Abstract
A significant proportion of mined natural corundum (ruby and sapphire) contain fractures, which negatively affects a gemstone’s clarity and value. Over the past decades, heat treatment techniques have been developed for either fracture healing, or filling to make such gems marketable. The clarity enhancement processes are mainly based on techniques which are either not durable, as in the case of lead silicate fillers, or do not yield perfect transmittance through a fracture, as in the case of borax based fluxes. Therefore, the gemstone treatment community is actively in pursuit of better techniques for clarity enhancement in corundum. Given that application of pressure is a recent advancement in the heat treatment processes of natural sapphire, it is essential to explore the possibilities regarding different outcomes such treatments can have. In this perspective paper, we have briefly described how application of pressure during heat treatments can lead to in-situ sintering of transparent polycrystalline ceramics within the fractures of corundum, which can result in clarity enhancement. Spinel-structure based fillers can be tailored to mimic corundum in terms of tribological, chemical and optical properties. Therefore, gemstones treated with such fillers will be durable, unlike currently used glass-based filler material. We also provide a possible explanation for ghost-fissures in sapphires heated under pressure, as being a by-product of in-situ sintering process of ceramic fillers that are thermodynamically compatible with Al2O3. The prospect of transparent polycrystalline ceramics in the gem and jewelry industry opens a new field of research in this area, given that ceramic fillers can outperform currently used methods and material for clarity enhancement in gemstones. In essence, we present a novel application for sintered transparent polycrystalline ceramics.
Highlights
Transparent ceramics Heat treatment Sapphire Transient liquid phase sintering High temperature + pressure (HT+P) High pressure high temperature (HPHT)
Hematite and ilmenite are often reported as Surface reaching fracture nano or micron size precipitates in sapphire [6, 8], dissolution of which can alter the overall color of a gemstone
Given the problems associated with present methods of clarity enhancement, the gemstone heat treatment community is actively in pursuit of better techniques, which makes it important to study the possibilities in this regard to safeguard the fine jewelry industry from treatments that may go undisclosed
Summary
Transparent ceramics Heat treatment Sapphire Transient liquid phase sintering High temperature + pressure (HT+P) High pressure high temperature (HPHT). The following paragraphs briefly introduce transparent ceramics and describe how an in-situ sintering of a spinel phase can be achieved within the fractures of such gemstones, through the application of pressure in the treatment process.
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