gemstone heat treatment
The history of heat treatment goes back to more than 2000 years ago as mentioned in Pliny's book. Heat treatment can be as simple as placing a specimen into a glass test-tube. Another simple technique is to place the specimen into the kitchen oven or bury it in the center of a charcoal brazier. To avoid fractures, heating process can be done at slower rate by burying the metal in some inert powder, for intance, iron filings and unburned lime of Porta. This way heat penetrates slowly to and into the gemstone.
Heat can have many different effects on gemstones; the nine most important effects of heat on gemstones are summarized in the table below:
Heat can have many different effects on gemstones; the nine most important effects of heat on gemstones are summarized in the table below:
| Effect | Mechanism | Examples |
|---|---|---|
| Darkening | Gentle charring and/or oxidation | Aged amber and ivory |
| Color change | Destruction of color center | Blue or brown topaz and zircon to colorless |
| Color change | Change in hydration, aggregation | Carnelian to orange, red, or brown; sapphire to deep yellow or orange |
| Color change | Oxidation state change, usually with oxygen diffusion | Green aquamarine to blue; amethyst to deep citrine |
| Structural change | Reverse the irradiation-induced metamict state | Low zircon to high zircon |
| Structural change | Solution or precipitation of a second phase | Development or removal of silk or asterism in corundum |
| Color addition | Impurity diffusion | Diffused color and asterism in sapphire |
| Cracking | Rapid change of temperature | Fingerprints in sapphire' crackled quartz |
| Reconstruction and clarification | Flow under heat and pressure | Reconstructed and clarified amber |
First item in the table is darkening. Heat can be used to darken amber or ivory to give them an aged look. This is equivalent of a gentle charring , at least in part.
Heating can produce "color centers" to change the color of the gemstone. This color change can be in two main forms 1) bleaching or fading 2) changing the actual color of the gemstone as when some amethyst turns to a very pale citrine.
The change of color due to hydration or aggregation usually involves iron impurities. If limonite (yellow, hydrated iron oxide), the pigment ‘ochre’ of the artist, is heated the deep orange, brown, or red color of rust or of burned ochre is obtained.
Color change due to oxidation state change (forth item in the table). When any oxygen-containing substance (gemstone crystal) is heated in an enclosed space to a very high temperature, a little oxygen gas is either emitted or absorbed. Absorption of oxygen is called "oxidizing" and this happens when extra oxygen enters the gemstone crustal. Alternatively if oxygen is emitted from crystal we speak of "reducing" conditions. Any non-reactive gas that can react with oxygen is reducing; one example is hydrogen, which forms water. This absorption or emitting of oxygen eventually changes the chemical and electrical properties of the gemstone. These changes are the reasons for changing the color of gemstones.
Cracking the gemstone; Although cracking during heating is usually avoided, there are some occasions when this effect is actually considered desirable. The cracks of crystalline quartz as the result of heating and dropping into water are considered attractive resembling the naturally occurring iris quartz.
Heating can produce "color centers" to change the color of the gemstone. This color change can be in two main forms 1) bleaching or fading 2) changing the actual color of the gemstone as when some amethyst turns to a very pale citrine.
The change of color due to hydration or aggregation usually involves iron impurities. If limonite (yellow, hydrated iron oxide), the pigment ‘ochre’ of the artist, is heated the deep orange, brown, or red color of rust or of burned ochre is obtained.
Color change due to oxidation state change (forth item in the table). When any oxygen-containing substance (gemstone crystal) is heated in an enclosed space to a very high temperature, a little oxygen gas is either emitted or absorbed. Absorption of oxygen is called "oxidizing" and this happens when extra oxygen enters the gemstone crustal. Alternatively if oxygen is emitted from crystal we speak of "reducing" conditions. Any non-reactive gas that can react with oxygen is reducing; one example is hydrogen, which forms water. This absorption or emitting of oxygen eventually changes the chemical and electrical properties of the gemstone. These changes are the reasons for changing the color of gemstones.
Cracking the gemstone; Although cracking during heating is usually avoided, there are some occasions when this effect is actually considered desirable. The cracks of crystalline quartz as the result of heating and dropping into water are considered attractive resembling the naturally occurring iris quartz.
Major changes of selected gemstones on heat treatment table
| Material | Changes |
|---|---|
| Amber | Darker; pressed (reconstructed) |
| Beryl, aquamarine | Green to blue; yellow to colorless; orange to pink |
| Corundum, blue sapphire | Darken blue, lighten blue; add or remove asterism; add color |
| Corundum, colorless | To yellow, green or blue; add color |
| Corundum, ruby | Purple or brownish to red; pink to orange; add or remove asterism |
| Diamond | Change color of irradiated stones |
| Ivory | Darker |
| Quartz, amethyst | To colorless, yellow, brown, green or milky |
| Tourmaline | Blue or blue-green to green; red to paler or colorless |
| Zircon | Brown to reddish, colorless, or blue; green to blue or yellow |
| Zoisite | To deep purple-blue |
Reference: Gemstone Treatment by Kurt Nassau, Butterworths, 1984.