As a metal clay artist, opening your kiln or inspecting your piece after torch firing is one of the most exciting steps in the creative process. However, sometimes results don’t turn out as expected. Whether you're working with silver clay or base metal clay, knowing how to diagnose and correct firing issues is key.
This comprehensive troubleshooting guide covers common firing problems, their causes, and how to correct them in future projects.
Problem: Silver Clay Turned Black, Distorted, or Melted in Kiln
Likely Cause: The firing temperature exceeded the sintering range, causing partial or full melting.
Solution:
- Double check the manufacturer's firing instructions specific to the clay type.
- Confirm that your kiln is properly calibrated using a digital pyrometer.
- Run test strips to find optimal firing settings. Fire the strips at several different temperatures until you find the "sweet spot".
- If firing multiple pieces, and only some pieces fail, you may have hot spots inside the kiln; rearrange pieces away from these zones.
Problem: Loss of Detail or Surface Pitting During Torch Firing
Likely Cause: Overheating caused by holding the flame too close or too long.
Solution:
- Monitor the glow color closely in dim lighting.
- Keep the flame moving and avoid placing the hottest part (inner blue cone) directly on the piece.
- Use a consistent sweeping motion to distribute heat.
Problem: Piece Broke After Sintering Without Physical Stress
Likely Cause: Incomplete sintering or improper binder burnout.
Solution:
- Make sure your kiln is accurate or if you are torch firing, that you are achieving the correct color glow and target temperature while firing.
- Use the longest and hottest recommended firing schedule unless you're firing with embedded elements like stones or glass.
- For open shelf firing pieces thicker than 3mm (12 cards), add a burnout phase at 600°F (316°C) for 30 minutes prior to the final firing. This will ensure the binder has burned out and the remaining silver particles can sinter.
- If you plan on bending your pieces after firing, an extended hold may also prove beneficial, extending the hold time at the final target temperature to 4 hours (again, so long as there are no stones or glass that cannot restrict temperature and time) will help ensure complete sintering in instances where kiln calibration may be slightly off.
- For base metal clays thicker than 3mm (12 cards), slow the burnout ramp to 300°F (149°C)/hour to 600°F (315°C) and hold for at least 15, up to 30 minutes.
Problem: Bubbling, Cracking, or Puffing During Firing
Likely Cause: Moisture wasn’t fully removed before firing.
Solution:
- It is hard to recommend times to ensure pieces are dry as qualities such as thickness, dimensionality, metal clay used, etc will influence the time needed to dry. Ensure pieces are completely dry using a hot plate, mug warmer, or dehydrator.
- Test dryness by placing the piece on a mirror for 20 seconds—if fog appears, it's still wet.
Problem: Warped or Distorted Shape After Firing
Likely Cause: Improper support during sintering.
Solution:
- Use compatible firing supports like alumina hydrate or vermiculite depending on the clay brand. Example: EZ960 fires at such a high temperature, it will take on texture from coarse vermiculite, and will stick to fiber blankets - so alumina hydrate is recommended.
- Dimensional pieces should be placed on something that can support their form during firing, such as alumina hydrate or vermiculite in a fused silica dish, or a fiber blanket shaped to support the form on a kiln shelf.
Problem: Blistering or Bubbling Surface on Silver Clay or Base Metal Clay
Likely Cause: Overheating led to partial melting.
Solution:
- Double check the manufacturer's firing instructions specific to the clay type.
- Confirm that your kiln is properly calibrated using a digital pyrometer.
- Run test strips to find optimal firing settings. Fire the strips at several different temperatures until you find the "sweet spot".
- If firing multiple pieces, and only some pieces fail, you may have hot spots inside the kiln; rearrange pieces away from these zones.
Problem: Cracked Base Metal Clay
Likely Cause: Uneven shrinkage or internal stress during the burnout phase or sintering. Especially in thin pieces or pieces that have areas of varying thickness. Or, if there is stress on the piece as a result of the form or shape.
Solution:
- Slow the ramp rate during both the burnout and sintering phases. We recommend 300°F (149°C)/hour.
- It never hurts to slow the ramp rate!
Final Tips from Your Metal Clay Supplier
- Maintain a firing log to track kiln settings and outcomes.
- When testing new clay formulas or complex pieces, fire test samples first.
- Choose high quality clay and firing tools for best results—Cool Tools offers a full range for silver clay, bronze clay, copper clay, and more.
Need help selecting the right clay or firing tools? Contact us for expert guidance and premium supplies tailored to your metal clay projects.
Email us at help@cooltools.us
