Gem Resources International

Gemstone Firing Tests

Gem Resources International
PO Box 62006
Minneapolis, MN55426
800-588-8515
612-664-0252
gri@gemresources.com
 
 
Gem Resources – Firing Tests
Lab Created Gemstones
Safe to fire or Fireable
Cubic Zirconia – white, pink, purple, orange, yellow, peridot, lavender, champagne, (black is usually safe-a few have changed to a icky burnt color)
Lab Created Spinel – Aquamarine, Zircon, Sapphire
Lab Created Corundum – Alexandrite color, Garnet Ruby Red
Synthetic Sapphire Pink
Synthetic Sapphire Blue
Lab Created Green Garnet
 
Lab Created Gemstones
Not safe to fire
Cubic Zirconia – Emerald Green, Tanzanite
 
Natural Gemstones
All Gems tested by firing on a ceramic fiber kiln shelf. Faceted gems were fired table down.
 
A. First firing at 1200º for 1 hour - fast ramp of 1798º per hour, left in kiln with door closed to cool to 300º
 
3 mm Orissa Garnet - Faceted        Survived
3 mm Orissa Garnet - Cab            Survived
4 mm Blue Sapphires - Cab           Survived
3 mm Black Sapphire - Faceted       Survived
3 mm Red Garnet - Cab               FAILED- hematite appearance
3 mm Red Garnet - Faceted           FAILED - Slight Hematite Burn
3 mm Peridot - Cab                  Survived
3 mm Peridot - Faceted              Survived
6 mm Champagne Topaz - Faceted      Survived - Color Change to White,
2 mm White Sapphire - Faceted       Survived
Ceylon Sapphire - Rough             Survived
Ant Hill Garnet - Rough             Survived
5 mm Black Spinel - Faceted         Survived
Montana Sapphire - Rough            Survived
Peridot - Rough                     Survived
3 mm Ant Hill Garnet - Faceted      Survived
 
B. Same stones fired at 1470º for 30 minutes - fast ramp of 1798* per hour, left in kiln with door closed to cool to 300º
3 mm Orissa Garnet - Faceted        Survived
4 mm Blue Sapphires - Cab           Survived
3 mm Black Sapphire - Faceted       Survived
3 mm Red Garnet - Cab               Not Tested
3 mm Peridot - Cab                  Survived - Color Change to Light Olive
3 mm Peridot - Faceted              Survived - Color Change to Light Olive
6 mm Champagne Topaz - Faceted      Survived - Color Change to White,
2 mm White Sapphire - Faceted       Survived
Ceylon Sapphire - Rough             Survived
Ant Hill Garnet - Rough             Survived
5mm Black Spinel - Faceted          Survived - Slight hematite burning
Montana Sapphire - Rough            Survived
Peridot - Rough                     Survived - Color change to Olive
3mm Ant Hill Garnet - Faceted       Survived
 
C. Fired at 1110º for 45 minutes - fast ramp of 1798º per hour, left in kiln with door closed to cool to 300º
3 mm Peridot - Faceted              Survived - No Color Change
Peridot - Rough                     Survived - No Color Change
3 mm Peridot - Cab                  Survived - No Color Change
5 mm Black Spinel - Faceted         Survived - Very Slight Color Change,
6 mm Champagne Topaz - Faceted      Survived - Color Lightened
Hessonite Garnet - Rough            Survived - No Color Change
Orissa Garnet - Cab                 Survived - No Color Change
Red Garnet - Faceted                Survived - No Color Change
 
D. Fired at 1650 for 10 minutes - fast ramp of 1798º per hour, left in kiln with door closed to cool to 300º
Hessonite Garnet - Rough            Survived - No Color Change
Black Sapphire - Faceted            Survived - No Color Change
Ant Hill Garnet - Faceted           Survived - No Color Change
Blue Sapphire - Faceted             Survived - No Color Change
Red Garnet - Faceted                Slight Hematite Burn
Ceylon Sapphire - Rough             Survived - No Color Change
Montana Sapphire - Rough            Survived - No Color Change
Ant Hill Garnet - Rough             Survived- No Color Change
Orissa garnet - Faceted             Survived - No Color Change
Peridot - Rough                     Color Change to Olive
Peridot - Faceted                   Color Change to Olive
Black Spinel - Faceted              Color Change to Definite Hematite Look
 
The garnets were all ok at lower temps, but above 1380 F they started to get a metallic sheen. 
 
By 1560 F the garnets also started to have color changes along with the metallic glaze, beginning to darken and turn muddy looking.
 
One of the White Topaz cabs cracked at the lowest firing temp (1110 F), but the others were fine up to 1650. I'm assuming that particular cab had a crack or flaw.
 
The Sunstone and Labradorite did great. 
 
*Natural stone is never "guaranteed" because flaws, cracks, cutting, different chemical compositions, etc. will affect outcomes.
 
**"Disclaimer" natural stones may not always behave the way we expect them to.