We tested these Cultured Opals with an azure, a hole behind the stone, with the following results:
 

Low Fire Fine Silver

• This Cultured Opal can be fired for 30 minutes at 1650ºF (900ºC) in a kiln with Art Clay Fine Silver and PMC3 Fine Silvers that can fire at temperatures up to 1650ºF and for shorter periods of time with no vermiculite burial.

• This Cultured Opal can be fired directly in a Torch on a bed of vermiculite. Stone facing down, culet or pointed side facing up.

• Long 2-hour firings need to be buried in fine-grain vermiculite. The cultured Opal is placed in the kiln,  the stone table up or down doesn't matter, with the jewelry sitting on top of a 1/2" inch bed of vermiculite, and with 3/4" inches of vermiculite covering it. (See Beware section below.)
   

Enriched Sterling Silver Clays

• Art Clay 950 and A Hand-mixed 960 (a 50/50 mix of Art Clay Fine Silver and 925 PMC Sterling) have been successfully fired in a kiln.

• Use fine-grain vermiculite to insulate the stone on the long firings, which protects it from hazing on the stone's surface. (We also found out that removing the haze with a diamond paste and a cotton handheld rotary tool is an option, but you probably want to avoid that.)

• The Cultured Opal is placed in the kiln, stone table down and pointed culet up, with the base of the jewelry sitting on top of a 1/2" inch bed of vermiculite, and with 3/4" inches of vermiculite covering it.

• Art Clay 950 - 1500°F ramp to temp 932ºF, Hold 30 minutes, 2nd ramp 1650ºF to 1625ºF, hold 2 hours; 960 - Successful firings at 1350ºF ramp to 1650ºF for 2 hours.

• Hand-mixed 960 (a 50/50 mix of Art Clay Fine Silver and 925 PMC Sterling) 1350ºF ramp to 1650ºF, hold 2 hours.
  (See Beware section below.)
  
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How to Fire Cultured Opals in Bronze Metal Clay

Karen Trexler tested these Cultured Opals in Aureus Bright Bronze with an azure, a hole behind the stone, with the following results:
 

• Placed with the table of the Heat Resistant Cultured Opal facing down on an inch of Coconut Carbon for the initial phase to burn out the binders.

• Set the kiln to a 1350℉ ramp.

• Fire for 30 minutes at 650℉.

• Once the first phase is finished, keep the Opal oriented with the culet pointed up and pour an additional inch of carbon over the top of the piece.

• Ramp at 1350℉.

• Set the target temperature for 1400℉ with a 2-hour hold.
 


  (See Beware section below.)
   

 How to Fire Cultured Opals in Copper Metal Clay

Karen Trexler tested these Cultured Opals in Cyprus Copper Clay with an azure, a hole behind the stone. This clay has a two-phase firing in Coconut Carbon:
 

• Placed with the table of the Heat Resistant Cultured Opal facing down on an inch of coconut carbon for the initial phase to burn out the binders.

• Set the kiln to a 1350℉ ramp.

• Fire for 30 minutes at 650℉.

• Once the first phase is finished, keep the Opal oriented with the culet pointed up and pour an additional inch of carbon over the top of the piece.

• Ramp at 1350℉.

• Set the target temperature for 1600℉ with a 3-hour hold.


   

 Beware of Cracking or Hazing

• Don't exceed temperatures above 1650º F / 900ºC
  If you exceed these temperatures you may get cracking or hazing on the stone.

• Re-firing these Cultured Opals - Proceed at your own risk. We've had mixed results.
  A long 2-hour refire cycle at 1650ºF in vermiculite was too much for a small stone and it cracked. Another larger Cultured Opal was fired twice with a short 30-minute firing at 1650ºF and it DID work. What's the difference? Either the size of the stone or the length of the firing or both affected the results. More tests are needed on this topic, and we'll see, then report back. Our best guess is when you refire to repair cracks, fire at shorter times, and temps when possible.

• A stone not set deep enough into the vermiculite when needed can get hazy on the front or back surface. The hazy can be removed with a tight cotton polishing head with a rotary tool and a small bit of diamond polishing paste.

• Not all cultured or lab-grown opals are heat resistant. Results may differ with different clays. Please see the list of clays tested on the right. Other clays "may" work fine, but they have not yet been tested. Christopher and Holly Gage, Gage Designs, and Healing Phoenix Lapidary can not be held responsible for individual results or operator error. We are providing this information to foster the love of using Cultured Opals and your enjoyment as a jewelry artisan.

• If you want to see the reports on tests performed, go to
  Holly's Blog.