9.2: Bisque Firing- Purpose and Process
- Page ID
- 299317
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The Role of Bisque Firing in the Ceramic Process
- Primary Purpose: Bisque firing transforms greenware into porous yet stable ceramic ware, making it easier to handle and glaze.
- Degassing: It eliminates organic materials and impurities from the clay body, ensuring a cleaner glaze application.
- Strengthening: Bisque-fired pieces are less fragile than greenware, reducing the risk of damage during glazing.
- Foundation for Glazing: The porous surface allows glazes to adhere effectively, making it a critical step in ceramic production.
Preparing Wares for Bisque Firing: Drying, Wadding, and Stacking
- Thorough Drying: Ensure pieces are completely dry to avoid steam explosions during firing. Preheating the kiln can help eliminate residual moisture.
- Wadding: In some cases, wads of clay are placed under pieces to prevent sticking to the kiln shelf.
- Stacking Strategies: Bisque pieces can be stacked tightly since they will not fuse during this firing stage, maximizing kiln efficiency.
- Inspection: Check for cracks, sharp edges, or inconsistencies before loading the kiln to ensure high-quality results.
Optimizing Bisque Firing Schedules for Different Clay Bodies
- Low-Fire Clay: Bisque firing typically reaches Cone 04–06 (1,820–1,940°F) for earthenware.
- Mid-Range and High-Fire Clay: Stoneware and porcelain are usually bisque fired at Cone 08–06 (1,730–1,830°F), depending on their specific requirements.
- Slow Ramps: Gradual temperature increases are vital for thicker or more complex pieces to avoid cracking.
- Hold Periods: A soak at the top temperature ensures complete burnout of organic material.
Assessing Bisque Ware: Identifying Issues Before Glaze Firing
- Cracking or Warping: Check for structural flaws caused by uneven drying or rapid temperature increases.
- Porosity: Ensure the bisque ware is porous enough for effective glaze application.
- Residual Carbon: Look for black spots or odors, indicating incomplete burnout of organic material.
- Surface Quality: Smooth out rough edges or surface imperfections before glazing.
Re-Bisque Firing: When and Why It Might Be Necessary
- Underglazed Pieces: Re-bisque firing can stabilize underglaze decorations before applying a glaze coat.
- Correcting Issues: If the original bisque firing was incomplete, a re-bisque ensures proper burnout of impurities.
- Low-Quality Bisque: Re-bisque firing can strengthen ware that feels overly fragile or brittle.
- Caution: Avoid multiple bisque firings on the same piece, as repeated heat exposure can weaken the clay body.