A new approach for detecting heat treatment of sapphire
- Author(s)
- T. Pluthametwisute, L. Nasdala, C. Chanmuang N., E. Libowitzky, G. Giester, Ch. Jakkawanvibul, W. Suwanmanee, T. Sripoonjan, Th. Tengchaisri, B. Wanthanachaisaeng, Ch. Sutthirat
- Abstract
Detecting heat treatment of sapphire has been a challenging issue for several decades. High-resolution optical microscopy, optical absorption (UV-VIS-NIR) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, Raman spectroscopy, and photoluminescence (PL) spectroscopy and imaging were applied in testing natural unheated sapphire before and after heat treatment.
Three groups of sapphire (high-density-silk, low-density-silk, and silk-free) were investigated. Most unheated sapphire specimens (except the high-density-silk group) showed orange to red luminescence under long-wave ultraviolet (LWUV) light, whereas under short-wave ultraviolet (SWUV) light, all specimens appeared inert. After heating at 1,650 °C for 10 hours under atmospheric conditions using an electric furnace, most of the sapphire specimens (except the high-density-silk group) showed strong purplish red luminescence under LWUV as well as blue luminescence under SWUV illumination. UV-excited PL indicated a link between the broad emission spectrum of the orange to red region and the orange to red luminescence presented in unheated sapphire under LWUV, as well as a link between the emission band of the green region and the blue luminescence of heated sapphire under SWUV illumination. Consequently,
occurrences of the orange to red emission band center around 630-650 nm and orange to red luminescence under LWUV that appeared in unheated sapphire specimens may serve as novel evidence of unheated sapphire, whereas the occurrence of the green emission band at the center around 525 nm and blue luminescence under SWUV that appeared in heated sapphire specimens2421
may serve as crucial evidence of sapphire that has undergone heat treatment. It is worth mentioning that the luminescence of purplish red under LWUV observed in heated sapphire specimens (except the high-density-silk group) could potentially function as an alternative means of detecting heated sapphire. It is noteworthy that sapphire belonging to the high-density-silk group appeared inert under LWUV and SWUV illumination, both before and after heating. This
may be attributed to the abundant presence of ferrous Fe (a luminescence quencher) derived from ilmenite. However, a more detailed investigation is recommended for the exact cause of such mechanism. In addition, fluid inclusions within dissolved silks may indicate high-temperature heat treatment of sapphire. It is also suggested here that applying Fourier-transform infrared (FTIR) spectroscopy alone is inadequate for distinguishing heated and unheated sapphire. Consequently, utilizing orange to red and blue luminescence, both prior to and after heat treatment, together with fluid inclusions, presents a novel and practical method for accurately distinguishing unheated sapphire and heated sapphire.- Organisation(s)
- Department of Mineralogy and Crystallography
- External organisation(s)
- Chulalongkorn University, Gem and Jewelry Institute of Thailand, G-ID Laboratories, Bangkok, 10120, Chiang Mai University
- Pages
- 2421-2422
- No. of pages
- 2
- Publication date
- 2024
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 105116 Mineralogy, 105113 Crystallography, 104026 Spectroscopy
- Keywords
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/92c454dd-c4de-4d2a-b5fc-52474d7ea2ee