Defect · Bisque firing
Black coring
Also called: black heart
Dark gray or black band visible in the cross-section of a fired piece, surrounded by normal color.
- Appears at
- Bisque firing
- Severity
- Structural
- How common
- uncommon
Causes
Most likely
- ·Insufficient oxygen during bisque firing to burn out organic carbon
- ·Fast bisque firing trapping carbon in the clay interior
Prevention
- ·Slow bisque firing with adequate ventilation
- ·Hold at 850-1100°F to allow carbon burnout
- ·Keep damper or vent open during bisque
Can it be fixed?
Not reliably — this one is prevented, not repaired. A cracked or bloated pot is a test tile now.
Materials involved
Glaze materials whose chemistry sits behind this defect — each links to its reference page.
Related defects
Questions
- Why does black coring happen?
- Most often: Insufficient oxygen during bisque firing to burn out organic carbon. Other causes: Fast bisque firing trapping carbon in the clay interior..
- Can black coring be fixed?
- Not reliably once it has happened; the fix is upstream, in prevention.
- How do I prevent black coring?
- Slow bisque firing with adequate ventilation. Hold at 850-1100°F to allow carbon burnout. Keep damper or vent open during bisque.
Sources. Compiled from Claybench's defect reference — the descriptions, causes and remedies that appear consistently across standard ceramics references and studio practice; no single source. It is the same reference the photo diagnosis tool reasons over. Where the Wikibooks Ceramicware Defects Handbook covers a defect it is linked above.
Not sure which defect you're looking at?
Claybench can read a photo of the pot against this same reference and rank the likely causes for your clay, glaze and firing.