Firing & Troubleshooting

Module 5: Firing & Troubleshooting

1. Optimizing Kiln Schedules

The firing process significantly impacts glaze results. Fine-tuning temperature curves and cooling rates unlocks advanced effects.

A. Slow vs. Fast Firing Effects

• Slow Firing (Conservative Ramp Rates)

• Best for: Thick or layered glazes, crystalline formations, and reducing thermal shock.

• Effects:

• Glazes mature evenly, minimizing pinholes/blisters.

• Encourages micro-crystal growth (e.g., matte textures).

• Typical Schedule:

• 150°C/hour to 600°C (water smoking phase).

• 100°C/hour to peak temperature.

• Fast Firing (Aggressive Ramp Rates)

• Best for: Commercial production, bright gloss glazes.

• Effects:

• Intensifies colors but risks bubbling or under-melted surfaces.

• Can create "reactive" effects where glazes intermix dynamically.

• Risks: Cracking if clay isn’t fully bisqued.

B. Reduction Cooling for Metallic Finishes

• Process: Introduce reduction (oxygen-starved) during cooling (900°C–700°C).

• How: Briefly starve gas kiln of air or use carbon-rich materials (sawdust).

• Effects:

• Iron glazes develop metallic luster (e.g., temmoku blacks).

• Copper glazes may flash red or purple.

• Key Tip: Over-reduction can muddy colors—limit to 10–15 minutes.

2. Identifying & Fixing Glaze Defects

Understanding defects ensures consistent results.

A. Common Defects & Solutions

1. Crazing (Fine Cracks in Glaze)

• Cause: Glaze shrinks more than clay (thermal expansion mismatch).

• Fix:

• Add silica (5%) to glaze or switch to higher-expansion clay.

• Slow-cool kiln below 800°C.

2. Shivering (Glaze Flakes Off)

• Cause: Glaze shrinks less than clay.

• Fix:

• Reduce silica in glaze or add flux (e.g., feldspar).

• Test on smaller clay batches.

3. Blistering (Bubbled Surface)

• Cause: Trapped gases (organics, carbonates) or over-firing.

• Fix:

• Extend bisque firing to burn out impurities.

• Hold at 800°C for 30 mins to release gases.

B. Adjusting Recipes

• For Better Melt:

• Increase fluxes (e.g., feldspar, whiting) by 5–10%.

• For Color Stability:

• Add 5% kaolin to prevent pigment burnout.

• For Matte Textures:

• Substitute dolomite for calcium (e.g., 10% swap).

Workshop Activity:

1. Firing Test:

• Fire identical glazes with slow vs. fast schedules, compare surfaces.

2. Defect Repair:

• Reglaze and refire a crazed piece with adjusted silica content.

Pro Tip:

• Keep a kiln log to track schedules, defects, and fixes.