WO2024095040A1 - Procédé d'amélioration des caractéristiques de couleur et/ou de la transparence d'au moins un corindon naturel et en particulier d'un rubis - Google Patents

Procédé d'amélioration des caractéristiques de couleur et/ou de la transparence d'au moins un corindon naturel et en particulier d'un rubis Download PDF

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Publication number
WO2024095040A1
WO2024095040A1 PCT/IB2022/060552 IB2022060552W WO2024095040A1 WO 2024095040 A1 WO2024095040 A1 WO 2024095040A1 IB 2022060552 W IB2022060552 W IB 2022060552W WO 2024095040 A1 WO2024095040 A1 WO 2024095040A1
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Prior art keywords
ruby
crucible
corundum
minutes
heating
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PCT/IB2022/060552
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English (en)
Inventor
Vladimir Viktorovich LEVCHENKO
Eduard Anvarovich AKMETSHIN
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Levchenko Vladimir Viktorovich
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Priority to PCT/IB2022/060552 priority Critical patent/WO2024095040A1/fr
Publication of WO2024095040A1 publication Critical patent/WO2024095040A1/fr

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/10—Inorganic compounds or compositions
    • C30B29/16—Oxides
    • C30B29/20—Aluminium oxides
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B33/00—After-treatment of single crystals or homogeneous polycrystalline material with defined structure
    • C30B33/02—Heat treatment

Definitions

  • the present invention relates to a method for improving colour characteristics and/or transparency of a natural corundum and in particular a ruby.
  • Corundum is a crystalline form of aluminium oxide (AI2O3) typically containing traces of iron, titanium, vanadium and chromium. Corundum is a naturally transparent material, but can have different colours depending on the presence of transition metal impurities in its crystalline structure. Corundum has two primary gem varieties: ruby and sapphire. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colours depending on what transition metal is present.
  • Ruby is the most sought-after variety of natural corundum on the jewellery market. As with other gemstones, rubies are graded using criteria known as the four Cs, namely colour, cut, clarity and carat weight. All natural rubies have imperfections in them, including colour impurities and inclusions of rutile needles known as "silk”. Gemmologists use these needle inclusions found in natural rubies to distinguish them from synthetics, simulants, or substitutes.
  • hue refers to colour as the everyday acceptance of the term.
  • Ruby refers to a corundum with red primary hue. All other hues of the gem species corundum are called sapphire.
  • Rubies may exhibit a range of secondary hues, including orange, purple, violet, and pink.
  • Tone refers to a gem’s relative lightness or darkness while saturation refers to the intensity of its primary hue (strong or soft).
  • High-grade rubies - distinguished by their bright red colour - are extremely rare and account for no more than 0.1 % of all rubies mined.
  • the reference stones are rubies from Burma, characterised by a pure red colour, without any secondary hues. Stones from other deposits - Mozambique, Madagascar, etc. - are characterised by the presence of secondary hues and a saturation of colour, either excessive or insufficient. Secondary hues, usually orange, purple, violet, and pink significantly reduce, sometimes tenfold, the value of the stone.
  • the main chemical element present as impurities in the crystalline structure and responsible for the variable colouring in a ruby are: Cr, Fe, Ti, V, known as chromophore elements. These chemical elements form the colour centres in the crystalline field of corundum by replacing Al 3+ ions in the form of Fe 3+ , Fe 2+ , Cr 3+ , Ti 3+ , Ti 4+ ions. Cr3 + ions are responsible for the bright red colour.
  • Iron ions form a group of colour centres depending on concentration, valence and structural localisation: iron III ion Fe 3+ induces an orange and greenish secondary hue; charge transfer [Fe 3+ -Fe 3+ ] induces a yellow secondary hue while charge transfer [Fe 2+ -Fe 3+ ] induces a blue-green secondary hue; Ti forms a chromophore association in the presence of divalent iron [Fe 2+ -Ti 4+ ] inducing a blue secondary hue. Vanadium ion V 3+ induces a greyish purple secondary hue.
  • ruby colouration is a variation function, where the primary bright red colouration is related to chromium and its concentration, and the secondary hues are determined by the above chromophore elements, their concentration and ratio.
  • Improvements used include colour alteration, improving transparency by dissolving rutile inclusions, healing of fractures (cracks) or even completely filling them.
  • the most common treatment is the application of heat.
  • Most rubies at the lower end of the market are heat treated to improve colour, mainly saturation, remove purple tinge, blue patches, and silk. These heat treatments typically occur around temperatures between 800° and 1800°C.
  • Some rubies undergo a process of low tube heat, when the stone is heated over charcoal at a temperature of about 1300°C for 20 to 30 minutes. The silk is partially broken, and the colour is improved.
  • a flux such as borax can be added to reduce the temperature. Fractures can then be healed with a lower temperature and clarity of the gem is improved.
  • the rough stone is pre-polished to eradicate all surface impurities that may affect the process
  • the second heating process occurs in an electrical oven with different chemical additives. Different solutions and mixes have shown to be successful, however mostly lead-containing glasspowder is used at present.
  • the ruby is dipped into oils, then covered with powder, embedded on a tile and placed in the oven where it is heated at around 900 °C for one hour in an oxidizing atmosphere.
  • the orange-coloured powder transforms upon heating into a transparent to yellow-coloured paste, which fills all fractures. After cooling the colour of the paste is fully transparent and dramatically improves the overall transparency of the ruby. If a colour needs to be added, the glass powder can be "enhanced" with copper or other metal oxides as well as elements such as sodium, calcium, potassium etc.
  • the second heating process can be repeated three to four times, even applying different mixtures. This treatment can be identified by noting bubbles in cavities and fractures using a 10x loupe. Rubies treated by lead-glass filling are not considered purely natural anymore and this type of treatment should be disclosed prior sale.
  • US 6 872 422 proposes a method for imparting colour to a gemstone consisting of coating the gemstone with one or more materials chosen depending on the desired colour to form of a thin film over the gemstone and heating the composition between approximately 700°C and 1200°C for 30 minutes to 10 hours in air or oxygen until the desired colour is achieved.
  • US 5 888 978 discloses a method for enhancing the colour of a gemstone such as a topaz, a quartz or a garnet consisting of subjecting a combination of a gemstone and at least one treating agent comprising a finely divided form of cobalt of cobalt oxide to a temperature in the range of about 900°C up to about 1250 °C, for a time in the range of about 3 up to about 200 hours, under conditions suitable to enhance the colour of said gemstone.
  • RU 2738536 discloses a method for colour enhancing garnet using a heat treatment.
  • the heat treatment is carried out a temperature from 700°C to 900°C for 20 to 120 minutes in a reducing atmosphere.
  • the heat treatment can be carried out in a corundum crucible and/or in the presence of powdered graphite.
  • the heat treatment can be repeated and can further comprise a preliminary heat treatment at a temperature from 700°C to 900°C for 20 to 120 minutes in neutral atmosphere.
  • the aim of the present invention is to provide a method for improving colour characteristics of a natural corundum and in particular ruby which overcomes the above-mentioned technical problems.
  • the object of the invention is a method according to claim 1 .
  • the method for improving colour characteristics of a natural corundum is in particular well suited for rubies, for example rubies with distinct orange and purple secondary hues and rutile micro inclusions.
  • rubies for example rubies with distinct orange and purple secondary hues and rutile micro inclusions.
  • One preferred embodiment of the method according to the invention is described in the following.
  • the first step of the method according to the invention is preferably to clean the said ruby to remove foreign mineral impurities and contaminants.
  • the now clean ruby is then subjected to a first heat treatment carried out in a reducing atmosphere.
  • the ruby is put in a corundum crucible with dispersed graphite powder.
  • the crucible is then tightly sealed or closed and is heated between 1580°C and 1630°C for at least 60 minutes but preferably 90 minutes.
  • the maximum heating rate is around 10°C/min, i.e., not faster than around 10°C/min.
  • carbon black can be used to establish a reducing atmosphere during the first heat treatment: carbon black, activated carbon, soot, nanotubes, fullerenes... All these forms of carbon are suitable to establish a reducing atmosphere at temperatures above 750°C-780°C through a two-stage reaction:
  • Reactions are heterogenous as there is an interaction process between the solid carbon form and the gas inside the crucible.
  • the solid carbon form graphite
  • the surface of the interacting phases increases and the process intensifies.
  • This first heat treatment allows to dissolve inclusions and increase the transparency. Indeed, when heating natural ruby between 1580°C and 1630°C, the rutile micro inclusions are dissolved at a microlevel, leading to a visual improvement of the transparency of the stone.
  • the chromophore elements Fe 3+ (responsible for the yellow-orange secondary hues) and Cr 3+ (responsible for the bright red and sark red primary hue) undergo the following changes in a reducing atmosphere of carbon monoxide generated by the interaction of the graphite and the atmospheric oxygen:
  • the iron colour centres in corundum such as ruby depend on the chromophore associates formed: charge transfer Fe 2+ - Fe 3+ induces greenish and blue green hues; charge transfer Fe 2+ - Ti 4+ induces a blue hue.
  • the first heat treatment allows to reduce the blue hues by dissolving rutile which is the source of Ti and allows to reduce the green hues by changing iron III ion in iron II ion.
  • the ruby gets more transparent and changes to a deep red colour with some purple secondary hue.
  • the crucible is cooled to room temperature, preferably at a rate of 3 to 10°C per minute.
  • the stone is removed from the crucible and is preferably rinsed with water to get rid of any impurities.
  • a second heat treatment occurs now.
  • the ruby is again put in a corundum crucible (the same as the first heat treatment or another one) and the crucible is heated to a temperature between 1300°C and 1580°C in an oxidizing environment (i.e., with access to air, no sealed crucible) for at least 60 minutes but preferably 180 minutes.
  • the heating rate is preferably of maximum 10°C per minute.
  • FeO interacts with O2 from ambient air to form Fe2Os forming new colour centres associated with iron III ions: Fe 3+ -Fe 3+ responsible for a yellowish-red-orange hue.
  • Fe 3+ -Fe 3+ responsible for a yellowish-red-orange hue.
  • These colour centres enhance the red colour centres associated with Cr 3+ : there is a transition of Fe 2+ to Fe 3+ and an increase in the absorption of Cr 3+ colour centres.
  • colour centres Fe 2+ -Ti 4+ are further destroyed during this second heat treatment ensuring a further reduction of the blue colour giving a purplish tint to the ruby.
  • the crucible is cooled to room temperature, preferably at a rate of 3 to 10°C per minute.
  • the stone is removed from the crucible and preferably rinsed with water to get rid of any impurities.
  • the rubies with a bright red colour can be sent for cutting.
  • rubies with remaining purple hues are sent for a third heat treatment.
  • the additional third heat treatment in case some purple hues remain, consists of heating the ruby in a corundum crucible in an oxidising environment (in air atmosphere) to a temperature of 1400°C for 180 minutes.
  • the heating rate is preferably 10°C per minute.
  • the crucible is cooled to room temperature, preferably at a rate of 3-10°C per minute. The ruby can then be sent for cutting.
  • the first heat treatment allows changes between iron III ions, iron II ions and Ti4+ ions to reduce green and blue hues and form a purple hue; •
  • the second and/or third heat treatment form a yellowish/orange/red hue enhancing the primary red hue of the ruby.
  • the embodiment above discloses a first cleaning step.
  • This step is optional and/or can be any suitable cleaning process known to the person of ordinary skills in the art.
  • any other pre-treatment step known could be applied like polishing, ...
  • the possible additional third heat treatment can be repeated until the desired colour enhancement is reached.
  • Any crucible suitable for a corundum can be used for each of the heating treatment and in particular ceramic crucible.
  • the enhanced ruby treated with the method according to the invention presents a bright red colour without undesirable hues and a high transparency. Both of these changes are stable and not superficial but allow the improved ruby to be cut as wished. The treatment does not alter any other qualities of the ruby and is not easily distinguishable on the finished product.
  • Samples of natural ruby from the Andilamena deposit in Madagascar were selected for this example.
  • a total of 120 crystals with dimensions from 3 to 12 mm, without visible cracks, with minor spinelide inclusions, light muntins due to microinclusions of needle-shaped rutile crystals were used for the experiments.
  • the colours of the samples range from orange-red (47 crystals) to pink-red (35 crystals) and purple-red (38 crystals).
  • the samples were placed in a 100 ml corundum crucible with 5 grams of graphite powder added and covered with a tight lid.
  • the corundum crucible was then placed in a SNOL 12/16 furnace with chromite lanthanum resistive elements and heated to 1600°C.
  • the heating rate was 10 °C per minute and the holding time was 90 min.
  • the oven was cooled down to room temperature.
  • the heat-treated rubies were washed after removal from the container - 40 of the original 120 crystals became markedly more transparent, 89 became distinctly purple, and were lighter in colour. Some of the crystals remained without visible changes.
  • the raw crystals were then put into an open crucible, also heated in the same furnace at 1500 °C for 180 min. After this second heat treatment, 32 crystals turned bright red - these stones were then sent for cutting. 88 remaining crystals were put into the crucible and subjected to a final third heat treatment - heated at 1400°C, in an open crucible, with a holding time of 180 min.
  • the method for improving colour characteristics and/or transparency of at least one natural corundum in particular a ruby comprises the following steps:
  • a first heat treatment consisting of heating at least one ruby in a tightly sealed conundrum crucible in a reducing environment to a temperature between 1580°C to 1630°C, preferably 1600°C for at least 60 minutes preferably 90 minutes, preferably at a maximum heating rate of 10°C per minute.
  • the crucible and its content are then cooled to room temperature preferably at a rate of 3 to 10°C per minute and the ruby is removed from the crucible and preferably rinsed with water to get rid of any impurities.
  • a second heat treatment consisting of heating the ruby in a corundum crucible with access to air to create an oxidising environment to a temperature between 1300°C and 1580°C, preferably 1500°C for at least 60 minutes but preferably 180 minutes, preferably at a maximum rate of 10°C per minute.
  • the method further comprises a cleaning step before the first heat treatment designed to clean the ruby of foreign mineral impurities and contaminants.
  • the method further comprises a sorting step before the first heat treatment or before the cleaning step and designed to sort corundum in different batch depending on their problematic secondary hues.
  • the method is then applied to each batch separately.
  • the crucible is a corundum crucible and graphite is added in the crucible to induce the reduced atmosphere.
  • the method further comprises a third heat treatment taking place after the second heat treatment: if the ruby still presents purple hues, it is heated in a crucible with access to air to create an oxidising environment to a temperature of 1400°C, for at least 180 minutes, preferably at a maximum rate of 10°C per minute. The crucible is then cooled to room temperature, preferably at a rate 3- 10°C per minute.
  • the treated ruby can then be cut and used to create jewellery.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Inorganic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystals, And After-Treatments Of Crystals (AREA)

Abstract

La présente invention concerne un procédé d'amélioration des caractéristiques de couleur et/ou de la transparence d'au moins un corindon naturel, en particulier d'un rubis, ce procédé comprenant les étapes suivantes : -chauffer au moins un rubis dans un creuset de corindon hermétiquement fermé dans un environnement réducteur à une température entre 1580 °C et 1630 °C, de préférence à 1600 °C, pendant au moins 60 minutes, de préférence pendant 90 minutes ;-refroidir le creuset et son contenu jusqu'à la température ambiante ;-chauffer le rubis dans un creuset de corindon avec accès à l'air pour créer un environnement oxydant à une température entre 1300 °C et 1580 °C, de préférence à 1500 °C, pendant au moins 60 minutes mais de préférence pendant 180 minutes.
PCT/IB2022/060552 2022-11-02 2022-11-02 Procédé d'amélioration des caractéristiques de couleur et/ou de la transparence d'au moins un corindon naturel et en particulier d'un rubis WO2024095040A1 (fr)

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PCT/IB2022/060552 WO2024095040A1 (fr) 2022-11-02 2022-11-02 Procédé d'amélioration des caractéristiques de couleur et/ou de la transparence d'au moins un corindon naturel et en particulier d'un rubis

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS57191299A (en) * 1981-05-20 1982-11-25 Natl Inst For Res In Inorg Mater Preparation of single crystal of corundum shedding asterism
US5888978A (en) 1995-09-12 1999-03-30 Trega Biosciences, Inc. Method for reducing the severity of gastro-intestinal damage
CN1266112A (zh) * 2000-03-03 2000-09-13 中国科学院上海光学精密机械研究所 蓝宝石晶体的脱碳去色退火方法
US6872422B2 (en) 2001-07-09 2005-03-29 Samir Gupta Process for imparting and enhancement of colours in gemstone minerals and gemstone minerals obtained thereby
RU2738536C1 (ru) 2019-09-30 2020-12-14 Алексей Владимирович Власов Способ термической обработки минерала и термически обработанный минерал

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS57191299A (en) * 1981-05-20 1982-11-25 Natl Inst For Res In Inorg Mater Preparation of single crystal of corundum shedding asterism
US5888978A (en) 1995-09-12 1999-03-30 Trega Biosciences, Inc. Method for reducing the severity of gastro-intestinal damage
CN1266112A (zh) * 2000-03-03 2000-09-13 中国科学院上海光学精密机械研究所 蓝宝石晶体的脱碳去色退火方法
US6872422B2 (en) 2001-07-09 2005-03-29 Samir Gupta Process for imparting and enhancement of colours in gemstone minerals and gemstone minerals obtained thereby
RU2738536C1 (ru) 2019-09-30 2020-12-14 Алексей Владимирович Власов Способ термической обработки минерала и термически обработанный минерал

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
TKACHENKO S ET AL: "Control of optical properties of YAG crystals by thermal annealing", JOURNAL OF CRYSTAL GROWTH, ELSEVIER, AMSTERDAM, NL, vol. 483, 5 December 2017 (2017-12-05), pages 195 - 199, XP085323998, ISSN: 0022-0248, DOI: 10.1016/J.JCRYSGRO.2017.12.002 *

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