Firing Cultured Opals in Silver Metal Clay

Since the introduction of Fire in Place Opals, several professionals within the Metal Clay industry, including Michael Glavin, Karen Trexler, Jade Reed, and Ann Adkins, have tested various clay. I focused on different Silver Metal brands. We will keep updating our findings on the Healing Phoenix Lapidary website as we make new discoveries and as new Metal Clays become available. The following examples illustrate the processes for both Silver and Base Metal Clays, which have slightly different methods and firing times.

Please refer to the following Firing Chart specifics on various clays. The main factors influencing clay performance include the firing medium—vermiculite for Fine Silver or carbon for Base Metal Clay like Bronze, Copper, or Sterling Silver.

Another critical factor is the clay's shrinkage. A shrinkage of 15-20% poses no issues, but higher shrinkage may require more loosely fitting bezels or prongs. This guideline applies to all fire in place gems, including these Cultured Opals.

Any clay with high shrinkage must allow room for shrinkage; if you notice around the stone, it may be due to a bezel that's too tight or pre-existing fissures in the clay.

Firing Cultured Opals in Silver Metal Clay

Our tests involved Cultured Opals set with an azure and a hole behind the stone, buried deep in vermiculite, yielding the following results: Low Fire Fine Silver can be fired for 30 minutes at 1650ºF (900ºC) in a kiln using Art Clay Fine Silver and PMC Fine Silvers, which can endure temperatures up to 1650ºF for shorter without vermiculite burial.

These Cultured Opals can be torch-fired directly on a bed of vermiculite, with the stone facing down and the pointed side facing up. Long firings of 2 hours require fine-grain vermiculite burial, with the jewelry placed on a ½ inch layer of vermiculite and covered with ¾ inch of vermiculite.

(Refer to the "Beware" section below.) Torch Firing Fine Silver allows for direct firing of Cultured Opals on vermiculite, with the same stone positioning.

Enriched Sterling Silver Clays

We tested these Cultured Opals similarly, ensuring a hole behind the stone and deep vermiculite burial: Enriched 950 and 960 Silver Metal Clay can be fired successfully in a kiln as long as temperatures stay below 1650ºF.

Using fine-grain vermiculite for long firings protects the stone from hazing. (While haze removal is possible with diamond paste and a rotary tool, it's better to avoid this if possible.) Place the Cultured Opal in the kiln, with the stone's orientation being inconsequential, on a ½ inch bed of vermiculite, and cover with ¾ inch of vermiculite.

(Refer to the "Beware" section below.)

Firing Cultured Opals in Base Metal Clays like Copper or Bronze

We also tested Cultured Opals with an azure and a hole behind the stone, beginning the burnout stage on a carbon bed and sintering with at least 1 inch of carbon covering the work.

Different Base Metal Clays have specific firing schedules; while some manufacturers suggest single-stage firings, all can be reliably two-stage fired, which is generally quicker. Always check manufacturers' firing recommendations, and refer to the tested Firing Chart for specifics regarding successful Cultured Opal parameters. The first and second stages should be fired without a cover, though a vented lid can be used for the second stage if desired. Using a lid can slightly reduce carbon consumption and slow heating, so consider adding 25% to the sinter times provided.

Start with the Heat Resistant Cultured Opal facing down on an of Coconut Carbon for the initial phase to burn out binders. Some tests showed no adverse effects with the culet down. Set the kiln to the suggested ramp, fire for the designated time temperature, and once the first phase is complete, add another inch of carbon over the piece. It’s best to avoid handling the work at this fragile stage, but careful movement is possible if necessary. A lid can be added, but it should be vented.

Ramp at the suggested temperature, targeting the recommended temperature and hold time.

(Refer to the "Beware" section below.)

Beware of Cracking or Hazing

Do not exceed temperatures above 1650ºF / 900ºC. Exceeding these temperatures may result in cracking or hazing of the stone. Re-firing these Cultured Opals is your own risk; we’ve had mixed results. A lengthy 2-hour refire cycle at 1650ºF in vermiculite proved too much for a smaller stone, leading to cracks. Conversely, a larger Cultured Opal fired twice with a short 30-minute session at 1650ºF was successful. The size of the stone or the duration of firing likely influenced the outcome, necessitating further tests.

A stone not deep enough in vermiculite can develop a hazy surface, which can be removed using a tight cotton polishing head with a rotary tool and a small amount of diamond polishing paste. Not all cultured or lab-grown opals are resistant; results may vary with different clays. Please consult the tested clays list in the chart.