Since working with the Fire in Place Opals, several professionals in the Metal Clay community and I have tested many clays. Michael Glavin, Karen Trexler, Jade Reed, and Ann Adkins tested various brands of Base Metal. I tested various Silver Metal brands. We will continue to report our findings on the Healing Phoenix Lapidary website as new discoveries come in, and as new Metal Clays appear on the market. The examples below give you an idea of the process for both Silver and Base Metal Clays with slightly different processes and firing times. Please see the Firing Chart that follows for specific clays.

The factors that make the majority of the difference between clays are the medium in which you fire in —vermiculite for Fine Silvers or carbon for any Metal Clay containing Base Metals such as Bronze, Copper, or Sterling Silver. The other factor is the amount of shrinkage of the clay. Up to 15 – 20% result in no problem, whereas higher shrinkage clays will require bezels fitting more loosely or using prongs to compensate for the shrinkage. This rule applies to all fire in place gems, not just these Fire in Place Cultured Opals. ANY high shrinkage clay needs room to shrink so if you are getting any cracking around the stone, it’s either a bezel that is too tight or fissures in the clay to begin with.
 

How to Fire Cultured Opals in Silver Metal Clay

We tested these Cultured Opals with an azure, a hole behind the stone, in a deep burial of vermiculite 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.

• Please see the Firing Chart that follows for specific clays.
  (See Beware section below.)

Torch Firing Fine Silver

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

Enriched Sterling Silver Clays

We tested these Cultured Opals with an azure, a hole behind the stone, in a deep burial of vermiculite with the following results:

• Enriched 950 and 960 Silver Metal Clay have been successfully fired in a kiln as long as they do not pass the 1650º F threshold.

• 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, the stone table up or down doesn’t matter, 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.

• Please see the Firing Chart that follows for specific clays.
  (See Beware section below.)
   

How to Fire Cultured Opals in Base Metal Clays such as Copper or Bronze

We tested these Cultured Opals with an azure, a hole behind the stone, with the burnout stage done on a bed of carbon, and the sinter stage done with a minimum of 1” of carbon covering the work. Different Base Metal Clays have specific firing schedules, and some manufacturers specify a single-stage firing, but all base metal clays can be two-stage fired, which is generally more reliable and quicker. Please check Manufacturers Firing recommendations, but you can also refer to the tested Firing Chart that follows for specific clays that were tested with the Cultured Opals successfully using the listed parameters. The First and second stages were fired without a lid or cover, but you can use a vented or askew lid for the second stage if you choose. Using a lid will reduce carbon consumption somewhat, and slow heating, so please add 25% to the sinter times in the table, and adding time will never hurt your project.

• Place the piece with the Heat Resistant Cultured Opal facing down on an inch of Coconut Carbon for the initial phase to burn out the binders. Some testing was also done with the culet down with no ill effects.

• Set the kiln to the ramp suggested.

• Fire for the designated time and temperature.

• Once the first phase is finished, pour an additional inch of carbon over the top of the piece. You do not need to handle the work, and it will be VERY fragile at this stage, so it is best left alone, but can be carefully moved if required. A lid can be added at this stage, but it should be vented or placed slightly askew on the container.

• Ramp at the suggested temperature.

• Set the target temperature and hold time suggested.

• (See Beware section below.)

​

Beware of Cracking or Hazing

• Don’t exceed temperatures above 1650º F / 900ºC
  If you exceed these temperatures, you may get cracking or hazing of 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 in the Metal Clay, 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 in the chart. Other clays “may” work fine, but they have not yet been tested.