How does the kiln atmosphere change the colour of fired clay products?

The colour of fired clay products is significantly influenced by the atmosphere within the kiln during the firing process.12

• Oxidizing Atmosphere: Under standard oxidizing conditions, the iron naturally present in clay oxidizes, resulting in colours that typically range from buff to dark red.2
• Reducing Atmosphere: In a reducing atmosphere—where a limited supply of air leads to the presence of carbon monoxide—the iron produces colours ranging from gray to black. Darker tones can also be achieved through the action of smoke.2
• Flashing: By regulating oxidizing conditions, manufacturers can produce browns, purples, and blacks through a process known as flashing. This technique generally alters only the surface colour of the brick, while the interior body retains its natural hue.1

These atmospheric changes work in conjunction with the inherent mineral composition of the clay to determine the final aesthetic of the building material.12