What Happens to Clay When It's Fired

Firing is a critical step in the ceramic process that irreversibly transforms clay into a hard, durable material. During firing, the clay pieces are subjected to intense heat, usually in a kiln, which causes chemical and physical changes. At lower temperatures (up to 2,000°F), water and chemically combined water are driven off. 

As the temperature increases, organic material burns away, and chemical changes begin to occur in the clay body resulting in new crystalline structures. Temperatures upwards of 2,300°F cause sintering, which fuses particles together. The final strength, hardness, and color of the ceramic depend on the clay composition and the maximum firing temperature.

What exactly happens on a molecular level when clay meets fire? Let's overview the key clay alterations that occur during firing:

Drying

As clay heats, any remaining moisture and water particles trapped within the clay structure evaporate and escape as vapor pressure builds within the clay body.

This drying effect accounts for the size reduction of the clay work as firing progresses. The temperature at which all moisture leaves the clay is around 150°C or 300°F.

Decomposition

Temperatures from 500-800°F begin decomposing organic matter within the clay-like binders, plasticizers, or added combustibles. These burn off through pyrolysis, leaving an increasingly pure ceramic material.

Gases released from decomposition disperse into the kiln atmosphere contributing to firing effects if done in reduction mode.

Sintering

From about 1100-1400°F, clay particles start softening and fusing together in sintering or initial vitrification. This welds clay particles together into a cohesive mass as pores collapse.

Sintering causes shrinkage as the material densifies, making the clay impervious to water infiltration.

Vitrification

As temperatures continue rising to around 2200°F or higher, full vitrification occurs. The melt phase bonds clay particles into a uniform glass-like matrix with no remaining porosity.

Vitrified clay becomes a strong, durable, engineered ceramic resistant to degradation, fracture, and environmental attack.

Crystallization

In high-fire kilns above 2200°F, mullite crystals can form within the clay. These needle-shaped microstructures further strengthen the ceramic.

However, excess crystallization can increase viscosity and melt fluidity - so temperatures must be controlled.

The transformations clay experiences through heat are remarkable. Mastering firing unleashes the full potential of this incredibly dynamic medium.

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