Many natural gemstones can be set into metal clay and fired in place. Other gemstones will not survive the heat of a kiln and should be set after firing. The charts below show the results of kiln and torch tests that have been performed on synthetic gemstones.
This information is for reference and should be used as a guide. There is always some risk of losing a natural gemstone even if others of it’s kind have survived in the past. Gemstones may have internal flaws that can be liquid or gaseous filled, or contain crystals of other materials that can cause the gemstone to fail where it usually does not. This guide aims to help metal clay artists sort out gemstones that are known to survive under fire from those that are not.
Gemstones are minerals that are classified into groups based upon the constancy of their major properties. Each mineral family has one or more varieties contained within the group. When we sort the tested gemstones according to their mineral group, it becomes clear that an easy way to gauge the survivability of a gemstone is to look at the results of other varieties within that same group. Aquamarine and emerald, for example, are both varieties of the beryl group of minerals. The result of tests done on aquamarine and
emerald indicate that minerals in the beryl group will not survive kiln heating. There are exceptions, as there always are in the natural world, but in general this method can be reliable for many varieties. For simplicity, many gems have been classified into their main group rather than their sub-variety.
These charts classify stones as No-Fire, Low-Fire, High-Fire, Torchable and Carbon-Safe. High-Fire stones are those that can withstand 1650F for at least 1 hour with no color change. Low-Fire stones are heat sensitive with a risk of color change. No-Fire stones are those that must be set after firing. Torchable stones are those that can survive at least a 2 minute torch firing, and Carbon-Safe stones are those that can survive a carbon firing
(carbon firing instructions are included at the end of this publication).The Moh’s hardness of each gemstone has been included to help dispell the myth that hardness determines
survivability (additional information on the Moh’s scale is provided on page 3 of this publication).
1.Temperature given is for hydrothermal grown gems.
2.This stone may darken slightly at 2 hours. The color may become a bit more orange, more intense.
3.This stone may darken slightly at 2 hours. The color may become a bit more orange, more intense.
4.All bright green CZ's are extremely oxygen sensitive. Fire at 1110F/10 minutes for any bright green stone, regardless of color name.
5.CZ's in the darker olive hue are stable up to 1650F/1H. Lighter olive colors are stable to 2 hours.
6.Orange CZ's can be fired up to 2 hours at 1650F, but color will fade slightly.
7.Tanzanite CZ's incur a very slight darkening, but no color loss at 1110F/10. Do not extend time. Extremely oxygen sensitive.
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