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  • Writer's pictureHolly Gage

New Cultured Opals can be Fired at 1650ºF in Metal Clay for 2 hours, but only if you do this...

Updated: Jan 4, 2023

Part 2 by Holly and Christopher Gage

Pictured in the Pendant is the Cool Mint Oval Cultured Opal

Holly did a bunch of extensive tests, and luckily, she's stubborn!!! Holly found a way to fire the New Cultured Opals in Fine Silver AND Enriched 950 Sterling and 960 Sterling at 1650ºF for 2 hours!!!! The results are so exciting, and after three prior near misses on the long firing, she discovered the secret. This video sums up the fabulous results of firing heat-resistant Opals in place. It CAN be done, and she's going to tell you how!

The Cultured Opal tests Holly ran are below for anyone who is like her and wants to know "why" and "how" something works. In addition, she'll sum up her findings — found below the test information, where she discusses the best handling and finishing jewelry tips, so those Cultured Opals look their best.

As reported in the first blog, "Exciting News: Fire-in-Place Opals for Silver Metal Clay! Part 1," you can achieve great results when you fire this particular type of Cultured Opal in Fine Silver Metal Clay at 1650ºF for 30 minutes in a kiln or torch. It's pretty straightforward.

Here are the Newest Tests

Working with Vermiculite

During Holly's first Enriched Sterling 960 Metal Clay test, she set the kiln at 1350ºF ramp, target temperature at 1650ºF for 2 hours (total time in the kiln was about 3 hours and 15 minutes). The Opal was placed in the kiln, the table of the faceted stone up, and the base of the jewelry sitting on a bed of vermiculite. The surface of the stone had a haze on it. As we mentioned in earlier tests, Holly was able to polish off the haze with a cotton buffing wheel and some diamond paste.

Because Holly "assumed" operator error of not cleaning the stone well enough, she reran the test. Because the piece was curved this time, she put the stone face down in the vermiculate. We were pleasantly surprised there was no haze on the front... Yahoooooooo! But wait, what??? There was a haze on the culet of the stone peeking out of the hole in the back. Only a bit was peeking out, which wasn't enough to affect the Opal's appearance, beauty, and sparkle.

On a crazy hunch, Holly performed another test. Same time, same clay, same temperature, but this time she buried the piece and Lab Grown Opal completely in vermiculite. She set it in the vermiculite and covered it, but it wasn't buried deeply. The front of the Cultured Opal facing down came out beautifully. The back, however, had a slight haze on the edges but not all over the back of the stone. She was now scratching her head.

The Winner!!!!! A Deep Burial of the Cultured Opal in Vermiculite

This time, Holly used Fine Silver Metal Clay as she ran out of the Enriched Sterling 960 Metal Clay. She buried a jewelry piece with a Cool Mint Oval Cultured Opal Cab, left a 1/2" inch of vermiculite on the bottom, added the jewelry Opal face down, and covered it with 3/4" inch of vermiculite.

Don't you know that did the trick!!! Let us be honest. We don't know why that made a difference. Vermiculite is a fire-resistant insulator. Does the extra insulation somehow protect the Opal from developing the haze on the surface? You bet it does!!! A 2-hour firing at 1650ºF works beautifully with Fine Silver!! I made several pieces, and it worked on all!! Consistency in results is the key.

Since this worked beautifully for a long Fine Silver and Opal firing, Holly felt confident in trying the Enriched Sterling 960 Metal Clay test with the Bermuda Ocean Faceted Cultured Opal. She set the kiln at 1350ºF ramp, target 1650ºF for 2 hours. The faceted stone was 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 she covered it with 3/4" inch vermiculite. The smile on Holly's face was priceless!! To be honest, she didn't know if the stone needed to be face down all the time, but that's what she did and so that's what we're reporting.

Art Clay 950 and a Black Cultured Opal Cab were fired without a hitch in a deep burial of vermiculite. This is a validation test for a 2-hour period and we're both smitten with these Opals and their firing success.

Lots of people are asking, "Can I replace the vermiculite with carbon?" As far as Holly is concerned the two items do different jobs. Vermiculite insulates the stone, whereas Carbon reduces the oxygen atmosphere. Early testing is indicating the stones can survive a carbon firing, but the testing clay was not holding up without cracks. So guess what, more testing will be done on carbon firing and there will be a Part 3. Sorry folks you just have to wait.

Summing It Up

How to Fire Cultured Opals in Silver Metal Clay

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 any Fine Silver 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.

  • Long 2-hour firings need to be buried in vermiculite. 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" inch vermiculite covering it.

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 successfully fired in a kiln. Use 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 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" inch 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, 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.

Additional Facts to Know When Working with these Fire-in-Place Cultured Opals

Setting Cultured Opals in Silver Metal Clay Setting the Opals is straightforward. The Opals have been set in clay with 18% shrinkage with no ill effects. If you have higher shrinkage clay, a looser setting is recommended to allow for shrinkage. This is no different than any other gemstone and Metal Clay and is a good rule of thumb. Clean well with alcohol on both sides before putting it in the kiln. Set the Firable Opal with an azure. A hole behind the stone is recommended to show off its beauty. Seeing the kiln white through any translucent stone will look cloudy or milky in Metal Clay, and this is also a good general practice for all stone settings.

Re-firing these Cultured Opals

Proceed at your own risk. We've had mixed results. Due to using old recycled Silver Metal in one of Holly's tests, the consistency of the clay wasn't great, but she used it anyway - it was a lousy choice, and the clay cracked, so she repaired and put it back in the kiln with another complete long 2-hour cycle at 1650ºF. It was too much for the small stone and it cracked. Holly also fired another larger Cultured Opal 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. However, my best guess is when you refire to repair cracks, fire at shorter times, and temps when possible.

Tumbling in a Tumbler

Don't do it! No steel shot and definitely no abrasive medium in a tumbler. Hand polishing or using a rotary tool is your best bet. We've heard some people "get away" with it, but for our money, we won't even risk it.

Rotary Finishing

Use quality masking tape to protect the Cultured Opal. Holly is a self-proclaimed klutz and will klutz proof everything, so she burnished the masking tape down so it really sticks to the Opal before using a rotary tool to do any polishing. These Cultured Opals are a bit more resilient than natural opals, but abrasive wheels will dull their shine.

Using Liver of Sulfur Holly doesn't dip her entire jewelry piece with stone in a warm diluted bath of Liver of Sulfur (LOS) because she doesn't want the LOS to get behind the stone. Any translucent stone will look dull or dirty if you can see through it to the back where the LOS can be seen. In this case, she might dip the Silver Jewelry just up to the stone and hand paint the rest with a brush. It takes longer, but it works. Putting it on a hotplate doesn't affect the Opal and speeds up the LOS if you are trying to achieve a rich black.

Polishing Behind the Stone It might be necessary to polish or burnish the azure or opening behind the stone with the sides of a pin tool or small ball stylus for similar reasons. One, to remove dirt and LOS, and two, to shine the kiln white, so the Beautiful Opal doesn't look cloudy with the white behind the stone. These techniques are great for the White Iridenscene Cultured Opals, the Bermuda Ocean Cultured Opals, or Cool Mint Cultured Opals.

Facts About this Type of Cultured Opal

  • These Cultured Opals are recognized by the GIA as synthetic opals having essentially the same chemical, physical, and optical properties, but are grown by experts in a laboratory.

  • The Opals in this article have the same chemical composition as natural Opals without the water content.

  • Natural Opals is 5 - 6.5. This Cultured Opal is 6- 7 on the Mohs scale making it more durable.

  • Cultured Opals are sustainable because they are not mined with forced labor and are renewable without being mined out of existence.

  • This Cultured Opal does not decompose or release any substances or gases when heated to very high temperatures.

Where to get the New Heat Resistant Opals

We know trying something new is always a gamble, but with such positive tests, Christopher is now carrying these new heat-resistant Cultured Opals on his website at He is excited to add them to his collection of Cultured Opals, but they are not to be confused with his artisan-cut Cultured Opal cabochons he offers on his Friday 10 to 10 Auctions, and he is sure to mark both clearly when making a purchase. We both want you to have success using them, so we'll be adding new information as testing continues.

Beware, not all cultured or lab-grown opals are heat resistant. Results may differ with different clays; therefore, 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.


Holly's Metal Clay and Jewelry Workshop Schedule

Live, Interactive Online Class for ALL Abilities

These small interactive, online group sessions provide expert learning and excellent connectivity with other artists; now, they are recorded! Click here for details or click on the Class Title.

Jazzy Jewelry (4 Spots)

Thursday, July 7- 28, 2022

1:00 PM - 2:00 PM Eastern

(5:00 Greenwich Mean, 12:00 PM Central, 11:00 AM Mountain, 10:00 AM Pacific)

(4) week group sessions

60 min. Live & Interactive on Go to Meeting

All levels of ability If you like textures, you'll love making your own Metal Clay texture plates. I used polymer plate making for many of my Cultured Opal samples in the article above.


Thursday, September 8 - October 13, 2022

6:00 PM to 7:00 PM, Eastern

(10:00 PM Greenwich, 5:00 PM Central, 4:00 PM Mountain, 3:00 PM Pacific)

(6) week group sessions

60 min. Live & Interactive on Go to Meeting Make interesting prongs from Metal Clay, and learn proper placement and structural consideration while designing with shrinkage in mind.


Tuesday, August 2 - October 25 (skipping August. 23 & 30, October 18)

1:00 PM - 2:00 PM Eastern

(5:00 Greenwich Mean, 12:00 PM Central, 11:00 AM Mountain, 10:00 AM Pacific) (10) week group sessions

90 min. Live & Interactive on Go to Meeting

All levels of ability

A soul-searching journey to creating a unique line of jewelry and artist statement for jewelry artisans using any jewelry material.


Wednesday, September 14 - October 5, 2022

1:00 PM - 2:00 PM Eastern

(5:00 Greenwich Mean, 12:00 PM Central, 11:00 AM Mountain, 10:00 AM Pacific)

(4) week group sessions • 60 min.

Live & Interactive on Go to Meeting

All levels of ability This 2-part firing method for adding a bezel to Metal Clay is clean and forgiving, so no soldering, no investment placeholder, no pull away from the embedded wire, no distortion, and the stone fits every time!

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