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How to Fire in Place Cultured Opals
in Metal Clay

Firing Healing Phoenix Lapidary Cultured Opals in Silver Metal Clay

Since I tested and introduced Fire in Place Opals (FIP), several professionals within the Metal Clay industry, including Karen Trexler, Jade Reed, Michael Glavin, and Ann Adkins, have tested various clays. I focused on different Silver Metal brands. We will keep updating our findings 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 Complete Fire in Place Guide specifics on various clays. The main factors influencing Metal lay performance include the firing medium—vermiculite for Fine Silver or carbon for Base Metal Clay like Bronze, Copper, or Sterling Silver.


What to Know About Metal Clay Shrinkage

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 splits and fractures around the stone, it may be due to a bezel that's too tight or pre-existing fissures in the clay.


The Importance of an Azure
An azure is a hole behind the stone. It is important when firing gems, especially transparent gems, so you don't see the fired unpolished Metal Clay behind the stone. The kiln white, for example, of Silver can make the gem look cloudy due to the milky white color. The hole lets the light through, and allows the Cultured Opals to shine beautifully.

Fine Silver Metal Clay and Cultured Opals

Short Low Fire Silver Clays Doesn't Need a Vermiculite Burial  
Most Low Fire Fine Silver Clays, such as Art Clay Fine Silver, can be fired for 30 minutes or shorter at 16750ºF/900ºC in a kiln. They can also endure temperatures up to 1650ºF for shorter without vermiculite burial.

 

Torch Firing Fine Silver Needs A Bed of Vermiculite

Healing Phoenix Lapidary Cultured Opals can be torch-fired directly on a bed of vermiculite, with the stone facing down and the culet facing up.


Long Fine Silver Firings Needs a Deep Vermiculite Burial

A long firing of Metal Clay of 2 hours is used for additional strength and requires fine-grain vermiculite burial, with the jewelry placed on an ½ inch layer of vermiculite and covered with ¾ inch of vermiculite.

Enriched Sterling Silver Clays - 950/960

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 an ½ inch bed of vermiculite and cover it with ¾ inch of vermiculite.
(Refer to the "Beware" section below.)


Firing Cultured Opals in Base Metal Clays like Copper or Bronze

We tested the Healing Phoenix Lapidary Cultured Opals with an azure 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
Complete Fire in Place Guide 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.

Phase 1 -- Binder Burn Out
Start with the Heat Resistant Cultured Opal facing up or down on an inch of Coconut Carbon. Set the kiln to the suggested ramp, and fire for the designated time and temperature.

 

Phase 2 -- Binder Burn Out

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 placed askew to vent.

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

 

Working with Fire in Place Cultured Opals

 

Handling

Cultured Opals are rated 6 - 7 on the Mohs scale, making them slightly more durable than Natural Opals, which are 5 - 6.5, but they still need to be handled with care. Opals are commonly used in necklaces, rings, and earrings. Poorly designed pieces may lead to scratching, cracking, or breaking due to force. For instance, rings experience a lot of wear and tear. Consider whether the gem might come into contact with other objects while in its setting, like accidentally knocking it against tools or counters. It's wise to set the Opal deeply in the design to prevent any contact with the stone's surface.
 

Tumbling in a Tumbler
Use Grafix® Incredible White Mask Liquid Frisket to protect and cushion the stone from damage in a tumbler. Apply the Frisket with a pin tool or small stylus, covering the stone's front and back. Once dry, it should look cloudy-translucent. After tumbling, peel off the Frisket and wipe off tools immediately after use. This technique works with any rotating, vibrating, or magnetic tumbler with stainless steel shot. Check the Frisket after 30 minutes; if it has come off, apply more. Avoid using coarse mediums like Hone and Shine, which can quickly remove the Frisket.

Using a Patina such as Liver of Sulfur

Liver of Sulfur (LOS) doesn't affect the stone itself, but it may alter the appearance of translucent stones. Test the color effect by placing the stone on different backgrounds. Some prefer the look of the darkened metal. Use Grafix® Incredible White Mask Liquid Frisket as a resist to prevent LOS from getting on unwanted areas. Cultured Opals are sensitive to extreme temperature changes, so use warm, not hot, water for the LOS solution. Allow the piece to reach room temperature before rinsing in cold water to avoid thermal shock.
 

Rotary Tool Finishing

Use quality masking tape to protect the Cultured Opal. Burnish the tape down firmly before using a rotary tool for polishing. These Opals are more resilient than natural opals, but abrasive wheels can dull their shine. If scratched, a muslin wheel with diamond paste can remove minor scratches if not too deep.

Caution Needed

Temperature

Do not exceed temperatures above 1650ºF / 900ºC. Exceeding these temperatures may result in cracking or hazing of the Cultured Opals.

 

If You Need to Refire Your Piece

Re-firing these Cultured Opals is your own risk; we’ve had mixed results. A lengthy 2-hour refire cycle at 1650ºF/900ºC in vermiculite proved too much for the 4 x 6 cabochon stones, leading to cracks. Conversely, a larger Cultured Opal fired twice with a short 30-minute session at 1650ºF/900ºF was successful. The stone's size or the firing duration likely influenced the outcome. Another possibility is that cabochons, in general, are poor conductors of heat, necessitating further tests.

Our recommendation is that when you need to repair cracks in your Metal Clay jewelry, fire at lower temperatures and shorter times when possible.

Vermiculite Burial

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.

 

Natural Opals,  Imitation, and Some Synthetic Opals Are Not Suitable for Heat Treatments.

Not all Opals are heat resistant, so please seek that precursor. Natural Opals will not survive. Imitation Opals are made of resin, polymers, glass, and plastic, so they are not suitable, and not all lab-grown Opals qualify.
 

Different Clay Brands

Please consult the Complete Fire in Place Guide for times, temperatures, and firing mediums. If you are using a Metal Clay brand that has not yet been tested, it may work, but there are no guarantees. Keep in mind the tips about shrinkage and firing medium mentioned above. If you dare to try a brand not listed, let us know the results.

Holly's Early Obession with Testing Cultured Opals in Place is Summed Up in the Video

Note: In the video, I state that the Cultured Opals don't like to go in the tumbler. However, they can go in with the Grafix® Incredible White Mask Liquid Frisket applied to the Cultured Opal to protect the stone.

Healing Phoenix Lapidary Fire in Place Cultured Opal (FIP) Firing Chart

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hgage1@ptd.net
Lancaster, PA USA
717-445-5755

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