Firing Troubleshooting Guide

Uneven Firing / Hot Spots

This can manifest as some pieces being overcooked while others are undercooked, or areas within a single piece appearing differently fired. Common causes include:

• Kiln Shelf Placement: Shelves too close together or too far apart can impede air circulation.
• Warped Shelves: Uneven surfaces lead to uneven heat distribution.
• Elements: Degraded or improperly functioning heating elements might not generate consistent heat.
• Loading Technique: Overcrowding the kiln or placing pieces too close to elements can cause localized overheating. Ensure adequate space for convection.

Solutions: Rotate shelves periodically, replace warped shelves, check and clean heating elements, and adjust kiln loading for better airflow.

Cracking or Explosions During Firing

Pieces breaking apart, sometimes dramatically, during the firing cycle is usually due to trapped moisture or stresses within the clay body.

• Insufficient Drying: Greenware that hasn't fully dried will release steam rapidly at higher temperatures, causing cracks or explosions.
• Thick/Thin Sections: Significant variations in thickness create uneven expansion and contraction.
• Air Bubbles: Trapped air pockets within the clay can expand and rupture.
• Thermal Shock: Rapid temperature changes, especially during cooling, can stress the ceramic.

Solutions: Ensure pieces are bone dry before bisque firing, aim for consistent wall thickness, wedge clay thoroughly to remove air bubbles, and follow slow cooling schedules for delicate pieces.

Glaze Defects (Crawling, Pinholing, Crazing)

These are issues with the glaze application or its interaction with the clay body during firing.

• Crawling: Glaze pulling away from the surface, exposing bare clay. Often caused by dust or grease on the bisqued surface or an incompatible glaze/clay body.
• Pinholing: Small holes appearing in the glaze. Can be due to trapped air or gases released from the clay or glaze during firing.
• Crazing: Fine cracks in the glaze surface. Usually due to a mismatch in the thermal expansion rates of the glaze and the clay body, causing the glaze to shrink more than the clay upon cooling.

Solutions: Thoroughly clean bisqueware before glazing, ensure proper glaze thickness, experiment with different glaze combinations or apply a ground coat glaze if crazing is persistent.

Underfired / Overfired Results

This is primarily a temperature control issue.

• Underfiring: Pieces are chalky, porous, and glazes are dull or not fully melted.
• Overfiring: Pieces can warp, melt excessively, or glazes may become too fluid and run off the piece.

Solutions: Calibrate your kiln's thermocouple, use witness cones to accurately gauge firing temperature, and ensure your firing schedule (ramp rates and hold times) is appropriate for your clay and glaze materials.