Firing faults

Centre for Mineral Products Professional Development Module: Clay Technology – Drying and Firing Unit Firing Faults

Important Note

  • This document serves as a reference for assessments.

  • All rights reserved; reproducing the document requires prior permission from the Internal Clay Technology Association (ICTa).

Overview of Firing Faults

  • The unit covers:

    • Introduction

    • Faults related to the kiln profile

    • Faults developed in the kiln

    • Cracking issues

    • Faults in delivered products

Classes of Firing Faults

  1. Pre-firing Faults: Issues arising before firing that only become apparent post-firing.

  2. Kiln Faults: Problems that occur during the firing process.

  3. Post-firing Faults: Faults revealed significantly after firing, often costly to rectify.

Faults Related to Kiln Profile

Scorching (Localized Overfiring)

  • Cause: Overfiring near burners resulting in:

    • Darker, melted, or excessively shrunk product.

    • Visible burner flame outline on fired packs.

  • Common Issues:

    • Cracking in areas near perforated bricks.

    • Causes include proximity of burners, high draught, excessive burner heat, and sudden changes in kiln throughput.

Underfiring Related Issues

  1. Side to Side Differences:

    • Caused by unequal draught or product bias in firing.

    • Can result from structural issues or burner setup differences.

  2. Underfiring at the Bottom of Setting:

    • Low kiln pressure leading to insufficient heat at pack bottoms.

    • Correct by adjusting fan settings and balancing exhaust.

  3. Differential Shrinkage Related to Body Fuel:

    • High carbon content leading to uneven shrinkage.

    • Solutions include reducing carbon proportion and adjusting firing curves.

Faults Developing in the Kiln

Dunting

  • Phase change in quartz causing invisible crack formation.

  • Identified by dull sound upon knocking together.

  • Best managed by slow cooling through critical temperatures.

Thermal Shock

  • Stress from uneven contraction results in cracking.

  • Most common in large solid pieces, leaving dome-shaped cores.

Shattering

  • Caused during firing due to rapid temperature rise inducing steam pressure.

  • High risks with wet products and extruded goods due to lower porosity.

Settling

  • Occurs in products with steep shrinkage versus temperature curves during the Firing Under Load (FUL) test.

  • Significant risk in tall settings due to weight on lower layers.

Liquefaction

  • Extreme case of settling leading to full melting.

  • Affects specific clay types prone to such transformations.

Overfiring, Fusing and Melting

  • Excessive heat leads to clay melting and changing structural integrity.

  • Resulting in products with high glass proportions and brittleness.

Coring and Bloating

  • Caused by insufficient carbon burn leading to trapped gases and deformation.

  • Risk is heightened with improper firing techniques.

Faults in Delivered Products

Durability Issues

  • Critical for construction materials; durability assessed via freeze-thaw cycles.

  • Salt crystallization can lead to surface spalling under wet conditions.

Efflorescence

  • Salt migration visible on surfaces due to moisture evaporation, resulting in unsightly staining.

  • Minimize risk by properly selecting clay and optimizing firing temperatures.

Vanadium and Manganese Staining

  • Similar to efflorescence; temporary if left alone but permanently fixed with harsh cleaning.

Lime and Quartz Blowing

  1. Lime Blowing:

    • Caused by volume increase of rehydrated calcium which leads to surface blemishes.

  2. Quartz Blowing:

    • Similar volume increase from quartz-rich clays, resulting in defects.

Metallic Element Contamination

  • Natural or incidental contaminants lead to dark spots, reducing product quality.

  • Careful management of raw materials is essential to prevent these faults.

Blistering

  • Results from low-melting materials in high-fired clays, leading to bubbling faults.

  • Unsightly but not structurally detrimental; prevent by eliminating problematic clays.

Flashing

  • Color changes caused by insufficient oxygen during firing, often due to burner setup.