
Chakkaphan Sutthirat
CORRESPONDING AUTHOR
Department of Geology, Faculty of Science, Chulalongkorn University, 10330 Bangkok, Thailand
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.
For years, mankind has heated sapphire to increase its blue color and value. Gemologists have...