the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Sep 2024
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.
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RC1: 'Comment on egusphere-2024-1529', Anonymous Referee #1, 22 Sep 2024
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 -
AC2: 'Reply on RC1', Teerarat Pluthametwisute, 15 Oct 2024
Detailed comments from RC1:
- 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.
- Answer: Our samples are of the A-type from Sri Lanka, whereas the type of untreated Madagascar sapphire presented by Lotus Labs remains unclear. Thus, comparing our samples to theirs would be inappropriate.
- 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.
- Answer: We do not agree that synthetic sapphire, which lacks important inclusions for detection, would be useful. Our study focuses on blue luminescence in natural sapphire with titanium-bearing inclusions before and after heat treatment.
- 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.
- Answer: Our samples are A-type samples.
- 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.
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RC2: 'Comment on egusphere-2024-1529', Anonymous Referee #2, 03 Oct 2024
Comments on " Luminescence and a New Approach for Detecting Heat Treatment of Sapphire" by Pluthametwisute et al.
This manuscript presents a study of the effects of heat treatment of geuda sapphires on their luminescence. It aims at establishing criteria for detection of heated gems with applications in gemology and jewel commerce. The study is of relevance to these topics and was conducted in a rational way. The presentation is somewhat hard to follow to the half-specialist that I am, and for the reader in general. The expression is convoluted and would gain using shorter sentences and being more factual. Detailed comments on those aspects are provided below. I believe once these are taken in to account, the text will be simpler, clearer, and substantially shorter, all requirements to making this publication acceptable for publication in Solid Earth. I also ponder whether Solid Earth is the appropriate medium for this article, which may better find its public in a gemology or mineralogy journal.
Detailed comments:
The expression is quite convoluted starting with the abstract.
"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."
The first part of the sentence is in passive form, the second in active form and is opposed to the first by whereas. This does not help the reader. There are two independent observations of equal value there.
"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."
I struggled and failed to understand what you mean there.
"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."
The first sentence is factual, the second could be more direct, and the third goes back to positive observations made at the beginning of the abstract. Going back and forth between observations is difficult to follow. Try to avoid the use of adverbs that is often useless.
The text should be better organized here and in the reminder of the manuscript. I will not go through all the paragraphs that need clarification and simplification, and trust the authors to do so. Sections 3, 4 and 5 are too long, a lot of space is devoted to lengthy descriptions and discussions. A shorter version would be much more appealing to the reader. The conclusion section is mostly a (long) repeat of the discussion.
Samples are different from one figure to the other and from one table to the other. Sample numbers are not a significant name, at least not one that makes sense for the reader. Having suffixes like HS, LS, and SF for high silk, low silk and silk-free, would be more useful, as done with N and H for natural and heated in figs 5-7. this should be used throughout the text not only in figures. Figures 2-4 have no sample names.
Introduction: this section is long and may be simplified. Lines 45-55 could better be used in the discussion. Same for lines 65-75 that dilute the message of the introduction.
Materials and methods: the first sentence rather belongs to the result section where it is repeated line 146. It can be suppressed here.
Section 3
lines 151 155 This paragraph goes back and forth from natural to heated sample description. You call figure 8 here for sample G02.
lines 281-289 This belongs to the discussion section not to the results. Please check through the text similar occurrences where you can re-organize, this will help avoiding repetition and would improve a lot the readability.
Section 4
lines 452-458: is this paragraph useful, it does not seem informative.
lines 459-473: I may have misunderstood but it seems you spend 15 lines plus ten in the former § to discuss a feature for which you have no current solid interpretation. There may be room for simplification, and using direct expression, like just a list of shorter potential explanations.
Citation: https://doi.org/10.5194/egusphere-2024-1529-RC2 -
AC1: 'Reply on RC2', Teerarat Pluthametwisute, 15 Oct 2024
Detailed comments from RC2:
- The expression is quite convoluted starting with the abstract.
"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."
The first part of the sentence is in passive form, the second in active form and is opposed to the first by whereas. This does not help the reader. There are two independent observations of equal value there.
- "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."
I struggled and failed to understand what you mean there.
- "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."
The first sentence is factual, the second could be more direct, and the third goes back to positive observations made at the beginning of the abstract. Going back and forth between observations is difficult to follow. Try to avoid the use of adverbs that is often useless.
- The text should be better organized here and in the reminder of the manuscript. I will not go through all the paragraphs that need clarification and simplification, and trust the authors to do so. Sections 3, 4 and 5 are too long, a lot of space is devoted to lengthy descriptions and discussions. A shorter version would be much more appealing to the reader. The conclusion section is mostly a (long) repeat of the discussion.
- Samples are different from one figure to the other and from one table to the other. Sample numbers are not a significant name, at least not one that makes sense for the reader. Having suffixes like HS, LS, and SF for high silk, low silk and silk-free, would be more useful, as done with N and H for natural and heated in figs 5-7. this should be used throughout the text not only in figures. Figures 2-4 have no sample names.
- Introduction: this section is long and may be simplified. Lines 45-55 could better be used in the discussion. Same for lines 65-75 that dilute the message of the introduction.
- Materials and methods: the first sentence rather belongs to the result section where it is repeated line 146. It can be suppressed here.
Section 3
- lines 151 155 This paragraph goes back and forth from natural to heated sample description. You call figure 8 here for sample G02.
lines 281-289 This belongs to the discussion section not to the results. Please check through the text similar occurrences where you can re-organize, this will help avoiding repetition and would improve a lot the readability.
Section 4
- lines 452-458: is this paragraph useful, it does not seem informative.
- lines 459-473: I may have misunderstood but it seems you spend 15 lines plus ten in the former § to discuss a feature for which you have no current solid interpretation. There may be room for simplification, and using direct expression, like just a list of shorter potential explanations.
Answer:
Dear Editor, we are grateful for a constructive review from Referee #2. We agree with comments 1, 3-8, and 10 and will revise the manuscript accordingly. For comment 2, we attempted to indicate that there might be a correlation between the broad emission obtained from UV-excited PL investigation and its luminescence in this experiment. This will be clarified in the revised manuscript. For comment 9, It can be removed.
Citation: https://doi.org/10.5194/egusphere-2024-1529-AC1
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AC1: 'Reply on RC2', Teerarat Pluthametwisute, 15 Oct 2024
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