Underfired Defect in Ceramics: Causes & Analysis

Underfired Defect in Ceramics: Causes & Analysis

Overview

Underfired wares refer to ceramic products that have not been exposed to enough heat during the firing process. These products lack proper strength, have high water absorption, and fail to meet quality standards, making them not suitable for use.

Causes of Underfiring

1. Low Kiln Temperature

The most common cause of underfiring is that the kiln does not reach the required peak temperature. This can happen due to:

• Fuel supply issues or burner problems in the kiln
• Incorrect temperature settings or thermocouple errors
• Power failures or sudden temperature drops during firing

2. Short Soaking Time

Even if the kiln reaches the correct temperature, if the soaking time (the period the kiln holds at peak temperature) is too short, the heat does not fully penetrate the ceramic body. This results in underfired products, especially for thick or large items.

3. Overloading the Kiln

When too many products are loaded into the kiln, the heat cannot circulate properly inside. Some products in the centre or bottom of the load may not receive sufficient heat, causing them to be underfired even when the kiln temperature is correct.

How to Identify Underfired Products

• The product has a dull glaze surface instead of a shiny finish
• The body is soft and less dense than a properly fired product
• Water absorption test shows values higher than the accepted standard
• The product has low mechanical strength and may break easily

How to Prevent Underfiring

• Regularly check and maintain kiln equipment, burners, and temperature controls
• Use the correct firing curve with proper soaking time for the ceramic body being fired
• Avoid overloading the kiln and ensure products are evenly spaced for proper heat distribution
• Monitor kiln temperature throughout the firing cycle using reliable thermocouples