Firing Opals in Metal Clay: Common Mistakes and How to Avoid Them

Whether you're brand new to metal clay or a seasoned artist venturing into fireable opals for the first time, getting that firing right can feel nerve-wracking. Opals are stunning — but they have a reputation for being temperamental. The good news? Most opal failures in metal clay are completely avoidable once you know what to watch for.

First, What Makes an Opal "Fireable"?

Not all opals can survive the heat of a kiln. Natural opals contain water within their silica structure — heat drives that moisture out rapidly, causing cracking, crazing, or a complete loss of play-of-colour.

Fireable opals (also called lab-created or synthetic opals) are engineered without that water content, making them stable at the temperatures required to sinter metal clay. They're available in a dazzling range of colours and fire patterns — and when used correctly, they produce results that rival (and often surpass) natural stones in jewellery making.

The Most Common Mistakes — and How to Fix Them

Mistake 1: Firing at Too High a Temperature

The problem: Even fireable opals have limits. Exceeding the recommended temperature can cause surface crazing, colour loss, or the opal turning milky or opaque.

The fix: Always check the manufacturer's recommended firing temperature for your specific opals. As a general guide, most fireable opals are safe up to around 800°C (1472°F), but this varies by brand and colour. When in doubt, fire lower around 650 C.

Always use a firing guide specific to your opals — not just your metal clay's standard schedule.

Mistake 2: Setting the Opal Before the Clay is Fully Dry

The problem: Moisture trapped beneath or around the opal can cause it to pop out during firing as steam escapes, or leave voids in the clay that weaken the setting.

The fix: Ensure your piece is completely bone dry before firing. Use a dehydrator, a mug warmer, or simply allow adequate drying time at room temperature. If you're setting the opal into a bezel or depression in the clay, dry the piece thoroughly after setting — don't rush this step.

Mistake 3: Thermal Shock — Heating or Cooling Too Quickly

The problem: Rapid temperature changes are one of the biggest causes of opal damage. Placing a cold piece directly into a hot kiln, or quenching a fired piece in water, can cause the opal to crack or shatter.

The fix:

• Ramp up slowly — use a gradual ramp rate in your kiln programme (e.g. 500°C/hour or slower).
• Cool naturally — never quench. Allow the kiln to cool with the piece inside, or open the door slightly once below 200°C to allow gradual cooling.
• Pre-warm if needed — if your kiln doesn't have a programmable ramp, place the piece on top of the kiln to warm gently before firing.

Mistake 4: Using the Wrong Metal Clay Type

The problem: Some metal clays require very high sintering temperatures that exceed what fireable opals can safely tolerate. Standard fine silver clay fired at full temperature, for example, may be too hot for some opals.

The fix: Choose a low-fire metal clay where possible, or use a clay that can be fired at a lower temperature with a longer hold time. Fine silver clays that sinter at 650°C–750°C are generally the safest choice for in-kiln opal setting. Always cross-reference your clay's firing schedule with your opal's safe temperature range.

Mistake 5: Setting the Opal Too Flush or Too Proud

The problem: If the opal sits too deep in the clay, it can be difficult to burnish the bezel over it after firing. Too proud, and it's vulnerable to knocks and may not be held securely.

The fix: Aim for the opal to sit so that approximately one-third of its depth is below the clay surface. This gives you enough clay to fold over and secure the stone post-firing while keeping the play-of-colour visible and protected.

Mistake 6: Skipping a Test Fire

The problem: Every kiln is different. Manufacturer temperatures are a starting point, not a guarantee — your kiln may run hot or cold, and colour batches can vary.

The fix: Always test fire a spare opal (or a less precious piece) before committing your best stones. Keep a firing log noting temperature, ramp rate, hold time, and results. Over time, you'll build a reliable profile for your specific kiln and opal combination.

After Firing: Setting and Finishing

Once your piece has cooled completely:

• Burnish the bezel carefully using a burnishing tool or the back of a spoon — work gradually around the opal, not all on one side at once.
• Polish gently — avoid ultrasonic cleaners and steam cleaners, which can damage opals. A soft cloth or light hand polishing is ideal.
• Inspect the play-of-colour — a well-fired opal should look as vibrant as before firing. Any milkiness or loss of fire may indicate the temperature was slightly too high.

A Note on Opals

If you're working with Opals — our own range of premium fireable lab opals — you'll find a dedicated firing guide included with your purchase (and available to download from your order). These opals have been specifically tested for use in metal clay and come with recommended temperature ranges, ramp rates, and hold times to take the guesswork out of firing.

Opals are available in a growing range of sizes and colours. New sizes, shapes and colours coming very soon!!

Browse the collection in our shop.

Want tutorials, firing tips, and first access to new opal releases? Join GalaxyVIP — our exclusive community for the full Fire Fusion opal range.

Quick Reference: Opal Firing Checklist

• Piece is completely bone dry before firing
• Using a low-fire or opal-compatible metal clay
• Kiln programmed with a slow ramp rate
• Firing temperature within opal's safe range
• Cooling slowly — no quenching
• Test fire completed on a spare opal
• Firing log updated with results

Have questions about firing Opals? Drop them in the comments below — we love hearing from the metal clay community.